Spur light exposure as a primary external cause for derivation of dris norms in walnut trees

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Abstract
Spur leaf macroelement profile of walnut (Juglans regia, cvs. ‘Hartley’ and ‘Serr') was characterized by a modified diagnostic and recommendation integrated system (DRIS), using canopy photosynthetic photon flux (PPF) density exposure as a primary external determinant (5) of leaf mineral content. Spur N, P, Ca and Mg content was linearly correlated with PPF and SLW when expressed on the basis of leaf area (A) while that of K was linearly correlated with SLW on % DW basis (W). Mineral ratios, relevant for the DRIS analysis, were calculated using all four possible combinations of Area and Weight expressions (A/A, A/W, W/A, W/W) and correlated with spur leaf SLW. The particular expressions chosen for the DRIS analysis were based on their highest correlation to spur SLW and included N/K and P/K, based on A/W expression of the respective nutrients, and the reciprocal (W/A) expression for all other ratios. The dimensionless mineral ratios based on Weight per Weight (W/W) or Area per Area (A/A), which eliminated the DW contribution, were not related to light exposure and SLW. Derivation of DRIS norms were based on the mineral profile of highly exposed spurs (10.8±3.1 and 8.8±3.9 mol m‐2d‐1 PPF in ‘Hartley’ and ‘Serr’, respectively), characterized previously to be highly productive. Calculated DRIS indices of gradually less exposed and less productive spurs revealed a strong exponential imbalance of K or K and N (increasingly positive) in ‘Hartley’ and ‘Serr’, respectively, vs Ca and Mg (increasingly negative). DRIS indices of P became slightly negative in ‘Hartley’ and positive in ‘Serr’, as spur light exposure decreased. The calculated Nutritional Imbalance Index (Nil) value of walnut spurs exposed to decreasing light intensities increased exponentially. The modification of the existing procedures of DRIS analysis that reflects the light exposure of the leaf and takes into account its DW component, is proposed.