Involvement of CO2 fixation products in the light-dark modulation of nitrate reductase activity in barley leaves

Nitrate reductase (NR, NADH:nitrate oxidoreductase, EC 1.6.6.1) activity from leaves of barley (Hordeum vulgare L. cv. Hassan) is rapidly and reversibly inactivated during a light-dark transition. A hyperbolic correlation exists between in vivo rates of CO 2 fixation and extractable NR activity from the leaves, and feeding hexose and hexose-phosphate protects against the dark-inactivation; indicating that carbon-assimilation products are regulatory factors of NR activity mediating both the light-dark modulation and its dependence upon CO 2 fixation. To conroborate this point, the effect of inhibiting CO 2 fixation on NR activity in barley leaves has been analyzed