A morning peak in acidity during phase II crassulacean acid metabolism in Opuntia erinacea, echinocereus echinocereus and Mammillaria vivipara

Abstract Previous investigations of metabolic activity in species utilizing crassulacean acid metabolism (CAM) have revealed a peak in the rate of CO 2 uptake following a brief decrease during the early morning period (Phase II). The present study demonstrates the existence of a similar peak in acid concentration for a native population of Opuntia erinacea (Cactaceae). Laboratory investigations of this phenomenon in O. erinacea, Echinocereus viridiflorus and Mammillaria vivipara indicate that Phase II acid concentration peaks within a maximum of 15–20 min after CO 2 uptake has peaked. The timing of these phenomena indicates a complexity of carboxylation and decarboxylation processes during CAM Phase II.