Canopy and leaf photosynthetic characteristics and water use efficiency of sweet sorghum under drought stress

The objective of this paper was to examine the relationship between Water Use Efficiency (WUE) at the canopy and leaf levels, to determine soil moisture conditions, which can optimize yield, and WUE of sweet sorghum (Sorghum bicolor (Linn.) Moench), thus providing some theoretical foundation for using marginal land effectively and developing production of sweet sorghum. Three levels of soil moisture conditions were established, and photosynthetic characteristics and yield were measured. The canopy apparent photo-synthetic rate (CAP) and leaf photosynthetic rate (PN) were reduced gradually with increased drought stress, and the CAP was lower than the PN under every soil moisture conditions. The PN had a midday depression phenomenon, but the CAP did not exhibit this midday depression phenomenon under severe drought stress. The linear regression relationship of CAP and PN was CAP = 1.5945 + 0.1496 PN. The canopy apparent WUEC and leaf WUEL were the highest under moderate drought stress. The first was 5.3 and 5.8 times higher than the WUEL in mid-July and late August, respectively. The stem fresh biomass yield was 77 tons/ha under moderate drought stress and WUE of aboveground biomass yield (WUEB) was also the highest. Our results showed that moderate drought stress did not result in a significant reduction in biomass yield but increased WUE significantly.