Photosynthetic and water-related physiological characteristics of Periploca sepium in response to changing soil water conditions in a shell sand habitat.

This study was performed to observe the effects of water on photosynthesis and water-related physiology in dominant shrubs in shell sand habitats. Four-year-old Periploca sepium seedlings were used as model species. A gradient of 12 water levels was established by artificially supplying the shell sand with water up to saturation and then allowing natural evapotranspiration to occur. The photosynthetic, chlorophyll fluorescence and stem sap flow parameters of P. sepium were measured under a range of water conditions. The different soil water conditions were classified according to the responses of these parameters. (1) With the increase in the relative water content (RWC) of the shell sand, the parameters of leaf photosynthesis, chlorophyll fluorescence and water-related physiology in P. sepium showed significant critical responses. The net photosynthetic rate (P_n), transpiration rate (T_r), instantaneous water use efficiency (WUE), potential water use efficiency (WUE_i), maximum photochemical efficiency (F_v/F_m), actual photochemical efficiency (Φ_PSII) and daily accumulation of stem sap flow all increased first and then decreased with increasing RWC, but the corresponding water conditions associated with their maximum values were not the same. An RWC of 69.40% was determined to be the optimal water condition for photosynthesis and water-related physiological activity in P. sepium. At an RWC of 36.61%, the mechanism of photosynthetic inhibition in P. sepium changed from stomatal limitation to nonstomatal limitation; this was also the minimum water requirement for maintaining normal photosynthetic processes. An RWC of 50.27% resulted in the highest WUE in P. sepium, indicating that moderate drought stress increased WUE. (2) Based on the quantitative relationship between the photosynthetic parameters of P. sepium and the shell sand water gradient, the soil water availability was classified into 6 water grades. The RWC range for maintaining strong photosynthesis and high WUE in P. sepium was 63.22–69.98%. (3) Gas exchange in P. sepium was inhibited under drought and waterlogging stresses. Under these conditions, the photosynthetic electron transport chain was blocked, and the dissipation of light energy as heat increased, which ultimately led to a decline in photosynthetic productivity; moreover, transpiration and dissipation were aggravated, and water transmission and utilization processes in P. sepium were hindered. A significant negative feedback regulation mechanism in the photosynthetic and water-related physiological processes of P. sepium was observed; this mechanism allowed P. sepium growing in shell sand to be highly adaptable to water stress. [ABSTRACT FROM AUTHOR]