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

Abstract

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 CO2 fixation and extractable NR activity from the leaves, and feeding hexose and hexosephosphate 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 CO2 fixation. To corroborate this point, the effect of inhibiting CO2 fixation on NR activity in barley leaves has been analyzed. Glycolaldehyde (50 mM), an inhibitor of the regeneration phase of the Calvin cycle, was fed through the transpiration stream and inhibited CO2 fixation by more than 80% at the same time as it produced a parallel inhibition of NR light‐activation. Feeding mannose (10 mM), inhibited CO2 fixation by 35% but did not affect NR activity in illuminated leaves and completely protected against dark‐inactivation. Interestingly, feeding inorganic phosphate, Pi, (10 mM) alone or together with mannose also protected NR activity against dark‐inactivation. The mannose effect could be interpreted in terms of accumulation of mannose 6‐phosphate, an analog of glucose 6‐phosphate. After feeding either 10 mM glucose or dihydroxyacetone phosphate, NR activity from darkened leaves was significantly higher than that of darkened control leaves fed with water (P< 0.03). These treatments, as well as Pi feeding, also produce some increase in extractable NR activity from illuminated leaves. The results indicate that factors increasing the levels of hexose‐ and triose‐phosphate have positive effects on NR activation, supporting the contention that the NR activation system is sensitive to carbon‐assimilation products.