Allocation of S in Generative Growth of Soybean.

Soybean plants (Glycine max L. Merr) were grown with 100 [mu]M S and 15 mM N and studied with respect to S allocation during grain development. The grains accounted for 87% of the S taken up after d 42, the balance coming from internal redistribution of S from leaves and pods. Detailed studies of the leaves, pods, and grains associated with leaf axils 6 and 7 showed that sulfate accumulated in the pods as they expanded to 50% of full length, ahead of grain enlargement, but declined to very low levels as grain growth commenced. Conversely, homoglutathione (hGSH), cysteine, and methionine increased. In developing grains, hGSH accounted for 60 to 90% of the soluble-S but sulfate was barely detectable. The data are consistent with a model in which, under S-limiting conditions, the pods act as sinks for sulfate and grain growth initiates the assimilation of sulfate into hGSH in the pods, and then into developing grains, where it is incorporated into grain proteins.