Aims Shell ridge islands are distinctive shell sand deposits lying on the upper surface of tidal flats where shellfish grow in abundance and fresh water discharge is minimal. The objective was to elucidate the critical effect of photosynthetic efficiency parameters in leaves of Salix matsudana on soil moisture, clarify the threshold range of photosynthetic efficiency to soil moisture, and define the water adaptability in shell ridge islands of Shandong Province, China, in the middle of the Yellow River Delta.
Methods Two-year-old S. matsudana grown on shell ridge islands was selected as the experimental material. Soil water gradients were obtained by providing water and by natural water consumption. ACIRAS-2 portable photosynthesis system was used to measure the photosynthetic efficiency parameters under different soil water conditions. The light response curves of net photosynthetic rate (P_n) and the water response curves of gas exchange parameters in leaves of S. matsudana were fitted and analyzed.
Important findings The P_n, transpiration rate (T_r), water use efficiency (WUE) and photosynthetic parameters of light response in leaves ofS. matsudana had significant critical effects on soil moisture. P_n, T_r, WUEand intrinsic water use efficiency first increased and then decreased with decreasing soil water, but their critical values were different. The critical value of relative soil water content (W_r) from stomatal limitation to non-stomatal limitation of P_n was 42.9%, and the water compensation point of P_n was 14.4%. The water saturation points of P_n and T_rwere 73.1% and 68.9%, respectively, and the water efficiency point of WUE was 80.1%. Salix matsudana appeared to have photo inhibition under drought stress and had the physiological strategy of weakening light utilization to counter stress. With increasing soil water, the apparent quantum yield (AQY), light saturation point (LSP) and maximum net photosynthetic rate (P_nmax) first increased and then decreased, while the light compensation point (LCP) first decreased and then increased. The values of P_n, AQY, LSP, P_nmax and dark respiration rate (R_d) under water logging stress were higher than under drought stress. When W_r was 69.1%, LCPreached a lower value with 18.6 µmol∙m ^-2∙s^-1, and AQY reached a higher value with 0.05, indicating that S. matsudana had strong ability to utilize weak light. When W_r was 80.9%, LSP reached the highest point with 18.6 µmol∙m ^-2∙s^-1, indicating that S. matsudana had wide light ecological amplitude and high light utilization efficiency. The compensatory effect on light intensity of soil water was significant. The soil water content was divided into five threshold grades by critical values to maintain photosynthetic efficiency of S. matsudana at different levels in shell sand soil. W_r of 73.1% to 80.1% was classified as high productivity and high efficiency; in this range, S. matsudana had high photosynthetic capacity and efficient physiological characteristics for water consumption. In conclusion, S. matsudana had the typical characteristics of water tolerance and no drought stress in shell sand, thus plantings should give full consideration to the soil water environment in shell ridge island.