ROLE OF LIGHT AND CO2 FIXATION IN THE CONTROL OF NITRATE-REDUCTASE ACTIVITY IN BARLEY LEAVES

Nitrate reductase (NR, NADH:nitrate oxidoreductase, EC 1.6.6.1) from b arley (Hordeum vulgare L. cv. Hassan) leaves was inactivated during a light-dark transition, losing approx. 50% of activity after 30 min of darkness. The dark inactivation was reversed by illumination of the se edlings, the kinetics of reactivation being similar to those of inacti vation. High extractable NR activity and significant differences betwe en illuminated and darkened leaves were observed in media containing E DTA and inorganic phosphate (P(i)). Addition of Ca2+ ions during extra ction and assay decreased NR activity from illuminated and darkened le aves, enhancing the light-dark difference. While no clear correlation could be found between irradiance and NR activity, a hyperbolic correl ation appeared between extractable NR activity and in-vivo rates of CO 2 fixation, indicating that NR activation follows saturation kinetics with respect to CO2 fixation. Furthermore, hexoses and hexose-phosphat es fed to the leaves via the transpiration stream protected against th e dark-inactivation of NR. The results indicate that carbon-assimilati on products are regulatory factors of NR activity in barley leaves, me diating both the light-dark modulation of NR and its dependence upon C O2 fixation.