Regulation of NO(3) Assimilation by Anion Availability in Excised Soybean Leaves

The regulation of NO(3) (-) assimilation by xylem flux of NO(3) (-) was studied in illuminated excised leaves of soybean (Glycine max L. Merr. cv Kingsoy). The supply of exogenous NO(3) (-) at various concentrations via the transpiration stream indicated that the xylem flux of NO(3) (-) was generally rate-limiting for NO(3) (-) reduction. However, NO(3) (-) assimilation rate was maintained within narrow limits as compared with the variations of the xylem flux of NO(3) (-). This was due to considerable remobilization and assimilation of previously stored endogenous NO(3) (-) at low exogenous NO(3) (-) delivery, and limitation of NO(3) (-) reduction at high xylem flux of NO(3) (-), leading to a significant accumulation of exogenous NO(3) (-). The supply of (15)NO(3) (-) to the leaves via the xylem confirmed the labile nature of the NO(3) (-) storage pool, since its half-time for exchange was close to 10 hours under steady state conditions. When the xylem flux of (15)NO(3) (-) increased, the proportion of the available NO(3) (-) which was reduced decreased similarly from nearly 100% to less than 50% for both endogenous (14)NO(3) (-) and exogenous (15)NO(3) (-). This supports the hypothesis that the assimilatory system does not distinguish between endogenous and exogenous NO(3) (-) and that the limitation of NO(3) (-) reduction affected equally the utilization of NO(3) (-) from both sources. It is proposed that, in the soybean leaf, the NO(3) (-) storage pool is particularly involved in the short-term control of NO(3) (-) reduction. The dynamics of this pool results in a buffering of NO(3) (-) reduction against the variations of the exogenous NO(3) (-) delivery.