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The regulation of N03-assimilation by xylem flux of N03-was studied in illuminated excised leaves of soybean (Glycine max L. Merr. cv Kingsoy). The supply of exogenous N03-at various concentrations via the transpiration stream indicated that the xylem flux of N03-was generally rate-limiting for N03-reduction. However, N03-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 N03-at low exogenous N03-delivery, and limitation of N03-reduction at high xylem flux of N03-, leading to a significant accumulation of exogenous N03-. The supply of 15NO3-to the leaves via the xylem confirmed the labile nature of the N03-storage pool, since its half-time for exchange was close to 10 hours under steady state conditions. When the xylem flux of 15N03-increased, the proportion of the available N03-which was reduced decreased similarly from nearly 100% to less than 50% for both endogenous 4NO3-and exogenous '5NO3. This supports the hypothesis that the assimilatory system does not distinguish between endogenous and exogenous N03-and that the limitation of N03-reduction affected equally the utilization of N03-from both sources. It is proposed that, in the soybean leaf, the N03-storage pool is particularly involved in the short-term control of N03-reduction. The dynamics of this pool results in a buffering of N03-reduction against the variations of the exogenous N03-delivery.Nitrate assimilation in many herbaceous plants with sufficient N nutrition occurs principally in the illuminated leaves (1, 2). In these organs, the availability of NO3-to the NR3 enzyme is the main factor limiting the entire NO3-assimilation process (2, 24). To explain the observed limited availability, even when the leaves have accumulated large amounts of NO3-, many authors postulate the compartmentation of NO3-between a large storage pool and a small metabolic pool which controls induction and activity of NR (2,8,18 (24), reduction of NO3-should be regulated primarily by the flux of NO3-into this pool. In leaves, this flux may originate from two separate sources: the endogenous NO3-present in the storage pool, probably vacuolar (2, 16), and the exogenous NO3-translocated from the roots and delivered to the leaf by the xylem. Shaner and Boyer (27) showed that, in excised corn shoots, in vitro NRA was much more dependent on the xylem flux of NO3-than on the NO3-content of the leaves. Moreover, they noticed that this NO3-content was not modified by large variations of the xylem flux of NO3-, implying that NO3-reduction equalled NO3-delivery from the xylem. These results indicate that, in corn shoots, the metabolic pool is predominantly supplied by the exogenous NO3-translocated from the roots. In accordance with this hypothesis, the NO3-accumulated in the storage pool was shown to be nearly unavailable for reduction (8). From these results and others (19, 30), the conclusion has emerged that NO3-reduct...

An in situ method, derived from an in vivo method, was used to determine nitrate reductase activity (NRA) in: i) excised barley and corn shoots and excised soybean leaves during a N-depletion experiment and; ii) roots and shoots of N-depleted barley and corn seedlings during induction of nitrate reductase (NR). Nitrate reduction, calculated from these in situ NRA measurements, was compared with estimates of each organ's nitrate reduction in light aerobic conditions from NO~-consumption and a ~SN model (Gojon et al., 1986b).The in situ NRA of roots strongly underestimated their ~5NO3 reduction. In contrast, in barley and corn shoots and in the first trifoliolate leaves from 26-day-old soybean, the in situ NRA assay gave a fair approximation of the true NO3 reduction rate (relative differences ranging from -14 to + 32%). In young soybean leaves (from 20-day-old plants), however, the in situ NRA strongly underestimated the actual NO~-reduction. The physiological significance of the in situ NRA assay in shoots and roots, and its value for field studies are discussed from these results.