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Early season leaf growth depends largely on nitrogen (N) provided by remobilization from storage, and many studies have tested the effect of N availability to roots on the amount of N provided for new leaf development by remobilization. Although it is well known that the light regime experienced by a leaf influences the amount of N per unit leaf area (LA), the effect of the local light regime on the amount of N derived either directly from root uptake or from remobilization for early season leaf growth has never been tested at an intra-canopy scale. The objective of this study was to quantify the relative importance of (1) N availability to roots, (2) local light regime experienced by the foliage (at the shoot scale) and (3) leaf rank along the shoot, on the total amount of N allocated to leaves and on the proportions of N provided by remobilization and root uptake. To quantify the importance of N uptake and remobilization as sources of leaf N, potted hybrid walnut trees ( Juglans nigra  L. ×  regia  L.) were grown outdoors in sand and fed with a labeled ( 15 N) nutrient solution. By removing the apical bud, the trees were manipulated to produce only two shoots. The experimental design had two factors: (1) high (HN; 8 mol N m −3 ) and low (LN; 2 mol N m −3 ) N availability; and (2) high (HL; 90% of incident photosynthetically active photon flux (PPF)) and low (LL; 10% of incident PPF) light.

tree physiology

Nitrogen availability, local light regime and leaf rank effects on the amount and sources of N allocated within the foliage of young walnut ( Juglans nigra  ×  regia ) trees