Uptake and allocation of nitrogen in young peach trees as affected by the amount of photosynthates available in roots

In trees, the regulation of the nitrogen (N) uptake and allocation seems not to be related directly to the photosynthetic activity of the leaves, but more probably to the result of the competition between the sinks which determines the amount of current photosynthates allocated to the roots. We intend here to evaluate the effect of the photosynthate supply to the roots on the (i) N uptake, (ii)N assimilation (i.e., nitrate reduction), and (iii)N allocation (i.e., N export from the roots). Therefore, three-year-old peach trees (Prunus persica L. Batsch) were girdled for 2 weeks at three different phenological stages to interrupt the transport of carbohydrates in the phloem from shoot to roots. A N-15 labeling of the nutrient solution was included in this treatment. Girdling strongly affected N uptake which decreased, according to the phenological stage to 19% of not-girdled trees. However, almost all the absorbed nitrate was reduced in the roots soon after its absorption in all the trees. The distribution of the absorbed N within the main plant parts was also strongly affected by girdling with N being allocated to the organs which continued to be well provided with current photosynthates, i.e., mostly to the trunks and shoots. In contrast, girdling did not affect the proportion of current N uptake incorporated into soluble N which represented, for both not-girdled and girdled trees, almost 60% of the current N uptake. It was concluded that the total N balance in trees was regulated by the current photosynthate supply to the roots.