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Xylem sap of poplar trees was collected by vacuum extraction. Samples were taken year round and concentrations of thiols as well as amino acids were determined. Throughout the season glutathione was the predominant thiol in the xylem sap of poplar trees with concentrations ranging from 0.1 to 13 μM. Cysteine and γ-glutamylcysteine concentrations were 0.1–4 and 0.01–3 μM, respectively. The glutathione content was subject to strong seasonal fluctuations. Highest amounts of glutathione (13 μM) were observed in March, immediately before flowering of the catkins. Like γ-glutamylcysteine and cysteine, glutathione values reached further maxima at the onset of budbreak and leaf expansion in April and May. Also the amino acids investigated (glutamine, asparagine, glutamic acid, methionine, arginine) showed strong seasonal fluctuations in the xylem sap of poplar, with glutamine being the most abundant organic nitrogen compound (max. 13 mM). Highest amounts of amino acids were detected during April and May, the period of budbreak and leaf expansion, presumably owing to an increased degradation of storage protein. The sulfur-containing amino acid methionine was present in high amounts during the early growing season. It appeared to be the dominant reduced sulfur compound translocated in the xylem sap of poplar trees in April and May, whereas glutathione was the predominant organic sulfur compound in the xylem sap in March. The significance of storage and mobilization of reduced sulfur and organic nitrogen compounds for development of reproductive (catkins) and vegetative tissue (leaves and sprouts) in poplar trees is discussed. Key words: xylem sap, amino acid, thiol, transport, storage, mobilization.

The levels of cysteine (Cys), -y-glutamylcysteine (yEC), and glutathione (GSH) were measured in the endosperms, scutella, roots, and shoots of maize (Zea mays L.) seedlings. GSH was the major thiol in roots, shoots, and scutella, Cys predominated in endosperms. The endosperm, scutellum, and functional phloem translocation were required for maintenance of GSH pools in roots and shoots of 6-day-old seedlings. and shoots in proportion to the tissue fresh weight. Taken together, the data supported the hypothesis that Cys from the endosperm is absorbed by the scutellum and used to synthesize GSH for transfer through the phloem to the root and shoot. The estimated flux of GSH to the roots was 35 to 60 nanomoles per gram per hour, which totally accounted for the small gain in GSH in roots between days 6 and 7. For Cd-treated roots the GSH influx was similar, yet the GSH pool did not recover to control levels within 24 hours. The estimated flux of GSH to the entire shoot was like that to the roots; however, it was low (11-13 nanomoles per gram per hour) to the first leaf and high (76-135 nanomoles per gram per hour) to the second and younger leaves.

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