Sulphur accumulation and redistribution in wheat (<i>Triticum aestivum</i>): a study using stable sulphur isotope ratios as a tracer system

Wheat plants were grown hydroponically and fed with two sulphate sources differing in stable isotope composition, one having a δ 34 S of 13·7‰ and the other 4·1‰. Plant sulphur (S) isotope ratios were determined using an on‐line continuous flow‐isotope ratio mass spectrometer. This method greatly simplified the procedure for the measurement of S isotope ratios, and was found to be precise for samples containing > 1 mg S g –1 dry weight. The δ 34 S values of plant shoots, which had been grown on a single sulphate source, were very close to the source values, suggesting little isotope fractionation during sulphate uptake and transport from roots to shoots. By changing the sulphate sources at different growth stages, it was possible to estimate S accumulation and redistribution within different plant parts. At maturity, wheat grain derived 14, 30, 6 and 50% of its S from the accumulation during the following successive growth stages: between emergence and early stem extension, between stem extension and flag leaf emergence, between flag leaf emergence and anthesis, and after anthesis, respectively. It was estimated that 39, 32 and 52% of the S present in the flag leaves, older leaves and stems, respectively, at anthesis, was exported during the postanthesis period. These results demonstrate considerable cycling of S within wheat plants, and highlight the importance of S uptake after anthesis to the accumulation of S in grain under the experimental conditions employed.