During vegetative growth (left), nitrate ions are rapidly absorbed by the root and transported via the xylem to the leaf. In the leaf, nitrate reduction produces organic acids (OA) and amino acids (AA). Most of the newly formed OA are translocated to the root where a carboxyl group is released in exchange for a nitrate ion, whereas the newly assimilated N is incorporated primarily into leaf N compounds (protein) (Ismande and Touraine, 1994).

During rapid pod fill (right), leaf proteolysis occurs and much of the amino N in the leaf is exported to the filling pods. Consequently the phloem is enriched with amino compounds, which repress nitrate uptake and consequently diminish the rate of nitrate reduction (Ismande and Touraine, 1994).

The amide asparagine inhibits both the uptake and reduction of nitrate, but glutamine inhibits only reduction. The inhibition of nitrate reduction by the amides is associated with inhibition of induction of both NR and NiR in maize seedlings (Sivasankar et al, 1997). Sucrose enhances expression of both NR and NiR (Sivasankar et al, 1997). Expressions of the root nitrate transporter genes Nrt2;1 and Nrt1 are diurnally regulated in photosynthetically active Arabidopsis thaliana plants (Lejay et al, 1999). Both increase during the light period and decrease in the first hours of the dark period (Lejay et al, 1999). Sucrose supply prevents the inhibition of Nrt2;1 and Nrt1 expression in the dark (Lejay et al, 1999).

Ismande J, Touraine B 1994 N demand and the regulation of nitrate uptake. Plant Physiol. 105: 3-7.

Lejay L, Tillard P, Lepetit M, Olive Fd, Filleur S, Daniel-Vedele F, Gojon A 1999 Molecular and functional regulation of two NO₃^- uptake systems by N- and C-status of Arabidopsis plants. Plant J. 18: 509-519.

Sivasankar S, Rothstein S, Oaks A 1997 Regulation of the accumulation and reduction of nitrate by nitrogen and carbon metabolites in maize seedlings. Plant Physiol. 114: 583-589.