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Heterotrophic assimilation of nitrate in roots and leaves in darkness is closely linked with the oxidative pentose phosphate pathway. The supply of glucose-6-phosphate to roots and chloroplasts in leaves in darkness is essential for assimilation of nitrite into atnino acids. When green leaves are exposed to light, the key enzyme, glucose-6-phosphate dehydrogenase, is inhibited by reduction with thioredoxin. Hence the dark nitrate assitnilatory pathway is inhibited under photoautotrophic conditions and replaced by regulatory reactions funetioning in light. On account of direct photosynthetic reduction of nitrite in chloroplasts and availability of excess NADH for nitrate reductase, the rate of nitrate assimilation is extremely rapid in light. Under dark anaerobic conditions also nitrate is equally rapidly reduced to nitrite on account of abolition of competition for NADH between nitrate reduetase and mitochondrial oxidation.

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