The below mentioned article provides an essay on xerophytes.
Xeric habitat characterizes xerophytes (xero = dry, phytes = plants). Xerophytes evolved to survive in an ecosystem where there is deficiency in available water. This includes the areas that are subjected to drought like deserts where low rainfall is the norm.
The areas may also include physiologically dry soil (not physically dry) where uptake of water becomes difficult due to salinity in soil water. Therefore xerophytes have evolved a wide variety of adaptations. These adaptations primarily aim to limit water loss, conserve water and obtain water as much as possible from the environment.
Xerophytes have developed the following three main adaptive strategies to survive drought avoidance, drought tolerance and succulence. Each of the adaptations is different but effective against extremely low water availability, desiccating winds, and high and fluctuating temperatures. Different types of plant have these adaptive strategies and among them there exist little morphological, physiological and taxonomical relationship.
The drought avoiding xerophytes are ephemeral-annuals. They live only for a short time period. Germination, growth, flowering, fruiting and dying occur after a heavy rain. These plants tide over drought by the dormant seeds. The seeds germinate only after a heavy rain. Growth occurs and leaves are formed. The leaves remain alive so long water is available.
They wither away after the soil dries out. So these plants remain leafless most of the time. These plants have large and brightly coloured flowers to attract pollinating insects. After fruiting the seeds enter into deep dormancy and thus these plants avoid drought. The drought avoiding plants do not need any xeromorphic adaptations because they have adequate water during their short period of activity.
Mention may be made of the following ephemeral-annuals:
Dithyrea californica, Atriplex pseudocampanulata (completes its life cycle within 70 days), Wahlenbergia communis (completes its life cycle within 36 days), Abronia villosa, Castilleja chromosa (Schrophulariaceae), C. exserta etc. The ephemeral-annuals, more accurately, are to be referred to as simply ‘ephemerals’ because most of them complete the entire life cycle within few months, some in just weeks.
The drought tolerant xerophytes are perennials. They have structural modifications that help them to adapt to the environment.
The roots are very well developed and spread out over a large area to collect maximum water. They may be laterally extensive and shallow. They may be radially elongated. The roots may also be long, vertically deep, e.g. Acacia. Most of the shallow and radial roots are within three feet of the surfaces, e.g. Cactus.
These roots allow quick acquisition, of large quantities of superficial water when it rains. Cacti store water both in root and stem. These plants have adaptations by means of which they can survive years of drought on the water collected from a single rainfall. Prosopis, a drought tolerant plant, develops long vertical deep root systems.
The vertical roots may be as long as 80 feet. These roots draw water from deep underground reservoirs. Some cacti develop special roots after rain to absorb more water. Many xerophytes have both shallow and deep taproots and respectively allow collecting surface water and absorbing water from water table or directly above or below it.
Many xerophytes have root hairs extended to the tip to increase water uptake. The root hairs of many xeric grasses, e.g. Andropogon foveolatus, Panicum turgidum etc. have rhizosheaths. Rhizosheaths are also known as sand grain root sheaths.
The root hairs secrete mucilage where the moving sand grains become attached. The attached sand grains form a sheath around the root hairs. With the increase in number of root hairs the sheath covers the whole root.
The following functions are attributed to rhizosheath:
(1) It is equivalent to cork layer that is formed in dicotyledonous root and prevents the loss of water from inner tissues.
(2) It accelerates water absorption as it contains mucilage that has high absorptive power for water.
(3) It is associated with nitrogen fixation The cortex is usually thin (Fig. 29.1) and therefore there exists small distance between xylem and soil water. The endodermis has wider caspanan bands. So endodermis is more efficient in its function of radial diffusion of water from stele to the cortex.
The xylem is well developed and as a result rapid transport of water occurs following absorption. Xerophytes have evolved methods by means of which they maintain a high osmotic pressure in root tissues. This enables them to increase water absorption. Many cacti accumulate hydrophilic colloids in their root cortex. This reduces the water potential of the root-tissues and thus accelerates the absorption of water by osmosis.
Some members of Juncaceae and many of the Restionaceae show hydromorphic features in the roots and xeromorphic features like abundant sclerenchyma in stems. The roots show air cavities in the cortex like hydrophytes.
This remarkable combination of dual adaptations of hydromorphic roots and xeromorphic stems is probably beneficial to the plant because the stems are sometimes exposed to strong drying winds when the roots become too cold to deliver enough water to meet evaporation losses.
One of the adaptations among the xerophytes is surface reduction. The leaves are very much reduced and absent in many xerophytes. The functions like transpiration and photosynthesis of leaf are surrendered to stem. Such stems grow by marginal growth like leaves and acquire the structure of leaves (ex. Rhipsalis, Ruscus etc.). Such stems are called phylloclades.
The bulbous habit is often associated with xerophytes. In many plants like Narcissus, Scilla, Tulipa, Haemanthus etc. leaves and flowers develop for a limited period in each year. The plants survive through underground stems. Swollen underground stems occur in many species of Asclepiadaceae.
Rhizomes occur in the species of Iris. Corms occur in Crocus, Watsonia etc. The above plants grow actively when water is available. The aerial organ like leaf remains alive so long water is available and withers away when the water is used up. So the aerial organs show little adaptations to xeric condition.
Internal adaptations in xerophytes include – mechanisms to provide for the storage of water and to develop mechanical tissues to resist collapse and tearing on drying.
The plants that are adapted to store water are described as being succulent (ex. many species of Crassulaceae, Aloe etc.). The plants that develop mechanical cells to resist tearing and disruption of tissues as a result of excessive desiccation are usually described as sclerotic (ex. Hakea, Leptocarpus, Ulex etc.).
Stems of woody xerophytes are efficiently insulated by periderm to limit water loss. The tissues of periderm may be lignified and often impregnated with resins. Spines may be present on the surfaces of stem. Many xerophytes have hypodermis, the cells of which are composed of chlorenchyma cells.
These cells, when enclosed in rigid and lined channels, add structural rigidity, e.g. Leptocarpus stem. Hypodermis may also be composed of thick walled cells like fibres and sclereids. These cells may be present as a continuous sheet or in patches. Hypodermal fibres may develop on the peripheral side of thin-walled chlorenchyma, e.g. Ecdeiocolea stem.
Hypodermal sclerenchyma protects the inner tissues from high intensity of light and thus limits the loss of water. It also provides mechanical support. Leafless xerophytes and xerophytes with reduced leaves have palisade like cells in the outer cortex. These cells are compactly arranged and compose the photosynthetic tissues, e.g. Casuarina (Fig. 29.2).
Xerophytes have well-developed vascular tissues with long xylem vessels. The tracheids have relatively thicker walls than those present in mesophytes. Annual rings are well developed. Wood of many xerophytes is ring porous. The conduction through this wood is ten times more than diffuse porous wood. In Pinus, oils and resins are produced. Latex develops in Euphorbia.
Leaf polymorphism is observed in many xerophytes. After heavy rain broad leaves are formed and narrow leaves follow when the soil dries out. Xerophytes have evolved mechanisms to limit water losses. Loss of water is due to transpiration from exposed aerial parts. The size of the transpiring surface determines the amount of water loss.
The smaller is the surface usually the lower is the transpiration. The reduction of transpiring surface can be accomplished in the following ways. The twigs, branches and mature leaves may be shed during drought. The plant-core remains alive. Immature leaves are more resistant to drought than mature leaves. So the former is seldom shed.
The leaves may be long, slender, dissected or greatly reduced to spines. The scale or needle-like leaves are common in Coniferae and Ericaceae. Leaves with reduced surface are noted in Calluna, Thuja and Asparagus. Such leaves are called microphylly. Leaves are entirely absent in some Cactaceae, Asclepiadaceae and Euphorbiaceae.
Most Restionaceae have non-functional leaves. During midday sun small and narrow leaves heat up less rapidly than larger ones. As a result transpiration decreases. The long, slender leaves are usually vertically oriented to reduce the amount of heat absorbed. Plants reduce the exposed surface to sun or drying winds by rolling or folding of leaves.
Plants also achieve this by rotating and orienting leaves away from maximum exposure to wind or sun. The leaves of xeromorphic species of Stipa and Ammophila remain folded constantly thus hiding the stomata. Ammophila arenaria (Fig. 29.3) leaf entirely rolls up towards the upper surface where stomata are located thus reducing the surface of moist tissue that is exposed to air and enclosing the stomata when dry condition prevails.
In many xeromorphic species dead hairs or hair-like projections cover the leaf surface and they form an insulating layer, e.g. Artemisia, Kleinia, Elaeagnaceae etc. Such structures are called trichophyllous. Such leaves are gray or white owing to the hair covers. The white colour aids in deflecting heat from leaves.
Moreover leaf hairs provide shade to leaves. It is interpreted that hairs create pockets on the leaf surface where water vapour accumulates. As a result the diffusion of water from the leaf is reduced. Most xerophytes have hairs that have thickened walls and some also have thick cuticle.
These hairs, e.g. Gahnia, Ammophila and Erica reduce water losses in contrast to thin-walled hairs that increase water loss under some conditions. Moreover hairs are good deterrents against insect feeding and egg laying. Hairs also give protection against predator when plants become the only source of moisture for animals during drought.
Transpiration may be cuticular or stomatal. The magnitude of cuticular transpiration is governed by the formation of cuticle and cuticular layers. Many xerophytes such as Cactaceae have a heavy cuticle and thick cuticular layers.
These cuticular layers almost entirely arrest the cuticular transpiration. Leaves are also covered with lipids to reduce moisture evaporation (ex. Ricinus, Calotropis etc.). Due to the presence of cutin and lipids the two epidermises of a leaf become impervious to water loss.
The thickness of cuticle is related to xeric conditions. In a study it is observed that Prosopis velutina, when grown in natural xeric condition, have cuticle ten times thicker in comparison to that grown in indoors.
Transpiration is a vital process in all plants. When the water potential inside a leaf is higher than the environment, the water vapour will diffuse out of the leaf. If a plant loses too much water, wilting will result. In case of permanent wilting plants will die. So the xeromorphic leaves have adaptations that decrease the water potential in order to reduce water loss.
The functioning and position of stomata govern the magnitude of stomatal transpiration. In many xeromorphs stomata open for a brief period only. In others stomata remain closed in the day. They open at night (ex. Camellia thea) when the relative humidity is high and temperature is low. As a result there is less transpiration.
Usually xeromorphs have well developed and often numerous stomata. It is interpreted that carbon dioxide enters rapidly through these stomata during rare wet periods. Xeromorphic leaves may be epi- or hypostomatic. The stomata are often protected to restrict water loss. An individual stoma may be sunken (ex. Aloe).
In Nerium oleander groups of stoma occur in a groove or depression on the abaxial surface of leaf (Fig. 29.4). The surface of groove is lined with hairs. In the groove the relative
humidity always remains high thus reducing the diffusion gradient within the chamber. This reduces evaporation of moisture.
In Ficus, Nerium etc. the stomata are restricted to well-protected crypts to reduce water loss. In Agave (Fig. 29.5), Dasylirion etc. cuticle forms ridges inside the pore of a stoma. Thus the canals that communicate between open air and intercellular space become narrower. As a result water loss is reduced.
Multiseriate epidermis occurs in Ficus elastica and Nerium. In mature leaves sometimes it is difficult to distinguish a multiple epidermis and a hypodermis. This structure reduces evaporation of water through epidermis and diminishes the intensity of light that reaches the photosynthetic tissue.
Xeromorphic leaves, like many mesophytes, have mesophyll differentiated into adaxial palisade and abaxial spongy tissue. But the palisade mesophyll is well developed and it is often correlated with high light intensity. Though the palisade mesophyll is typically adaxial, they may occur on abaxial side also, e.g. Nerium, Ficus, Atriplex portulacoides, Artemisia, Myoporum, Sonneratia Alba etc.
In these leaves spongy tissue occurs in between the adaxial and abaxial palisade tissue. In many xeromorphs the mesophyll palisade replaces spongy tissue where the leaves have palisade mesophyll only, namely Greggia camportum, Sphaeralcea incana etc. Due to the loss of spongy mesophyll, the palisade parenchyma becomes smaller and packed together.
As a result the volume and surface area of leaf apoplast diminish. So each cell loses less water to the apoplast. The mesophyll cells of Pinus have peg-like ingrowths (plicate) to increase the surface of photosynthesis. Xeromorphic leaves have larger bundles of vascular tissues as compared to mesophytes.
Drought-resistant leaves are hard and rigid. This structure is called sclerophylly. Sclerophylly prevents leaf tissues from mechanical deformation during shrinkage. Thick cuticle occurs on the epidermises. As a result the leaves become leathery with a hard and glossy surface. The tissues of scleromorphic leaves are small-celled and dense.
The palisade mesophyll may be multiseriate. The spongy tissues have little intercellular spaces. Development of sclerenchyma is very common. The hypodermis of Pinus needle is composed of sclerenchyma. In many leaves lignification occurs in mesophyll. In Stipa, the major part of the leaf tissue is composed of sclerenchyma.
The abaxial epidermis of Ammophila arenaria is without stomata and lignified as the major portion of mesophyll. The collenchyma cells are restricted to small strands. Sclerenchyma provides mechanical support to the leaves and protects the inner tissues from high intensity of light. Thus loss of water is reduced. The thickening of cell wall is caused by the conversion of polysaccharides into celluloses and other materials.
Succulence in stems and leaves is common within xerophytes. The succulent organs thicken due to water storage. The succulent stem may attain a spherical shape, e.g. Mamillaria. A sphere has the smallest surface area in relation to tissue volume. So by increasing the thickness of an organ the relative surface is reduced. The surface reduction is favourable for the water balance.
Succulent stems have little differentiation of ground tissue. Parenchymatous spherical cells compose the ground tissue. Stem succulence occurs as a result of proliferation of xylem parenchyma and by primary thickening growth, e.g. Cactaceae. The mechanical tissue and vascular tissue are poorly developed.
The cell walls are often mucilaginous. The succulent leaves, also referred to as malacophyllous xerophytes, are similar on all sides. The epidermis of such leaves is multilayered, large-celled and occupies the major portion to the leaf volume e.g. Begonia, Zebrina (Fig. 29.6), and other members of Commelinaceae Piperaceae etc.
It is interpreted that the water present in these layers protects the central mesophyll against water losses during dry periods. The succulent leaves of Aloe and Haworthia have epidermis with thick outer walls. The epidermis has cuticle and cuticular waxes. The stomata are sunken. In Aloe, the stoma (Fig. 29.7) is variously protected thus regulating and minimizing water loss during dry periods.
Above each stoma there is raised rim that forms a suprastomatal cavity. The cavity has a constricted opening to the atmosphere. It is interpreted that during favorable conditions of growth the cavity has a role in enhancing evaporation. Due to the presence of narrow opening in the cavity the structure has a venturi effect that lowers pressure above the stoma and assists transpiration.
Some leaves, e.g. Haworthia and Lithops have characteristic features. The leaves have chlorenchyma and water storing mesophyll. The leaf tips are translucent. The leaves remain underground, the tips being above the ground level only. The photosynthetic tissues perceive the light stimulus through the cells present on the leaf tips. These are often referred to as ‘window plants’.
In succulent leaves the vascular tissues are poorly developed. The mechanical cells are also scanty. Most of the succulent species fix carbon dioxide in the dark. The stomata of such plants remain closed during day. The stomata open at night. During high temperature the stomata may also remain closed at night when carbon dioxide exchange becomes nil.
Such plants survive by fixing carbon dioxide made available internally through respiration. These plants have adapted a specialized photosynthetic process, called Crassulacean Acid Metabolism (CAM). Plant having Crassulacean Acid Metabolism is referred to as CAM plant. In this plant carbon dioxide is fixed through phosphoenolpyruvate in the dark.
Phosphoenolpyruvate carboxylase catalyzes the carboxylation. As a result oxaloacetic acid is formed. It is then reduced to malate. During daytime malate is decarboxylated. The released carbon dioxide is fixed in ribulose-1, 5-bisphosphate and enters the Calvin cycle. Ribulose-1, 5-bisphosphate carboxylase catalyzes the reaction.
These plants have sufficient ribulose-1, 5-bisphosphate carboxylase activity. CAM plants can exist for long periods without any carbon dioxide uptake in light. CAM plants are adapted to store carbon dioxide in malate during night. Carbon dioxide is made available during day when malate is decarboxylated.
Plants belonging to the family Crassulaceae have CAM. The term CAM derives from the family name. Certain members of other families like Bromeliaceae, Euphorbiaceae, Aizoaceae, Liliaceae etc. also exhibit CAM.
Many xerophytes have an alternative pathway of photosynthetic carbon fixation in contrast to Calvin cycle that operates in mesophyte. This alternate pathway is known as Hatch-Slack pathway or C4-dicarboxylic acid pathway or simply C4-pathway. Plants having this pathway are referred to as C4-plants. This pathway is characterized by the formation of dicarboxylic acids of 4-carbon compounds as primary products of photosynthesis in contrast to 3-carbon compounds of the Calvin cycle.
The chloridoid—eragrostoid and panicoid taxonomic divisions of the Gramineae, Centrospermae, Compositae, Euphorbiaceae etc. carry out photosynthesis by using Hatch-Slack pathway. These plants have specific type of anatomy, referred to as ‘Kranz’ anatomy. Kranz is a German word—meaning garland or wreath.
In this type specialized bundle-sheath cells surround the vascular bundles of leaf. Chlorenchyma cells encircle the bundle-sheath. These radially aligned chlorophyllous mesophyll cells were given the German name ‘Kranz’. The cells of bundle-sheath contain large chloroplasts and starch.
The essence of Hatch-Slack pathway is that C4-compounds carry carbon dioxide from mesophyll cells to bundle-sheath cells where photosynthesis occurs. The C4-pathway starts (Fig. 29.8) in the mesophyll cells for the transport of carbon dioxide. Carboxylation occurs in the mesophyll cells where carbon dioxide and its receptor phosphoenolpyruvate condense to transient 4-carbon compound—oxaloacetate.
The enzyme phosphoenolpyruvate carboxylase catalyzes the condensation. Oxaloacetate is rapidly reduced to malate or aminated to aspartate depending upon species. C4-compounds, formed in the mesophyll, enter into bundle-sheath cells where they are decarboxylated. The released carbon dioxide enters the Calvin cycle.
Pyruvate formed during decarboxylation returns to the mesophyll cell for another round of carboxylation. It is to note that the decarboxylation of the C4-compounds in the bundle-sheath cells maintains a high concentration of carbon dioxide at the site of photosynthesis.
In Calvin cycle carboxylation occurs by the condensation of carbon dioxide and ribulose-1, 5-bisphosphate to form a transient 6-carbon compound, which rapidly hydrolyzes to two molecules of 3-phosphoglycerate. Ribulose-1, 5- bisphosphate carboxylase catalyzes the condensation.
RibuIose-1, 5-bisphosphate carboxylase is also an oxygenase when it catalyzes the addition of oxygen to ribulose-1, 5-bisphosphate to form phosphoglycolate and 3-phosphoglycerate. The oxygenase and carboxylase reactions occur at the same site and compete with each other. Under normal atmospheric conditions at 25°C the rate of carboxylase activity is four times greater than oxygenase.
In higher temperature and when there is high oxygen and low carbon dioxide the Ribulose-1, 5-bisphosphate carboxylase switches to oxygenase activity. This enzyme catalyzes the oxygenation of Ribulose-1, 5-bisphosphate thus forming phosphoglycolate and 3-phosphoglycerate.
The recycling of phosphoglycolate leads to the release of carbon dioxide and consumption of oxygen in a process called photorespiration. It is interpreted that photorespiration is seemingly a wasteful process because the organic carbon is converted to carbon dioxide without the production of adenosine triphosphate (ATP).
Xerophytes have evolved method to minimize the wasteful reaction of photorespiration. They have adapted C4-pathway. This pathway maintains a high concentration of carbon dioxide in the bundle-sheath cells at the site of Calvin cycle.
This accelerates the carboxylase reaction of Ribulose-1, 5-bisphosphate carboxylase in relation to oxygenase reaction. Thus the C4-plants take advantage of high temperature and minimize the oxygenation of Ribulose-1, 5-bisphosphate.
Plants growing on the mountainous parts have xeromorphic adaptation, e.g. Pycnophyllum molle and P. micronatum etc. belonging to the family Caryophyllaceae. They have cylindrical stems and very much reduced leaves. Stomata are present only on the adaxial surface and they are sunken and protected amongst papillae.
But the species like Oxalis exidua show little xeromorphic adaptation. In O. exidua the stomata are superficial; hairs and papillae occur on the epidermal surface. The chlorenchymatous mesophyll cells are not compact. The leaves have all the characteristics similar to those of the mesic members of the genus.
But the anatomy of stem have characteristic of a liana. The vascular bundles are separate. During secondary growth the interfascicular cambium produces parenchyma only instead of secondary xylem and phloem. By this device the stem can twist and deform without compressing the vascular bundles when the stem penetrates the cracks between rocks.
Azorella compacta, another plant growing on mountainous environment, has very shiny leaves. The shiny leaves reflect ultraviolet light. The plant possesses contractile roots, which help the plant to be firmly anchored in the frost heave. Many plants growing in mountainous environment have sap that is mucilaginous nature. This acts as a kind of antifreeze.
Halophyte often shows xeromorphic adaptations. They grow on locations with a high content of salts that make the soils entirely sterile. Many halophytes exhibit succulence that is normally associated with drought. The leaf area is strongly reduced, e.g. Salsola, Glaux, Mesembryanthemum etc. In some plants leaves are absent, e.g. Salicornia.
The succulent leaves have salt glands through which excretion of salt occurs. Salt gland regulates the salt content of plants and prevents salts accumulating in the protoplasm. Moreover salt glands protect the leaves against strong sunlight and insect feeding.
Succulence and hard-leaf characters are combined in the tree and shrubby species of halophytes. The leaves have thick cuticle and the palisade tissue is many layered. The mesophyll tissue is weakly differentiated.