Xerophyte

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. 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.^[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. Their waxy, thorny leaves prevent loss of moisture.

Introduction

Haberlea rhodopensis

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

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. Different plant species possess different qualities and mechanisms to manage water supply, enabling 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

Haberlea rhodopensis and Ramonda serbica.^[2]

In environments 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. 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, has specialised protein transporters in its cells which allows storage of excess ions in their vacuoles 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

Cactaceae, 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

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 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

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 typically have less

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

Reflective features

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

reflectivity of any known naturally-occurring biological substance.^[9]

Cuticles

Many xerophytic species have thick

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.^[citation needed] 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. ^[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.

Water storage

Some plants can store water in their

caudiciforms

.

Production of protective molecules

Plants may secrete

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]

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]

Xanthophyll 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

Pineapple plant

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

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.

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.

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.^[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.

ocotillo

which will shed its leaves during prolonged dry seasons in the desert, then re-leaf when conditions have improved.

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

Mechanism	Adaptation	Examples
Water uptake	Extensive root system	Acacia, Prosopis
Water storage	Succulence	Kalanchoe, Euphorbia
Water storage	Fleshy tuber	Raphionacme
Reduce water loss	Surface area reduction	Basal rosette, Eriogonum compositum
	Sunken stomata and hairs	Bromeliads
	Waxy leaf surface	Malosma laurina, Dudleya pulverulenta
	Nocturnal stomata	Tea plant, Alfalfa, Brachychiton discolor, Quercus trojana
	CAM photosynthesis	Sansevieria trifasciata
	Curled leaves	Esparto grass
Dormancy and reduced photosynthesis	Resurrection plants	Ramonda nathaliae, Ramonda myconi, Haberlea rhodopensis, Anastatica, Craterostigma pumilum
	Dormant seeds	Californian poppy
	Leaf abscission	Coastal sage scrub, Wiliwili, Geoffroea decorticans

Uses

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.^{[citation needed]}

A study has shown that xerophytic plants which employ the

Nerium oleander

on the left during autumn and on the right during summer.

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