THIOL COMPOSITION OF XYLEM SAP OF BEECH TREES

The thiol composition of the xylem sap of beech trees (Fagus sylvatica L.) was analysed in ca 40-year-old trees at two forest stands differing in soil characteristics. Five trees from an acidic and five from a calcareous soil were felled bimonthly from October 1988 to October 1989. Xylem sap was taken from 100 cm long segments of the trunk base by means of water displacement. Cysteine and glutathione were the only thiols found in the xylem sap. During the entire season the cysteine concentration of the xylem sap was higher than that of glutathione. The cysteine concentrations ranged from 5 to 2600 nmol l-1 xylem sap and the glutathione concentrations from 2 to 400 nmol l-1, with peak values for both thiols in the spring. During winter, cysteine contents were significantly higher in the xylem sap of trees grown on acidic soil than on calcareous soil. The origin of reduced sulphur in the xylem sap and its possible contribution to sulphur nutrition are discussed.