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.