Sulphate supply and its regulation of transport in roots of a tropical legumeMacroptilium atropurpureum cv. Sirato

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During a period of sulphate deprivation, roots of Macroptilium atropurpureum responded by increasing their uptake capacity at the plasma membrane. This effect was apparent both in intact plants and in tissues excised prior to uptake. In experiments using excised root systems previously labelled with 35 SO 4 2− , the rate of tracer transport to the xylem was much greater in roots subsequently deprived of external sulphate than in those supplied with unlabelled sulphate. Removing the external sulphate supply also greatly reduced the loss of sulphate to the external solution. Additionally, compartmental analysis of tracer exchange kinetics showed that the flux of sulphate from the cytoplasm to the xylem (Φ cx ) was much larger than that from the vacuole to the cytoplasm (Φ vc ) in roots supplied with 0.25 mol m −3 SO 4 2− . Therefore, once the external sulphate supply is removed, the flux of sulphate out of the cytoplasm (to the xylem) is likely to be much greater than the flux into the cytoplasm (from the vacuole), especially if the xylem flux is increased. Thus, even though the total sulphate content of roots fell by less than half during the first 24 h of sulphur deprivation the cytoplasmic concentration may have fallen substantially, resulting in a 3-fold increase in the uptake capacity of intact roots. The results are discussed in relation to the control of sulphate transport across the plasmalemma and the redistribution of sulphur during sulphur stress