Sulphur Distribution and Redistribution in Vegetative and Generative Plants

Abstract

Plants normally acquire sulphur (S) from soil as SO₄ ²⁻. It is then loaded into the xylem, and directed in the transpiration stream into the shoot. S is not distributed in proportion to leaf area but is preferentially directed into leaves that are 60–80% expanded. This implies that the short-term delivery of SO₄ ²⁻ involves extensive xylem/phloem transfer. In S-adequate plants, most of the SO₄ ²⁻ that is initially delivered to the leaves is gradually redistributed, mostly to younger leaves where it is assimilated into S-amino acids. This results in the formation of long term products, the most important being protein-S, which is not readily reexported, and GSH. Redistribution of S between leaves is achieved by loading S, principally SO₄ ²⁻, into the phloem. This implies the expression of SO₄ ²⁻ transporters which support net export of S when the leaf is about 70–80% expanded. Organic S is redistributed principally as GSH or SMM in the phloem, and the amount of each depends on the tissue, species and possibly growth conditions. Phloem transport provides organic S to the root system. In S-inadequate plants, newly acquired S is retained in the root and S in existing leaves is not redistributed in response to the demand of young leaves.

Redistribution of endogenous S occurs during various phases of plant development. Germinating seeds redistribute S from storage proteins to the growing seedling in the absence of exogenous S. Recent studies with germinating legumes indicate that sulphate, rather than GSH/hGSH, is the principal form in which protein-S is transported to the seedling, implying that S-amino acids, from the hydrolysis of storage proteins, are catabolised to SO₄ ²⁻ Develo** seeds require S for the synthesis of storage proteins. The S demand during early seed development in soybean is largely supplied by redistributing pools of SO₄ ²⁻ and hGSH that accumulated previously in the pod, but later most of the demand is supplied by exogenous S. Develo** cotyledons contain both SO₄ ²⁻ and hGSH. It is yet to be established whether exogenous SO₄ ²⁻ delivered to the pods is metabolised to hGSH in the pod or in develo** cotyledons. In wheat, seed growth normally takes place when water, and hence uptake of exogenous S, is restricted. Under these circumstances, S for grain growth is normally recruited from sources within the plant. Plants which receive adequate S during vegetative growth recruit S principally as SO₄ ²⁻ from the roots and, to a lesser extent, as GSH from the leaves. SMM has also been found in the phloem sap travelling to wheat ears. Develo** grains have active mechanisms for the assimilation of SO₄ ²⁻ into Cys and Met and catabolism of GSH and presumably SMM. Wheat plants that receive inadequate S during vegetative growth do not contain significant pools of soluble S to support grain growth. Instead S is recruited from protein-S in the leaves, especially the flag leaf. Since GSH accounts for most of the soluble S in both the endosperm and the endosperm cavity it is concluded that protein-S is metabolised to GSH in leaves and transported in the phloem to the endosperm cavity where it is recruited into the endosperm and serves as the source of S for the production of S-amino acids for incorporation into grain proteins.