Crassulacean acid metabolism and fitness under water deficit stress: if not for carbon gain, what is facultative CAM good for?

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, C-3-CAM) and CAM-cycling plants drought-induced dark CO2 fixation may only be, with few exceptions, a small proportion of C-3 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, in 19 species, drought-induced dark CO2 fixation represents on average 11 % of C-3 CO2 assimilation of watered plants. Evidence is discussed on the impact of the operation of CAM in facultative and CAM-cycling plants on their survival - carbon balance, water conservation, water absorption, photo-protection of the photosynthetic apparatus - and reproductive effort. It is concluded that in some species, but not all, facultative and cycling CAM contribute, rather than to increase carbon balance, to increase water-use efficiency, water absorption, prevention of photoinhibition and reproductive output.