Partitioning and redistribution of sulphur during S-stress in Macroptilium atropurpureum cv. Siratro

During the first 7 d of sulphate-deprivation stored SO 4 2− was redistributed and assimilated into organic forms in the tropical legume Macroptilium atropurpureum cv. Siratro. However, whilst the sulphate content of all tissues declined after removing the external SO 4 2− supply this was slowest in mature leaves. By contrast, the total S content of mature leaves declined markedly in the absence of external sulphate whilst that of both young leaves and roots increased. Furthermore, when radiolabelled SO 4 2− was applied to abraded surfaces of mature leaves, most of the translocated label was recovered in the root following 2 d SO 4 2− deprivation. By contrast, radiolabelled SO 4 2− applied to young leaves was mostly retained in these tissues and not translocated. Within 3 d of removing the SO 4 2− supply there was a large increase in extractable APS-sulphotransferase activity in roots accompanied by a decline in nitrate reductase activity, but these effects were not seen in leaves. Five days after the removal of SO 4 2− there was a large increase in the content of asparagine in roots. The results are discussed in relation to the co-ordination of NO 3 − and SO 4 2− uptake and assimilation and the partitioning of sulphur during S-stress