ASSESSMENT OF NITROGEN NUTRITION OF WALNUT TREES USING FOLIAR ANALYSIS AND CHLOROPHYLL MEASUREMENTS

The response of walnut trees to nitrogen (N) availability was studied on 3-year-old pot-grown trees (Juglans major MJ209 L. Juglans regia L.) in a greenhouse and on 10-year-old field grown trees. The greenhouse pot trial included 5 N levels (0.25, 0.5, 1, 2.5 and 5 g N per plant), and the field trial compared walnut trees with or without N₂-fixing intercrops. In the greenhouse trial, the growth of both root and aerial parts was found to be significantly dependent on N availability. The deficiency treatment (0.25 g N/plant) reduced leaf and root dry weights to 30 and 54% of those of the sufficiency treatment (5 g N/plant) values. The analysis of the youngest fully expanded leaf after the end of shoot elongation period was a good indicator of the tree N nutrition level. Severe deficiency corresponded to leaf N contents below 17 g N kg^?1 DM (mean dry matter), light deficiency between 17 and 24, and maximum growth was obtained with 26 g N kg^?1 DM contents. Terminal leaflet chlorophyll measurements were linearly correlated with N concentration expressed on a leaf area basis (g N m^?2, r²=0.73 for greenhouse trees and r²=0.86 for field trees). The chlorophyll meter readings divided by the specific leaf weight were also correlated with N expressed on a weight basis (g N kg^?1 DM, r²=0.73 for greenhouse trees and r²=0.89 for field trees). However, these relationships were found to be dependent of environmental conditions and were different between greenhouse and field grown trees. Relative indices of chlorophyll measurements and of N concentrations were however consistent over the two sites. Chlorophyll meter measurements could therefore be a useful method to assess the N nutrition status of walnut trees at a low cost, provided that some reference trees with sufficient N nutrition are locally available.