Drought tolerance refers to the degree to which a plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

 is adapted to arid

Arid

A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...

 or drought

Drought

A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...

 conditions. Desiccation tolerance

Desiccation tolerance

Desiccation tolerance refers to the ability of an organism to withstand or endure extreme dryness, or drought-like conditions. Plants and animals living in arid or periodically arid environments such as temporary streams or ponds may face the challenge of desiccation, therefore physiological or...

is an extreme degree of drought tolerance. Plants naturally adapted to dry conditions are called xerophyte

Xerophyte

A xerophyte or xerophytic organism is a plant which has adapted to survive in an environment that lacks water, such as a desert. Xerophytic plants may have adapted shapes and forms or internal functions that reduce their water loss or store water during long periods of dryness...

s.

Adaptations to dry conditions

Drought tolerant plants typically make use of either C4 carbon fixation

C4 carbon fixation

C4 carbon fixation is one of three biochemical mechanisms, along with and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon molecule present in the first product of carbon fixation in these plants, in contrast to the 3-carbon molecule products in plants. fixation is an...

 or crassulacean acid metabolism

Crassulacean acid metabolism

Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. The stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide...

 (CAM) to fix carbon

Carbon fixation

In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...

 during photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. Both are improvements over the more common but more basal

Basal (phylogenetics)

In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 C3

C3 carbon fixation

carbon fixation is a metabolic pathway for carbon fixation in photosynthesis. This process converts carbon dioxide and ribulose bisphosphate into 3-phosphoglycerate through the following reaction:...

 pathway in that they are more energy efficient. CAM is particularly good for arid conditions because carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 can be taken up at night, allowing the stoma

Stoma

In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...

ta 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 stoma
  Stoma
  In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...
  
  ta to reduce water loss, such as reduced numbers or waxy surfaces.
• Water storage in succulent
  Succulent plant
  Succulent plants, also known as succulents or fat plants, are water-retaining plants adapted to arid climates or soil conditions. Succulent plants store water in their leaves, stems, and also in roots...
  
   above-ground parts or water-filled tuber
  Tuber
  Tubers are various types of modified plant structures that are enlarged to store nutrients. They are used by plants to survive the winter or dry months and provide energy and nutrients for regrowth during the next growing season and they are a means of asexual reproduction...
  
  s.
• Adaptations in the root
  Root
  In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
  
   system to increase water absorption
  Water absorption
  During the transmission of electromagnetic radiation through a medium containing water molecules, portions of the electromagnetic spectrum are absorbed by water molecules...
  
  .
• Trichome
  Trichome
  Trichomes are fine outgrowths or appendages on plants and certain protists. These are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae.- Algal trichomes :...
  
  s(small hairs) on the leaves
  Leaf
  A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
  
   to absorb atmospheric water.

Importance in agriculture

Arid conditions can lower the yield

Crop yield

In agriculture, crop yield is not only a measure of the yield of cereal per unit area of land under cultivation, yield is also the seed generation of the plant itself...

 of many crops

Crop (agriculture)

A crop is a non-animal species or variety that is grown to be harvested as food, livestock fodder, fuel or for any other economic purpose. Major world crops include maize , wheat, rice, soybeans, hay, potatoes and cotton. While the term "crop" most commonly refers to plants, it can also include...

. Plant breeding

Plant breeding

Plant breeding is the art and science of changing the genetics of plants in order to produce desired characteristics. Plant breeding can be accomplished through many different techniques ranging from simply selecting plants with desirable characteristics for propagation, to more complex molecular...

 programs for improved yield during drought conditions have great economic importance, and these programs may be broad in scope. For example, one study on soybean

Soybean

The soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...

s currently being conducted by the United States Department of Agriculture

United States Department of Agriculture

The United States Department of Agriculture is the United States federal executive department responsible for developing and executing U.S. federal government policy on farming, agriculture, and food...

 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

Wilting

Wilting refers to the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non-lignified plant cells falls towards zero, as a result of diminished water in the cells...

 and of the specific gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s for drought tolerance.

Importance in horticulture

In landscapes in arid or drought-prone regions, drought tolerance is an important consideration in plant selection. Xeriscaping

Xeriscaping

Xeriscaping and xerogardening refers to landscaping and gardening in ways that reduce or eliminate the need for supplemental water from irrigation...

 is an approach to landscaping first developed in Denver

Denver, Colorado

The City and County of Denver is the capital and the most populous city of the U.S. state of Colorado. Denver is a consolidated city-county, located in the South Platte River Valley on the western edge of the High Plains just east of the Front Range of the Rocky Mountains...

, Colorado

Colorado

Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...

, a region with hot, dry summers. The use of drought tolerant plants is essential to a successful xeriscape, which ideally requires no supplemental irrigation

Irrigation

Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...

.