CO2‐concentrating: consequences in crassulacean acid metabolism

The consequences of CO2‐concentrating in leaf air‐spaces of CAM plants during daytime organic acid decarboxylation in Phase III of CAM (crassulacean acid metabolism) are explored. There are mechanistic consequences of internal CO2 partial pressures, piCO2. These are (i) effects on stomata, i.e. high piCO2 eliciting stomatal closure in Phase III, (ii) regulation of malic acid remobilization from the vacuole, malate decarboxylation and refixation of CO2 via Rubisco (ribulose bisphosphate carboxylase/oxygenase), and (iii) internal signalling functions during the transitions between Phases II and III and III and IV, respectively, in the natural day/night cycle and in synchronizing the circadian clocks of individual leaf cells or leaf patches in the free‐running endogenous rhythmicity of CAM. There are ecophysiological consequences. Obvious beneficial ecophysiological consequences are (i) CO2‐acquisition, (ii) increased water‐use‐ efficiency, (iii) suppressed photorespiration, and (iv) reduced oxidative stress by over‐energization of the photosynthetic apparatus. However, the general potency of these beneficial effects may be questioned. There are also adverse ecophysiological consequences. These are (i) energetics, (ii) pH effects and (iii) Phase III oxidative stress. A major consequence of CO2‐concentrating in Phase III is O2‐concentrating, increased piCO2 is accompanied by increased piO2. Do reversible shifts of C3/CAM‐intermediate plants between the C3–CAM–C3 modes of photosynthesis indicate that C3‐photosynthesis provides better protection from irradiance stress? There are many open questions and CAM remains a curiosity.