Preferential allocation of sulphur into y‐glutamylcysteinyl peptides in wheat plants grown at low sulphur nutrition in the presence of cadmium

Plants respond to Cd by synthesising phytochelatins (PC) and similar S‐rich peptides which are important in alleviating Cd toxicity. The hypothesis that S nutrition influences the sensitivity of plants to Cd was examined by measuring the growth, PC and Cd content of wheat plants ( Triticum aestivum cv. Condor) grown at 10–1000 µ M S with and without 30 µ M Cd. In the absence of Cd, 100 µ M S was marginally S‐limiting. Cd severely inhibited root growth at 100 µ M S but the concentrations of PC, Cd–PC complexes and Cd in the root were similar to those of plants grown at 1000 µ M S which exhibited no evidence of Cd toxicity. Plants grown at low S (10 and 30 µ M ) contained lower concentrations of Cd. However, they produced very low amounts of PC and were very sensitive to Cd. At 10–300 µ M S (but not at 1000 µ M ), Cd enhanced the concentration of S in the root. The data are consistent with a model in which plants preferentially allocate S to PC synthesis. When S was marginally limiting (100 µ M ), the S supply was sufficient for PC synthesis but not root growth, effectively causing Cd‐induced S deficiency. This did not occur at high S (1000 µ M ). Conversely, at low S (10 and 30 µ M ), the synthesis of PC₂–PC₄ was decreased by 87 and 66%, respectively, thereby resulting in decreased Cd uptake while also making the plants especially sensitive to Cd.