Effect of Photosynthetic Inhibitors and Uncouplers of Oxidative Phosphorylation on Nitrate and Nitrite Reduction in Barley Leaves

Abstract

The effects of several herbicides that act as photosynthetic inhibitors and uncouplers of oxidative phosphorylation on NO3- and NO2- assimilation were studied using detached barley (H. vulgare L. cv. Numar) leaves in which only endogenous NO3- or NO2- were available for reduction. Uncouplers of oxidative phosphorylation greatly increased NO3- reduction in light and darkness, while photosynthetic inhibitors did not. The NO2- concentration in the control leaves was very low in light and darkness; 98% or more of the NO2- formed from NO3- was further assimilated in control leaves. More NO2- accumulated in the leaves in light and darkness in the presence of photosynthetic inhibitors. Of this NO2-, 94% or more was further assimilated. Apparently, metabolites, either external or internal to the chloroplast, capable of reducing NADP (which, in turn, could reduce ferredoxin via NADP reductase) might support NO2- reduction in darkness and light when photosynthetic electron flow is inhibited by photosynthetic inhibitors. Nitrite assimilation was much more sensitive to uncouplers in darkness than in light: in darkness, 74% or more of NO2- formed from NO3- was further assimilated, whereas in light, 95% or more of the NO2- was further assimilated.