Physiological and metabolic origin of sulphur for the synthesis of seed storage proteins

Wheal: plants were grown with adequate sulphur (S) during vegetative growth but the supply of nutrients (including S) was terminated during generative growth. The grain yield and the S content of plants that did not receive n utrients after anthesis were similar to plants that received S throughout g enerative growth. When the S supply was terminated at anthesis the S for gr ain growth was derived principally from sulphate in the root and glutathion e in the flag leaf; insoluble S was not an important source. Plants that re ceived inadequate S prior to terminating the S supply at anthesis produced a lower yield of grain with a lower S content indicating the production of low-S storage proteins. The internal pool of soluble S in these plants was negligible. Furthermore, the S for the synthesis of grain proteins was deri ved principally from protein-S in the flag leaf rather than from soluble so urces. Since glutathione was the main source of S in the endosperm cavity t he protein-S must have been metabolised to glutathione in the flag leaf. En dosperm extracts from S-inadequate plants catalysed the hydrolysis of gluta thione at rates consistent with those required to supply S for grain growth . The extracts also contained enzymes of methionine synthesis and reductive sulphate assimilation. Extracts from S-adequate plants also exhibited thes e activities but the enzymes of sulphate assimilation were more active, con sistent with the production of S-rich proteins in these plants. Soybeans, u nlike wheat. acquired most of the S for grain growth during generative grow th. Prior to the onset of grain growth. sulphate and homoglutathione accumu lated in the pod. Sulphate declined rapidly when grain growth commenced but homoglutathione showed a transitory increase. Developing grains contained homoglutathione but negligible sulphate. The most likely explanation is tha t sulphate is metabolised to homoglutathione in the pod but. uptake of sulp hate and assimilation within the cotyledons cannot be ruled out,