Direct evidence for the involvement of the root in the redistribution of sulfur between leaves

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Abstract
The hypothesis that the root is involved in redistributing sulfur (S) between leaves was examined. Soybean plants were grown in 20 μM sulfate and part of the root system (donor root) was pulsed with [35S]sulfate for 4–24 h. Immediately after the pulse, most of the 35S‐label occurred in the donor root (44–46%) and the expanding leaf, L3 (24–26%). When growth was continued in unlabeled sulfate (chase), 35S‐label was redistributed from the donor root. This confounded the study of redistribution from L3. When the donor root was excised immediately after the pulse, the recipient root and the new leaves acquired 35S‐label during the chase; redistribution of label from L3 accounted for the gains in the root and L4 and, eventually, L5 and L6. The gain of label in the root was transient. Complete removal of the root system after the pulse decreased the export of label from L3 to L4 and resulted in the recovery of label in the nutrient solution and excision of L3 prior to pulsing the plants decreased the short‐term net gain of label in the root during the chase. Collectively, the data are consistent with the proposal that some of the S that is redistributed between leaves is recycled via the root. Other aspects of the data indicate that short‐term distribution, and subsequent redistribution, of S adjusts to alterations to the endogenous S sinks and sources.