Aims Warm-temperate deciduous broad-leaved forest plays an important role in maintaining regional ecosystem function and balance. To explore the growth and eco-physiological response to nitrogen (N) addition will deepen our understanding of its dynamic development under the scenario of global N deposition.

Methods A simulated N deposition experiment was established with four N addition plots (100 kg·hm ^-2·a ^-1) and four control plots in a deciduous broad-leaved forest in Dongling Mountain, Beijing. The responses of branch growth, photosynthesis, chlorophyll fluorescence and seed mass of dominant species Quercus wutaishanica and associate species Acer pictum subsp. mono to N addition were investigated.

Important findings Net photosynthetic rate, transpiration rate, and chlorophyll content were enhanced by the N addition. Furthermore, the N addition also enlarged the electron transport pool in photosystem II reaction center and increased the turnover number of the primary quinone acceptor (QA) reduction expressed by fast chlorophyll fluorescence method of JIP test in both species. The N addition also increased the length and biomass of the current year branches of the two tree species and improved the seed quality of Q. wutaishanica to a certain extent. In general, we observed more pronounced responses of photosynthetic nitrogen use efficiency, and branch biomass formation to N addition in Q. wutaishanica compared with A. pictum subsp. mono. Our results suggested that the dominant position of Q. wutaishanica should be further highlighted under increased N deposition scenario in the warm-temperate deciduous broad-leaved forest in Dongling Mountain.

Aims The response of plant leaf functional traits to flooding in salt marshes is not only helpful in exploring the internal correlation between leaf plasticity mechanism and photosynthetic characteristics, but also of vital significance for gaining a better understanding on the stress resistance strategies of plants in salt marsh wetlands. The aim of this study is to investigate the responses of leaf functional traits of Saussurea salsa to flooding with the changes of water-logging durations.
Methods The research site was located in provincial migratory bird nature reserve in Xiao Sugan Lake, Gansu Province, China (39.22°-39.35° N, 94.45°-94.59° E). A sample belt was selected from the edge of the lakeshore to the end of the perennial tidewater line in the low-lying area along the north side of Xiao Sugan River. The duration of water-logging in the salt marsh wetland was measured base on the water level marked by the flood rise and retreat marks in Xiao Sugan Lake over the years. The sample belt was divided into 3 plots according to the water-logging duration: low flooding area (water-logging duration: 30-90 days); medium flooding area (water- logging duration: 90-150 days); and deep flooding area (water-logging duration: 150-210 days). Six subplots (2 m × 2 m) of S. salsa were selected from each of the three plots for a total of 18 subplots. We investigated community characteristics and population traits of S. salsa, soil moisture and soil electrical conductivity (EC). Six plants per subplot were selected for photosynthesis and chlorophyll fluorescence measurements. Foliar samples collected from each of the six S. salsa were taken to the laboratory for measurements of leaf traits (leaf area, thickness, dry mass and chlorophyll content).
Important findings The results showed that S. salsa changed the covariation strategy of S. salsa foliar morphology and photosynthesis with the extension of water-logging duration. In the low flooding area, S. salsa adopted fleshy lobular pattern with small specific leaf area (SLA), high effective quantum yield of photosynthetic system II (PSII) photochemistry in light (Y(II)) and low quantum yield of regulated energy dissipation (Y(NPQ)). However, S. salsa grew in the deep flooding area adopted completely opposite covariation strategy in foliar morphology and photosynthesis compared with those grew in the low flooding area. We observed a significant correlation between SLA and Y(II), photochemical quenching (QP), and Y(NPQ), as well as a significant positive correlation between the quantum yield of non-regulated energy dissipation (Y(NO)) and chlorophyll a content (C_a), chlorophyll b content (C_b) in all of the three plots. Under the influence of the spatio-temporal evolution pattern of still water in the flooded area of Xiao Sugan Lake, S. salsa population achieved the balance of the photosynthetic carbon budget by changing the morphological characteristics of leaves, such as leaf area, leaf thickness and SLA, timely adjusting the photosynthetic characteristics, such as Y(NPQ) and Y(II). Saussurea salsa showed strong tolerance to water and salt heterogeneity, reflecting the leaf plasticity and resistance mechanism of salt marsh wetland plant under extreme environments.

Aims Liana is an important component of tropical forest, and exert a significant impact on community structure and function. Previous studies have found significant differences in hydraulic traits between lianas and trees, as indicated that lianas tended to have large and long vessels to compensate hydraulically to their thin stems, resulting in high hydraulic conductivity but low resistant to drought-induced cavitation. In order to reduce the influence of different genotypes on the comparative results, we aimed to compare the differences in hydraulic and photosynthetic characteristics between the two life forms from two genera Millettia and Gnetum.
Methods We measured branch and leaf hydraulic properties, sapwood density, gas exchange rates in the dry season for nine tree and liana species grown in common garden. We compared the hydraulic and photosynthetic traits between each species using one-way ANOVA. In addition, we analyzed hydraulic efficiency-safety trade-off, and the relationship between dry-season photosynthetic rates and hydraulic traits.
Important findings (1) There was a significant variations in hydraulic traits in genus Millettia, which was related to their light requirements and life forms. Compared with trees, the shade-tolerant liana species had lower hydraulic conductivity and higher resistance to cavitation. (2) Despite angiosperm-like characteristics such as vessels and broad pinnate-veined leaves, the Gnetum tree species had the lowest hydraulic conductivity among the nine species. However, the Gnetum liana species had higher hydraulic conductivity, comparable to light-demanding angiosperm species in this study. (3) There was no significant trade-off between hydraulic conductivity efficiency and hydraulic safety in both branch- and leaf-level across all the species, or within each plant group. (4) Compared to co-generic tree species, liana species’ leaves were more resistant to cavitation than branches, as indicated by higher maximum net photosynthetic rates and stomatal conductance during the dry season. These results support the hypothesis of “growth advantages at dry season” for liana species. This study reveals the high diversity and significance of hydraulic functioning in tropical lianas. Extensive measurements of hydraulic properties are needed to promote understanding of tropical species response to environmental change.

Aims In this study, changes in growth, photosynthesis and root structure in response to drought were tested in transgenic poplar 84K (Populus alba × P. glandulosa) seedlings with different expression levels of aquaporin gene (PtPIP2;8). The function of aquaporin gene PtPIP2;8 and its response to drought stress were analyzed.
Methods We selected PtPIP2;8 silencing line of poplar 84K, PtPIP2;8 overexpressing line of poplar 84K and wildtype (WT) as the experimental materials. The Real-time fluorescence quantitative PCR technique was used to detect the PtPIP2;8 expression in roots, stems and leaves. Root hydraulic conductance was measured by high pressure liquid flow meter. The photosynthetic light-response curve, and gas exchange parameters were measured by a LI-6400 photosynthetic system. Growth indexes were determined, and the root length, root surface area, root volume and total root tips were scanned and then analyzed with the root analysis software.
Important findings The results showed that: (1) The gene PtPIP2;8 was mainly expressed in the root system in WT, while its significant expression occurs not only in roots, but also in stems and leaves in PtPIP2;8 overexpressing poplar lines. The PtPIP2;8 RNAi-silence poplar lines only showed weak expression of PtPIP2;8 in the root, and the expression level were 1/20 and 1/80 of WT and overexpression line, respectively. (2) The root structure analysis showed that overexpression lines had significantly lower total root length, total root surface area, total root volume and total root tips than RNAi-silence line and WT, but higher root hydraulic conductance compared with RNAi-silence line and WT. These results showed that the aquaporin gene PtPIP2;8 participated in plant water transport and improved water transport efficiency. (3) Under normal water conditions, RNAi-silence lines showed lower plant height and leaf area but higher root-shoot ratio compared with overexpression line and WT. After drought stress, RNAi-silence lines only slightly decreased the net photosynthetic rate (P_n) and stomatal conductance (G_s), and maintained a relatively high P_n. Diurnal changes of P_n and G_s in RNAi-silence lines showed a single-peak pattern, in which the decrease of photosynthesis was caused by stomatal limitation. Diurnal changes of P_n in both overexpression lines and WT had a two-peak pattern, indicating the non-stomatal limitation of photosynthesis. Drought stress slightly decreased P_n of RNAi-silence lines, while largely decreased P_n of overexpression line and WT decreased, especially at 13:00 and 15:00, indicating that overexpression line and WT were more sensitive to drought stress compared with the RNAi-silence lines. (4) Under drought stress, RNAi-silence line showed the least decline in relative growth rate and total biomass, and the highest root-shoot ratio among the three poplar lines. The total root surface area, total root volume and total root tips of RNAi-silence line were significantly higher than those of WT. The results suggest that aquaporin gene PtPIP2;8 directly participates in the water transport and helps to improve the water transport efficiency, thus the transformation of aquaporin PtPIP2;8 gene may affect root development and growth of plants. Overexpression lines weaken their drought resistance with decreased root development and increased leaf area, while RNAi-silence line increases its drought resistance with reduced leaf area, increased root growth and root-shoot ratio. The results of this study indicate that aquaporin improves the efficiency of water transport across membranes, while the non-aquaporin water-conducting mechanism has greater tolerance to drought.

Aims The rapid economic growth has led to serious atmospheric compound pollution in China. Ozone and aerosol are the two main air pollutants that could greatly alter the photosynthesis of plants. Ozone often inhibits leaf photosynthesis through phototoxic effect, while aerosol could increase leaf photosynthesis through diffuse fertilization effect. This paper aims to investigate the relative impact of ozone and aerosol on leaf photosynthesis under atmospheric compound pollution.
Methods We planted Populus × canadensis at three sites along the air pollution gradient around Beijing metropolitan area. During the growing season of 2012-2013, ozone concentration (AOT40), aerosol optical depth (AOD), air temperature, photosynthetically active radiation (PAR) and photosynthetic rates of leaves were measured at these sites.
Important findings The results showed that: (1) AOT40 increased with increasing air temperature and AOD. Aerosol loadings reduced the external canopy PAR but increased internal canopy PAR. (2) The photosynthesis of sun leaves positively correlated with AOD and air temperature, and negatively correlated with AOT40, while the photosynthetic of shade leaves was positively correlated with AOD. (3) For sun leaves, ozone was the most important factor affecting their photosynthetic rates. Aerosol’s positive effect on photosynthesis of sun leaves was compensated by the negative effect of ozone. Meanwhile, by scattering solar radiation, aerosol contributed the most to the increased photosynthesis of the shade leaves. Overall, our results indicated that ozone and aerosol induced different impacts on the photosynthesis of sun and shade leaves, suggesting that plants with different canopy structures and ozone sensitivity may respond differently to compound pollution. Our results also indicated that aerosol and ozone pollution need to be controlled simultaneously to protect the productivity and function of ecosystem.

Aims To quantify the responses of stomatal conductance (g_s) to CO₂ concentration (g_s-C_a) under current and future climate conditions, it is necessary to build a generally applicable model suitable for simulating this process at plant leaf levels.

Methods The response curves (A_n-C_a) of photosynthesis of soybean (Glycine max) and wheat (Triticum aestivum) to CO₂ were fitted using data collected from a portable photosynthetic apparatus (LI-6400). Based on the comparison between the traditional Michaelis-Menten model (M-M model) and the CO₂ response model developed by Ye, a new g_s-C_a response model was proposed. Then, the measured g_s-C_a curves of soybean and wheat were fitted with the new model. The model results were compared with those of the traditional model and the corresponding observation data to judge the rationality of the model.

Important findings The A_n-C_a model developed by Ye could fit well the A_n-C_a curve of soybean and wheat, and the coefficient of determination (R²) is as high as 0.999. Although the R ² values of M-M model fitting the A_n-C_a curves of soybean and wheat were also high, the fitting curves deviated from the observation at higher CO₂ concentrations. Meanwhile, M-M model greatly overestimated the maximum photosynthetic rate and could not estimate the saturation CO₂ concentrations. Therefore, it was more feasible to developg_s-C_a model based on the A_n-C_a model of Ye. The new model of g_s-C_a could fit well the g_s-C_a curves of soybean and wheat, and the R ² were 0.995 and 0.994, respectively. Moreover, the maximum stomatal conductance (g_s-max), the minimum stomatal conductance (g_s-min) and the CO₂ concentration corresponding to g_s-min (C_s-min) could also be generated directly. g_s-max of soybean and wheat fitted by the g_s-C_a model was 0.686 and 0.481 mol·m^-2·s^-1, respectively, and there was no significant difference between the fitted values and corresponding observation values (0.666 and 0.471 mol·m^-2·s^-1, respectively). The new model of g_s-C_a could also obtain the minimum g_s (g_s-min) of soybean and wheat (0.271 and 0.297 mol·m^-2·s^-1, respectively), and there was also no significant difference between the fitted values and corresponding observation values (0.279 and 0.293 mol·m^-2·s^-1, respectively). In addition, the new model of g_s-C_a generated the C_s-min values of 741.45 and 1 112.43 μmol·mol ^-1for soybean and wheat, respectively, and also showed no significant difference from the observed value (732.78 and 1 200.34 μmol·mol^-1, respectively). Consequently, the g_s-C_a model developed in this paper can be used as an effective mathematical tool to quantitatively study the effect of stomatal conductance on CO₂ concentration.

Aims Photodamage to a shade-tolerant species is common due to a sudden increase in growth light intensity. However, it is unknown about the underlying mechanism of the sensitivity of the shade-tolerant species to high light. The objective of the present study was to elucidate the mechanisms involved in the inability of the typically shade-tolerant species Panax notoginseng to survive under natural full-light condition.

Methods The relative chlorophyll content (SPAD), photosynthetic parameters and chlorophyll fluorescence parameters were continuously examined in P. notoginseng when transferred from shade (10% of full sunlight) to full sunlight for three days.

Important findings The net photosynthetic rate (P_n) of P. notoginseng exposed to full sunlight condition showed a “double-peaked” diurnal curve, and P_n decreased with the prolonged days of full light treatment. The SPAD value, water utilization efficiency and light use efficiency were significantly decreased under full sunlight condition. Furthermore, the maximum fluorescence signal of the P700 reaction center, electron transfer rate of photosystem II (PSII), the maximum quantum efficiency of PSII under dark adaptation and maximum quantum efficiency of PSII under light adaptation were significantly lower in full sunlight than those under shading condition, while the fraction of energy passively dissipated in the forms of heat and fluorescence, energy dissipation due to acceptor side limitation of PSI, and cyclic electron flow were significantly higher under the full light condition. Moreover, the sudden increase in growth light intensity caused a significant change in the fluorescence induction kinetic curve and significantly increased the fluorescence yield on the donor and acceptor side of PSII. The oxygen-evolving complex activity in the donor side of PSII was impaired under full sunlight. Furthermore, the electron transfer in the acceptor side of PSII was inhibited and the over-reduction of the acceptor side of PSI was caused by PSI photoinhibition. The study reveals that the full sunlight might induce the irreversible damage to PSII and the moderate photoinhibition to PSI in shade-tolerant species, and it may be an important underlying mechanisms why the shade-tolerant speciesP. notoginseng cannot survive under full sunlight.

Aims This study is to examine the relative changes and regulation in photosynthetic energy partitioning components of a typical desert shrub Artemisia ordosica in Northwest China.
Methods The minimum fluorescence yield (F_o), maximal fluorescence yield (F_m), actual fluorescence yield (F_s), maximal fluorescence yield in the light-adapted state (F_m′), net photosynthetic rate (P_n), dark respiration (R_d), transpiration rate (E) and stomatal conductance (g_s) of A. ordosica were investigated from April to October, 2018 using a multi-channel fluorometer and a portable photosynthesis analyzer. The specific leaf area (SLA), nitrogen per leaf area (N_area), chlorophyll content (C_Chl) and the ratio of chlorophyll a to chlorophyll b content (Chl a/b) were also measured. We analyzed the fraction of energy used for CO₂ assimilation (Φ_A), the fraction of energy used for photorespiration (Φ_PR), the fraction of regulate heat dissipation (Φ_NPQ) and the fraction of non-regulate heat dissipation in energy allocation (Φ_NO) during the photosynthetic process in A. ordosica and their responses to fluctuating environment.
Important findings The photochemical reaction component (Φ_A and Φ_PR) and the heat dissipation component (Φ_NPQ and Φ_NO) showed a negative competition relationship, and exhibited positive synergistic relationship within components. E was positively correlated with Φ_A and Φ_PR, and negatively correlated with Φ_NPQ and Φ_NO. Under low soil water content (SWC) and high saturated water vapor pressure (VPD) conditions, Φ_A, Φ_PR and SLA were significantly decreased, while Φ_NPQ and Φ_NO were significantly increased. The results indicate that A. ordosica avoids excessive loss of water under the condition of the long drought or high evapotranspiration via reducing SLA. Synergistically, excessive light energy is transferred from photorespiration to thermal dissipation pathway. The relative changes and long-term regulation of photosynthetic energy partitioning in A. ordosica under fluctuating environment, fully reflect the acclimation of plants to adversity via its synergistic morphologic and physiological plasticity.

Aims Early spring rainfall event is an important factor affecting the growth of desert ephemeral plants. This study aims to understand the effects of the first rainfall timing and amount on the photosynthesis and growth of desert ephemeral plants in early spring.

Methods Two dominant ephemeral species, Erodium oxyrhinchum and Centaurea pulchella were selected in the Gurbantünggüt Desert, and treatments of three times of the first rainfall (10, 20 and 30 days after the snow melted completely) and three amounts of rainfall (5, 10 and 15 mm) were conducted. The plant morphology, biomass and chlorophyll fluorescence of the two plants were determined.

Important findings When the rainfall occurred on the 10th day after the snow melted completely, the leaf area and total biomass of the E. oxyrhinchum increased significantly with the rainfall amount, while the maximum photochemical efficiency decreased. If the rainfall was applied on the 20th and 30th day after the snow melted completely, the leaf area of the E. oxyrhinchum and C. pulchellaincreased significantly with the rainfall amount. On the other hand, if the treatments were applied with the addition of 5 mm rainfall, the maximum photochemical efficiency of the E. oxyrhinchum and C. pulchella decreased firstly and then increased significantly with the delay of the first rainfall timing. If the treatments were applied with the addition of 10 mm rainfall, the shoot to root ratio (S/R), root length and total biomass of the E. oxyrhinchum increased firstly and then decreased significantly with the delay of the first rainfall timing, but the root length of the C. pulchella increased significantly. If the treatments were applied with the addition of 15 mm rainfall, the leaf area, S/R and maximum photochemical efficiency of the E. oxyrhinchum increased with the delay of the first rainfall timing. For the C. pulchella, total biomass increased significantly, maximum photochemical efficiency and S/R decreased significantly with the delay of the first rainfall timing. The root length of the E. oxyrhinchum and C. pulchella were significantly positively correlated with soil moisture content of 0-5 cm and 5-10 cm, respectively. In general, the effects of the first rainfall amount on plant growth are much larger than effects of the first rainfall timing in early spring. However, different ephemeral species showed different trends with the changes in rainfall pattern. The variation in early spring rainfall caused by climate change can affect the morphological characteristics of desert ephemeral plants in desert, and then may affect the stability of desert ecosystem in early spring.

Aims Differences in number of ramets or biomass ratio might have remarkable effects on the adaptability of plant clonal systems to resource heterogeneity. Connected-ramets of bamboo, with lignified rhizomes, usually grow in heterogeneous light environment, but little is known about the response of photosynthetic physiological characteristics in the ramet leaves to heterogeneous light environment and the effects of ramet ratio. Methods The clonal system with four-connected ramets of Indocalamus decorus was grown in the two different shading rates (50% ± 5% and 75% ± 5%) and in the three levels of ramet ratios (the ratios of shaded to unshaded ramets are 1:3, 2:2 and 3:1, respectively). The light response characteristics, gas exchange parameters and photosynthetic pigment content of shaded and unshaded ramets under heterogeneous light conditions were measured at 30, 90 and 150 days after shading treatment, respectively. Important findings The ramet ratio, and its interaction with shading and treatment time had significant effects on photosynthetic physiology of I. decorus. With the increase in the proportion of shaded ramets, leaves apparent quantum efficiency (AQE), light saturation point, maximum net photosynthetic rate (P_{n max}), net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r), and water use efficiency increased, but light compensation point and dark respiration rate decreased, indicating higher photosynthetic efficiency and light utilization of shaded ramets, whereas the reverse was true for connected unshaded ramets. Furthermore, leaves chlorophyll a and chlorophyll b contents of the shaded ramets firstly increased and then decreased, while both the carotenoid content of shaded ramet and photosynthetic pigment (chlorophyll a, chlorophyll b and carotenoid) contents of the unshaded ramets all decreased continuously. Besides, under the same ramet ratio, AQE, P_{n max}, P_n, G_s, C_i and photosynthetic pigments content of leaves in shaded ramets increased with the increment of shading rate, while leaves P_{n max}, C_i and carotenoid contents of the unshaded ramets decreased. Our results indicated that photosynthetic efficiency and light utilization of shaded ramets significantly increased in the heterogeneous light environment, whereas the reverse was true for unshaded ramets. These exhibited obvious division of labor in the clonal system, and ramet ratio of 2:2 and 3:1 showed the better adaptability to heterogeneous light environment than that of 1:3. Therefore, those findings highlighted I. decorus could largely enhance the fitness of the clonal system to adapt to the heterogeneous light environment by modifying the leaf photosynthetic physiology and photosynthetic pigments content, which improved the light utilization and photosynthetic efficiency of the shaded ramets.

Aims With fast urbanization in China, ground-level ozone (O₃) has become the major atmospheric pollutant in summer. It was documented that O₃ enters leaves through stomata, inhibits photosynthesis, and alters the carbon and water cycles in terrestrial ecosystems. However, few studies have investigated the key parameters of photosynthetic and stomatal conductance models in response to elevated O₃ concentration.
Methods In this study, plants of four common tree species (Camellia sinensis, Acer negundo, Koelreuteria paniculata and Quercus mongolica) were exposed to two O₃ treatments (CF, charcoal-filtered air; E-O₃, ambient air + 60 nmol·mol^-1 O₃) in open top chambers. The effects of elevated O₃ concentration on key parameters of photosynthetic and stomatal conductance models were explored using data from leaf gas exchange measurements.
Important findings Our results indicated that elevated O₃ concentration significantly decreased the light-saturated photosynthesis and mesophyll conductance in the four species. However, species showed distinct responses of the maximum rate of Rubisco carboxylation and the maximum rate of electron transport to elevated O₃ concentration. The response of stomatal conductance to O₃ was also different among species. We found that elevated O₃ concentration significantly increased the slope parameters (g₁) of Q. mongolica and A. negundo; however, the intercept parameter of stomata model in C. sinensis was decreased in A. negundo. The intrinsic water- use efficiency of the four species was negatively correlated with g₁ across O₃ treatments. All in all, elevated O₃ concentration significantly affected key parameters of the photosynthetic and stomatal conductance models. This study could provide the foundation and support for improving the accuracy of terrestrial ecosystem models under elevated O₃ concentration.

Owing to the high carbon capture and storage capacity, salt marshes are considered an effective blue carbon sink for mitigating global warming. In addition, salt marshes are likely to increase their carbon sink capacity in the future in response to climate warming and sea level rise. Therefore, the blue carbon sink function of salt marshes has received increasing attention from the international research community. This study reviewed the five aspects comprising the key processes of blue carbon formation, photosynthetic carbon allocation, burial fluxes and sources of sedimentary organic carbon, stability of soil carbon pools and the associated microbial mechanisms, and the simulation and assessment of blue carbon sequestration potentials in salt marshes. On this basis, concerning the main knowledge gaps, this paper proposes further research on the effect of vegetation distribution pattern along the land-to-sea hydrologic gradient on photosynthetic carbon fixation and allocation, the response of soil organic carbon deposition and burial to global change, the stability of soil carbon pools and its lateral exchange, blue carbon simulation and assessment of blue carbon sink potential in the context of climate change and sea level rise, and technologies and approaches of blue carbon sequestration in salt marshes. Prioritizing these research topics may elucidate the formation processes and mechanisms of blue carbon, predict the changing trend of blue carbon sequestration potential under global changes, and offer new insights into achieving the goal of “carbon peak and carbon neutrality”.

Aims Based on the background of increasing drought frequency and intensity globally, investigating the photosynthetic properties and growth adaptation of annual grasses differed in photosynthetic pathway (C₃ and C₄) in response to drought-rehydration conditions is useful to predict the changes of grassland ecosystem composition, structure and function in the context of global climate change. Methods The study was conducted as a pot experiment with four annual C₃ and three C₄ grasses widely grown in the Songnen Grassland. Three water levels were set up including control, moderate drought and severe drought. Plant photosynthetic (leaf gas exchange) and growth (biomass, leaf mass per area etc.) parameters were measured at the end of the drought and during the rehydration periods. Important findings The net photosynthetic rate (A) and stomatal conductance (G_s) of all studied species showed a decreasing trend and water use efficiency showed an increasing trend under the drought conditions. There were functional group differences in the effects of drought on photosynthetic properties of the studied plants. The effects of the severe drought on net photosynthetic rate of the C₄ plants are more pronounced than that of the C₃ plants, with the C₄ plants gradually losing their photosynthetic advantage as drought severity increases. The A of the C₄ plants recovered more slowly than that of the C₃ plants after the rehydration because of carbon assimilation in the C₃ plants was mainly restricted by stomata limitation while it in the C₄ plants was mainly restricted by metabolic limitation. The biomass of all studied species decreased, but the root to shoot ratio and leaf mass per area increased, under the drought conditions. The effects of drought on all growth indicators were greater in the C₃ plants than in the C₄ plants. After the rehydration, the biomass of C₃ plants showed a decreasing trend with increasing drought intensity, while the biomass of C₄ plants was not significantly different from the control treatment.

Aims Myriophyllum aquaticum has high tolerance to high concentration of ammonium (NH₄⁺) and thus becomes the preferred species for swine wastewater treatment. It is of great significance to explore the effects of exogenous NH₄⁺ on the photosynthetic pigment composition and the stoichiometric characteristics of nitrogen (N) and phosphorus (P) of the M. aquaticum for improving the purifying efficiency of artificial wetlands system of M. aquaticum.

Methods Six NH₄⁺ concentration levels (0, 0.1, 1, 5, 15, 30 mmol·L^-1) were set up in this study. After 21 days of indoor cultivation, the contents of chlorophyll, N and P of M. aquaticum were measured to analyze their characteristics of changes.

Important findings The results showed that the relative stem height and biomass increased initially and then decreased, which were well fitted by a curve equation. Moreover, the peak of them appeared at 16.22 and 12.58 mmol·L^-1 exogenous NH₄⁺, respectively, via fitting non-linear Gaussian equation. With the increase of exogenous NH₄⁺ concentrations, the chlorophyll content in the leaves decreased significantly, but increased in the stems. In addition, there was a wide variation of chlorophyll a than chlorophyll b. The chlorophyll a/b did not change significantly among different NH₄⁺ treatments, except the value in the stems of 5 mmol·L^-1 NH₄⁺ treatment, which was significantly decreased. With the increase of exogenous NH₄⁺ concentrations, the N contents in the leaves and stems were significantly increased by 85%-235% and 127%-373%, respectively, and the P content in the leaves was increased by 49%-51% in comparison to the control (CK). When the concentrations of exogenous NH₄⁺ was no more than 15 mmol·L^-1, the N content and N:P ratio of the leaves and stems increased rapidly, so were relative stem height and relative biomass. Correlation analysis showed that the contents of N and P, and N:P ratio were negatively correlated with total chlorophyll content in the leaves, but positively correlated with total chlorophyll content in the stems. In conclusion, M. aquaticum grew well with larger biomass and higher absorption of N and P, when the concentrations of exogenous NH₄⁺ were in the range of 12-16 mmol·L^-1. Therefore, the constructed wetlands system planted with M. aquaticum can effectively remove N and P from polluted wastewater and achieve the purpose of efficient water purification.

Aims Diffuse radiation is one of the important factors affecting forest carbon uptake. However, the response of gross primary productivity (GPP) of planted forest ecosystems to diffuse radiation in China is still unclear. We explored the effects of diffuse radiation on GPP at 6 plantation ecosystems in eastern China during the growing season.

Methods Based on carbon flux data and meteorological data during the growing season of 2019-2020, we estimated the diffuse radiation and identified the direct and diffuse conditions. The important light response parameters of plantation ecosystems were obtained by the rectangular hyperbolic curve. Meanwhile, we quantified the variations of GPP responding to diffuse and direct radiation. The contribution of light and environmental factors to the diurnal variation of GPP was analyzed by partial correlation method.

Important findings Diffuse radiation can effectively promote canopy photosynthesis. The values of light response parameter canopy quantum efficiency (α) and photosynthesis at photosynthetically active radiation of 1 000 µmol·m^-2·s^-1 (P₁₀₀₀)increased by 47%-150% and 2%-65%, respectively. Compared with direct sky conditions, GPP increased by 0.86%-1.70% in response to 1 μmol·m^-2·s^-1 enhancement of photosynthetically active radiation (PAR) under diffuse sky conditions, which was affected by forest type and vegetation phenology. In diffuse skies, the increment of the variation of GPP under increasing per unit PAR (0.86%-1.00%) at Pinus sylvestrisvar. mongolica and P. tabuliformissites with lower normalized difference vegetation index (NDVI) value was significantly lower than other plantation sites (1.04%-1.70%), and there was a significant positive correlation between NDVI and P₁₀₀₀. Under low light level, PAR controlled the averaged gross primary productivity (GPP_a), but diffuse fraction (DF) mainly regulated GPP_ain middle and high light level. The photosynthesis corresponding to diffuse radiation under moderate light was roughly equal to photosynthesis corresponding to total radiation under high light. Under middle light conditions, the GPP_a value in medium and high DF (≥0.5) at Cunninghamia lanceolata, Populusspp., Quercus variabilis and Larix gmeliniiwas about 27%-50% higher than under low DF condition (<0.5), and the GPP_a value at high DF was about 2% more than under low DF conditions at Pinus sylvestrisvar. mongolica and P. tabuliformis sites. Under diffuse radiation conditions, diffuse photosynthetically active radiation (PAR_dif) explained 16%-45% of the variation of GPP. Air temperature (T_a) and vapor pressure deficit (VPD) explained 10%-19% of the variation of GPP at Cunninghamia lanceolata, Quercus variabilis and Larix gmelinii sites. Under diffuse radiation conditions, the P₁₀₀₀ will be the highest when T_a is 15-25 °C and VPD is 0-1 kPa.

Aims Ecosystem apparent quantum yield (α) and maximum photosynthetic rate (P_max) are important parameters reflecting the photosynthetic characteristics of ecosystem, and also important physiological parameters in ecosystem model simulation and remote sensing inversion. The objectives of this study were to: (1) analyze the characteristics and spatial-temporal variations of the light response parameters of ecosystems in arid and semi-arid areas; and (2) reveal the key factors affecting the photosynthetic parameters and its underlying mechanisms in arid and semi-arid areas, so as to provide a scientific basis for the study of ecosystem photosynthesis and response to climate change on a regional scale.

Methods The observational fluxes and synchronous meteorological data of 9 stations in arid and semi-arid area were integrated from ChinaFLUX. The non-rectangular hyperbolic equations were used to fit the light response parameters and which influencing factors were identified by linear regression, multiple stepwise regression and path analysis.

Important findings There were obvious spatial and temporal variations in ecosystem photosynthetic parameters in arid and semi-arid areas. The photosynthetic parameters increased gradually from desert, desert grassland, typical grassland to meadow grassland. Precipitation was the dominant environmental factor of the spatial variations of photosynthetic parameters, and it also affected the spatial variation of leaf area index, both of them jointly determine the spatial variation of photosynthetic parameters. α and P_max have an obvious increasing trend with the increase of precipitation, and there was a significant negative correlation between temperature and α, but the effect of radiation on the spatial variation of photosynthetic parameters was not significant. In the growing season, P_max and α increased first and then decreased, but the monthly variability and peak time of different vegetation types were different, the photosynthetic parameters of meadow grassland had the greatest monthly variability. The monthly dynamics of α was mainly controlled by temperature and radiation, while P_max was regulated by temperature and radiation in desert and desert grassland, and by soil water content in typical grassland and meadow grassland. Ecosystem α were 0.000 47-0.002 12 mg·μmol^-1, and P_max were 0.11-0.78 mg·m^-2·s^-1 in arid and semi-arid area, which were at a low level compared with other grassland ecosystems. High temperature and low soil water supply were likely the main factors restricting the photosynthetic parameters in arid and semi-arid areas.

Aims Nitrogen (N) is essential for the photosynthesis of plants. Shade plants are mostly exposed to dynamic light under natural growth conditions. However, little is known about the role of N levels in the photosynthetic regulation of shade plants under dynamic light. The objective of present study was to elucidate the mechanisms involved in the effect of N on dynamic photosynthesis in the typically shade-tolerant species Panax notoginseng.

Methods The gas exchange parameters and the activity and amount of Calvin cycle enzyme/proteins were examined under dynamic and steady light conditions in P. notoginseng grown under low N (LN, 112.5 kg·hm^-2) and high N (HN, 450.0 kg·hm^-2), respectively.

Important findings N content per unit of leaf area (N_area) was negatively correlated with the induction state at 60s of light (IS₆₀) and positively correlated with the time required to reach 90% of photosynthetic steady state (t_P90) and 100% of photosynthetic steady state (t_P-steady), suggesting that N_area does not regulate the photosynthetic induction during dynamic light by the total activity of ribulose-1,5-bisphosphate carboxylase (Rubisco). Moreover, a short low light interval only slightly decreased the activity of Rubisco, but significantly reduced the activity of fructose-1,6-bisphosphatase (FBPase) and sedoheptulose-1,7-bisphosphatase (SBPase); when the sunfleck of high intensity appeared suddenly, Rubisco was still highly active, but SBPase and FBPase need to be reactivated to match Rubisco activity, so that photosynthetic induction after low light intervals is largely limited by reactivation of SBPase and FBPase. Furthermore, the content of Rubisco was higher than that of FBPase and SBPase. During the high light period of dynamic light, HN leaves need to activate a higher proportion of FBPase and SBPase and a longer period of time to resume photosynthesis. The results of present study reveal that under dynamic light condition, LN could alleviate the decline of photosynthetic induction rate, while HN exacerbated the decline of photosynthetic induction rate. The limitation in enzymes related to the ribulose-1,5-disphosphate (RuBP) may be one of the reasons why HN exacerbates the decline of photosynthetic induction rate under dynamic light condition.

Aims The plant mortality induced by drought has significant impact on forest ecosystems around the world. It thus has brought intensive research attention on the plant adaptive strategy to drought in the field of physiological ecology. This study aims to investigate the differences in adaptation strategies to drought between two dominant species in arid desert areas, i.e., Haloxylon ammodendron and Alhagi sparsifolia.

Methods Three types of functional traits (i.e., leaf, photosynthetic and hydraulic traits) of A. sparsifolia and H. ammodendron were measured in response to a natural drought gradient (mild, moderate and severe) in Ebinur Lake Nature Reserve in Xinjiang, China. The changes of functional traits with drought gradient, and the differences of functional traits and adaptation strategies to drought between the two species were analyzed.

Important findings The functional traits of A. sparsifolia and H. ammodendron changed differently across drought gradient. All the functional traits were significantly different between the two species except for leaf dry matter content. However, the differences in functional traits between the two species showed a decrease due to the synergetic influence of drought stress and species convergence. Pearson correlation among the traits for the two species indicated that only 10 pairs of functional traits are significantly correlated for A. sparsifolia, while 15 pairs were significantly correlated for H. ammodendron. Principal component analysis (PCA) showed that two typical trait combinations related to drought resistance can be obtained from 11 functional traits of H. ammodendron, namely drought resistance-carbon acquisition group and drought resistance group. However, the trait combinations in coping with drought were not identified for A. sparsifolia. The results suggested that A. sparsifolia,was a conservative species, having greater drought tolerance than H. ammodendron. The traits in A. sparsifolia was less associated than that in H. ammodendron. In contrast, H. ammodendron used the trade-offs and compensatory relationships among functional traits to reduce drought stress. This study provided insights into the relationship between functional traits and drought adaptation strategies of different plant life forms, which advanced the fundamental theories of plant physiological ecology, and provided implications and references for the protection and diversity maintenance of desert ecosystem.

Aims The aims of this study were to investigate the effects of different stubble height on non-structural carbohydrate metabolism of Cyperus esculentus, to further clarify the relationship between stubble height and aboveground biomass of C. esculentus, and to seek the best clipping height.
Methods The growth physiology parameters, non-structural carbohydrates and aboveground biomass of C. esculentus leaves at six stubble heights (10, 20, 30, 40, 50 cm and uncut) were determined.
Important findings The results showed that clipping can stimulate the photosynthesis of C. esculentus. The peak of regeneration and growth was reached from 1 to 14 days after clipping. The contents of soluble sugars in leaves of 30 cm stubbles (7th, 21st and 28th days) were higher than those in leaves of other treatments, which were 9.22%, 10.83% and 9.07%, respectively, and the starch contents (14th and 21st days) were 4.88% and 4.11%, respectively. The sucrose contents in leaves of 40 cm stubbles (21st and 28th days) were higher than those in leaves of other treatments, which were 7.88% and 11.38%, respectively. The fructose contents (14th and 21st days) were also higher than those in leaves of other treatments, which were 5.29% and 6.40%, respectively. The activities of sucrose phosphate synthase and sucrose synthase in 30 cm stubbles and 40 cm stubbles were higher than those in other treatments in different periods after clipping. Stubble 10 cm inhibited the increase of sucrose content and related enzyme activities of C. esculentus. The total amount of forage at clipping and harvest, the hay mass of 30 cm stubbles was up to 10 605.11 kg·hm^-2, 19.93% higher than that of uncut. The hay mass of 40 cm stubbles was up to 8 976.93 kg·hm^-2, 1.52% higher than that of uncut. In addition, through redundancy analysis, soluble sugar content, the activities of sucrose synthase and sucrose phosphate synthase were important factors affecting the forage yield and regeneration rate. In the long period of cutting (day 7-28), 30-40 cm stubbles were more conducive to regeneration and growth, accumulation and synthesis of non-structural carbohydrate, as well as related enzyme activities and forage yield. Therefore, the suitable stubble height range was 30-40 cm.

Aims The purpose of this study was to investigate the regulatory effect of exogenous H₂S on the photosynthetic carbon metabolism of plants under saline-alkali stress.

Methods The Avena nude was selected and a potted soil culture experiment was conducted to study the effects of spraying 50 µmol·L⁻¹ H₂S donor sodium hydrosulfide (NaHS) solution on photosynthetic and fluorescence parameters, the contents of monosaccharides, oligosaccharides and starch, the activities of key enzymes in Calvin cycle and sugar metabolism in leaves, and yield components under 3.00 g·kg^-1 saline-alkali stress.

Important findings (1) Spraying NaHS significantly decreased chlorophyll content, intercellular CO₂ concentration, photosystem II (PSII) initial fluorescence, maximum fluorescence, regulatory energy dissipation, photochemical quenching, non-photochemical quenching, and significantly increased Hill reaction activity, net photosynthetic rate, transpiration rate, stomatal conductance, apparent CO₂ utility efficiency, PSII maximum photochemical efficiency, and the activities of ribulose-1,5-bisphophate carboxylase (Rubisco), Rubisco activase, glyceraldehyde-3-phosphate dehydrogenase and transketolase in A. nude leaves under salt-alkali stress. These indicated that exogenous H₂S could alleviate saline-alkali stress-induced photoinhibition and photosynthetic rate decrease by reducing light energy absorption and the portion of light energy absorbed by the PSII antenna pigment which was used for photochemical electron transfer, improving primary photochemical efficiency and promoting water photolysis of PSII, regulating key enzyme activities in the Calvin cycle, and enhancing CO₂ utility efficiency. (2) Under saline-alkali stress, the activities of α-amylase and sucrose phosphate synthase were significantly increased and the contents of starch and reducing sugar and neutral invertase activities were significantly decreased in A. nude leaves on the 7th day after spraying NaHS. On the 14th day, the contents of total soluble sugar and reducing sugar, and the activities of sucrose synthase and sucrose phosphate synthase decreased significantly, and the activity of neutral invertase increased significantly. The contents of glucose, fructose and galactose on the 7th and 14th days increased to varying degrees, and the content of raffinose decreased significantly, while the activities of total amylase, β-amylase, starch phosphorylase, adenosine diphosphate glucose pyrophosphorylase and acid invertase and the contents of fucose, trehalose and sucrose did not change significantly. These changes suggested that exogenous H₂S was involved in the regulation of starch and sucrose metabolism and the conversion between polysaccharides and oligosaccharides in A. nude under saline-alkali stress. (3) Spraying NaHS had no significant effect on plant height, spike numbers per plant, boll numbers per spike, thousand-grain weight and biological yield per plant of A. nude under salt-alkali stress, while grain numbers per spike and grain yield per plant were increased significantly. In summary, exogenous H₂S participates in the regulation of photosynthetic carbon metabolism in A. nude under salt-alkali stress, and it can enhance the tolerance of A. nude to saline-alkali stress.

Aims Heavy metal chromium (Cr) contamination has toxic effects on crops and will disrupt soil microbial community homeostasis in agricultural soils. Nevertheless, the mechanisms underlying the responses of different crops and their rhizosphere soil microbial communities to Cr stress were different. By using time series data, this study analyzed the effects of Cr stress on ‘Jingu 21’ growth, functional pathways of differentially expressed genes (DEGs) in cereals and soil microbial community structure and function. Our objective was to elucidate the response mechanism of millet (Setaria italica) and soil microbial community, and provide a theoretical basis for the growth of cereals under Cr stress and the ecological restoration of the contaminated soil.

Methods We collected millet seedlings and soil samples from the pot experiments with planted millet before (CK) and 6-hour and 6-day after Cr stress (Cr_6h, Cr_6d), and determined the physiological traits of seedlings and soil physicochemical properties. The gene expression and the functional pathways enriched in the seedlings were investigated by transcriptome analysis; the dynamics of microbial community composition, structure, diversity as well as function in time series and their correlations with soil physicochemical properties were studied by high-throughput sequencing analysis.

Important findings 1) Transcriptome analysis showed that Cr stress induced up-regulation of gene expression (54% for up-regulated DEGs); GO enrichment analysis showed that DEGs significantly down-regulated the expression of photosynthesis-related genes in CK & Cr_6h, Cr_6h & Cr_6d samples, and also significantly up-regulated the expression of defense and damage regulation-related genes, and down-regulated the expression of cell wall and cell membrane and cell division-related genes in Cr_6h & Cr_6d samples. 2) High-throughput sequencing revealed a significant change in the composition of soil bacterial and fungal communities at the phylum and genus level during the time series of Cr stress. The α diversity of bacterial communities showed a phase change from stress to stability (Shannon-Wiener diversity index for CK, Cr_6h, Cr_6d were 6.09, 5.93, 6.05, respectively. Simpson diversity index for CK, Cr_6h, Cr_6d were 0.006 8, 0.007 8, 0.006 8, respectively; Chao diversity index for CK, Cr_6h, Cr_6d were 2 818.49, 2 630.73, 2 769.38, respectively), while the α diversity of fungal community decreased significantly (Shannon-Wiener diversity index for CK, Cr_6h, Cr_6d were 4.17, 3.81, 3.23, respectively). The distribution of β diversity of bacterial community in the Cr stress time series was significantly different from that of fungal community. 3) Correlation analysis between soil physicochemical properties and microbial communities showed that soil physicochemical factors were significantly correlated with a variety of fungal flora, but weakly correlated with bacterial flora. Cr stress significantly inhibited the photosynthesis of millet seedlings by reducing chlorophyll content, photosystem activity and affecting structural components such as thylakoid, and inhibited the proliferation and differentiation of leaf cells by down-regulating the expression of cell wall and microtubule-related components. At the same time, Cr stress also activated the plant defense system to overcome toxicity. Meanwhile, soil bacterial and fungal communities adapted to Cr stress through changing community composition and diversity, and their response levels and strategies differed in the stress time series.

Aims Understanding and quantifying the variability of drought tolerance and the potential driving mechanism in urban trees are critical to the prediction and management of urban ecosystem stability under global climate change. The objectives of this study were: 1) to identify the branch hydraulic traits of trees at urban sites with different percentages of impervious pavements in Beijing, and 2) to investigate if the drought tolerance of urban trees is adapted to urbanization.

Methods The investigated species in the study was Fraxinus velutina. This species is widely applied to street planting in Beijing. We selected six sites along the north-south axis of the city with different percentages of impervious pavements as represented by normalized difference built-up index (NDBI). The NDBI and monthly surface temperature at each site were obtained by remote sensing. The bench dehydration technique was used to assess site-specific branch vulnerability to drought-induced xylem cavitation. Net photosynthesis rate, stomatal conductance, and maximal efficiency of PSII photochemistry (F_v/F_m) were also measured with a photosynthesis instrument.

Important findings The percentage of impervious pavements was positively correlated with the water potential corresponding to 50% loss of hydraulic conductivity (Ψ₅₀), while Ψ₅₀ was found to correlate with pre-dawn xylem water potential (Ψ_pd) and vapor pressure deficit. A significant trade-off relation was found between specific conductivity and Ψ₅₀, but not between leaf specific conductivityand Ψ₅₀. The embolism repair ability was significantly positively correlated with Ψ_pd. The net photosynthetic rate decreased with the increase in percentage of impervious pavement, whereas the F_v/F_m did not show significant difference among sites. The results suggest that the percentage of impervious pavements is one of the key urban environmental indicators affecting the drought tolerance of urban trees. The hydraulic architecture of F. velutinashowed adaptability to the urban environment in the city. The study not only provides important research data for evaluation of the health, resilience, and stability of the urban ecosystems under the scenarios of rapid urbanization and global climate change, but also a theoretical support for decision-makers to formulate practical and feasible management strategies for street planting in Beijing.

Aims In recent years, the aquatic ecosystem of Caohai Lake, in Guizhou Province in southwest China, has shown a trend of phase shift from macrophyte- to phytoplankton-dominated states, which indicates the weakening ecological functioning of the lake. The chlorophyll a (Chl a) concentration is an important indicator of phytoplankton biomass. It is of great significance to clarify the variation of Chl a concentration in comprehensively understanding the ecological status and developing feasible restoration measures for the Lake.

Methods Surface water samples were collected from the Lake in summers and autumns of 2020-2021 to analyze the variation of Chl a concentration. The generalized additive model (GAM) was applied to explore the relationship between each environmental factor and Chl a concentration.

Important findings The results showed that, the Chl a concentration in surface water of Caohai Lake increased during phase shift, with an average up to (17.96 ± 10.62) μg·L^-1(during the monitoring period in 2021), which was 2.5 times greater than that before the phase shift (during monitoring period in 2020). The comprehensive trophic state of the Lake changed from mesotrophic state to eutrophic state, and the water quality presented an obvious trend of deterioration. Results from the single-factor GAM analysis showed that permanganate index (COD_Mn), total nitrogen (TN) concentration and water temperature (WT) had significant effects on Chl a concentration, which among all investigated environmental factors, account for 26.70%-33.30% of the concentration variation of Chl a, with WT, COD_Mn, TN concentration exhibiting the largest, intermediate, and smallest impact respectively. Based on the present study, it is concluded that COD_Mnand TN concentration were important factors driving the variation of Chl a concentration in Caohai Lake, and WT was an inducing factor. The massive extinction of submerged macrophyte and the significant increase of Chl a concentration indicated that the Lake is in phase shift from macrophyte- to phytoplankton-dominated state. Feasible measures should be developed timely to promote the restoration of submerged macrophyte, and further studies on phase shift mechanisms and restoration measures for the Lake should be carried out.

Recent advances in solar-induced chlorophyll fluorescence (SIF), which is a complement to optical remote sensing based on greenness observation, have made it possible to monitor the photosynthesis of plants in terrestrial ecosystems using state-of-the-art technologies. With the rapid development of tower-based, unmanned aerial vehicle (UAV), airborne and space-borne SIF observation technology and improving understanding of SIF mechanism, SIF is providing essential data support and mechanism understanding for the estimation of biological traits and gross primary production of terrestrial ecosystem, early detection of abiotic stress, extraction of photosynthetic phenology and monitoring of transpiration. In this review, we first introduce the fundamental theory, the observation systems and technologies and the retrieval method of SIF. Then, we review the applications of SIF in terrestrial ecosystem monitoring. Finally, we propose a roadmap of activities to facilitate future directions and discuss critical emerging applications of SIF in terrestrial ecosystem monitoring that can benefit from cross-disciplinary expertise.

Aims Based on the solar-induced chlorophyll fluorescence (SIF), this study was conducted to reveal the benefit of vegetation productivity in the revegetation region with a significant increase in land surface greenness under the large-scale implementation of ecological programs in the Loess Plateau.

Methods By interpreting satellite-observed terrestrial greenness changes and land use/cover dynamics, we first identified the spatial distribution of revegetation and existing vegetation in the Loess Plateau in the last 20 years. Then, using SIF and meteorological data, the gross primary productivity (GPP) of the revegetation and existing vegetation was calculated according to the revised mechanistic light response (rMLR) model. Finally, we adopted the comparative analysis approach to compare the differences in GPP of the revegetation based on the SIF observations.

Important findings Our results indicated that the ecological programs have made a widespread increase in land surface greenness in the Loess Plateau. In the period 2001 to 2020, the area of revegetated forest was 35 000 km², accounting for 7.42% of the total area, whereas revegetated grassland area was 110 000 km², accounting for 25.25% of the total area. Overall, the photosynthetic capacity and vegetation productivity of the revegetated forests were lower than that of existing forests in the Loess Plateau, while revegetated grassland was higher. GPP of the revegetated forest was equivalent to 83.86% of the existing forest, and GPP of the revegetated grassland was equivalent to 121.10% of that of the existing grassland. At the same leaf area index (LAI) level, GPP of revegetation and existing vegetation showed differences that GPP gap increased as LAI became higher. Revegetation transformed from bare land showed the lowest vegetation productivity, whereas forest growth and grassland restoration from cropland were the optimal land use/cover transition pattern for the revegetated forest and revegetated grassland, respectively. LAI increasing rate and restoration time also affected the productivity of revegetation, revegetated areas with higher LAI increasing rate showed more extensive productivity benefits. Vegetation productivity of revegetated forest increased with standage, while revegetated grassland with shorter restoration periods showed higher productivity. Overall, although ecological programs have widely increased vegetation cover and biomass in the Loess Plateau, however, there exists a certain gap in GPP between the revegetation and existing vegetation areas (especially in forests), thereby affecting the ecological benefits of the ecological programs.

Aims This study investigated the spatial and temporal variation of spring and autumn photosynthetic phenology of vegetation in the Dongting Lake basin and revealed its response to climate change, and provides a useful reference for the establishment of model of subtropical vegetation phenology and the evaluation of carbon budget.

Methods Using solar-induced chlorophyll fluorescence (SIF) data, we extracted spring photosynthetic phenology (the start date of photosynthesis) and autumn photosynthetic phenology (the end date of photosynthesis) of vegetation in Dongting Lake basin, and evaluated temporal and spatial patterns of vegetation spring and autumn photosynthetic phenology and its response to climate change.

Important findings (1) From 2000 to 2018, the vegetation spring photosynthetic phenology was significantly advanced at the rate of 0.75 d·a^-1, the autumn photosynthetic phenology was delayed at the rate of 0.17 d·a^-1, and the vegetation growing season length was significantly prolonged at the rate of 0.90 d·a^-1. (2) The preseason maximum air temperature and minimum air temperature were the main factors affecting the advance of spring photosynthetic phenology. The autumn photosynthetic phenology of vegetation was positively correlated with preseason precipitation, minimum air temperature and radiation intensity, but negatively correlated with preseason maximum air temperature. (3) In addition, we found that the spring photosynthetic phenology of vegetation in the study area was more sensitive to climate change, especially the increase of preseason minimum air temperature led to the significant advance of spring photosynthetic phenology of evergreen needleleaf forest, evergreen broadleaf forest, bush and grassland. In conclusion, the advance of vegetation spring photosynthetic phenology in Dongting Lake basin played a dominant role in prolonging the growth season, indicating that spring photosynthetic phenology plays a more important role in enhancing the carbon sink function than the autumn photosynthetic phenology in the context of global warming. The vegetation spring photosynthetic phenology was more sensitive to climate change and the air temperature was the main factor controlling the vegetation spring photosynthetic phenology, which provides a scientific basis for the simulation and prediction of evergreen vegetation phenology.

Aims As anisohydric plants lacking vascular tissue, mosses are thought to be influenced on their water movement and photosynthesis mainly by the water status and light of the ambient environment. Do the mosses develop an intrinsic circadian regulation mechanism similar to that of vascular plants as these environmental conditions change with day and night alteration? Understanding its rhythmic response characteristics is of great significance for the conservation and utilization of the mosses.

Methods The Brachythecium thraustum, Hypnum plumaeforme and Mnium lycopodioides were trained to 12 h/12 h light-dark (LD) cycles before transfer to 24 h/0 h constant light. We measured the three mosses’ net photosynthetic rate (P_n) and transpiration rate (T_r) on the constant light and last day of the light-dark cycles. Subsequently, non-structural carbon (NSC) content was measured for Brachythecium thraustum. Finally, we sequenced the Brachythecium thraustum transcriptomes in a time series in constant light conditions.

Important findings We found the significant rhythmic changes in the P_n and T_r of the three mosses when excluding environmental influences such as light and vapor pressure difference (VPD), and rhythmic regulation could explain 23.4% and 30.2% of the diurnal changes in P_n and T_r. Moreover, the rhythmic effect of T_r, which is rarely observed in vascular plants, appeared in all the mosses tested, and the rhythmic responses of P_n and T_r were different among species. The species with the strongest rhythm in this study was Brachythecium thraustum, and it was further analyzed for the rhythmic responses of time-series transcriptome and its NSC content under full light. The results showed that 35.1% of starch and carbohydrate biosynthesis-related genes showed rhythmic expression, which was closely related to NSC content change. In addition, the rhythmic regulation of T_r was associated with the transcript changes of abscisic acid signaling and stomatal regulation. Furthermore, the photosynthesis rhythms were associated with transcript changes of photosynthesis-related protein, like photosynthetic electron transport and carbon sequestration proteins. Although the CCA1/LHY genes, the core feedback loop of the circadian clock, have been lost in the mosses, the core circadian clock function concentrates in the PRRs family. The rhythmic response to carbon sequestration and water consumption is highly conserved.

Aims Global climate change has aggravated the effects of drought which is one of the major factors restricting the sustainable development of agriculture and forestry. It is important to study the growth performance and the changes of physiological mechanism of dioecious plants during drought and rewatering process, which could help to understand the difference of adaptability and stress tolerance to the unfavorable environment in dioecious plants. And this paper also provides a theoretical basis for the selection of tree species for afforestation in the context of global climate change.

Methods Male and female cuttings of Populus deltoides were planted in the pots in a greenhouse, and were treated by drought stress and rewatering. The growth, leaf water parameters, and photosynthetic parameters were measured to analyze the physiological adaptability and stress tolerance of males and females under drought-rewatering conditions.

Important findings Drought stress showed negative effects on plant growth by reducing the growth of plant height and basal diameter, with decreased relative water content, water potential, net photosynthetic rate, stomatal conductance, transpiration rate, photosynthetic electron yield, photochemical quenching coefficient and electron transfer rate of leaves of males and females. There were no significant sexual differences in all parameters between males and females under sufficient water supply. Under drought stress, the growth of male plants was better than that of females, with higher growth rate of plant height and more root biomass accumulation in males. Drought resulted in the decrease of the maximum photochemical efficiency and the potential activity of photosystem II (PSII), and the increase of the intercellular CO₂ concentration of females. PSII of male plants was less damaged under drought conditions, and the photosynthetic reaction center still maintained a high light-harvesting efficiency. Meanwhile, alternating oxidase (AOX) activity was significantly increased in roots and leaves of male seedlings, which could alleviate the effect of photoinhibition. All indexes recovered after 30 days of rewatering. However, the growth rate of plant height and ground diameter, and net photosynthetic rate of males and females under drought stress were significantly lower than those of the control group without drought stress. The results showed that the growth of male and female seedlings of P. deltoides was inhibited by drought stress, and the females were more likely affected by water deficit. Water stress induced a series of adaptive physiological effects in males, including decreased leaf relative water content, decreased photosynthetic and chlorophyll fluorescence parameters, and increased activity of alternating oxidase. Therefore, males had a more effective protective mechanism than females, which was also conducive to the recovery of various functions after rewatering.

Aims Poplars (Populusspp.) are main timber and greening trees in the Northern Plains of China, and remains unclear whether nitrogen form defines the effects of nitrogen on poplar growth and physiology. Hence, this study aimed at investigating the effects of different nitrogen forms (ammonium nitrogen, nitrate nitrogen, ammonia-nitrogen mixed state and amido nitrogen) on photosynthesis and growth of poplar seedlings.

Methods Two poplar clones (Populus deltoides cv. ‘55/56’ × P. deltoides cv. ‘Imperial’, clone ‘546’ and P. euramericana cv. ‘74/76’, clone ‘107’) were used as test materials. Poplar ‘546’ has lower plant height and larger single leaf area and is more sensitive to low temperature than poplar ‘107’. An experiment was conducted to determine gas exchange, chlorophyll fluorescence, plant height and branch diameter, specific leaf area, biomass and root-shoot ratio in seedlings treated with different forms of nitrogen.

Important findings The results showed that leaf photosynthesis characteristics of clones ‘546’ and ‘107’ were similar, but their growth parameters differed. Nitrogen application significantly increased leaf net photosynthetic rate, stomatal conductance, electron transport rate of photosystem II (PSII), PSII actual photochemical quantum yield, photosynthetic electron transport rate, branch diameter, plant height, specific leaf area and plant biomass, but reduced root-shoot ratio. Except for leaf net photosynthetic rate, there were no significant differences in other photosynthesis characteristics and growth parameters among different nitrogen forms. The response significantly differed between the two clones. Relative to other nitrogen treatments, specifically, leaf net photosynthetic rate was significantly increased by ammonia-nitrogen mixed state treatment in clone ‘107’, while most of the indexes of ‘546’ poplar were not significantly different among different nitrogen treatments. These results suggested that the application of ammonia-nitrogen mixed state and amido nitrogen can promote the photosynthesis, growth and biomass accumulation of poplar seedlings. However, clone ‘107’ has a higher nitrogen use efficiency under amide nitrogen treatment, whereas the best suitable nitrogen application form for clone ‘546’ was nitrate nitrogen and ammonium nitrogen.

Aims Leaf temperature is one of the important microenvironmental parameters for energy exchange and physiological processes of plants. Leaf thermal traits can regulate leaf temperature so as to relieve heat damage to some extent. However, systematic studies on leaf thermal traits are rare.
Methods In the present study, 43 dominant canopy species of four typical vegetation types with varying temperature and precipitation from tropical to temperate zones in Yunnan Province were selected: savanna vegetation, tropical rain forest, subtropical evergreen broadleaf forest, and temperate mixed forest. We selected 23 thermal traits that might have influence on leaf temperature, including leaf morphological, optical, material property, anatomical and physiological traits.
Important findings The results showed that plants in savanna vegetation mainly relied on transpiration for cooling. Savanna species have thin leaves and short life span. They are mainly “quick investment-return” species. Tropical rain forest plants developed large leaves, with low transpiration rates, which have no advantage of leaf cooling. Thick leaves and high water content can alleviate high temperature to some extent. They adopted “slow investment-return” strategy. Subtropical evergreen broadleaf forest was rarely exposed to extreme temperatures. The species had thick leaves, long leaf life span, and adopted “slow investment-return” strategy. They did not show obvious thermal adaptation traits. Temperate mixed forest had small and thick leaves, growing in clusters which provides benefits for thermal insulation. Photosynthetic rate of canopy evergreen plants in this forest was low, adopting “slow investment-return” strategy, while photosynthetic rate of deciduous plants was high, presenting “quick investment-return” characteristics. This study systematically investigated the variation of leaf thermal traits and plant adaptation strategies along temperature and precipitation gradients, providing a theoretical basis for further understanding of plant adaptation to the environment.

Aims The objectives of this study were to reveal the changing trends and regional differences of vegetation fractional coverage on the Loess Plateau 20 years after the implementation of the “Grain for Green (GFG)” policy, and to quantify the contribution of climate and human activities to the change of vegetation fractional coverage and its spatial distribution in the region.

Methods The spatial and temporal variation of photosynthetic vegetation (PV) fractional coverage on the Loess Plateau from 2001 to 2020 and its drivers and contributions were analyzed based on MODIS-PV and meteorological data, and using the methods of the Mann-Kendall method, the Sen estimator, and multivariate residual trend analysis.

Important findings Regional vegetation fractional coverage increased from 40% in 2001 to 60% in 2020. Vegetation fractional coverage of the Loess Plateau showed a significant increasing trend over 20 years, with an increasing rate of 0.8%·a^-1. The proportion of the area with an increasing trend of vegetation fractional coverage for the entire region was 90%, and the proportion of the area with a significant increase was 71%. The contribution to the increase of vegetation fractional coverage in the region was mainly in the loess hilly region (2/5), followed by the sandy hilly region (1/4) and the rocky mountain region (1/5). Within the different geomorphology divisions, vegetation fractional coverage in the loess hilly region increased rapidly in Yulin and Yanʼan in Shaanxi. Vegetation fractional coverage in Ordos, Nei Mongol, changed the fastest in the sandy hilly region. Human activities and climate change contributed 76% and 24%, respectively, to the increase of vegetation fractional coverage on the Loess Plateau during the study period. The areas where human activities contributed positively to vegetation fractional coverage were mainly located in the loess hilly and sandy hilly regions in the northern part of Yanʼan in Shaanxi, the southern part of Taiyuan in Shanxi, the southern part of Tongxin in Ningxia, and the hills and plateaus of Pingliang and Qingyang in Gansu where the ecological projects funded by the Chinese government have been well implemented.

Aims Kobresia pygmaea is a perennial cushion herb from the Cyperaceae family with a height of 1-3 cm and small linear leaves about 1 mm wide. It is mainly distributed on the low slopes of the high mountains ranging from 3 000 m to 5 960 m on the Qingzang Plateau. Its habitat is harsh, and extreme climate conditions such as low temperature, strong wind, and high sunlight intensity are the main abiotic stresses during plants growing season. The objectives of this study were to analyze the photochemical and non-photochemical energy distribution of the photosystem II (PSII) reaction center in K. pygmaea leaves, and their quenching protection mechanism after nocturnal low-temperature (NLT) treatment.

Methods Kobresia pygmaea meadow turfs (30 cm × 15 cm) were collected from the Alpine Grassland Ecosystem Research Station of the Resource of Three Rivers. The turf blocks were separated into two groups, one group was kept in a culture room with a temperature of 24/18 °C (day/night) as a control treatment, and another was kept in an artificial climate chamber with 0 °C in the evening as an NLT treatment. During the daytime, the NLT group was moved back to the culture room and irradiated together with the control group. On day 0, day 1, day 3, and day 5 after NLT treatment, the chlorophyll fluorescence of K. pygmaea leaves including, the light-response curve, PSII photochemical efficiency at 400 and 1 500 μmol·m⁻²·s⁻¹ steady-state light intensities, and dark relaxation were monitored using CF imager. Then, based on the “Lake Model”, the variation of the PSII actual photochemical efficiency (Φ_PSII), the quantum yield of non-regulated energy dissipation (Φ_NO) and regulated energy dissipation (Φ_NPQ) were explored. Additionally, the fast and slow relaxation components of PSII non-photochemical quenching were determined.

Important findings Nocturnal low temperature had limited effects on the rapid light-response curves of PSII relative electron transfer rate through PSII (rETR), the fraction of open PSII centers (q_L), and PSII non-photochemical quenching coefficient (q_NP). The comparison of chlorophyll fluorescence between 400 and 1 500 μmol·m⁻²·s⁻¹ steady-state light intensities confirmed that NLT treatment did not affect the activity of the PSII reaction center and the process of non-photochemical quenching of K. pygmaea. On the third day after NLT treatment, under high light intensity, the ratios of Φ_PSII:Φ_NO:Φ_NPQ were 36:19:45 and 38:19:43 in the control and NLT groups, respectively; while under lower light intensity, they were 66:22:12 and 66:23:11, respectively. The fast relaxation component (NPQ_f) was the main component in non-photochemical quenching (NPQ); the proportion of the slow relaxation component in non-photochemical quenching was 11% and 10% on day 1 and day 3 in control group, and 13% and 12% in NLT group, respectively. Our results indicated that the probability of photoinhibition of the PSII reaction center in K. pygmaeawas increased after NLT treatment; low light intensity and NLT led to the prolongation of photosynthetic induction time. Overall, the NLT treatment did not increase the tendency of excess excitation energy to be difficult to regulate and dissipate in K. pygmaea leaves, since PSII photochemical energy dissipation and protective regulation mechanism still effectively distributed the absorbed light energy.

Aims The growth and survival of plants are being threatened by more frequent extreme high temperature events under global climate change. Therefore, for species conservation and adaptability prediction, it is essential to accurately evaluate the physiological heat tolerance across plant species. Magnoliaceae, as a typical basal angiosperm lineage, is one of the representative lineages in subtropical evergreen forests in China, with high ecological and evolutionary significance. However, the photosynthetic heat tolerance of Magnoliaceae plants is still unclear.

Methods In this study, we used chlorophyll fluorescence method to exam the photosynthetic heat tolerance of 23 species of Magnolicaeae plants in a common garden. In order to analyze the relationships of photosynthetic heat tolerance with leaf morphology and temperature niche, for each species, we measured leaf morphological traits and calculated the temperature niche across their geographic distribution areas.

Important findings The temperature that causes 50% decrease of the maximum photochemical quantum efficiency of photosystem II (T50) ranged from 46.1 to 56.7 °C, with significantly higher T50 of evergreen species than that of deciduous species. For leaf morphology, T50 was positively correlated with the leaf area but was not significantly correlated with the leaf thickness. For temperature niches, T50 was positively correlated with mean annual air temperature and minimum air temperature of the coldest month but was not significantly correlated with maximum air temperature of the warmest month. A weak phylogenetic signal was found in T50, indicating that T50 was influenced by leaf morphology and environment conditions, rather than phylogeny. Our results suggested that photosynthetic heat tolerance of Magnoliaceae species were generally strong, but its adaptation may not be driven by high temperature environment. Future extreme high temperature events may severely threaten deciduous Magnoliaceae plants living in the warmer areas.

Aims The morphology and photosynthetic functions of compound leaf are one of the most intuitive manifestations of plant response to habitats. The changes of photosynthetic capacity and number of leaflets of compound leaf directly reflect the adaptability of plants to habitats. There are some morphological differences between male and female Fraxinus mandschurica, and their compound leaf may have different adaptabilities in various habitats.
Methods This experiment took compound leaves male and female mature F. mandschurica growing in the drought saline-alkali habitat and suitable habitat as the research materials. The gender differences between female and male plants and the differences of morphology and photosynthetic function between compound leaf with different numbers of leaflets under two habitats were compared and analyzed.
Important findings The results showed that (1) compared with the suitable habitat, the morphology and photosynthetic capacity of compound leaf with the same number leaflets of female and male F. mandschurica under the drought saline-alkali habitat did not show gender difference, but the increment of percentages of compound leaf with 9 leaflets of female plants was 29.13% higher than that of the male plants, the decrement of percentages of compound leaf with 11 leaflets was 33.74% higher than that of the male plants, and the decreament of photosynthetic rate of compound leaf with 9 leaflets of female plants was 10.98% higher than that of the male plants. (2) Under the suitable habitat, the proportion of compound leaves of male plants was mainly concentrated within 9 and 11 leaflets, while the proportion of compound leaf of female plants was mainly concentrated within 11 leaflets. Meanwhile, the specific leaf area and leaf dry mass of compound leaves with 11 leaflets of female and male plants were greater than that of compound leaf with 7 and 9 leaflets. Under the drought saline-alkali habitat, the proportion of compound leaf of female and male plants was mainly concentrated in 9 leaflets. The specific leaf area of compound leaf with 9 leaflets of male and female plants was less than that of 11 leaflet compound leaf, whereas the leaf dry mass showed the opposite trend. There was also no significant difference in the net photosynthetic rate of compound leaf with 9 and 11 leaflets between female and male plants in the two habitats. (3) Among the indexes of compound leaf morphology and stomatal gas exchange capacity, the percentages of compound leaf with different number of leaflets, leaf dry mass, specific leaf area and net photosynthetic rate of compound leaf have high plasticity. Therefore, under the suitable habitat, there is no gender difference in the photosynthetic function between male and female plants, but there are gender differences in the morphology of compound leaf of male and female F. mandschurica, which are shown as follows: the female plants mainly developed the compound leaf with 11 leaflets, and the male plants predominantly developed the compound leaf with 9 leaflets and 11 leaflets. However, under the drought saline-alkali habitat, the morphology and photosynthetic function of compound leaf of male and female F. mandschurica did not show gender differences, and both female and male plants mainly developed the compound leaf with 9 leaflets. This study provides a theoretical basis for the gender difference in the growth and development of compound leaf in different habitats, and also provides data support for the ecological adaptability of compound leaf of dioecious trees.

Aims Water is the main limiting factor for revegetation and ecorestrotion in arid sandy areas. Soil available water content directly affects the xylem water transport capacity of plant. However, the differences in xylem hydraulic traits and leaf gas exchange of different shrub species with different ages under varied soil water content are still unclear.

Methods The 10-year-old and 30-year-old Caragana korshinskii and C. liouana in the sand-binding area were employed to explore the differences and relationships between their hydraulic traits and photosynthetic traits during the dry and rainy season.

Important findings The results showed that the shrub age has no significant effect on xylem hydraulic conductivity, degree of natural embolism, leaf water potential and relative water content of C. korshinskii and C. liouana, while soil water content significantly effects on these functional traits. Both shrub age and soil water content have significant effects on leaf photosynthesis of two shrub species, but the effect of shrub age on them is little significant during the rainy season. In addition, there was a significant positive linear relationship between soil water content with leaf water content and xylem-stem specific hydraulic conductivity. Xylem hydraulic conductivity also had a significant positive correlation with leaf water content and stomatal conductance, whereas net photosynthetic rate was also positive related to xylem hydraulic conductivity and leaf water content. Results indicated that soil water content directly affected leaf water status and the capacity of photosynthetic carbon assimilation by affecting xylem hydraulic conductivity and embolization degree. In conclusion, the change of soil water content significantly affected the xylem hydraulic conductivity and leaf photosynthetic carbon assimilation capacity of C. caraganaand C. Liouana.

Aims Understanding the differences in leaf functional traits and their correlations in ferns at different vertical heights can provide a scientific basis for revealing the resource utilization strategies of large fern fronds.

Methods Individuals of three fern species in a mixed broadleaved-Korean pine (Pinus koraiensis) forest, i.e., Dryopteris crassirhizoma, Athyrium brevifrons and Matteuccia struthiopteris were divided into upper, middle, and lower layers according to the vertical height of leaves. We measured specific leaf area, leaf dry matter content, net photosynthetic rate, instantaneous water use efficiency, leaf nitrogen content and leaf phosphorus content, as well as the light environment and soil factors of each individual plant to reveal the vertical variation patterns and correlations of leaf functional traits.

Important findings (1) Leaf dry matter content of the three fern species increased with the vertical height of the fronds, but specific leaf area showed no variation. The net photosynthetic rate of A. brevifrons and M. struthiopteris showed an increasing trend with the increases of vertical height of fronds, the instantaneous water use efficiency of M. struthiopteris increased and then decreased with the vertical height of the fronds, and leaf nitrogen content gradually decreased; leaf phosphorus content of D. crassirhizoma showed an increase and then decreased trend. (2) There were positive correlations between leaf nitrogen content and specific leaf area, and also between instantaneous water use efficiency and net photosynthetic rate. There were negative correlations between leaf nitrogen content and leaf dry matter content, and between leaf dry matter content and specific leaf area. The correlations among those leaf functional traits did not differ significantly among different vertical heights. (3) Soil available phosphorus content and soil pH were the main factors affecting the variation of leaf functional traits at different vertical heights, with soil available phosphorus content having the highest explanatory degree to the variation of leaf functional traits. Our results indicated that there were vertical differences in leaf functional traits of large ferns in the mixed broadleaved-Korean pine forest, but the rate of change among individual characters was basically constant, the effects of light environment and soil factors on the variation of leaf functional traits differed among vertical heights. This study provided reference for further research on the mechanism of leaf functional traits variation in different vertical heights of ferns in understory.

Aims The purpose of this study is to investigate the relationship between net photosynthetic rate (P_n) and nitrogen allocation in Gossypium spp. leaves, to analyze the limiting factors that affect the improvement of photosynthetic nitrogen use efficiency (PNUE), and to reveal the photosynthetic physiological regulation mechanism of nitrogen use efficiency.

Methods In this study, the gas exchange parameters, nitrogen content and proportion of different components, cell wall content per unit area (CW_area) of 16 genotypes of Gossypium spp., including G. hirsutum, G. barbadense, G. arboreum and G. herbaceum, were measured at flowering and bolling stages. The relationship between P_n,CW_area, PNUE and nitrogen content of different components was analyzed.

Important findings The results showed that when nitrogen content per unit area (N_area) was less than 3.75 g·m^-2, the P_n was very significant positively correlated with N_area, When N_area exceeds this value, the P_n was not correlated with N_area, while P_n had a significant positive correlation with Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) nitrogen content (N_r) and bioenergetics nitrogen content (N_b), a marginal positive correlation with light harvesting system nitrogen content (N_L) in photosynthetic apparatus, indicating the nitrogen content of photosynthetic apparatus (N_p) plays more important role than N_area in the photosynthetic capacity. In addition, P_n was also positively correlated with stomatal conductance (G_s). The low P_n of genotype ‘CHANG ZI 1 HAO’ was mainly related to low N_p, while low P_n of genotype ‘MOC-620’ was mainly related to low G_s. The proportion of nitrogen components in photosynthetic apparatus was also significantly and inversely correlated with PNUE. However, the fractions of nitrogen allocated to photosynthetic apparatus components were affected by N_area and CW_area. The correlation analysis showed that the fraction of nitrogen allocation to bioenergetics (P_b) and light harvesting system (P_L) was inversely correlated with N_area; the fraction of nitrogen allocation to Rubisco (P_r), P_L and P_b were significantly, extremely significant and marginally negatively correlated with CW_area, respectively. The results showed that with the increase of N_area and/or CW_area, leaves tended to allocate nitrogen to non-photosynthetic apparatus, including stored nitrogen component and cell wall component. In addition, cell wall may reduce PNUE by influencing mesophyll conductance (g_m). It’s likely to improve the photosynthetic nitrogen use efficiency in genotype ‘CHANG ZI 1 HAO’ and ‘MOC-620’ by increasing the fraction of N allocation to photosynthetic apparatus and decreasing allocation to non-photosynthetic apparatus.

The light response curve of electron transport rate is an important tool to investigate plant physiology and ecology. It can provide a theoretical basis for quantifying the absorption and transmission of light energy in primary reaction. In this paper, the mathematical characteristics, the advantages, and the potential weaknesses in practical application and research trends of the light response models of electron transport rate are reviewed and discussed. The primary reaction, which includes absorption of light energy, excitation and de-excitation of photosynthetic pigment molecules (including photochemical reaction, fluorescence emission and heat dissipation), and electron transport rate stemming from charge separation in the photosystem II (PSII) reaction center caused by exciton resonance, is consisted by a series of complex physical and biochemical reactions. The classical and semi-mechanistic models of light response curve of electron transport rate are difficult to explain the dynamic down-regulation of PSII, light adaptation and light protection of algae and higher plant, because they did not involve or only partly involved the primary reaction process. However, the mechanism model takes into account the important role of the physical parameters of the photosynthetic pigment molecule (e.g., the eign-absorption cross-section of light energy (σ_ik), the average life-time of the molecule in the lowest excited state (τ_min), the energy level degeneracy of the molecule and the number of photosynthetic pigment molecules in the excited state (N_k)) in the whole primary reaction process. This model can not only obtain the maximum electron transfer rate and its corresponding saturation light intensity of algae and higher plant, but also get some important physical parameters such as σ_ik and τ_min. Meanwhile, it also can obtain the laws about light-response of the effective light energy absorption cross-section ($\sigma_{\mathrm{ik}}^{\prime}$) and of N_k. It may be the developmental direction of the mechanism model of electron transport rate response to light in the future when the mechanistic model was coupled with some environmental factors (e.g., temperature and CO₂ concentration) and the relationship between light intensity and $\sigma_{\mathrm{ik}}^{\prime}$ and N_k were determined.

Aims As the ocean warms, the upper mixed layer becomes shallower, increasing nutrient limitation and sunlight exposure for diatoms. The photosynthetic yield of diatoms was affected by the dual stress of high light and nutrient limitation. This study mainly explored the photophysiological regulation of diatoms in response to phosphorus starvation and high light stress to further understand the effects of marine environmental changes on diatom photosynthesis.
Methods We cultured the two different-sized diatom species Thalassiosira pseudonana and T. weissflogii under the condition of phosphorus starvation to monitor the changes of photosystem II (PSII) function and to investigate their photophysiological responses to high light.
Important findings Under the condition of phosphorus starvation, the PSII activity of smaller T. pseudonana gradually declined, the electron transport efficiency from plastoquinone Q_A^- which binds to D2 protein to plastoquinone Q_B which binds to D1 protein descended. Thus, the energy captured for electron transport per unit reaction center decreased, and the non-photoquenched was induced, while the PSII activity of larger T. weissflogii could be maintained for a longer time; T. pseudonana had higher value of PSII photoinactivation cross section (σ_i) under phosphorus sufficient condition than T. weissflogii, which was prone to photoinhibition and exhibited a higher repair rate for PSII. Phosphorus starvation had no significant effect on its sensitivity to photoinhibition, while T. weissflogii had significantly higher σ_i under phosphorus starvation condition, and its tolerance to high light intensity was significantly reduced. Under the condition of nutrient limitation and increased light exposure, the larger T. weissflogii may tend to distribute in the lower euphotic layer. In summary, this study suggests that marine environmental changes may change the niche of diatoms with different cell sizes and affect their contribution to primary production.

Aims Meteorological data show faster warming of the global land surface during the night than during the day in the past 50 years. However, most of the previous studies were focused on the effects of whole-day equivalent warming, and the understanding of effects of diurnal asymmetric warming remains elusive.
Methods This study evaluated the effects of diurnal asymmetric warming on carbon sink capacity using a optimization model considering photosynthetic gain and hydraulic cost in a broadleaf Korean pine (Pinus koraiensis) forest in Changbai Mountains.
Important findings Results show that the model simulations matched well with observations of net primary production based on the data measured from eddy covariance flux towers. Warming promoted carbon sequestration (11.2%-13.8%) in our study area but did not significantly affect the water use efficiency, and the positive effects on annual carbon sequestration had no statistical difference among different warming scenarios. In addition, warming increased the water stress for forest plants, subsequently increasing the loss percentage of conductivity (PLC, hydraulic vulnerability; 1.1%). In conclusion, all warming scenarios significantly enhanced the current carbon sink capacity of forests compared with ambient condition, but warming may increase the risk of forest death through hydraulic failure, which would significantly affect the future forest carbon sink.