Sulfate and thiol levels in roots and shoot of sulfur-deprived spinach plants as affected by high pedospheric sulfate levels

Sulfur-deprivation of spinach resulted in a reduced growth, a decreased shoot/root ratio and an increase in dry matter content. The content of sulfur, thiols and soluble proteins was strongly decreased, whereas that of nitrate and free amino acids was increased. When sulfur-deprived plants were transferred to a nutrient solution containing a strongly elevated sulfate level (10 mM), it resulted in gradual but rapid increase in the sulfate content, simultaneously in both roots and shoot. The sulfate accumulation was accompanied by a rapid increase in thiol and cysteine content, illustrating that a proportion of the sulfate taken up in sulfur-deprived plants was rapidly reduced and assimilated into cysteine and its metabolites. During the first hours after the transfer the actual sulfate uptake exceeded a rate of 0.2 mu mol g(-1) fresh weight plant h(-1). In sulfur-sufficient plants a transfer to 10 mM sulfate solely resulted in a increase in sulfate content in the roots, whereas the shoot sulfate content and the plant thiol content remained unaffected. The transfer of excised roots of sulfur-deprived spinach plants to 10 mM sulfate also resulted in an increase in both the sulfate and thiol content, which demonstrated that excised roots of sulfur- deprived plants were able to reduce and assimilate sulfate. The sulfate and thiol content of roots of excised sulfur-sufficient plants were hardly affected by a transfer to 10 mM sulfate.