Distribution of Nitrate Reductase and Nitrite Reductase

Let us see whether nitrate assimilation depends upon the reductants produced in photosynthesis or in oxidative metabolism. It is now known that nitrate reductase and nitrite reductase are located in the photosynthetic membranes of cyanobacteria. In higher plants (C₃) nitrate reductase has been shown to be a cytoplasmic enzyme and nitrite reductase a chloroplastic enzyme.

In C₄ plants nitrate assimilation and carbon dioxide assimilation are compartmentalised the former located in mesophyll cells and the latter is present in bundle sheath cells. It is observed that C₄ plants have greater efficiency of nitrate assimilation, in comparison to C₃ plants. This is due to the special separation of nitrate and nitrite reduction from the site of CO₂ reduction. Cytoplasmic nitrate reductase and chloroplastic nitrite reductase are characteristic of mesophyll cells and not of bundle sheath cells. In non-green tissues, like roots of higher plants nitrate and nitrite reductase also occur in active form.