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Xerophyte: the story on HearLore | HearLore
— Ch. 1 · Defining Xerophytic Survival —
Xerophyte.
~5 min read · Ch. 1 of 6
A xerophyte is a species of plant that has adaptations to survive in an environment with little liquid water. Examples of xerophytes include cacti, 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 metabolic activity may effectively shut down. Plants with such morphological and physiological adaptations are said to be xeromorphic. If placed in a dry environment, a typical mesophytic plant would evaporate water faster than the rate of water uptake from the soil, leading to wilting and even death. 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.
Morphological Adaptations To Drought
Xerophytic plants typically have less surface to volume ratio than other plants, so as to minimize water loss by transpiration and evaporation. An example of leaf surface reduction is the spines of a cactus, while the effects of compaction and branching can be seen in 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. Some xerophytes have tiny hair on their surfaces to provide a wind break and reduce air flow, thereby reducing the rate of evaporation. When a plant surface is covered with tiny hair, it is called tomentose. Stomata is located in these hair or in pits to reduce their exposure to wind. The color of a plant, or of the waxes or hair on its surface, may serve to reflect sunlight and reduce transpiration. An example is the white chalky epicuticular wax coating of Dudleya brittonii, which has the highest ultraviolet light (UV) reflectivity of any known naturally occurring biological substance.
Physiological Mechanisms And Metabolism
When in heat shock, for example, protein molecule structures become unstable, unfold, or reconfigure to become less efficient. Membrane stability will decrease in plastids, which is why photosynthesis is the first process to be affected by heat stress. Heat shock proteins 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. 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. Prime examples of plants employing the CAM mechanism are the pineapple, Agave americana, and Aeonium haworthii. The plasma membrane of cells are made up of lipid molecules called phospholipids. These lipids become more fluid when temperature increases. Saturated lipids are more rigid than unsaturated ones i.e. unsaturated lipids becomes fluid more easily than saturated lipids.
Diversity Of Xerophyte Types
Succulent plants store water in their stems or leaves. These include plants from the family Cactaceae, which have round stems and can store a lot of water. 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. Ephemerals 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. During dry times, resurrection plants look dead, but are actually alive. Some examples of resurrection plants include the Anastatica hierochuntica plant or more commonly known as the Rose of Jericho, as well as one of the most robust plant species in East Africa, Craterostigma pumilum.
Ecological Roles In Arid Zones
Land degradation is a major threat to many countries such as China and Uzbekistan. The major impacts include the loss of soil productivity and stability, as well as the loss of biodiversity due to reduced vegetation consumed by animals. Xerophytic plants are used widely to prevent desertification and for fixation of sand dunes. In northwest China, the seeds of three shrub species namely Caragana korshinskii, Artemisia sphaerocephala, and Hedysarum scoparium are dispersed across the region. These shrubs have the additional property of being palatable to grazing animals such as sheep and camels. H. scoparium is under protection in China due to it being a major endangered species. Haloxylon ammodendron and Zygophyllum xanthoxylum are also plants that form fixed dunes. Plants that live under arctic conditions 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.
Human Applications And Economic Value
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. 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. 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. 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.
A xerophyte is a species of plant that has adaptations to survive in an environment with little liquid water. Examples of xerophytes include cacti, pineapple and some gymnosperm plants.
How do xerophytic plants conserve water during dry periods?
Xerophytic plants typically have less surface to volume ratio than other plants so as to minimize water loss by transpiration and evaporation. They may use water from their own storage or lose less water to the atmosphere to channel a greater proportion of water from the soil to photosynthesis and growth.
Which plants are examples of resurrection plants?
Some examples of resurrection plants include the Anastatica hierochuntica plant or more commonly known as the Rose of Jericho, as well as one of the most robust plant species in East Africa, Craterostigma pumilum. These plants look dead during dry times but are actually alive.
Why are xerophytic plants used to prevent desertification in China?
In northwest China, the seeds of three shrub species namely Caragana korshinskii, Artemisia sphaerocephala, and Hedysarum scoparium are dispersed across the region to fix sand dunes. Haloxylon ammodendron and Zygophyllum xanthoxylum are also plants that form fixed dunes.
How does Agave americana produce nectar and beverages?
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.