ABSTRACT
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.
ABSTRACT
Nitrate reduction in roots and shoots of 7-day-old barley seedlings, and 9-day-old corn seedlings was investigated. The N-depleted seedlings were transferred for 24 h or 48 h of continuous light to a mixed nitrogen medium containing both nitrate and ammonium. Total nitrate reduction was determined by (15)N incorporation from (15)NO(3) (-), translocation of reduced (15)N from the roots to the shoots was estimated with reduced (15)N from (15)NH(4) (+) assimilation as tracer, and the translocation from the shoots to the roots was measured on plants grown with a split root system. A model was proposed to calculate the nitrate reduction by roots from these data. For both species, the induction phase was characterized by a high contribution of the roots which accounted for 65% of the whole plant nitrate reduction in barley, and for 70% in corn. However, during the second period of the experiment, once this induction process was finished, roots only accounted for 20% of the whole plant nitrate reduction in barley seedlings, and for 27% in corn. This reversal in nitrate reduction localization was due to both increased shoot reduction and decreased root reduction. The pattern of N exchanges between the organs showed that the cycling of reduced N through the plant was important for both species. In particular, the downward transport of reduced N increased while nitrate assimilation in roots decreased. As a result, when induction was achieved, the N feeding of the roots appeared to be highly dependent on translocation from the leaves.
