Cacti possess a number of interesting metabolic features, most of which are related to the fact that the plants usually occur in regions with a limited supply of water for at least part of the year. Sue-
culence is one such feature, and some cacti, when fully hydrated, consist of almost 95% water. Remarkably, they can become desiccated and survive to the point at which water content is as low as 20% (Gibson and Nobel 1986,9).
Cacti also have the capability to absorb water very quickly once rain has fallen. Gibson and Nobel (1986, 9-10) report that Ferocactus cylin-draceus can take up noticeable quantities of water within 12 hours of as little as 7 mm (0.3 in) of rain. In a few days the stem becomes fully hydrated.
The great majority of cacti carry out the special photosynthetic process called crassulacean acid metabolism (cam), a phenomenon also found in the stonecrop family Crassulaceae, an unrelated family of succulents, and other plants that must cope with a limited supply of water, including agaves, yuccas, bromeliads, and many orchids (Mauseth 1991,268-269). It is a kind of photosynthesis that minimizes the use of water by the plant.
Gas-exchanging stomata may be opened or closed by the plant. Most plants open stomata during the day so that carbon dioxide may be taken in and combined with hydrogen obtained from the splitting of water using sunlight to make sugars in the process of photosynthesis. Oxygen, a by-product of photosynthesis, is released into the atmosphere through the open stomata. Water vapor is also lost when stomata are open, however, creating a problem for desert plants, especially when stomata are open during the hot time of the day. Desiccation can occur rapidly, with serious consequences. In cam photosynthesis the stomata open only at night, thus conserving water because much less water vapor is lost during cooler night hours. During the night, carbon dioxide enters the chlorophyll-containing cells of the stem and is converted to organic acids that are briefly stored in the cell vacuole. At the same time, oxygen is released and only a small amount of water vapor is lost. The following morning, the stomata close and the cells begin to photosynthe-size as the sun rises. Carbon dioxide is removed from the organic acids and used to make sugars. Thus efficient photosynthesis can occur during the daylight hours with the stomata closed. Non-cam plants, including the majority of flowering plants, conduct gas exchange during the day. The metabolism of the great majority of those is described as C3 because carbon dioxide is captured as three-carbon acids. Interestingly, cacti that have some of the most primitive characteristics in the family have C3 metabolism: Pereskia, which has persistent leaves, has cam in the stems and C3 in the leaves, and Maihuenia is completely C3.
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