Where water is plentiful and temperatures are moderate, plants have broad, thin leaves with a great deal of surface area for maximum photosynthesis. Plants adapted to drought conditions, however, exhibit several structural characteristics that prevent water loss, helping them live hot, dry conditions. In addition they have physiological mechanisms which are not directly observable and that will help them conserve water. A number of these plants are suitable drought-tolerant landscaping subjects.
Reduced Leaves
A typical leaf has three chief layers. The lower and upper layers consist of skin, which is usually one cell thick. The mesophyll is in the leaf’s center; it is moist and is where photosynthesis happens. The skin contains breathing holes, called stomata, where gas exchange occurs. On leaves using thin skin and numerous stomata, water escapes through the skin and stomata. Some arid-climate plants have the ability to conserve water because of the reduced leaf size. Less leaf surface area results in low water loss through the skin. Small leaves have fewer stomata than larger leaves, which variation also reduces water loss. Several dry-land plants possess stomata only on the bottom epidermis, which further reducing water loss, and a few have several layers of epidermal cells. An example of a plant with small leaves is “Rosy Dawn” manzanita (Arctostaphylos edmundsii “Rosy Dawn”), that is perennial in U.S. Department of Agriculture plant hardiness zones 8b through 10; it’s gray-green leaves tinged with pink and pink blossoms. Prickly pears (Opuntia spp., USDA zones 3b through 11) possess very reduced, cylindrical, fleshy leaves which happen on only new development.
Water Storage
Succulent plants have developed several structural mechanisms which prevent water loss. When water is available, they absorb it through their origins and bind it in place in interior water storage cells. The water is held there with no danger of being discarded until the plants require it. Succulent plants incorporate golden barrel cactus (Echinocactus grusonii, USDA zones 9 through 11), which stores water in its stem, and coral aloe (Aloe striata, USDA zones 9 through 11), that stores water in its leaves.
Coated Leaves
Coatings of wax or hairs also help avoid water loss in crops. Waxy layers may provide either a sheen or dull, grayish or bluish cast to your leaf surface. Century plant (Agave americana, USDA zones 8 through 11) has a dull, waxy coating that adds a grey color to its long leaves. A shiny wax and a coating of hairs are on the leaves of carob tree (Ceratonia siliqua, USDA zones 9 through 11), also a native of the Eastern Mediterranean. The hairs assist slow air movement over the tree’s leaves, reducing transpiration and water loss. Brittlebush (Encelia farinosa, USDA zones 8 through 11) has silvery white leaves covered with reflective, white hairs which may reduce the plant’s temperature by several degrees.
Physiological Mechanisms
Several succulents and arid-climate plants possess a specialized kind of photosynthesis called Crassulacean acid metabolism. Their stomata open at night and shop the carbon dioxide that they absorb; the plants use the carbon dioxide for photosynthesis during daylight hours. During extended droughts, these crops may decrease their metabolism rate, maintaining their stomata closed night and day, and maintaining moist internal tissues that the very low amount of action sufficient to prolong life.