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

Abstract

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.

ACKNOWLEDGMENTS

We thank A. Gavaland for his advice, and J. Willm and H. Duval for technical assistance during the greenhouse experiment, which was carried out at the Agroforestry and Farm Forestry station, INRA Toulouse Auzeville. We thank sincerely A. Bricka for allowing us to work in his walnut plantations.