Brassinolide alleviated the adverse effect of water deficits on photosynthesis and the antioxidant of soybean (Glycine max L.)

Brassinolide (BR) is a relatively new plant growth regulator. To test whether BR could be used to increase tolerance to water deficits in soybean, the effects of BR application on photosynthesis, assimilate distribution, antioxidant enzymes and seed yield were studied. BR at 0.1 mg l(-1) was foliar applied at the beginning of bloom. Two levels of soil moisture (80% field capacity for well-watered control and 35% for drought-stressed treatment) were applied at pod initiation. BR treatment increased biomass accumulation and seed yield for both treatments. Drought stress inhibited translocation of assimilated C-14 from the labeled leaf, but BR increased the translocation for both treatments. Drought stress depressed chlorophyll content and assimilation rate (A), while chlorophyll content and A of BR-treated plants were greater than that of drought-stressed plants. BR treatment increased maximum quantum yield of PS II, the activity of ribulose-1,5-bisphosphate carboxylase, and the leaf water potential of drought-stressed plants. Treatment with BR also increased the concentration of soluble sugars and proline, and the activities of peroxidase and superoxide dismutase of soybean leaves when drought-stressed. However, it decreased the malondialdehyde concentration and electrical conductivity of leaves under drought stress. This study show that BR can be used as a plant growth regulator to enhance drought tolerance and minimize the yield loss of soybean caused by water deficits.