Influence of assimilate demand on photosynthesis, diffusive resistances, translocation, and carbohydrate levels of soybean leaves.

Rates of net photosynthesis and translocation, CO(2) diffusive resistances, levels of carbohydrates, total protein, chlorophyll, and inorganic phosphate, and ribulose 1,5-diphosphate carboxylase activity were measured in soybean (Glycine max L. Merrill) leaves to ascertain the effect of altered assimilate demand. To increase assimilate demand, the pods, stems, and all but one leaf (the "source leaf") of potted plants were completely shaded for 6 or 8 days and the responses of the illuminated source leaf were monitored. Rate of net photosynthesis in the source leaf of the shaded plants was found to increase curvilinearly to a maximum on the 8th day. The source leaf of the control plants (no sink shading) maintained a constant photosynthetic rate during this period. Vapor-phase resistance to CO(2) diffusion did not vary with treatment, but mesophyll (liquid phase) resistance was significantly lower in the source leaf of the shaded plants.Starch concentration in the source leaf of shaded plants decreased more than 10-fold during the 8-day shading period. In this same period, sucrose concentration rose nearly 3-fold. Conversely, in the source leaf of the unshaded plants, starch concentration remained high (23% of leaf dry weight) and sucrose concentration remained very low (1.2%). When measured on the 8th day of treatment, translocation rate, ribulose 1,5-diphosphate carboxylase activity, and inorganic phosphate concentration were found to be significantly higher in the source leaf of the shaded plants than in the control source leaf.When shaded plants were again illuminated, all measured response trends in the source leaf were reversed. These data indicate that assimilate demand has a marked influence on source-leaf photosynthesis and carbohydrate formation and export.