Compartmental analysis of^{35}\mathrm{S}{\mathrm{O}}_{4}^{2-}$exchange kinetics in roots and leaves of a tropical legume Macroptilium atropurpureum cv. Siratro

Compartmental analysis of ^{35}\mathrm{S}{\mathrm{O}}_{4}^{2-}$ tracer exchange kinetics has been used to estimate unidirectional fluxes and compartment contents in excised root and leaf tissue of the tropical legume, Macroptilium atropurpureum. In excised root tissue only 5% of the sulphate taken up across the plasmalemma was reduced to organic forms whereas in excised leaf tissue approximately 20% was reduced. It was necessary, therefore, to incorporate the metabolism of sulphate during the course of the experiment into the compartmental models. In root tissue, wash-out data was fitted by three exponentials, assumed to correspond to exchange in the extracellular spaces, cytoplasm and vacuole, but in leaf tissue two large, slowly exchanging compartments have been postulated in order to achieve a fit to the data. It is likely that differences in leaf cell populations cause the 'anomalous' tracer exchange kinetics and the justification of this assumption is discussed. The fluxes of sulphate at the plasmalemma were greater than the corresponding fluxes at the tonoplast in both roots and leaves. The flux of $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ from the cytoplasm to the external solution did not appear to limit the loss of $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ from the vacuole. At an external $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ concentration of 0.25 mol m-3 the rate constants for exchange in the vacuole were two orders of magnitude greater in roots than in the slowest exchanging leaf cell population. It is possible, therefore, that the slow loss of $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ from leaf cell vacuoles may limit the redistribution of sulphate during S-stress.