Distribution of NO3− reduction between roots and shoots of peachtree seedlings as affected by NO3− uptake rate

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Abstract
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.