Immediately after unfolding, cotyledons of the tropical platyopuntoid cactus, Opuntia elatior Mill., exhibited a C3-type diel CO2 exchange pattern characterized by net CO2 uptake in the light. Significant nocturnal increases in titratable acidity typical of crassulacean acid metabolism (CAM) were not detected at this early developmental stage. As cotyledons matured and the first cladode (flattened stem) developed, features of CAM were observed and the magnitude of CAM increased. Nonetheless, in well-watered seedlings up to 10 cm tall, C3 photosynthetic CO2 fixation in the light remained the major pathway of carbon fixation. Reduced soil water availability led to an up-regulation of net dark CO2 fixation and greater nocturnal increases in tissue acidity, consistent with facultative CAM. These observations demonstrate that C3 photosynthesis, drought-stress-related facultative CAM, and developmentally controlled constitutive CAM can all contribute to the early growth of O. elatior. The strong C3 component and facultative CAM features expressed in young O. elatior contrast with mature plants in which obligate CAM is the major pathway of carbon acquisition.
