Xerophyte

The Joshua tree is an example of a xerophyte.

A xerophyte (from Greek xero dry, phuton plant) is a species of plant that has adapted to survive in an environment with little water, such as a desert or an ice- or snow-covered region in the Alps or the Arctic.

The morphology and physiology of xerophytes are variously adapted to conserve water, and commonly also to store large quantities of water, during dry periods. Other species may be adapted to survive long periods of desiccation of their tissues, during which their metabolic activity may effectively shut down. Plants with such morphological and physiological adaptations are xeromorphic.^[1]

Introduction

Plants absorb water from the soil, which then evaporates from their outer surfaces; this process is known as transpiration. In dry environments, a typical (mesophytic) plant would evaporate water faster than the rate at which water was replaced in the soil, leading to wilting. To reduce this effect, xerophytic plants exhibit a variety of specialized adaptations to survive in such conditions. Xerophytes may use water from their own storage, allocate water specifically to sites of new tissue growth, or lose less water to the atmosphere and so convert a greater proportion of water in the soil to growth, or have other adaptations to manage water supply and enable them to survive.

Cacti and other succulents are commonly found in deserts, where there is little rainfall. Other xerophytes, such as certain bromeliads, can survive through both extremely wet and extremely dry periods and can be found in seasonally moist habitats such as tropical forests, exploiting niches where water supplies are too intermittent for mesophytic plants to survive. Likewise, chaparral plants are adapted to Mediterranean climates, which have wet winters and dry summers. Plants that live under arctic conditions also have a need for xerophytic adaptations, since water is unavailable for uptake when the ground is frozen.

Types of xerophytes

Succulent plants store water in their stems or leaves. They include the Cactaceae family, which has round stems and can store a lot of water. The leaves are often vestigial, as in the case of cacti, wherein the leaves are reduced to spines, or they do not have leaves at all.

Water is stored in the bulbs of some plants, at or below ground level. They may be dormant during drought conditions and are, therefore, known as drought evaders.

Importance of water conservation

If the water potential (or strictly, water vapour potential) inside a leaf is higher than outside, 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 the open stomata. Transpiration is natural and inevitable for plants, and much water is lost through this process. However, it is vital that plants living in dry conditions are adapted so as to reduce this water loss and decrease the size of the open stomata, in order to reduce unnecessary 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. This is known as plasmolysis. If the plant loses too much water, it will pass its permanent wilting point, and die.

In brief, the rate of transpiration is governed by the number of stomata, leaf area (allowing for more stomata), temperature differential, the relative humidity, the presence of wind or air movement, the light intensity, and the presence of a waxy cuticle. It is important to note, that whilst it is vital to keep stomata closed, they have to be opened for gaseous exchange in photosynthesis.

Morphological adaptations

Cereus peruvianus

Euphorbia virosa

The cactus Cereus peruvianus looks superficially very similar to Euphorbia virosa due to convergent evolution.

Xerophytic plants may have similar shapes, forms, and structures and look very similar, even if the plants are not very closely related, through a process called convergent evolution. For example, some species of cacti (members of the family Cactaceae), which evolved only in the Americas, may appear similar to Euphorbias, which are distributed worldwide. An unrelated species of caudiciforms, plants with swollen bases that are used to store water, may also display such similarities.

Reduction of surface area

Xerophytic plants can have less overall surface area than other plants, so reducing the area that is exposed to the air and reducing water loss by evaporation. Xerophytes can have smaller leaves or fewer branches than other plants. An example of leaf surface reduction are the spines of a cactus. An example of compaction and reduction of branching are the barrel cacti. Other xerophytes may have their leaves compacted at the base, as in a basal rosette, which may be smaller than the plant's flower. This adaptation is exhibited by some Agave and Eriogonum species, which can be found growing near Death Valley.

Reduction in air flow

Some xerophytes have tiny hairs on their surface to provide a wind break and reduce air flow, thereby reducing the rate of evaporation. When a plant surface is covered with tiny hairs, it is called tomentose.

In a still environment, the areas under the leaves/spines where transpiration is taking place form a small localised environment that is more saturated than normal with water vapour. If this is not blown away by wind, the water vapour potential gradient is reduced and so is transpiration. Thus, in a windier situation, this localization is not held and so the gradient remains high, which aids the loss of water vapour. Spines trap a layer of moisture and also slow air movement over tissues.

Reflectivity

The color of a plant, or of the waxes or hairs on its surface, may serve to reflect sunlight and reduce evaporation. An example is the white chalky wax (epicuticular wax) coating of Dudleya brittonii, which has the highest ultraviolet light (UV) reflectivity of any known naturally occurring biological substance.^[2]

Physiological adaptations

Some plants can store water in root structures, trunk structures, stems, and leaves. Water storage in swollen parts of the plant is known as succulence. A swollen trunk or root at the ground level of a plant is called a caudex and plants with swollen bases are called caudiciforms.

Tiny pores on the surface of a xerophytic plant called stomata may open only at night, so as to reduce evaporation.

Plants may secrete resins and waxes (epicuticular wax) on their surfaces, which reduce evaporation. Examples are the heavily scented and flammable resins (volatile organic compounds) of some chaparral plants, such as Malosma laurina, or the chalky wax of Dudleya pulverulenta.

Plants may drop their leaves in times of dryness (drought deciduous), or modify the leaves produced so that they are smaller.

During dry times, xerophytic plants may stop growing and go dormant, or change the allocation of the products of photosynthesis from growing new leaves to the roots.^[3]

Seeds may be modified to require an excessive amount of water before germinating, so as to ensure a sufficient water supply for the seedling's survival. An example of this is the California poppy, whose seeds lie dormant during drought and then germinate, grow, flower, and form seeds within four weeks of rainfall.

Modification of environment

The leaf litter on the ground around a plant can provide an evaporative barrier to prevent water loss. Waxes shed from some plants coat the ground, so reducing evaporation in the immediate vicinity of the plant, as in the case of Dudleya pulverulenta. A plant’s root mass itself may also hold organic material that retains water, as in the case of the arrowweed (Pluchea sericea).

Mechanism table

Mechanism	Adaptation	Example
Limit water loss	waxy stomata	prickly pear
	few stomata
	sunken stomata	pine
	stomata open at night	tea plant
	CAM photosynthesis	cactus
	large hairs on surface	Bromeliads
	curled leaves	esparto grass
Storage of water	succulent leaves	Kalanchoe
	succulent stems	Euphorbia
	fleshy tuber	Raphionacme
Water uptake	deep root system	Acacia, Prosopis
	below water table	Nerium oleander
	absorbing surface moisture from leaf hairs or trichomes	Tillandsia

References

↑ ”Xeromorphic”, The Cambridge Illustrated Glossary of Botanical Terms, Michael Hickey, Clive King, Cambridge University Press, 2001
↑ Mulroy, Thomas W. (1979). "Spectral properties of heavily glaucous and non-glaucous leaves of a succulent rosette-plant". Oecologia 38 (3): 349–357. doi:10.1007/BF00345193.
↑ "Plant Adaptations". University of New Mexico. Retrieved December 2, 2014.