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 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 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.

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 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 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 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.

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 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 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 photosynthetic electron flow 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 photosynthetic electron flow 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 flow (J) steming from charge separation in the 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 photosynthetic electron flow are difficult to explain the dynamic down-regulation of PSII, light adaptation and light protection of algae and 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(?ik), the average lifetime of the molecule in the lowest excited state (?min), the energy level degeneracy of the molecule and the number of photosynthetic pigment molecules in the excited state (Nk)) in the whole primary reaction process, it can not only obtain the Jmax and PARsat of algae and plant, but also get some important physical parameters such as ?ik and ?min. Meanwhile also it can obtain the laws about light-response of the effective light energy absorption cross-section ( ) and of Nk. It may be the development direction of the mechanism model of photosynthetic electron flow 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 and Nk were determined.