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In obligate Crassulacean acid metabolism (CAM), up to 99 % of CO₂ assimilation occurs during the night, therefore supporting the hypothesis that CAM is adaptive because it allows CO₂ fixation during the part of the day with lower evaporative demand, making life in water-limited environments possible. By comparison, in facultative CAM (inducible CAM, C₃-CAM) and CAM-cycling plants drought-induced dark CO₂ fixation may only be, with few exceptions, a small proportion of C₃ CO₂ assimilation in watered plants and occur during a few days. From the viewpoint of survival the adaptive advantages, i.e. increased fitness, of facultative CAM and CAM-cycling are not obvious. Therefore, it is hypothesized that, if it is to increase fitness, CAM must aid in reproduction.

An examination of published reports of 23 facultative CAM and CAM-cycling species finds that …