The mobility of sulphur in higher plants

Sulphuris is available to plants predominantly in the form of sulphate to the roots, though most of it is needed in its reduced form (oxidation state -2). As major parts of the reduction of sulphate take place in the leaves, not only translocation of sulphate from the roots to the leaves, but also translocation of thiol from the leaves to the roots is essential to meet the plant's needs for reduced sulphur. The long-distance transport processes involved in the sulphur nutrition of plants include sulphate transport in xylem and phloem, indicating a cycling of sulphate in plants. Reduced sulphur is translocated from the leaves to the roots and to the growing parts of the stem mainly in the form of glutathione in the phloem. Membrane transport processes involved are the influx of sulphate into the roots and into the leaf mesophyll cells; the efflux of glutathione from the leaves; and the influx of glutathione into root cells, where degradation of glutathione has to take place to make redu ced sulphur available for protein synthesis. A possible mechanism regulating this complex system of long-distance - and membrane transport processes is discussed. (IFU