DeJong, T.M.; Weinbaum, S.A.; Muraoka, T.T.- Department of Pomology, University of California, Davis, CA 95616

Exposure to photosynthetically active radiation and the consequent effect on leaf mass per unit leaf area (SLW) and nitrogen (percent dry weight and μg·mm^-2) allocation within tree canopies was investigated in walnut (Juglans regia `Serr' and `Hartley') trees. Percent contribution of discrete light flux densities below light saturation (100-700 μmol·s^-1·m^-2) to the total light exposure of individual spurs, exposed up to 9 hour·day^-1 to saturating light (>700 μmol·s^-1·m^-2), was minimal (<1 hour), indicating that individual spurs were either exposed or shaded most of the day. SLW and N content per unit leaf area of individual spurs were highly correlated (second-order polynomial curve fit) with light exposure within the tree canopy, indicating uneven allocation of available N for optimal utilization. Nitrogen expressed as percent dry weight was not correlated with light exposure and SLW. Leaf N content per leaf area was highly correlated (linear fit) with SLW.