Photosynthetic characteristics and water use efficiency of sweet sorghum under different watering regimes

摘要

Sweet sorghum (Sorghum bicolor (L.) Moench) has been recognized as an important ethanol crop which can be planted in marginal lands in China, but little is known about its photosynthetic characteristics in this environment. We investigated gas exchange parameters and biomass yield under different watering regimes along the oases chain of the Hexi Corridor in China. The results showed that the net photosynthetic rate (Pn) exhibited a mid-day depression under serious drought stress (SD). However, during the soft dough stage, the diurnal changes in maximum photochemical efficiency of photosystem II, PSII (Fv/Fm) showed an obvious decline, indicating the existence of photoinhibition. Under normal water (NW) and moderate drought stress (MD) conditions, there was a unimodal pattern except in the jointing stage under MD, and there was no evidence of photoinhibition. The highest water use efficiency (WUE) occurred under MD in the early and middle growth stages, while it was highest under SD in the late growth stage. With increasing drought stress, the light compensation point (LCP) increased, whereas the light saturation point (LSP), the apparent quantum yield (AQY) and dark respiration rate (Rd) declined. The stem fresh biomass was highest under MD (77 t·hm-2). The main conclusion of the study was that SD caused photoinhibition of sweet sorghum and decreased WUE and stem biomass. Under NW, photoinhibition was avoided and stem biomass increased, however, WUE decreased. As a result, the highest WUE and stem biomass of sweet sorghum was achieved under MD.