XYLEM SAP COMPOSITION OF BEECH (FAGUS-SYLVATICA L) TREES - SEASONAL-CHANGES IN THE AXIAL DISTRIBUTION OF SULFUR-COMPOUNDS

During different phases of the annual growth cycle, xylem sap was coll ected from trunk segments of adult beech (Fagus sylvatica L.) trees by die water displacement technique. Irrespective of the height of the t runk, both sulfate and reduced sulfur compounds were detected in the x ylem sap throughout the year. Sulfate was the predominant sulfur compo und in all samples analyzed. Its concentration in the xylem sap varied between 10 and 350 mumol l-1, with highest concentrations in April, s hortly before bud break. In contrast to other tree species, cysteine a nd not glutathione was the predominant thiol transported in the xylem sap of beech trees. The cysteine concentration ranged between 0.1 and 1 mumol l-1. As observed for sulfate, maximum cysteine concentrations were found in April. Apparently, both sulfate and cysteine transport c ontribute to the sulfur supply of the developing leaves. Seasonal chan ges in the axial distribution of cysteine and sulfate differed, indica ting differences in the source-sink relations of these sulfur compound s. High, but uniform, xylem sap sulfate concentrations in April may or iginate from balanced sulfate uptake by the roots, whereas high cystei ne concentrations in April, increasing with increasing height of the t runk, may originate in part from protein breakdown in the trunk. Rever sal of the axial distribution of xylem sap cysteine in late summer-ear ly fall to higher concentrations in the lower part of the trunk than i n the upper part of the trunk suggests that the upper part of the trun k becomes a sink for cysteine as a result of the synthesis of storage proteins at this time of the year.