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 significantly reduced the efflux of foliar-applied N-15-enriched urea from treated spurs over a 33-day period in comparison with similarly-treated defruited spurs. Those data suggest that local aboveground demand for N influences vascular transport of amino N. In another experiment, a 1% urea solution 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 uptake of soil-applied labelled N were determined by replicated whole tree excavation, 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 foliar-applied urea were also determined. In foliar urea-treated trees, the amino acid concentrations increased significantly in leaf and bark phloem exudate, within 24 and 96 h, respectively. Foliar-applied urea N was translocated to the roots of almond trees over the experimental period and decreased soil N uptake. The results of these experiments are consistent with the hypothesis that aboveground N demand affects the amount of amino N cycling between shoots and roots, and may be involved in the regulation of soil N uptake.