Regulation of Sulphate Transport in a Tropical Legume,Macroptilium atropurpureum, cv. Siratro

Abstract

When young plants of Macroptilium atropurpureum, cv. Siratro were deprived of external sulphate (-S plants) growth of shoots and roots continued at rates comparable to those in plants well supplied with sulphate (control) for 3 d and 5 d respectively. Dilution of internal sulphur therefore took place and redistribution of sulphur occurred between inorganic and organic forms and between roots and younger leaves. Even when S-deficiency limited growth, plants contained 16% of their total sulphur as sulphate, but most of this was retained in old leaves and redistributed slowly to growing zones. The capacity for sulphate uptake increased in roots of –S plants very soon after they were deprived of external sulphate; within 24 h the absorption from 0.25 mol m⁻³ SO₄²⁻ was more than five times that of control roots. Maximum increased capacity was reached after 2–3 d stress when the V_max of system 1 was 1948 nmol h⁻¹g^–1root fr. wt. in –S plants and 337 nmol h⁻¹g⁻¹root fr. wt. in controls. The K^mfor system 1 did not change significantly with S-stress being between 5–8 μM in both sets of plants. Absorption of L-cysteine was not stimulated by S-stress. There was a close, positive relationship between plant growth rate and the rate at which sulphate uptake capacity was enhanced by withholding sulphate from culture solutions. When –S plants were replaced in sulphate-containing solution their capacity for SO₄²⁻ declined to the control level within 24 h. Very marked repression of capacity was also found when –S plants were treated with L-cysteine, but there was no immediate effect with methionine. Roots of this species appear to have a very active system for degrading L-cysteine to sulphate, 30% of the label in ³⁵S-cysteine absorbed by roots was recovered in ³⁵SO₄²⁻ after 20 min or 2 h incubation. By contrast, roots had a very weak ability to reduce sulphate. When part of the root system was in solution lacking sulphate there was enhanced uptake of sulphate by other parts which themselves were amply supplied with sulphate. This is seen as an example of compensatory absorption. The response to S-stress is specific and there were no positive interactions between S-stress and the absorption of phosphate, or P-stress and the uptake of sulphate. The results are discussed in relation to the close control of sulphate uptake by internal sulphate concentration, redistribution of forms of sulphur during stress and mobility of sulphate in the phloem.