The Effect of Nitrogen Nutrition on Cytokinin Activity and Free Amino Acids inBetula pendulaRoth, andAcer pseudoplatanusL.

Abstract

A comparison was made of the levels of extractable cytokinin-like compounds and free nitrate in the tissues of Betula pendula and Acer pseudoplatanus at two levels of nitrogen nutrition. In further studies comparisons were made of the levels of cytokinin-like compounds and free amino acids extracted from the tissues of B. pendula supplied with nitrogen as ammonium sulphate, ammonium nitrate, or calcium nitrate. These results were then related to growth differences between the treatments. B. pendula supplied with a low level of nitrogen as ammonium nitrate continued to make slow growth whereas A. pseudoplatanus formed terminal resting buds after 2–3 weeks, further growth being arrested. The low levels of extractable cytokinin-like compounds found in the leaves of B. pendula receiving a low nitrogen supply were in contrast to the results from A. pseudoplatanus, where only a small reduction was found in response to low nitrogen supply. Analysis of the plant tissue showed that free nitrate and amino acids were depleted to a greater extent from B. pendula than from A. pseudoplatanus under conditions of low nitrogen supply. It is suggested that the ability of B. pendula to continue active growth with low nitrogen supply may be related to the readiness with which nitrogen can be mobilized and re-circulated in the plant, a process including sequential leaf senescence. Contrasting growth habits were seen in B. pendula in ammonium nitrate- and ammonium sulphate-fed plants. Lateral shoot growth in ammonium sulphate-fed plants was completely inhibited and no detectable cytokinin activity was found in tissue extracts, whereas ammonium nitrate-fed plants grew rapidly, and showed considerable development of laterals. Changes in the levels of individual amino acids show that the form in which nitrogen is taken up by plants affects the subsequent pathways of metabolism.