Effect of photosynthetic inhibitors and uncouplers of oxidative phosphorylation on nitrate and nitrite reduction in barley leaves

The effects of several photosynthetic inhibitors and uncouplers of oxidative phosphorylation on NO₃⁻ and NO₂⁻ assimilation were studied using detached barley (Hordeum 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 both light and darkness, while photosynthetic inhibitors did not.

The NO₂⁻ concentration in the control leaves was very low in both 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. It appears that 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.