Physiological indicators of nitrogen response in a short rotation sycamore plantation. II. Nitrogen metabolism

American sycamore (Platanus occidentalis L.) seedlings were grown in the field under different urea–nitrogen fertilization regimes to identify nitrogen variables that characterize the growth response. Treatments included fertilization with 50, 150, and 450 kg N/ha, periodic fertilization (three times during the growing season) at 37.5 kg N/ha, and an unfertilized control. Leaf total nitrogen concentration was a poor indicator of plant growth response to nitrogen fertilization. Salt-extractable protein, nitrate, and free amino acid concentrations all trended upward by the end of the growing season as relative growth rate declined, but treatment differences were minimal. Leaf nitrate reductase activity was consistently higher in fertilized trees before substantial leaf senescence had occurred. When leaf loss was evident, all treatments had high levels of nitrate reductase activity. Aspartic acid and glutamic acid were the most prevalent free amino acids in leaves, whereas concentrations of amine-rich amino acids were low. Although several nitrogen variables, including foliar asparagine and glycine concentrations, were positively correlated with relative growth rate (r ≥ 0.7), no single variable closely reflected treatment differences in growth response. Key words: amino acids, nitrogen, nitrate reductase, sycamore.