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

Wheat plants were grown hydroponically and fed with two sulphate sources di ffering in stable isotope composition, one having a delta(34)S of 13.7 part s per thousand and the other 4.1 parts per thousand. Plant sulphur (S) isot ope ratios were determined using an on-line continuous flow-isotope ratio m ass spectrometer, This method greatly simplified the procedure for the meas urement of S isotope ratios, and was found to be precise for samples contai ning >1 mg S g(-1) dry weight. The delta(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 differe nt growth stages, it was possible to estimate S accumulation and redistribu tion within different plant parts. At maturity, wheat grain derived 14, 30, 6 and 50% of its S from the accumulation during the following successive g rowth stages: between emergence and early stem extension, between stem exte nsion and flag leaf emergence, between flag leaf emergence and anthesis, an d 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 ant hesis, was exported during the postanthesis period. These results demonstra te considerable cycling of S within wheat plants, and highlight the importa nce of S uptake after anthesis to the accumulation of S in grain under the experimental conditions employed.