Physiological adaptation to arid environments in many desert succulents involves crassulacean acid metabolism (CAM) with modified photosynthetic carbon acquisition aimed at improving water use efficiency. The leafless perennial Caralluma acutangula is an abundant stem succulent in the arid regions of subtropical zones of the Arabian Peninsula. These arid regions southwest of Saudi Arabia are characterized by a short three months wet season (June–August) and a long dry season. The wet season is characterized by high temperature, high evaporation, and strong sand storms. Research presented in this paper involved water status, diffusive conductance, and crassulacean acid metabolism (CAM) of both seedlings and mature C. acutangula plants growing under natural conditions. Plant water status was determined by monitoring changes in stem water content, diffusive conductance was monitored using porometry technique, and CAM was detected by determining chlorenchyma titratable acidity. Seedling establishment of C. acutangula takes place in the short wet season during which seedlings depend on the high productivity C3 mode of photosynthesis. Mature plants are obligate CAM plants exhibiting this mode of carbon fixation during both the wet and the dry seasons. As the dry season progresses dampening of CAM acidification-deacidification cycles and low stomatal conductance denote the switch of mature plants from CAM to CAM-idling in response to protracted water stress. Our results show that the C3 mode of photosynthesis in C. acutangula seedlings during early ontogeny is due to exposure of seedlings to suboptimal irradiance in the shade of other plant species at the study site. This C3 mode of photosynthesis is beneficial for fast seedling establishment. Our results also show that the shift from CAM to CAM-idling in mature C. acutangula plants occurs in response to protracted water stress, which is favourable for survival of this succulent in its inimical arid habitat.
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