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

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Abstract
The nature and sequence of metabolic events during phase II (early morning) Crassulacean acid metabolism in O. 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 min 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 percent 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 Ki (malate) for PEPC decreased markedly with time. Sensitivity of PEPC to malate inhibition was considerably greater at pH 7.5 than at 8.0. Decarboxylation and synthesis of malate occur simultaneously during the early morning period and 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.