Response of growth, photosynthetic gas exchange, translocation of 13C-labelled photosynthate and N accumulation in two soybean (Glycine max L.) Merrill) cultivars to drought stress

Two soybean (Glycine max L. Merrill) cultivars, Shin-Tanbakuro and Midori, were grown under mild water stress conditions for 11 days at the vegetative growth stage in order to examine the effect of drought stress on plant biomass production, photosynthetic gas exchange, chlorophyll fluorescence and photoassimilate translocation along with changes in carbohydrates and nitrogen. Relative growth rate (RGR), net assimilation rate (NAR), plant growth rate (GR) and relative leaf area expansion rate (RLER) were decreased by drought and this reduction was greater in cultivar Shin-Tanbakuro than in Modori. Photosynthetic gas exchange (Pn), stomatal conductance (gs), intercellular CO2 concentration (Ci) and transpiration rate (E) in both cultivars were severely decreased by drought; however, the maximum photochemical efficiency of photosystem 11 (Fv/Fm) and apparent photosynthetic electron transport rate (ETR) were not affected by drought. Drought stress did not affect the export of 13C-labelled photosynthate from leaves to stem and roots. Drought decreased the starch concentration in leaves of both cultivars, but the soluble sugar concentration increased more in Midori than in Shin-Tanbakuro. The nitrogen accumulation in plants was decreased by drought in both cultivars, but the reduction in N accumulation in leaves and roots was higher in Shin-Tanbakuro. Results suggested that Midori is more tolerant to drought stress than Shin-Tanbakuro by virtue of greater nitrogen uptake and supply to developing leaves in improving growth.