Reduced sulphur allocation from three-year-old needles of Norway spruce (Picea abies [Karst] L.)

Abstract

The 35S-labelled sulphur compounds glutathione, cysteine, and γ-glutamylcysteine were fed to 6-year-old spruce trees via the cut surface of a single 3-year-old needle. After 1-3 h exposure, uptake of the radiotracer into the fed needle, export into other parts of the plant and distribution between needles, bark and wood along the transport path were analysed. Uptake of cysteine into the exposed needle was one order of magnitude and uptake of γ-glutamylcysteine two orders of magnitude lower than that of glutathione. Independent of the thiol applied, the current year's sprouts were the preferential sinks of exported 35S. Transport towards basipetal parts of the twig amounted to less than 10% of total 35S export in all cases. After feeding 35S-cysteine and 35S-γ-glutamylcysteine, 35S-glutathione was found along the transport path, in particular in distant parts of the twig. This was also observed when 35S-GSH was fed. This result confirms the significance of glutathione as the major long-distance transport form of reduced sulphur in spruce twigs. In xylem sap of trunk sections of spruce, cysteine rather than glutathione was the main thiol. Cysteine concentrations in the xylem sap of the trunk amounted to 260-500 nmol l-1. Glutathione concentrations were 2-5 times and γ-glutamylcysteine concentrations 4-16 times lower than those of cysteine.