What Group of Plants Use Crassulacean Acid Metabolism?

Plant Groups Using CAM Photosynthesis

• No single plant group uses CAM photosynthesis; many different plants evolved the process convergently depending on specific ecological needs. CAM is named for the family of desert plants in which it was initially discovered, Crassulaceae, which comprises familiar houseplants such as the jade plant (Crassula argentea) and hen-and-chicks (Sempervivum tectorum). Plants that use this type of photosynthesis, including many members of the cactus and agave family as well as the odd collection of plants that make up the spurge family, usually have succulent leaves. However, several nonsucculents also use CAM, including some species of orchids, ferns, grapes and lilies. The total number of plants using CAM photosynthesis is unknown, but biologists estimate roughly 1,000 species in 17 different genera use CAM photosynthesis.

How CAM Photosynthesis Works

• Most plants use one of two photosynthetic processes that take place during daylight hours, taking up carbon dioxide through pores in the leaves called stomata. The third group, or CAM plants, usually grow in harsher settings and are able to close these pores during the day, instead taking up carbon dioxide at night when conditions are generally milder. This carbon dioxide is converted to the 4-carbon malic acid, which is stored in special cells called vacuoles. When morning arrives and plant stomata close, CAM plants retrieve the malic acid and convert it back into carbon dioxide, which is then transformed through the normal photosynthetic process to produce sugar and oxygen.

Advantages and Disadvantages

• CAM plants are often found in areas with extremes in temperature, salinity and atmospheric humidity, such as the desert, in coastal areas, windy sites and rocky, elevated zones. Some plants are able to switch to CAM from regular photosynthesis during unusually harsh conditions, such as the ice plant (Mesembryanthemum spp.) This adaptability comes at a cost, however: CAM plants are typically less cold resistant because of their tendency to store quantities of water in their leaves and also usually grow much more slowly than other types of plants.

Familiar Plants

• Probably the largest group of plants with CAM photosynthesis recognizable to the average gardener is the sedum family (Sedum spp.). Many sedums are hardy in North America, including many colorful, frost-tender sedum species. Bromeliads, including the pineapple (Ananas comosus) and the tree-draping Southern stalwart, Spanish moss (Tillandsia usneoides), are also CAM plants. Often used as a houseplant or office accent, the glossy, dark-leaved kalanchoe (Kalanchoe spp.) is also classified as a CAM plant.