A xerophyte or xerophytic organism (xero meaning dry, phyte meaning plant) is a plant which is able to survive in an ecosystem with little available water or moisture, usually in environments where potential evapotranspiration exceeds precipitation for all or part of the growing season. Plants like the cactus and other succulents are typically found in deserts where low rainfall amounts are the norm, but xerophytes such as the bromeliads can also be found in moist habitats such as tropical forests, exploiting niches where water supplies are limited or too intermittent for mesophytic plants. Plants that live under arctic conditions may also have a need for xerophytic adaptations, as water is unavailable for uptake when the ground is frozen. Their leaves are covered with silvery hairs (creates wind break & light reflective surface).
Adaptations of xerophytes include reduced permeability of the epidermal layer, stomata and cuticle to maintain optimal amounts of water in the tissues by reducing transpiration, adaptations of the root system to acquire water from deep underground sources or directly from humid atmospheres (as in epiphytic orchids), and succulence, or storage of water in swollen stems, leaves or root tissues. The typical morphological consequences of these adaptations are collectively called xeromorphisms.
If the water potential inside the leaf is higher than outside the leaf, the water vapour will diffuse out of the leaf down this gradient. This loss of water vapour from the leaves is called transpiration, and the water vapour diffuses through open stomata in the leaf. Although this is a normal and important process in all plants, it is vital that plants living in dry conditions have adaptations that decrease this water potential gradient, and decrease the size of open stomata, in order to reduce water loss from the plant. It is important for a plant living in these conditions to conserve water because without enough water, plant cells lose turgor and the plant tissue wilts. If the plant loses too much water, it will pass its permanent wilting point, where the plant will die.
Types of xerophytic plants are:
- Succulent plants - typically store water in stems or leaves. They include the Cactaceae family which typically have stems that are round and store a lot of water. Often, as in cacti where the leaves are reduced to spines, their leaves are vestigial, or they do not have leaves.
- Bulbs - water is stored in their bulbs, at or below ground level. They may spend a period of dormancy during drought conditions underground, and are therefore known as drought evaders.
- Short-lived annuals can often germinate following rainfall. An example of this is the California poppy whose seeds lie dormant during drought and then, flower and form seeds within four weeks of rainfall.
Drought tolerant plants typically make use of either C4 carbon fixation or crassulacean acid metabolism (CAM) to fix carbon during photosynthesis. Both are improvements over the more common but more basal C3 pathway in that they are more energy efficient. CAM is particularly good for arid conditions because carbon dioxide can be taken up at night, allowing the stomata to stay closed during the heat of day and thus reducing water loss.
Many adaptations for dry conditions are structural, including the following:
- Adaptations of the stomata to reduce water loss, such as reduced numbers or waxy surfaces.
- Water storage in succulent above-ground parts or water-filled tubers.
- Adaptations in the root system to increase water absorption.
- Trichomes (small hairs) on the leaves to absorb atmospheric water.
Arid conditions can lower the yield of many crops. Plant breeding programs for improved yield during drought conditions have great economic importance, and these programs may be broad in scope. For example, one study on soybeans currently being conducted by the United States Department of Agriculture is scheduled to span several years, with research taking place across that country, and has among its goals the identification of specific mechanism by which soybeans resist wilting and of the specific genes for drought tolerance.
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