Résumé
The regulation of NO3− assimilation by xylem flux of NO3− was studied in illuminated excised leaves of soybean (Glycine max L. Merr. cv Kingsoy). The supply of exogenous NO3− at various concentrations via the transpiration stream indicated that the xylem flux of NO3− was generally rate-limiting for NO3− reduction. However, NO3− assimilation rate was maintained within narrow limits as compared with the variations of the xylem flux of NO3−. This was due to considerable remobilization and assimilation of previously stored endogenous NO3− at low exogenous NO3− delivery, and limitation of NO3− reduction at high xylem flux of NO3−, leading to a significant accumulation of exogenous NO3−. The supply of 15NO3− to the leaves via the xylem confirmed the labile nature of the NO3− storage pool, since its half-time for exchange was close to 10 hours under steady state conditions. When the xylem flux of 15NO3− increased, the proportion of the available NO3− which was reduced decreased similarly from nearly 100% to less than 50% for both endogenous 14NO3− and exogenous 15NO3−. This supports the hypothesis that the assimilatory system does not distinguish between endogenous and exogenous NO3− and that the limitation of NO3− reduction affected equally the utilization of NO3− from both sources. It is proposed that, in the soybean leaf, the NO3− storage pool is particularly involved in the short-term control of NO3− reduction. The dynamics of this pool results in a buffering of NO3− reduction against the variations of the exogenous NO3− delivery.