© 2005 Heron Publishing—Victoria, Canada
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
Ela Frak (1, 2, 3), Xavier Le Roux (1, 4), Peter Millard (5), Sabine Guillaumie (1, 6) and Renate Wendler (5)
1. UMR-PIAF (INRA-Université Blaise Pascal), 234 avenue du Brézet, 63039 Clermont-Ferrand cedex 02, France / 2. Unité d’Ecophysiologie des Plantes Fourragères (INRA-UEPF), Route de Saintes, 86600 Lusignan, France / 3. Corresponding author (ela.frak@lusignan.inra.fr) / 4. Laboratoire d’Ecologie Microbienne (UMR 5557 CNRS-Université Lyon I-USC INRA 1193), bat 741, 43 bd du 11 novembre 1918, 69622
Villeurbanne, France / 5. Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, Scotland / 6. Unité de Génétique et d’Amélioration des Plantes Fourragères (INRA-UGAPF), Route de Saintes, 86600 Lusignan, France / Received April 26, 2004; accepted April 9, 2005; published online October 3, 2005
Summary
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 (15N) 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.
Total leaf N per tree was unaffected by either N availability or irradiance. The HN treatment increased the amount of leaf
N derived from root uptake at the whole-tree scale (typically around 8 and 2% in the HN and LN treatments, respectively).
Nitrogen allocation within foliage of individual trees was controlled by the local light regime, which strongly affected individual
leaf characteristics as leaf mass per unit LA and area- based amount of leaf (Na). Decreasing the light availability to a branch decreased the amount of N allocated to it, benefiting the less shaded branches.
In contrast, shading of the lower branch did not affect the fraction of total leaf N remobilized for either the lower, shaded
branch or the upper, unshaded branch. The relevance of these findings for tree growth modeling is discussed.
Keywords:
branch, leaf N, N allocation, N remobilization, N uptake, 15N, shading, shoot.
