Thiol and amino acid composition of the xylem sap of poplar trees (Populus x canadensis 'robusta')

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 myM Cysteine and gamma-glutamylcysteine concentrations were 0. 1 -4 and 0.01 -3 myM respectively. The glutathione content was subject to strong seasonal fluctuations. Highest amounts of glutathione (13 myM) were observed in March, immediately before flowering of the catkins. Like gamma-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 detect ed 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.