Characterization of Early Morning Crassulacean Acid Metabolism in Opuntia erinacea var Columbiana (Griffiths) L. Benson

The nature and sequence of metabolic events during phase II (early morning) Crassulacean acid metabolism in Opuntia erinacea var columbiana (Griffiths) L. Benson were characterized. Gas exchange measurements under 2 and 21% O₂ revealed increased O₂ inhibition of CO₂ fixation with progression of phase II. Malate and titratable acidity patterns indicated continued synthesis of C₄ acids for at least 30 minutes into the light period. Potential activities of phosphoenolpyruvate carboxylase (PEPC) and NADP-malic enzyme exhibited little change during phase II, while light activation of NADP-malate dehydrogenase, pyruvate, orthophosphate dikinase, and ribulose-1,5-bisphosphate carboxylase was apparent. Short-term ¹⁴CO₂ fixation experiments showed that the per cent of ¹⁴C incorporated into C₄ acids decreased while incorporation into other metabolites increased with time. PEPC exhibited increased sensitivity to 2 millimolar malate, and the K_i(malate) for PEPC decreased markedly with time. Sensitivity of PEPC to malate inhibition was considerably greater at pH 7.5 than at 8.0. The results indicate that decarboxylation and synthesis of malate occur simultaneously during the early morning period, and that phase II acid metabolism is not limited by CO₂ diffusion through stomata. With progression of phase II, CO₂ fixation by PEPC decreases while fixation by ribulose-1,5-bisphosphate carboxylase increases.