Photosynthetic performance of Setaria viridis to soil drought and rewatering alternations

The ability of psammophyte (a plant that grows in sand or sandy soil) to withstand and recover from severe droughts is crucial for desertification control and restoration of degraded vegetation in semi-arid sandy land in northern China. Responses of the photosynthetic gas exchange and chlorophyll fluorescence of an annual (Setaria viridis) were measured through three cycles of soil drying and rewatering. Results showed that the net photosynthesis rate (P-n) decreased by 95 percent, 91 percent, and 61 percent at end of three drought periods; the stomatal conductance (g(s)) decreased by 72 percent, 73 percent, and 53 percent; the transpiration rate (T-r) decreased by 70 percent, 69 percent, and 37 percent; and water-use efficiency (WUE) decreased by 83 percent, 69 percent, and 38 percent. At the same time, the trapping probability with no dark adaptation (F-V'/F-M') decreased by 70 percent, 12 percent, and 10 percent, while the electron transport per cross-section (ET0'/CS'(0)) decreased by 80 percent, 45 percent, and 13 percent. The intercellular CO2 (C-i) increased by 1.77, 1.02, and 0.66 times, and the dissipation per cross-section (DI0'/CS0') increased by 98 percent, 28 percent, and 22 percent. These results indicated that S. viridis was subjected to photoinhibition and some nonstomatal limitation of photosynthesis under drought. However, the above photosynthetic characteristics were restored to control values after three or four days of rewatering. The capability to recover from drought may contribute to the plant's use of water as efficiently as possible. Furthermore, during the subsequent drought spells, suppression of Pn, gs, Tr, F-V'/F-M', and ET0'/CS0' decreased or slowed down following the drying and rewatering alternations, especially in the third drought spell.