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

Early season leaf growth depends largely on N provided by remobilization from storage and many studies have tested the impact of current-N availability to roots on the amount of leaf N provided by remobilization. While it is well known that the light regime experienced by a leaf influences the amount of N per unit leaf area, the effects on N derived from direct root uptake or remobilization have never been tested at an intra-canopy scale. The objective of this study was to quantify the relative importance of (i) N availability to roots, (ii) local light regime experienced by the foliage (here at shoot scale), and (iii) leaf rank along the shoot, on the total amount of N allocated to leaves and on the proportions of N provided by remobilization and uptake. Potted hybrid walnut trees (Juglans nigra x regia L.) were grown outdoors in sand and fed with a labelled (15N) nutrient solution in order to quantify the importance of uptake and remobilization as sources for leaf N. The trees were manipulated to produce only two shoots by removing the apical bud and an experimental design with two factors was used: (1) N availability for the tree (8 mol N m-3, HN, or 2 mol N m-3, LN), and (2) light level of treated (lower) branch (90%, HL, or 10%, LL, of incident light). Total leaf N per individual tree was not influenced by either N availability or light level. Higher N availability increased the amount of leaf N derived from root uptake at the whole - tree scale (typically around 8% and 2% under HN and LN, respectively). N allocation within the foliage of individual trees was controlled by the local light regime that strongly affected individual leaf characteristics (Ma and Na). Decreasing the light availability to a branch decreased the amount of N allocated to this branch to the benefit of the less shaded branch. In contrast, the local light regime (shading) experienced by the lower branch did not affect the fraction of total leaf N provided by remobilization for both lower, shaded or upper, unshaded branches. These results are discussed in terms of the modelling of tree growth