INFLUENCE OF GLUTATHIONE (GSH) ON SULFATE INFLUX, XYLEM LOADING AND EXUDATION IN EXCISED TOBACCO ROOTS

In short-term experiments sulphate influx of excised tobacco roots (Nicotiana tabacum L. var. 'Samsun') followed monophasic Michaelis-Menten kinetics with an approximate K(m) of 12 +/- 4-mu-M and V(max) of 657 +/- 211 nmol g-1 FW h-1. An inhibition of sulphate influx, xylem loading and exudation of more than 70% was achieved with 0.1 mM GSH within 1 h. Cysteine was two orders of magnitude more effective as an inhibitor than GSH. An inhibition of more than 75% was already obtained with 1.0-mu-M cysteine. It may, therefore, be assumed that GSH is decomposed to yield cysteine concentrations that may inhibit sulphate influx, xylem loading and exudation. When BSO, a specific inhibitor of the initial step of GSH synthesis, was added, cysteine-mediated inhibition on sulphate influx, xylem loading and exudation was strongly diminished. Apparently, GSH synthesis is required to obtain inhibition of these processes by cysteine. The physiological mechanisms that may cause the inhibition of sulphate influx, xylem loading and exudation by glutathione are discussed.