INTRACANOPY VARIATION IN NITROGEN CYCLING THROUGH LEAVES IS INFLUENCED BY IRRADIANCE AND PROXIMITY TO DEVELOPING FRUIT IN MATURE WALNUT TREES

Intracanopy variation in net leaf nitrogen (N) resorption and N cyclin g through leaves in mature walnut (Juglans regia L. cv Hartley) trees were monitored in 3 different years. Differential irradiance among the spurs sampled was inferred from differences among leaves in dry weigh t per unit area (LW/LA) which varied from 4.0 mg . cm-2 to 7.0 mg . cm -2 in shaded (S) and exposed (E) canopy positions, respectively. Our r esults, using N-15-depleted (NH4)2SO4 validated the concept that N inf lux and efflux through fully expanded leaves occurred concurrently dur ing the period of embryo growth. Additionally, it also suggested that N influx into leaves was substantially greater in exposed as compared with shaded canopy positions. Because of its well documented phloem im mobility, leaf Ca accumulation was used to better estimate the relativ e influx of N into exposed and shaded leaves. N cycling varied locally within the tree canopy, i.e. Ca (and presumably N) influx was 100% gr eater in exposed than shaded tree canopy positions, but influx was not influenced significantly by the proximity of developing fruit. In con trast, both the amount and percentage N efflux was significantly great er during embryo growth in fruit-bearing than defruited spurs. Net lea f N resorption averaged 2-4 times greater (25-30%) in fruit-bearing sp urs than the 5-10% decrease in the leaf N content in defruited spurs. Since about 90% of leaf N content reportedly occurs as protein, fruit N demand apparently influenced protein turnover and catalysis in assoc iated spur leaves. The amount of leaf N resorption was greater in expo sed than shaded positions in the tree canopy in 2 of the 3 years of da ta collection. Our data show that like leaf N content, N influx, N eff lux and net leaf N resorption vary throughout mature walnut tree canop ies under the combined local influences of fruiting and irradiance.