Exchange between phloem and xylem during long distance transport of glutathione in spruce trees [Picea abies[Karst.] L.)

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Abstract
Radioactively labelled glutathione (35S-GSH) was fed to a single 1-year-old spruce needle on a twig girdled apical to the application site. Uptake and translocation of the radiotracer were investigated after 3 h incubation in the light. Experiments were performed with 4-5-year-old trees in July and October. In both sets of experiments 3-16% of the applied radioactivity was taken up by the needle and resulted in 54-77% export to the preferential sinks, i.e. the current year's sprouts. Apparently, the girdle imposed apical to the application site did not hinder long distance transport. A radial transfer of 35S from the bark to the xylem tissue basal to the girdle and a redistribution from the xylem to the bark apical to the girdle was observed. This observation indicates that GSH can readily be exchanged between phloem and xylem in both directions. Basipetal transport of 35S towards the stem was higher in October, but never exceeded 7% of total uptake (13% of total export) of S-GSH. Preferential incorporation of the 35S into protein was observed in July, whereas the main portion of the radiolabel remained in the acid soluble fractions of needles, bark and wood in October.

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