REGULATION OF NO3- ASSIMILATION BY ANION AVAILABILITY IN EXCISED SOYBEAN LEAVES

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 (NO3)-N-15- 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 (NO3)-N-15- increased, the proportion of the available NO3- which was reduced decreased similarly from nearly 100% to less than 50% for both endogenous (NO3)-N-14- and exogenous (NO3)-N-15-. 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.