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A xerophyte () is a species of plant that has to survive in an environment with little liquid water. Examples of xerophytes include cactus, pineapple and some gymnosperm plants. The morphology and physiology of xerophytes are adapted to conserve water during dry periods. Some species called resurrection plants can survive long periods of extreme dryness or desiccation of their tissues, during which their metabolism may effectively shut down. Plants with such morphological and physiological adaptations are said to be .” Xeromorphic”, The Cambridge Illustrated Glossary of Botanical Terms, Michael Hickey, Clive King, Cambridge University Press, 2001 Xerophytes such as cacti are capable of withstanding extended periods of dry conditions as they have deep-spreading roots and capacity to store water. Their waxy, thorny leaves prevent loss of moisture.
Xerophytic plants exhibit a diversity of specialized adaptations to survive in such water-limiting conditions. They 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 channel a greater proportion of water from the soil to photosynthesis and growth. Different plant species possess different qualities and mechanisms to manage water supply, enabling them to survive.
Cactus and other succulent plant are commonly found in deserts, where there is little rainfall. Other xerophytes, such as certain Bromeliaceae, 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 also have a need for xerophytic adaptations, since water is unavailable for uptake when the ground is frozen, such as the European resurrection plants Haberlea rhodopensis and Ramonda serbica.
In environments with very high salinity, such as and semi-deserts, water uptake by plants is a challenge due to the high salt ion levels. Such environments may cause an excess of ions to accumulate in the cells, which is very damaging. and xerophytes evolved to survive in such environments. Some xerophytes may also be considered halophytes; however, halophytes are not necessarily xerophytes. The succulent xerophyte Zygophyllum xanthoxylum, for example, has specialised protein transporters in its cells which allows storage of excess ions in their to maintain normal pH and ionic composition.
There are many factors which affect water availability, which is the major limiting factor of seed germination, seedling survival, and plant growth. These factors include infrequent raining, intense sunlight and very warm weather leading to faster water evaporation. An extreme environmental pH and high salt content of water also disrupt plants' water uptake.
Non-succulent perennials successfully endure long and continuous shortage of water in the soil. These are hence called 'true xerophytes' or euxerophytes. Water deficiency usually reaches 60–70% of their fresh weight, as a result of which the growth process of the whole plant is hindered during cell elongation. The plants which survive drought are, understandably, small and weak.
Ephemeral plant are the 'drought escaping' kind, and not true xerophytes. They do not really endure drought, only escape it. With the onset of rainfall, the plant seeds germinate, quickly grow to maturity, flower, and set seed, i.e., the entire life cycle is completed before the soil dries out again. Most of these plants are small, roundish, dense shrubs represented by species of Faboideae, some inconspicuous Asteraceae, a few Zygophyllaceae and some grasses. Water is stored in the of some plants, or at below ground level. They may be dormant during drought conditions and are, therefore, known as drought evaders.
Shrubs which grow in arid and semi-arid regions are also xeromorphic. For example, Caragana korshinskii, Artemisia sphaerocephala, and Hedysarum scoparium are shrubs potent in the semi-arid regions of the northwest China desert. These psammophile shrubs are not only edible to grazing animals in the area, they also play a vital role in the stabilisation of desert sand dunes.
Bushes, also called semi-shrubs often occur in sandy desert region, mostly in deep sandy soils at the edges of the dunes. One example is the Reaumuria soongorica, a perennial resurrection semi-shrub. Compared to other dominant arid xerophytes, an adult R. soongorica, bush has a strong resistance to water scarcity, hence, it is considered a super-xerophytes.
In brief, the rate of transpiration is governed by the number of stomata, stomatal aperture i.e. the size of the stoma opening, 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 respiration and photosynthesis.
Under conditions of water scarcity, the seeds of different xerophytic plants behave differently, which means that they have different rates of germination since water availability is a major limiting factor. These dissimilarities are due to natural selection and eco-adaptation as the seeds and plants of each species evolve to suit their surrounding.
In a still, windless environment, the areas under the leaves or spines where transpiration takes place form a small localised environment that is more saturated with water vapour than normal. If this concentration of water vapour is maintained, the external water vapour potential gradient near the stomata is reduced, thus, reducing transpiration. In a windier situation, this localisation is blown away and so the external water vapour gradient remains low, which makes the loss of water vapour from plant stomata easier. Spines and hair trap a layer of moisture and slows air movement over tissues.
A. miersiana has thick cuticle as expected to be found on xerophytes, but H. disermifolia and G. africana have thin cuticles. Since resources are scarce in arid regions, there is selection for plants having thin and efficient cuticles to limit the nutritional and energy costs for the cuticle construction.
In periods of severe water stress and stomata closure, the cuticle's low water permeability is considered one of the most vital factors in ensuring the survival of the plant. The rate of transpiration of the cuticles of xerophytes is 25 times lower than that of stomatal transpiration. To give an idea of how low this is, the rate of transpiration of the cuticles of mesophytes is only 2 to 5 times lower than stomatal transpiration.
In regions continuously exposed to sunlight, Ultraviolet can cause biochemical damage to plants, and eventually lead to mutation and damages in the long run. When one of the main molecules involved in photosynthesis, photosystem II is damaged by UV rays, it induces responses in the plant, leading to the biosynthesis of protectant molecules such as and more wax. Flavonoids are UV-absorbing and act like sunscreen for the plant.
Heat shock proteins (HSPs) are a major class of proteins in plants and animals which are synthesised in cells as a response to heat stress. They help prevent protein unfolding and help re-fold denatured proteins. As temperature increases, the HSP protein expression also increases.
As compared to other plants, xerophytes have an inverted stomatal rhythm. During the day and especially during mid-day when the sun is at its peak, most stomata of xerophytes are closed. Not only do more stomata open at night in the presence of mist or dew, the size of stomatal opening or aperture is larger at night compared to during the day. This phenomenon was observed in xeromorphic species of Cactus, Crassulaceae, and Liliaceae.
As the epidermis of the plant is covered with water barriers such as lignin and waxy cuticles, the night opening of the stomata is the main channel for water movement for xerophytes in arid conditions. Even when water is not scarce, the xerophytes A. Americana and pineapple plant are found to utilise water more efficiently than mesophytes.
If the membrane integrity is compromised, there will be no effective barrier between the internal cell environment and the outside. Not only does this mean the plant cells are susceptible to disease-causing bacteria and mechanical attacks by herbivores, the cell could not perform its normal processes to continue living - the cells and thus the whole plant will die.
Under high light, it is unfavourable to channel extra light into photosynthesis because excessive light may cause damage to the plant proteins. Zeaxanthin dissociates light-channelling from the photosynthesis reaction - light energy in the form of will not be transmitted into the photosynthetic pathway anymore.
Many succulent xerophytes employ the Crassulacean acid metabolism or better known as CAM photosynthesis. It is also dubbed the "dark" carboxylation mechanism because plants in arid regions collect carbon dioxide at night when the stomata open, and store the gases to be used for photosynthesis in the presence of light during the day. Although most xerophytes are quite small, this mechanism allows a positive carbon balance in the plants to sustain life and growth. Prime examples of plants employing the CAM mechanism are the pineapple, Agave Americana, and Aeonium haworthii.
Although some xerophytes perform photosynthesis using this mechanism, the majority of plants in arid regions still employ the C3 and C4 photosynthesis pathways. A small proportion of desert plants even use a collaborated C3-CAM pathway.
The wilting of leaves is a reversible process, however, abscission is irreversible. Shedding leaves is not favourable to plants because when water is available again, they would have to spend resources to produces new leaves which are needed for photosynthesis. Exceptions exist, however, such as the ocotillo which will shed its leaves during prolonged dry seasons in the desert, then re-leaf when conditions have improved.
| Water uptake | Extensive root system | Acacia, Prosopis |
| Water storage | Succulent plant | Kalanchoe, Euphorbia |
| Fleshy tuber | Raphionacme | |
| Reduce water loss | Surface area reduction | Barrel cactus, Basal rosette, Eriogonum compositum |
| Sunken stomata and hairs | Pine, Nassauvia falklandica, Bromeliads | |
| Waxy leaf surface | opuntia, Malosma laurina, Dudleya pulverulenta | |
| Nocturnal stomata | Tea plant, Alfalfa, Brachychiton discolor, Quercus trojana | |
| CAM photosynthesis | Cactus, Pineapple, Agave Americana, Aeonium haworthii, Sansevieria trifasciata | |
| Curled leaves | Esparto grass | |
| Dormancy and reduced photosynthesis | Resurrection plants | Ramonda nathaliae, Ramonda myconi, Haberlea, Anastatica, Craterostigma pumilum |
| Dormant seeds | Californian poppy | |
| Leaf abscission | Coastal sage scrub, Wiliwili, Geoffroea decorticans | |
A more well-known xerophyte is the succulent plant Agave americana. It is cultivated as an ornamental plant popular across the globe. Agave nectar is garnered from the plant and is consumed as a substitute for sugar or honey. In Mexico, the plant's sap is usually fermented to produce an alcoholic beverage.
Many xerophytic plants produce colourful vibrant flowers and are used for decoration and ornamental purposes in gardens and in homes. Although they have adaptations to live in stressful weather and conditions, these plants thrive when well-watered and in tropical temperatures. Phlox sibirica is rarely seen in cultivation and does not flourish in areas without long exposure to sunlight.
A study has shown that xerophytic plants which employ the CAM mechanism can solve micro-climate problems in buildings of humid countries. The CAM photosynthetic pathway absorbs the humidity in small spaces, effectively making the plant such as Sansevieria trifasciata a natural indoor humidity absorber. Not only will this help with cross-ventilation, but lowering the surrounding humidity increases the thermal comfort of people in the room. This is especially important in East Asian countries where both humidity and temperature are high.
Recent years has seen interests in resurrection plants other than their ability to withstand extreme dryness. The metabolites, sugar alcohols, and sugar acids present in these plants may be applied as natural products for medicinal purposes and in biotechnology. During desiccation, the levels of the sugars sucrose, raffinose, and galactinol increase; they may have a crucial role in protecting the cells against damage caused by reactive oxygen species (ROS) and oxidative stress. Besides having anti-oxidant properties, other compounds extracted from some resurrection plants showed anti-fungal and Antibiotic properties. A glycoside found in Haberlea rhodopensis called myconoside is extracted and used in cosmetic creams as a source of anti-oxidant as well as to increase elasticity of the human skin. Although there are other molecules in these plants that may be of benefit, it is still much less studied than the primary metabolites mentioned above.
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