REGULATION OF RESERVE CARBOHYDRATE METABOLISM IN MESEMBRYANTHEMUM CRYSTALLINUM EXHIBITING C3 AND CAM PHOTOSYNTHESIS

Abstract

SummaryDifferences in carbohydrate metabolism between the C3 and CAM forms of Mesembryanthemum crystallinum L. were demonstrated and studied. The CAM form accumulated less starch (defined as boiling water‐insoluble polyglucan) than the C3 form at high irradiances, but at limiting light diverted a greater proportion of photosynthate to starch. Starch accumulation was more sensitive to the effects of exogenous inorganic phosphate in the C3 form than in the CAM form.Following induction of CAM by salt treatment, the CAM form ceased to accumulate glucose and fructose and commenced accumulating pinitol. Pinitol is not involved directly in CAM but may act as a compatible osmoregulatory solute.Concentrations of fructose 2,6‐bisphosphate in the C3 and CAM forms were similar, but the cytosolic fructose 1,6‐bisphophatase extracted from the two forms was markedly different in sensitivity to this effector. The two enzymes also differed both in their sensitivity to inorganic phosphate and in their affinity for fructose 1,6‐bisphosphate.The concentrations of key intermediates of carbohydrate metabolism, fructose and glucose 6‐phosphate and fructose 1,6‐bisphosphate were measured in the C3 and CAM forms. Concentrations of fructose 1,6‐bisphosphate were higher in the CAM form, whereas those of glucose and fructose 6‐phosphate were higher in the C3 form.The differences observed are discussed in relation to the regulation of storage carbohydrate metabolism in the two forms. A partial explanation is offered for the differences in the characteristics of starch accumulation between the C3 and CAM forms through the influence of the effector fructose 2,6‐bisphosphate on the enzyme fructose 1,6‐bisphosphatase.