The Effect of the Autumn Senescence of Leaves on the Internal Cycling of Nitrogen for the Spring Growth of Apple Trees

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Abstract
The internal cycling of nitrogen (N) has been studied in apple rootstocks grown in sand culture and subjected to a constant N supply, or defoliation, or withholding the N supply in the autumn in order to manipulate the amount of N stored over the winter. The trees subsequently received either no N or 8–0 mol N m−3 (labelled with 15N to 498 atom%) with the irrigation the following spring in order to determine the effect of the current N supply on the remobilization of N for leaf growth. Provision of an autumnal N supply delayed leaf senescence and reduced the amount of N withdrawn from leaves from 156 mg N plant−1 to 91 mg N plant−1. Loss of protein ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO) accounted for 83–87% of the soluble protein N lost during leaf senescence, there being a preferential loss of RUBISCO compared with other soluble leaf proteins. Remobilization of N from perennial woody tissues (stems and roots) in the spring was used predominantly for leaf growth. The amount of N remobilized depended upon the size of the N store, but was unaffected by the current N supply, demonstrating that fertilization of trees does not alter the efficiency with which they cycle N. Degradation of RUBISCO in the autumn accounted for between 32% and 48% of the N subsequently remobilized for leaf growth the following spring, suggesting that RUBISCO has a role as a summer store for N.

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