Regulation of nitrogen partitioning in field-grown almond trees: Effects of fruit load and foliar nitrogen applications

Two treatments were employed to influence the amount of amino nitrogen (N) transport in phloem. In walnut trees (Juglans regia L.), developing fruit s ignificantly reduced the efflux of foliar-applied N-15-enriched urea from t reated spurs over a 33-day period in comparison with similarly-treated defr uited spurs. Those data suggest that local aboveground demand for N influen ces vascular transport of amino N. In another experiment, a 1% urea solutio n was applied foliarly to 5-year old 'Mission' almond trees [Prunus dulcis (Mill.) D. A. Webb] to increase the concentration of amino N in the phloem. The effect of foliar N treatments on a) the transport and distribution of labelled urea N within the trees over the experimental period and b) the up take of soil-applied labelled N were determined by replicated whole tree ex cavation, fractionation into various tree components and mass spectrometric analyses of the N-14/N-15 ratios. Concentrations and composition of amino acids in the phloem and xylem saps of control trees and trees receiving fol iar-applied urea were also determined. In foliar urea-treated trees, the am ino acid concentrations increased significantly in leaf and bark phloem exu date, within 24 and 96 h, respectively. Foliar-applied urea N was transloca ted to the roots of almond trees over the experimental period and decreased soil N uptake. The results of these experiments are consistent with the hy pothesis that aboveground N demand affects the amount of amino N cycling be tween shoots and roots, and may be involved in the regulation of soil N upt ake.