Aims Our objectives were to find the optimal models for different drought stresses by comparing four light response models of photosynthesis in leaves of Periploca sepium growing in sand formed from seashells, investigate the adaptability law of parameters of light response models to drought stresses, and define the adaptability of P. sepium to the shell sand moisture conditions.
Methods We used three-year-old P. sepium grown in shell ridge island of China’s Yellow River Delta as experimental material, set up four moisture conditions simulating those in shell sand, measured light responses of photosynthesis in leaves of P. sepium under different drought stresses using CIRAS-2 portable photosynthesis system, and fitted and analyzed the light response curves by four models: rectangular hyperbola, non-rectangular hyperbola, exponential and modified rectangular hyperbola.
Important findings The sequence of fitting effect of the four light response models was in descending order: modified rectangular hyperbola model > non-rectangular hyperbola model > exponential model > rectangular hyperbola model. This result was related to the latter three models not having extreme values, not fitting the process of light response curve declining after light saturation point (LSP), and not directly and accurately solving maximum net photosynthetic rate (Pnmax) and LSP. Among them, the non-rectangular hyperbola model best fit dark respiration rate (Rd), the rectangular hyperbola model best fit light compensation point (LCP) and modified rectangular hyperbola model best fit Pnmax and LSP. The adaptabilities of the four models to different drought stresses varied. The modified rectangular hyperbola model well fit the curves under all the drought stresses, the rectangular hyperbola model and exponential model suited mild drought stress and the non-rectangular hyperbola model suited severe drought stress. The light response parameters, including apparent quantum yield (AQY), Rd, LCP, Pnmax and LSP, responded to drought stresses differently. AQY, Rd and LCP first increased and then decreased with the aggravation of drought stress, while Pn, Pnmax and LSP decreased gradually. LSP decreased by 5.2%, 16.3% and 34.55% under light stress, moderate stress and severe stress, respectively, in comparison to the control (1 556 μmol·m–2·s–1). Pnmax decreased by 17.8%, 39.0% and 59.0% compared to the control (22.58 μmol·m–2·s–1). Light use capability of P. sepium leaf was strongest and light ecological amplitude (1 520 μmol·m–2·s–1) was widest under sufficient water condition; photo-saturation and photo-inhibition occurred significantly, light use capability weakened, and photosynthetic capacity was inhibited seriously under severe stress.