Seasonal Variation of Amino Acids and Amides in the Xylem Sap of Salix

Abstract

Summary Amino acids and amides have been analysed in the xylem sap obtained by vacuum extraction from the vessels of one- and two-year-old shoots of willow. Besides glutamine and asparagine, 25 different amino acids have been identified and measured quantitatively with an amino acid analyser. Their seasonal variation has been followed over a two year period. During the development of the male catkins the content of amides and amino acids increases drastically in the tracheal sap, reaching up to 2 to 3.7 mg/ml sap (= 0.2 to 0.37% w/v) at its maximum. A second maximum is observed during the outgrowth of vegetative buds. Glutamine is clearly dominant and constitutes up to 75% of these nitrogen compounds. During summer, fall and winter, the total content of amides and amino acids of the tracheal sap stays below 50 to 200 μg/ml. Experiments with isolated shoots indicate that these compounds are mobilized in early spring at least in part within the shoots themselves and that they permeate into the vessels in which they are translocated towards the outgrowing buds. In isolated shoots a permeation rate of at least 4 to 16 pmol cm -2 min -1 has been obtained. The high content of nitrogen compounds emphasizes the particular role of the xylem pathway for the translocation of organic compounds in trees that flower in early spring.