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Metabolism and Enzymology

Regulation of NO₃^– Assimilation by Anion Availability in Excised Soybean Leaves

Biochimie et Physiologie Végétales, INRA-ENSA URA CNRS 573, 34060 Montpellier Cedex 01, France

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.

² Present address: Station d'Agronomie et de Physiologie Végétale, INRA, 45 Boulevard du Cap, 06606 Antibes, France.

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