Table I.

NR activity in wild-type and NR mutants

Genotype	NR Activity Shoot	NR Activity Root
Wild type	28.1 ± 0.3	4.7 ± 1.5
G5 (chl3-5)	2.6 ± 0.7	ND
G′4-3(chl3-5/nia1-1)	0.14 ± 0.02	ND
NR-null mutant (chl3-5/nia1-2)	0.00 ± 0.00	0.00 ± 0.00

Arabidopsis were grown in pots with vermiculite soil irrigated with medium containing 2.5 mm (ammonium)₂succinate. After 10 d, plants were irrigated with medium containing 5 mm NH₄NO₃. After 24 h leaves were collected and assayed for NR activity as described by Redinbaugh and Campbell (1983). NR activity is expressed as nmol nitrite/(mg protein min). ND, Not determined. For root measurements, plants were grown hydroponically with 2.4 mm (ammonium)₂succinate for 10 d, then irrigated with 5 mm NH₄NO₃ for 6 h before harvesting roots.