Assessment of nitrogen nutrition of walnut trees using foliar analysis andchlorophyll measurements

The response of walnut trees to nitrogen (N) availability was studied on 3- year-old pot-grown trees (Juglans major MJ209 L. x Juglans regia L.) in a g reenhouse and on 10-year-old field grown trees. The greenhouse pot trial in cluded 5 N levels (0.25, 0.5, 1, 2.5 and 5 g N per plant), and the field tr ial compared walnut trees with or without N-2-fixing intercrops. In the gre enhouse trial, the growth of both root and aerial parts was found to be sig nificantly 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 suff iciency 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 co ntents below 17 g N kg(-1) DM (mean dry matter), light deficiency between 1 7 and 24, and maximum growth was obtained with 26 g N kg(-1) DM contents. T erminal leaflet chlorophyll measurements were linearly correlated with N co ncentration expressed on a leaf area basis (gN m(-2), r(2)=0.73 for greenho use trees and r(2)=0.86 for field trees). The chlorophyll meter readings di vided by the specific leaf weight were also correlated with N expressed on a weight basis (g N kg(-1) DM, r(2) = 0.73 for greenhouse trees and r(2)=0. 89 for field trees). However, these relationships were found to be dependen t of environmental conditions and were different between greenhouse and fie ld grown trees. Relative indices of chlorophyll measurements and of N conce ntrations were however consistent over the two sites. Chlorophyll meter mea surements could therefore be a useful method to assess the N nutrition stat us of walnut trees at a low cost, provided that some reference trees with s ufficient N nutrition are locally available.