On the evolutionary pathways resulting in C₄ photosynthesis and crassulacean acid metabolism (CAM)

Plants with C₄ photosynthesis have a net photosynthetic rate which continues to increase with increasing radiation flux rather than levelling off above a saturation level, as it does in C₃ plants. The initial evolutionary stages of C₄ photosynthesis are argued to have been driven by selection for reduced photorespiration rates and the influence of such reduction on net CO₂ assimilation rate. Plants with CAM conserve water by maintaining their stomata open at night, and absorbing CO₂, but closed during the day; this is done by dissociating CO₂ fixation from that of energy capture. Assimilation of respired CO₂ during the night, with assimilation of atmopsheric CO₂ during the day is proposed as a precursor to the evolution of fully-expressed CAM in some species. Both C₄ and CAM plants have arisen from C₃ ancestors. Thus both C₄ photosynthesis and CAM may both have originated from mechanisms that improved the plant's carbon balance by assimilating internally-generated CO₂. Comments are included on costs associated with each pathway. -P.J.Jarvis