• Nitrogen transport;
  • NO3 reduction;
  • NO3 uptake;
  • peach-tree;
  • Prunus persica;
  • root;
  • shoot

The distribution of NO3 reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings (Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO3, together with transport through xylem and phloem of the newly reduced N were estimated, using 15N labellings, in intact plants supplied for 90 h with 0.5 mM NH4+ and 0.5, 1.5 or 10 mM NO3. Xylem transport of NO3 was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO3 reduction at all 3 levels of NO3 nutrition. However, the contribution of the shoots to the whole plant NO3 reduction increased with increasing external NO3 availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 mM NO3, respectively. Both 15N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO3 translocation from roots to shoots. It is proposed that the lack of NO3 export to the shoots at low NO3 uptake rate resulted from a competition between NO3 reduction in the root epidermis/cortex and NO3 diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO3 nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO3 assimilation, principally occurring in the roots, was mainly incorporated in the shoots.