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

During the first 7 d of sulphate-deprivation stored SO42- was redistributed and assimilated into organic forms in the tropical legume Macroptilium atropurpu-reum cv. Siratro. However, whilst the sulphate content of all tissues declined after removing the external SO42- 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 SO42- was applied to abraded surfaces of mature leaves, most of the translocated label was recovered in the root following 2 d SO42- deprivation. By contrast, radiolabelled SO42-applied to young leaves was mostly retained in these tissues and not translocated.

Within 3 d of removing the SO42- 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 SO42- there was a large increase in the content of asparagine in roots.

The results are discussed in relation to the co-ordination of NO3- and SO42- uptake and assimilation and the partitioning of sulphur during S-stress.