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) activi ty from leaves of barley (Hordeum vulgare L. cv. Hassan) is rapidly an d reversibly inactivated during a light-dark transition. A hyperbolic correlation exists between in vivo rates of CO2 fixation and extractab le NR activity from the leaves, and feeding hexose and hexose-phosphat e protects against the dark-inactivation; indicating that carbon-assim ilation products are regulatory factors of NR activity mediating both the light-dark modulation and its dependence upon CO2 fixation. To cor roborate this point, the effect of inhibiting CO2 fixation on NR activ ity in barley leaves has been analyzed. Glycolaldehyde (50 mM), an inh ibitor of the regeneration phase of the Calvin cycle, was fed through the transpiration stream and inhibited CO2 fixation by more than 80% a t the same time as it produced a parallel inhibition of NR light-activ ation. Feeding mannose (10 mM) inhibited CO2 fixation by 35% but did n ot affect NR activity in illuminated leaves and completely protected a gainst dark-inactivation. Interestingly, feeding inorganic phosphate. P-i, (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 B- phosphate. After feeding either 10 mM glucose or dihydroxyacetone phos phate, NR activity from darkened leaves was significantly higher than that of darkened control leaves fed with water (P<0.03). These treatme nts, as well as P-i feeding, also produce some increase in extractable NR activity from illuminated leaves. The results indicate that factor s increasing the levels of hexose- and triose-phosphate have positive effects on NR activation, supporting the contention that the NR activa tion system is sensitive to carbon-assimilation products.