DIRECT EVIDENCE FOR THE INVOLVEMENT OF THE ROOT IN THE REDISTRIBUTIONOF SULFUR BETWEEN LEAVES

The hypothesis that the root is involved in redistributing sulfur (S) between leaves was examined. Soybean plants were grown in 20 mu M sulf ate and part of the root system (donor root) was pulsed with [S-35]sul fate for 4-24 h. Immediately after the pulse, most of the S-35-label o ccurred in the donor root (44-46%) and the expanding leaf, L3 (24-26%) . When growth was continued in unlabeled sulfate (chase), S-35-label w as redistributed from the donor root. This confounded the study of red istribution from L3. When the donor root was excised immediately after the pulse, the recipient root and the new leaves acquired S-35-label during the chase; redistribution of label from L3 accounted for the ga ins in the root and L4 and, eventually, L5 and L6. The gain of label i n the root was transient. Complete removal of the root system after th e pulse decreased the export of label from L3 to L4 and resulted in th e 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 th e root during the chase. Collectively, the data are consistent with th e proposal that some of the S that is redistributed between leaves is recycled via the root. Other aspects of the data indicate that short-t erm distribution, and subsequent redistribution, of S adjusts to alter ations to the endogenous S sinks and sources.