NON-PHOTOSYNTHETIC FIXATION OF CARBON DIOXIDE AND POSSIBLE BIOLOGICAL ROLES IN HIGHER PLANTS

1. Non-photosynthetic fixation of CO₂/HCO₃^- occurs both under light and dark conditions and involve the addition of carbon to substrates which in higher plants are derived originally from carbon reduced to carbohydrates during photosynthesis. Despite the endergonic nature of these carboxylations, the advantages offered seem to be sufficient to outweigh the disadvantages of energy loss. 2. Non-photosynthetic carbon incorporation into metabolism is dealt mainly in relation to PEP carboxylase, acetyl-CoA carboxylase, carbamoyl phosphate synthetase and phosphoribosylaminoimidazole carboxylase while other carboxylases await further characterization or discovery. The extent to which a carboxylase participates depends upon the need for products of its activity in metabolism. 3. Non-photosynthetic carbon fixation is intricately involved in several pathways of metabolism throughout the ontogeny of plants. The roles in relation to leaf carbon metabolism, respiratory metabolism, nitrogen metabolism, lipid and isoprenoid biosynthesis, purine and pyrimidine metabolism and metabolism associated with the action of growth regulators have been described. The fixation reactions appear to be largely concerned with the production of intermediary metabolites, circumvention of energy barriers in metabolism and regulation of plant metabolism. In addition, the activity of PEP carboxylase is involved in ionic balance and pH-stat. 4. Malate derived by way of PEP carboxylase and NAD-malate dehydrogenase acts as an effective osmoticum and a counter-ion for K⁺ accumulation in actively growing plant cells. In addition, malate may enter the TCA cycle or can be decarboxylated by cytoplasmic NADP-malic enzyme converting NADH to NADPH. Wherever it has been sought in different plant tissues, some evidence for PEP carboxylase and metabolism of malate has always been found. 5. Almost every plant process spanning from seed development and germination to flowering and fruit-set requires the essential participation of non-photosynthetic carbon fixation in regulating certain metabolic and cellular functions but it does not contribute in a major way to the carbon nutrition of plants. It is largely the tissue type that appears to determine which of the roles is predominant at any one time.