UPTAKE AND ALLOCATION OF NITROGEN IN YOUNG PEACH-TREES AS AFFECTED BYTHE AMOUNT OF PHOTOSYNTHATES AVAILABLE IN ROOTS

In trees, the regulation of the nitrogen (N) uptake and allocation see ms not to be related directly to the photosynthetic activity of the le aves, but more probably to the result of the competition between the s inks which determines the amount of current photosynthates allocated t o the roots. We intend here to evaluate the effect of the photosynthat e supply to the roots on the (i) N uptake, (ii)N assimilation (i.e., n itrate reduction), and (iii)N allocation (i.e., N export from the root s). Therefore, three-year-old peach trees (Prunus persica L. Batsch) w ere girdled for 2 weeks at three different phenological stages to inte rrupt the transport of carbohydrates in the phloem from shoot to roots . A N-15 labeling of the nutrient solution was included in this treatm ent. Girdling strongly affected N uptake which decreased, according to the phenological stage to 19% of not-girdled trees. However, almost a ll the absorbed nitrate was reduced in the roots soon after its absorp tion in all the trees. The distribution of the absorbed N within the m ain plant parts was also strongly affected by girdling with N being al located to the organs which continued to be well provided with current photosynthates, i.e., mostly to the trunks and shoots. In contrast, g irdling did not affect the proportion of current N uptake incorporated into soluble N which represented, for both not-girdled and girdled tr ees, almost 60% of the current N uptake. It was concluded that the tot al N balance in trees was regulated by the current photosynthate suppl y to the roots.