Drought stress effect on carbohydrate concentration in soybean leaves and pods during early reproductive development: its implication in altering pod set

To advance our understanding of the physiological mechanisms by which pod abortion is regulated in soybean (Glycine max L. Merr.), leaf photosynthetic rates, carbohydrate concentrations and soluble invertase activities in leaves, flowers and pods were determined in plants subjected to drought stress during early reproductive development. Soybeans were grown in pots in an environmentally controlled glasshouse. Drought stress was imposed from -11 to 10 days after anthesis (DAA). Drought decreased photosynthetic rates and water potentials in leaves, flowers and pods. Drought decreased leaf sucrose and starch concentrations but increased hexose (glucose + fructose) concentrations. Drought did not affect the activity of soluble invertase in leaves. In flowers and pods, sucrose concentrations were higher under drought as compared with well-watered controls. Hexose and starch concentrations of flowers and pods were also higher under drought until 7 and 3 DAA, respectively; thereafter they were significantly lower than those of the well-watered controls. Soluble invertase activity was decreased by drought in pods 5-10 DAA. Although the concentrations of non-structural carbohydrate (sucrose + hexose + starch) were higher in droughted flowers and pods, the total amount of non-structural carbohydrate accumulated in pods during the sampling periods was substantially reduced by drought. Pod growth was decreased by drought 3-5 DAA, which coincided with a decrease in hexose to sucrose ratio, when a significant reduction of pod set was occurred. Collectively, the results indicate that both source and sink restrictions are involved in regulating pod set in drought-stressed soybeans. It is suggested that a low availability of current and reverse photosynthate in leaves coupled with an impaired ability to utilize the incoming sucrose by pods resulted in a decreased carbohydrate flux from leaves to pods, together with a decreased hexose to sucrose ratio in pods are potential factors contributing to pod abortion in drought-stressed soybeans. (C) 2003 Elsevier B.V. All rights reserved.