Photosynthesis of Arid and Subtropical Succulent Plants

Abstract

The hypothesis of Crassulacean acid metabolism (CAM) is that it is a physiological adaptation to arid or otherwise dry habitats. Stomata are closed during the day and open at night when the evaporative demand is low. Thus exogenous CO2 is fixed at night with relatively little water loss. CAM is typically found in succulents occurring in desert and dry Mediterranean regions, but not in the cold deserts of Asia. Recently, it has become known that many arid tropical succulent plants are CAM as well, particularly those growing epiphytically. The vegetation of St. John, U.S. Virgin Islands, ranges from desertlike cacti at the windward, dry eastern side of the island to near tropical rainforest at the higher elevations. Native CAM plants are found in families Agavaceae, Bromeliaceae, Cactaceae, Clusiaceae, Orchidaceae, Piperaceae and Vitaceae. Exotic CAM species are in the families Aizoaceae, Crassulaceae, and Liliaceae. The distribution of these plants is entirely consistent with the hypothesis of CAM being an adaptation to arid habitats. All species in the Agavaceae and Cactaceae are CAM. All of the Bromeliaceae with the exception of the terrestrial Pitcairnia and the epiphytic Catopsis are CAM. The epiphytic and dry habitat orchids occurring in the genera Epidendrum, Oncidium, Vanilla, and Tetramicra are CAM. Of three species of Cissus, only the species occurring in the most arid zones, C. trifoliata, is CAM. Clusia rosea is enigmatic in that it is epiphytic when young and a rooted tree when mature. It is the only known genus of true dicotyledonous trees to have CAM. Two of the most serious exotic weeds naturalized in arid scrub areas are CAM, Sansevieria trifasciata and Bryophyllum pinnatum.