Photosynthesis of Digitaria ciliaris during repeated soil drought and rewatering

The ability of psammophyte photosynthesis to withstand and recover from severe droughts is crucial for vegetation stability in semi-arid sandy lands. The responses of gas exchange and chlorophyll fluorescence of an annual grass, Digitaria ciliaris, were measured through three soil drought and rewatering cycles. Results showed that the net photosynthesis rate(P n) decreased by 92%, 95%, and 63% at end of the three drought periods, respectively, water use efficiency(WUE) decreased by 67%, 54%, and 48%, while the constant intercellular CO2 concentration(C i) increased by 1.08, 0.88, and 0.45 times. During those three cycles, the trapping probability with no dark adaptation(F v′/F m′) decreased by 55%, 51%, and 9%, the electron transport per cross section(ET0′/CS0′) decreased by 63%, 42%, and 18%, and the dissipation per cross section(DI0′/CS0′) increased by 97%, 96%, and 21%. These results indicated that D. ciliaris was subjected to photoinhibition and some non-stomatal limitation of photosynthesis under drought. However, after four days of rewatering, its photosynthetic characteristics were restored to control values. This capability to recover from drought may contribute to making the plant’s use of water as efficient as possible. Furthermore, the photosynthesis decreased more slowly in the subsequent drought cycles than in the first cycle, allowing D. ciliaris to enhance its future drought tolerance after drought hardening. Thus, it acclimatizes itself to repeated soil drought.