The Influence of Nitrogen Supply on the Uptake and Remobilization of Stored N for the Seasonal Growth of Apple Trees

Abstract

M26 apple rootstocks were grown in sand culture and supplied with three rates of nitrogen (N) with the irrigation: none, 0·8 mol N m−2 or 8·0 mol N m−2. All the N supplied to the trees was labelled with 15N at 5·0 atom percent enrichment. The effect of N supply on tree growth, N uptake and the remobilization of N from stems for the annual growth of the trees was measured. Increasing the N supply increased leaf growth, but had no effect upon root mass and so altered the root/leaf dry matter ratio Plants receiving no fertilizer N had to rely entirely upon stored reserves of N for their seasonal growth. Initially this N was used for leaf growth, which stopped after a few weeks. Thereafter the N-deficient plants retranslocated some of the N from their leaves to support root growth. Increasing the N supply had little effect upon the amount of N remobilized for growth, although well-fertilized plants accumulated N in their leaves and did not retranslocate any to support root growth. The partitioning of N between roots and shoots was, therefore, altered by increasing the N supply. Amino acid analysis of stems showed that the major forms of N remobilized during growth were protein rich in asparagine and arginine The results show the importance of internal N cycling for the growth of young apple trees, and are discussed in relation to other studies of N cycling in deciduous trees