Crassulacean Acid Metabolism. A Plastic Photosynthetic Adaptation to Arid Environments

Extract

Crassulacean acid metabolism (CAM) is an important elaboration of photosynthetic carbon fixation that allows chloroplast-containing cells to fix CO₂initially at night using phosphoenolpyruvate carboxylase (PEPC) in the cytosol. This leads to the formation of C₄ organic acids (usually malate), which are stored in the vacuole. Subsequent daytime decarboxylation of these organic acids behind closed stomata creates an internal CO₂ source that is reassimilated by Rubisco in the chloroplast. The refixation of this internal CO₂ generates carbohydrates via the conventional photosynthetic carbon reduction cycle. Thus, CAM involves a temporal separation of carbon fixation modes in contrast to the spatial separation found in C₄ plants. The first recognition of the nocturnal acidification process can be traced to the Romans, who noted that certain succulent plants taste more bitter in the morning than in the evening (Rowley, 1978). However, formal descriptions of the ability of succulent plants to conduct nocturnal CO₂ fixation or to acidify photosynthetic tissues at night and deacidify them during the day did not appear until the early 19th century (de Saussure, 1804; Heyne, 1815). The term CAM was coined to give credit to Heyne's observations that were made usingBryophyllum calycinum, a succulent member of the Crassulaceae.