Xerophyte

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A xerophyte (from Greek ξηρός xeros dry, φυτόν phuton plant) is a species of plant that has adaptations to survive in an environment with little liquid water, such as a desert or an ice- or snow-covered region in the Alps or the Arctic. Popular examples of xerophytes are cacti, pineapple and some Gymnosperm plants.

Plant multicellular eukaryote of the kingdom Plantae

Plants are mainly multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi were treated as plants. However, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes. By one definition, plants form the clade Viridiplantae, a group that includes the flowering plants, conifers and other gymnosperms, ferns and their allies, hornworts, liverworts, mosses and the green algae, but excludes the red and brown algae.

Adaptation Trait with a current functional role in the life history of an organism maintained and evolved by natural selection

In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the population during that process. Thirdly, it is a phenotypic trait or adaptive trait, with a functional role in each individual organism, that is maintained and has evolved through natural selection.

Desert Area of land where little precipitation occurs

A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation. About one-third of the land surface of the world is arid or semi-arid. This includes much of the polar regions where little precipitation occurs and which are sometimes called polar deserts or "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location.

Contents

The structural features (morphology) and fundamental chemical processes (physiology) of xerophytes are variously adapted to conserve water, also common to store large quantities of water, during dry periods. Other species are able to 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 xeromorphic. [1] Xerophytes such as cacti are capable of withstanding extended periods of dry conditions as they have deep-spreading roots and capacity to store water. The leaves are waxy and thorny that prevents loss of water and moisture. Even their fleshy stems can store water.

Morphology (biology) In biology, the form and structure of organisms

Morphology is a branch of biology dealing with the study of the form and structure of organisms and their specific structural features.

Physiology science of the function of living systems

Physiology is the scientific study of the functions and mechanisms which work within a living system.

Desiccation state of extreme dryness, or the process of extreme drying

Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.

Introduction

Ramonda Serbica.jpg
Ramonda serbica a.k.a. Serbian phoenix flower
The structural adaptations of these two resurrection plants are very similar. They can be found on the grounds of Bulgaria and Greece.

Plants absorb water from the soil, which then evaporates from their shoots and leaves; this process is known as transpiration. In dry environments, a typical mesophytic plant would evaporate water faster than the rate of water uptake from the soil, leading to wilting and even death.

Transpiration process of water movement through a plant and its evaporation from aerial parts

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth and metabolism. The remaining 97–99.5% is lost by transpiration and guttation. Leaf surfaces are dotted with pores called stomata, and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered by guard cells and their stomatal accessory cells that open and close the pore. Transpiration occurs through the stomatal apertures, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants, changes osmotic pressure of cells, and enables mass flow of mineral nutrients and water from roots to shoots. Two major factors influence the rate of water flow from the soil to the roots: the hydraulic conductivity of the soil and the magnitude of the pressure gradient through the soil. Both of these factors influence the rate of bulk flow of water moving from the roots to the stomatal pores in the leaves via the xylem.

Wilting Reduced plant functioning caused by dehydration

Wilting is 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. The rate of loss of water from the plant is greater than the absorption of water in the plant. The process of wilting modifies the leaf angle distribution of the plant towards more erectophile conditions.

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.

Photosynthesis Biological process to convert light into chemical energy

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", and σύνθεσις, synthesis, "putting together". In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth.

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.

Cactus Family of mostly succulent plants, adapted to dry environments

A cactus is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. The word "cactus" derives, through Latin, from the Ancient Greek κάκτος, kaktos, a name originally used by Theophrastus for a spiny plant whose identity is not certain. Cacti occur in a wide range of shapes and sizes. Most cacti live in habitats subject to at least some drought. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth. Cacti show many adaptations to conserve water. Almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. In the absence of leaves, enlarged stems carry out photosynthesis. Cacti are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north—except for Rhipsalis baccifera, which also grows in Africa and Sri Lanka.

Succulent plant plants having some parts that are more than normally thickened and fleshy

In botany, succulent plants, also known as succulents, are plants that have some parts that are more than normally thickened and fleshy, usually to retain water in arid climates or soil conditions. The word "succulent" comes from the Latin word sucus, meaning juice, or sap. Succulent plants may store water in various structures, such as leaves and stems. Some definitions also include roots, thus geophytes that survive unfavorable periods by dying back to underground storage organs may be regarded as succulents. In horticultural use, the term "succulent" is sometimes used in a way which excludes plants that botanists would regard as succulents, such as cacti. Succulents are often grown as ornamental plants because of their striking and unusual appearance.

Bromeliaceae family of plants

The Bromeliaceae are a family of monocot flowering plants of 51 genera and around 3475 known species native mainly to the tropical Americas, with a few species found in the American subtropics and one in tropical west Africa, Pitcairnia feliciana.

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 . [2]

Arctic polar region on the Earths northern hemisphere

The Arctic is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Alaska, Finland, Greenland (Denmark), Iceland, Northern Canada, Norway, Russia and Sweden. Land within the Arctic region has seasonally varying snow and ice cover, with predominantly treeless permafrost -containing tundra. Arctic seas contain seasonal sea ice in many places.

Resurrection plant

A resurrection plant is any poikilohydric plant that can survive extreme dehydration, even over months or years.

<i>Haberlea</i> genus of plants

Haberlea is a monotypic genus of flowering plants in the family Gesneriaceae. The only member of this genus, Haberlea rhodopensis, is endemic to several mountains in Bulgaria, especially the Rhodope Mountains, and a small part of northern Greece. Common names include Orpheus flower and resurrection plant because of the remarkable ability of Haberlea to survive very long periods of desiccation.

In an environment with very high salinity such as mangrove swamps and semi-deserts, water uptake by plants is a challenge due to the high salt ion levels. Besides that, such environments may cause an excess of ions to accumulate in the cells, which is very damaging. [3] Halophytes 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, have specialised protein transporters in their cells which allow storage of excess ions in their vacuole to maintain normal cytosolic pH and ionic composition. [4] [5]

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.

Types

Cistus albidus is a xerophyte which grows in European countries such as France, and Italy and North African countris like Morocco. Cistus albidus in Sainte Lucie Island.jpg
Cistus albidus is a xerophyte which grows in European countries such as France, and Italy and North African countris like Morocco.

Succulent plants store water in their stems or leaves. These include plants from the Cactaceae family, which have 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. These include the C4 perennial woody plant, Haloxylon ammodendron which is a native of northwest China.

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.

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. Most of these plants are small, roundish, dense shrubs represented by species of Papilionaceae, some inconspicuous Compositae, a few Zygophyllaceae and some grasses. Water is stored in the bulbs 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. [6]

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. [6]

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; a significant amount of water is lost through this process. However, it is vital that plants living in dry conditions are adapted so as to decrease the size of the open stomata, lower the rate of transpiration, and consequently reduce water loss to the environment. Without sufficient 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. [7]

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.

Morphological adaptations

Cereus peruvians (cropped).jpg
Cereus peruvianus
Euphorbia-virosa.jpg
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 some similarities.

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. [8]

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 transpiration and evaporation. They can also have smaller leaves or fewer branches than other plants. An example of leaf surface reduction are the spines of a cactus, while the effects of compaction and reduction of 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.

Forming water vapour-rich environment

Some xerophytes have tiny hairs 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 hairs, it is called tomentose. Stomata are located in these hairs or in pits to reduce their exposure to wind. This enables them to maintain a humid environment around them.

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 hairs trap a layer of moisture and slows air movement over tissues.

Reflective features

The succulent leaves of Dudleya brittonii are visibly coated with a 'powdery' white which is the epicuticular wax. Dudleya Brittonii.jpg
The succulent leaves of Dudleya brittonii are visibly coated with a 'powdery' white which is the epicuticular wax.

The color of a plant, or of the waxes or hairs 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. [9]

Cuticles

Many xerophytic species have thick cuticles. Just like human skin, a plant's cuticles are the first line of defense for its aerial parts. As mentioned above, the cuticle contains wax for protection against biotic and abiotic factors. The ultrastructure of the cuticles varies in different species. Some examples are Antizoma miersiana , Hermannia disermifolia and Galenia africana which are xerophytes from the same region in Namaqualand, but have different cuticle ultrastructures.

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 as one of the most vital factor 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. [10]

Physiological adaptations

There are many changes that happen on the molecular level when a plant experiences stress. When in heat shock, for example, their 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. [11] Despite the many stresses, xerophytes have the ability to survive and thrive in drought conditions due to their physiological and biochemical specialties.

Dudleya pulverulenta is called 'chalk lettuce' for its obvious structures. This xerophyte has fleshy succulent leaves and is coated with chalky wax. Dudleya pulverulenta 1.jpg
Dudleya pulverulenta is called 'chalk lettuce' for its obvious structures. This xerophyte has fleshy succulent leaves and is coated with chalky wax.

Water storage

Some plants can store water in their 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.

Production of protective molecules

Plants may secrete resins and waxes (epicuticular wax) on their surfaces, which reduce transpiration. 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 .

In regions continuously exposed to sunlight, UV rays can cause biochemical damage to plants, and eventually lead to DNA mutations and damages in the long run. When one of the main molecules involved in photosynthesis, photosystem II (PSII) is damaged by UV rays, it induces responses in the plant, leading to the synthesis of protectant molecules such as flavonoids 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. [11]

Evaporative cooling

Evaporative cooling via transpiration can delay the effects of heat stress on the plant. However, transpiration is very expensive if there is water scarcity, so generally this is not a good strategy for the plants to employ. [11]

Line 1 represents typical mesophytic plants and line 2 represents xerophytes. The stomata of xerophytes are nocturnal and have inverted stomatal rhythm. Diffrences in Stomata Opening Throughout the Day for C3 plants and CAM plants (1).svg
Line 1 represents typical mesophytic plants and line 2 represents xerophytes. The stomata of xerophytes are nocturnal and have inverted stomatal rhythm.

Stomata closure

Most plants have the ability to close their stomata at the start of water stress, at least partially, to restrict rates of transpiration. [12] They use signals or hormones sent up from the roots and through the transpiration stream. Since roots are the parts responsible for water searching and uptake, they can detect the condition of dry soil. The signals sent are an early warning system - before the water stress gets too severe, the plant will go into water-economy mode. [11]

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 close. 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 Cactaceae, 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. [12] Even when water is not scarce, the xerophytes A. Americana and pineapple plant are found to utilise water more efficiently than mesophytes. [12]

Phospholipid saturation

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. Plant cells undergo biochemical changes to change their plasma membrane composition to have more saturated lipids to sustain membrane integrity for longer in hot weather. [11]

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. [13]

Xanthopyll cycle

Light stress can be tolerated by dissipating excess energy as heat through the xanthophyll cycle. Violaxanthin and zeaxanthin are carotenoid molecules within the chloroplasts called xanthophylls. Under normal conditions, violaxanthin channels light to photosynthesis. However, high light levels promote the reversible conversion of violaxanthin to zeaxanthin. These two molecules are photo-protective molecules.

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 photons will not be transmitted into the photosynthetic pathway anymore. [11]

CAM mechanism

Aeonium haworthii.jpg
Aeonium haworthii a.k.a. Haworth's pinwheel
Plants utilising the CAM photosynthetic pathway are generally small and non-woody.

Stomata closure not only restricts the movement of water out of the plant, another consequence of the phenomenon is that carbon dioxide influx or intake into the plant is also reduced. As photosynthesis requires carbon dioxide as a substrate to produce sugar for growth, it is vital that the plant has a very efficient photosynthesis system which maximises the utilisation of the little carbon dioxide the plant gets.

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 . [12]

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. [14]

Delayed germination and growth

The surrounding humidity and moisture right before and during seed germination play an important role in the germination regulation in arid conditions. An evolutionary strategy employed by desert xerophytes is to reduce the rate of seed germination. By slowing the shoot growth, less water is consumed for growth and transpiration. Thus, the seed and plant can utilise the water available from short-lived rainfall for a much longer time compared to mesophytic plants. [6]

Resurrection plants and seeds

A Rose of Jericho plant in dormancy re-flourishes when its roots are placed in a bowl of water.
A Geoffroea decorticans tree is both a winter and drought deciduous tree. Arbolchanar.JPG
A Geoffroea decorticans tree is both a winter and drought deciduous tree.

During dry times, resurrection plants look dead, but are actually alive. Some xerophytic plants may stop growing and go dormant, or change the allocation of the products of photosynthesis from growing new leaves to the roots. [11] [15] These plants evolved to be able to coordinately switch off their photosynthetic mechanism without destroying the molecules involved in photosynthesis. When water is available again, these plants would "resurrect from the dead" and resume photosynthesis, even after they had lost more than 80% of their water content. [16] A study has found that the sugar levels in resurrection plants increase when subjected to desiccation. This may be associated with how they survive without sugar production via photosynthesis for a relatively long duration. [17] 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, the Craterostigma pumilum. [18] [19] 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.

Leaf wilting and abscission

If the water supply is not enough despite the employment of other water-saving strategies, the leaves will start to collapse and wilt due to water evaporation still exceeding water supply. Leaf loss (abscission) will be activated in more severe stress conditions. Drought deciduous plants may drop their leaves in times of dryness.

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. [11]

Modification of environment

The leaf litter on the ground around a plant can provide an evaporative barrier to prevent water loss.[ citation needed ] 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

MechanismAdaptationExamples
Water uptakeExtensive root system Acacia , Prosopis
Water storage Succulence Kalanchoe , Euphorbia
Fleshy tuber Raphionacme
Reduce water lossSurface area reduction Barrel cactus, Basal rosette, Eriogonum compositum
Sunken stomata and hairs Pine, Nassauvia falklandica, Bromeliads
Waxy leaf surface Prickly pear, Malosma laurina , Dudleya pulverulenta
Nocturnal stomata Tea plant, Alfalfa, Brachychiton discolor , Quercus trojana
CAM photosynthesis Cactus, Pineapple plant, Agave Americana , Aeonium haworthii , Sansevieria trifasciata
Curled leaves Esparto grass
Dormancy and reduced photosynthesisResurrection plants Ramonda nathaliae , Ramonda myconi , Haberlea rhodopensis , Anastatica , Craterostigma pumilum
Dormant seeds Californian poppy
Leaf abscission Coastal sage scrub, Wiliwili, Geoffroea decorticans

Uses

Agave Americana is a versatile xerophyte. All parts of the plant can be used either for aesthetics, for consumption, or in traditional medicine. Agave americana 3zz.jpg
Agave Americana is a versatile xerophyte. All parts of the plant can be used either for aesthetics, for consumption, or in traditional medicine.

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. [20] In arid regions where water is scarce and temperatures are high, mesophytes will not be able to survive, due to the many stresses. Xerophytic plants are used widely to prevent desertification and for fixation of sand dunes. In fact, 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. [6] Haloxylon ammodendron and Zygophyllum xanthoxylum are also plants that form fixed dunes. [21]

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. [22]

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 trifasciatas 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. [23]

Batalha.Nerium oleander01.jpg
"Beautiful but deadly". The Nerium oleander plant on the left was taken during Portugal's Autumn, while the one on the right was taken during summer in Italy.
Fliederbaum.JPG

Herbal extracts from plants of the Craterostigma genus are traditionally used by Kenyan natives as remedies to reduce inflammation and to relieve pain relating to the muscle and joints. However, scientific research on this practice revealed that these herbal extracts could cause hyperglasia. [24]

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 anti-bacterial 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. [25] 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. [26]

See also

Related Research Articles

Deciduous trees or shrubs that lose their leaves seasonally

In the fields of horticulture and botany, the term deciduous (/dɪˈsɪdʒuəs/) means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, after flowering; and to the shedding of ripe fruit.

Stoma part of a plant

In botany, a stoma, also called a stomate, is a pore, found in the epidermis of leaves, stems, and other organs, that facilitates gas 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 stomatal opening.

<i>Ammophila</i> (plant) genus of grasses

Ammophila is a genus of flowering plants consisting of two or three very similar species of grasses. The common names for these grasses include marram grass, bent grass, and beachgrass. These grasses are found almost exclusively on the first line of coastal sand dunes. Their extensive systems of creeping underground stems or rhizomes allow them to thrive under conditions of shifting sands and high winds, and to help stabilize and prevent coastal erosion. Ammophila species are native to the coasts of the North Atlantic Ocean where they are usually the dominant species on sand dunes. Their native range includes few inland regions, with the Great Lakes of North America being the main exception. The genus name Ammophila originates from the Greek words ἄμμος (ámmos), meaning "sand", and φίλος (philos), meaning "friend".

Crassulacean acid metabolism metabolic process

Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect and allow carbon dioxide to diffuse into the mesophyll cells. The CO
2
is stored as the four-carbon acid malate in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to CO
2
, which is then used during photosynthesis. The pre-collected CO
2
is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. The mechanism was first discovered in plants of the family Crassulaceae.

Abscisic acid chemical compound

Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental stresses, including drought, soil salinity, cold tolerance, freezing tolerance, heat stress and heavy metal ion tolerance.

Mesophytes are terrestrial plants which are neither adapted to particularly dry nor particularly wet environments. An example of a mesophytic habitat would be a rural temperate meadow, which might contain goldenrod, clover, oxeye daisy, and Rosa multiflora. Mesophytes prefer soil and air of moderate humidity and avoid soil with standing water or containing a great abundance of salts. They make up the largest ecological group of terrestrial plants, and usually grow under moderate to hot and humid climatic regions.

Ecohydrology is an interdisciplinary scientific field studying the interactions between water and ecological systems. It is considered a sub discipline of hydrology, with an ecological focus. These interactions may take place within water bodies, such as rivers and lakes, or on land, in forests, deserts, and other terrestrial ecosystems. Areas of research in ecohydrology include transpiration and plant water use, adaption of organisms to their water environment, influence of vegetation and benthic plants on stream flow and function, and feedbacks between ecological processes and the hydrological cycle.

Epidermis (botany) outermost single layer of cells in plants

The epidermis is a single layer of cells that covers the leaves, flowers, roots and stems of plants. It forms a boundary between the plant and the external environment. The epidermis serves several functions: it protects against water loss, regulates gas exchange, secretes metabolic compounds, and absorbs water and mineral nutrients. The epidermis of most leaves shows dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. Woody stems and some other stem structures such as potato tubers produce a secondary covering called the periderm that replaces the epidermis as the protective covering.

Moisture stress occurs when the water in a plant's cells is reduced to less than normal levels. This can occur because of a lack of water in the plant's root zone, higher rates of transpiration than the rate of moisture uptake by the roots, for example, because of an inability to absorb water due to a high salt content in the soil water or loss of roots due to transplantation. Moisture stress is more strongly related to water potential than it is to water content.

Transpiration stream

In plants, the transpiration stream is the uninterrupted stream of water and solutes which is taken up by the roots and transported via the xylem to the leaves where it evaporates into the air/apoplast-interface of the substomatal cavity. It is driven by capillary action and in some plants by root pressure. The main driving factor is the difference in water potential between the soil and the substomatal cavity caused by transpiration.

Guard cell

Guard cells are specialized cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are largest when water is freely available and the guard cells turgid, and closed when water availability is critically low and the guard cells become flaccid. Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream, with water taken up by the roots. Plants must balance the amount of CO2 absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor and stomatal pore size.

Plant cuticle

A plant cuticle is a protecting film covering the epidermis of leaves, young shoots and other aerial plant organs without periderm. It consists of lipid and hydrocarbon polymers impregnated with wax, and is synthesized exclusively by the epidermal cells.

Ecophysiology, environmental physiology or physiological ecology is a biological discipline that studies the adaptation of an organism's physiology to environmental conditions. It is closely related to comparative physiology and evolutionary physiology. Ernst Haeckel's coinage bionomy is sometimes employed as a synonym.

<i>Selaginella lepidophylla</i> species of plant

Selaginella lepidophylla is a species of desert plant in the spikemoss family (Selaginellaceae). Known as a "resurrection plant", S. lepidophylla is renowned for its ability to survive almost complete desiccation. During dry weather in its native habitat, its stems curl into a tight ball and uncurl only when exposed to moisture.

Drought tolerance is the ability to which a plant maintains its biomass production during arid or drought conditions. Some plants are naturally adapted to dry conditions, surviving with protection mechanisms such as desiccation tolerance, detoxification, or repair of xylem embolism. Other plants, specifically crops like corn, wheat, and rice, have become increasingly tolerant to drought with new varieties created via genetic engineering.

Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration of electrolytes to keep the fluids from becoming too diluted or concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the osmotic pressure of a solution, the more water tends to move into it. Pressure must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water by osmosis from the side containing pure water.

Photosynthesis systems are electronic scientific instruments designed for non-destructive measurement of photosynthetic rates in the field. Photosynthesis systems are commonly used in agronomic and environmental research, as well as studies of the global carbon cycle.

By definition, stomatal conductance, usually measured in mmol m⁻² s⁻¹, is the measure of the rate of passage of carbon dioxide (CO2) entering, or water vapor exiting through the stomata of a leaf. Stomata are small pores on the top and or bottom of a leaf that are responsible for taking in CO2 and expelling water vapour. The rate of stomatal conductance, or its inverse, stomatal resistance, is directly related to the boundary layer resistance of the leaf and the absolute concentration gradient of water vapor from the leaf to the atmosphere. It is under direct biological control of the leaf through the use of guard cells, which surround the stomatal pore (Taiz/Zeiger 1991). The turgor pressure and osmotic potential of guard cells is directly related to the stomatal conductance. Stomatal conductance is a function of stomatal density, stomatal aperture, and stomatal size. Stomatal conductance is integral to leaf level calculations of transpiration (E). Multiple studies have shown a direct correlation between the use of herbicides and changes in physiological and biochemical growth processes in plants, particularly non-target plants, resulting in a reduction in stomatal conductance and turgor pressure in leaves.

References

  1. ” Xeromorphic”, The Cambridge Illustrated Glossary of Botanical Terms, Michael Hickey, Clive King, Cambridge University Press, 2001
  2. Gechev, Tsanko S.; Hille, Jacques; Woerdenbag, Herman J.; Benina, Maria; Mehterov, Nikolay; Toneva, Valentina; Fernie, Alisdair R.; Mueller-Roeber, Bernd (2014-11-01). "Natural products from resurrection plants: Potential for medical applications". Biotechnology Advances. 32 (6): 1091–1101. doi:10.1016/j.biotechadv.2014.03.005. ISSN   0734-9750. PMID   24681091.
  3. Liu, Hua; Wang, Qiuqing; Yu, Mengmeng; Zhang, Yanyan; Wu, Yingbao; Zhang, Hongxia (2008). "Transgenic salt-tolerant sugar beet (Beta vulgaris L.) constitutively expressing an Arabidopsis thaliana vacuolar Na/H antiporter gene, AtNHX3, accumulates more soluble sugar but less salt in storage roots". Plant, Cell & Environment. 31 (9): 1325–1334. doi:10.1111/j.1365-3040.2008.01838.x. ISSN   1365-3040. PMID   18518917.
  4. Wu, Guo-Qiang; Wang, Qian; Bao, Ai-Ke; Wang, Suo-Min (1 March 2011). "Amiloride Reduces Sodium Transport and Accumulation in the Succulent Xerophyte Zygophyllum xanthoxylum Under Salt Conditions". Biological Trace Element Research. 139 (3): 356–367. doi:10.1007/s12011-010-8662-9. ISSN   0163-4984. PMID   20352373.
  5. McNair, J.B. "Hydrophytes, xerophytes and halophytes and the production of alkaloids, cyanogenetic and organic sulphur compounds". Journal of Natural Products. 6: 1–17.
  6. 1 2 3 4 Zeng, Yan Jun; Wang, Yan Rong; Zhang, Ju Ming (April 2010). "Is reduced seed germination due to water limitation a special survival strategy used by xerophytes in arid dunes?". Journal of Arid Environments. 74 (4): 508–511. doi:10.1016/j.jaridenv.2009.09.013.
  7. "3.1.4 - Turgor loss, cytorrhysis, and plasmolysis | Plants in Action". plantsinaction.science.uq.edu.au. Retrieved 2018-03-21.
  8. Ibañez, A.N.; Passera, C.B. (February 1997). "Factors affecting the germination of albaida (Anthyllis cytisoidesL.), a forage legume of the Mediterranean coast". Journal of Arid Environments. 35 (2): 225–231. doi:10.1006/jare.1995.0142.
  9. 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. PMID   28309493.
  10. Jordaan, A.; Kruger, H. (February 1998). "Notes on the cuticular ultrastructure of six xerophytes from southern Africa". South African Journal of Botany. 64 (1): 82–85. doi:10.1016/S0254-6299(15)30829-2.
  11. 1 2 3 4 5 6 7 8 Turnbull, C. (2017) LS1-OB.34 - Plant stress.
  12. 1 2 3 4 GINDEL, I. (April 1970). "THE NOCTURNAL BEHAVIOUR OF XEROPHYTES GROWN UNDER ARID CONDITIONS". New Phytologist. 69 (2): 399–404. doi:10.1111/j.1469-8137.1970.tb02438.x.
  13. McNeil, Paul L.; Steinhardt, Richard A. (7 April 1997). "Loss, Restoration, and Maintenance of Plasma Membrane Integrity". The Journal of Cell Biology. 137 (1): 1–4. ISSN   0021-9525. PMC   2139853 . PMID   9105031.
  14. Atia, Abdallah; Rabhi, Mokded; Debez, Ahmed; Abdelly, Chedly; Gouia, Houda; Haouari, ChirazChaffei; Smaoui, Abderrazak (1 December 2014). "Ecophysiological aspects in 105 plants species of saline and arid environments in Tunisia". Journal of Arid Land. 6 (6): 762–770. doi:10.1007/s40333-014-0028-2. ISSN   1674-6767.
  15. "Plant Adaptations". University of New Mexico. Archived from the original on January 4, 2015. Retrieved December 2, 2014.
  16. Schwab, K. B.; Schreiber, U.; Heber, U. (1989-02-01). "Response of photosynthesis and respiration of resurrection plants to desiccation and rehydration". Planta. 177 (2): 217–227. doi:10.1007/bf00392810. ISSN   0032-0935. PMID   24212344.
  17. Muller, J., Sprenger, N., Bortlik, K., Boller, T., & Wiemken, A. (1997). Desiccation increases sucrose levels in Ramonda and Haberlea, two genera of resurrection plants in the Gesneriaceae. Physiologia Plantarum, 100(1), 153-158. http://dx.doi.org/10.1111/j.1399-3054.1997.tb03466.x
  18. Zia, Ahmad; Walker, Berkley J.; Oung, Hui Min Olivia; Charuvi, Dana; Jahns, Peter; Cousins, Asaph B.; Farrant, Jill M.; Reich, Ziv; Kirchhoff, Helmut (September 2016). "Protection of the photosynthetic apparatus against dehydration stress in the resurrection plant". The Plant Journal. 87 (6): 664–680. doi:10.1111/tpj.13227. PMID   27258321.
  19. "Craterostigma pumilum - Alpine Garden Society - Plant Encyclopaedia". encyclopaedia.alpinegardensociety.net.
  20. Toderich, K. N.; Shuyskaya, E. V.; Rajabov, T. F.; Ismail, Shoaib; Shaumarov, M.; Yoshiko, Kawabata; Li, E. V. (2013). Combating Desertification in Asia, Africa and the Middle East. Springer, Dordrecht. pp. 249–278. doi:10.1007/978-94-007-6652-5_13. ISBN   9789400766518.
  21. Kang, J., Duan, J., Wang, S., Zhao, M., & Yang, Z. (2013). Na compound fertilizer promotes growth and enhances drought resistance of the succulent xerophyte Haloxylon ammodendron. Soil Science And Plant Nutrition, 59(2), 289-299. http://dx.doi.org/10.1080/00380768.2012.763183
  22. Belaeva, Tatiana Nikolaevna; Butenkova, Alina Nikolaevna; Astafurova, Tatiana Petrovna (2014). "Phlox sibirica L. in South Siberia". Biosciences Biotechnology Research Asia. 11 (Spl Edition Nov. 14): 371–376. doi:10.13005/bbra/1488.
  23. Prijambada, Erlina; Sudikno, Antariksa; Murti Nugroho, Agung; Leksono, Amin (2016-03-01). "Sansevieria trifasciatas, xerophyte as indoor humidity absorber of small type residences 1". Ecology, Environment and Conservation. 22.
  24. John, Mwonjoria; Aliyu, Umar; Kevin, Juma; Titus, Kahiga; Piero, Ngugi; David, Mburu; Alphonse, Wanyonyi; Charles, Githinji; Joseph, Ngeranwa (25 August 2016). "Anti-inflammatory activity of craterostigma pumilum (hochst) is associated with hyperalgesia". International Journal of Pharmacology and Toxicology. 4 (2): 169. doi:10.14419/ijpt.v4i2.6497.
  25. Dell’Acqua, G.; Schweikert, K. (April 2012). "Skin benefits of a myconoside-rich extract from resurrection plant Haberlea rhodopensis". International Journal of Cosmetic Science. 34 (2): 132–139. doi:10.1111/j.1468-2494.2011.00692.x. PMID   22023081.
  26. Gechev, Tsanko S.; Hille, Jacques; Woerdenbag, Herman J.; Benina, Maria; Mehterov, Nikolay; Toneva, Valentina; Fernie, Alisdair R.; Mueller-Roeber, Bernd (1 November 2014). "Natural products from resurrection plants: Potential for medical applications". Biotechnology Advances. 32 (6): 1091–1101. doi:10.1016/j.biotechadv.2014.03.005. ISSN   0734-9750. PMID   24681091.

Further reading