Sources of reductant for nitrate assimilation in non-photosynthetic tissue: a review

Abstract. Potential sources of reductant for the pathway of nitrate assimilation in non-photosynthetic tissue are considered in terms of the intracellular distribution of the enzymes involved (relative to those of nitrate assimilation), their maximum catalytic capacity, the availability of their respective substrates, and the form in which the reductant is produced. According to these criteria, the principal sources appear to be the reactions catalysed by NAD-glyceraldehyde-3-phosphate dehydrogenase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, NAD-malate dehydrogenase and NADP-malic enzyme. Mitochondrial metabolism may also make a limited contribution, although the evidence is inconclusive. The capacity to generate reductant by carbohydrate oxidation within the proplastids appears insufficient to meet the demand, and an influx of reducing power from the cytoplasm in the form of dihydroxy acetone phosphate may be necessary to support nitrite reduction and the glutamate synthase reaction which occur there.