Distribution and Redistribution of Sulfur Supplied as [35S]Sulfate to Roots during Vegetative Growth of Soybean

Soybean (Glycine max L.) plants were grown in nutrient solution containing 10 [mu]M sulfate and were treated at various times with [35S]sulfate for 48 h. Growth was then continued in unlabeled solution. The sulfur content of each leaf increased rapidly until it was about 40% expanded; small, additional increases occurred until the leaf was about 70% expanded after which the sulfur content decreased by about 50%. Leaves that were about 60 to 70% expanded during the pulse were strongly labeled but then underwent a significant loss of 35S label. Leaves that were in the early stages of expansion imported little 35S label during the pulse but acquired 35S label during the chase period as they expanded (i.e. redistribution). Most of the redistributed 35S label was derived from other leaves. The rates of both sulfur import and sulfur export by a leaf were greatest at about 70% expansion. Leaves that acquired 35S label during early development retained a much higher proportion of their label than leaves that were more developed, suggesting that the sulfur acquired by leaves during early development is preferentially incorporated into a pool that is less mobile than the sulfur acquired in the later stages of leaf growth.