Effect of root temperature on the amino‐nitrogen composition of leaves in successive segments of apple shoots

Young apple trees ( Malus pumila Mill. cv. Cox's Orange Pippin) given nitrogen either at or 40 days after bud‐break were kept at a root temperature of 6, 18 or 30°C under otherwise constant conditions. Twelve weeks after the start of the experiment leaves from successive shoot segments and roots were collected and in most cases analysed to assess total nitrogen, protein nitrogen, and the main amides and amino acids. The percentage composition of the amino‐nitrogen fraction of the roots was not or was hardly at all affected by the treatments; asparagine predominated, followed by arginine. In contrast, in the leaves the share of arginine dropped from about 90% at 6°C to about 30% at 30°C in favour of especially asparagine. This pattern was mainly attributable to the situation in the basal sections of the shoot. In the middle and top segments the temperature effects were small. In general, a high level of amino nitrogen corresponded to a high contribution of arginine. Soluble nitrogen was higher after the late than after the early application of nitrogen. Shoot growth was reduced at 6°C root temperature, but little difference was seen between 18 and 30°C. It was concluded that with respect to nitrogen metabolism roots and shoots function more or less independently of each other. The hypothesis that the roots affect leaf nitrogen metabolism via the supply of growth substances produced in the roots, presumably cytokinins, is discussed.