Amino acids during development of mountain birch leaves

Summary. The nutritive value of tree foliage for herbivores decreases rapidly with leaf maturation, due in particular to the decline in leaf nitrogen content. Since the amino acid content of plants differs from the need of herbivores for individual amino acids, we examined developmental changes in the contents of amino acids throughout the growth season of mountain birch. The contents of free and protein-bound amino acids, as well as essential and nonessential ones, displayed different patterns with leaf maturation, suggesting that total nitrogen or protein levels are poor predictors of the nutritive status of leaves. The contents of protein-bound amino acids were 100 times higher than those of free amino acids, indicating that the role of free amino acids in nutrition of herbivores is probably less important than that of protein-bound amino acids. Among protein-bound amino acids, both the absolute and the relative contents of two nitrogen-rich essential amino acids, lysine and arginine, decreased during early leaf growth, presumably reducing nitrogen availability in developing leaves. Essential amino acids were mainly positively related to each other, suggesting the co-ordinated regulation of their synthesis. Changes in correlations among individual free amino acids reflected developmental changes in allocation preferences between biosynthesis pathways with leaf growth. Received 31 January 2003; accepted 17 March 2003. R1D=" Correspondence to: Teija Ruuhola, e-mail: teiruu@utu.fi