Drought and Plant Adaptations Towards It

Plants live with environmental adjustment in nature which is never stable. Water deficit occurs in most natural and agricultural habitats and is caused mainly by intermittent to continuous periods without precipitation. Drought is the meteorological term for a period of insufficient precipitation that results in plant water deficit. Water deficit can affect plants differently during vegetative versus reproductive growth. When plant cells experience water deficit, cell dehydration occurs. Cell dehydration adversely affects many basic physiological processes. By far it is one of the most important factors which adversely affect the crop production. Drought results in high soil salinity.

Drought severity is changeable as it depends on numerous factors like soil structure and it is water reserve capacity, average rainfall, and evapotranspiration rate. Drought has been classified into two broad categories:

• Soil drought
• Atmospheric drought.

Soil drought often leads to atmospheric drought and if both combine, it becomes disastrous. Otherwise, for plants it is more difficult to struggle against soil drought than against atmospheric drought. Atmospheric drought occurs due to low atmospheric humidity, high wind velocity, and high temperature which cause a plant to lose most of its water.

Stress sensing mechanisms in plants

Plants use a variety of mechanisms to sense abiotic stress. Stresses disrupt or changes many physiological processes in the plants by affecting cellular functions like protein or RNA stability, ion transport etc. These changes indicate the plant that a change in the environment has occurred and it’s time to respond to it accordingly. At least five different types of stress- sensing mechanisms can be distinguished.

• Physical sensing refers to the mechanical effects of stress on the plant or cell structure like the contraction of plasma membrane during drought stress.
• Biophysical sensing involves changes in protein structure or enzymatic activity.
• Metabolic sensing that results from the detection of by-products that accumulates in the cell due to stress condition. Example includes accumulation of Reactive Oxygen Species (ROS).
• Biochemical sensing that involves the presence of specialized proteins that have the capability to sense a particular stress.
• Epigenetic sensing refers to the modification of DNA or RNA structure due to changes in environmental conditions.

Each of these mechanisms can act individually or in combination to activate downstream signal transduction pathway.

Effect of Drought on Plants

Drought is harmful to the normal growth of plant and yield of crop. A few of them are summarized here.

1. Under prolonged drought, many plants dehydrate and die.
2. It reduces the water potential inside the plant cell.
3. Drought results in the accumulation of abscisic acid (ABA).
4. Drought stress often leads to the generation of Reactive Oxygen Species (ROS), including superoxide anion radicals, hydroxyl radicals, hydrogrn peroxide, alkoxy radicals and singlet oxygen. ROS may react with proteins, lipids and DNA, causing oxidative damage and impairing normal functions of the cell.
5. Drought stress conditions acidify the chloroplast stroma and affects the carbon fixation process by causing inhibition to the RuBisCO activity
6. It also affects stomatal closure, limits gaseous exchange, prevents the transpirational loss of water and arrests carbon assimilation photosynthetic rates.
7. It leads to the disruption of homeostasis and ion distribution in the cell.
8. Plant growth is retarded.
9. Drought leads to decrease in chlorophyll content and also its synthesis.
10. Water stress alters gene expression and the proteins in the plant leaves decreases during water deficiency due to the suppressed synthesis.
11. Drought causes disturbance of the association between membrane lipids and proteins as well as enzymes activity and transport capacity of membranes. Drought results in the variation of fatty acid composition.
12. It results in the decrease in K concentration mainly due to membrane damage and disruption in ion homeostasis.

Drought tolerance

Drought tolerance is the ability of the plants to grow, flower and maintain the economic yield under arid conditions. The drought tolerant plants undergo various changes to maintain their biomass during the water stress conditions. It is a complex mechanism which involves mechanisms like early maturity, avoidance, tolerance and desiccation tolerance. In plants, a wide range of morphological and physiological traits have been linked to drought tolerance which includes morphology of root and rooting depth, plant architecture, leaf area, stomatal conductance, osmotic conductance, antioxidant defense etc. Plant also expresses a variety of genes associated with morphological and physiological traits contributing to abiotic stress tolerance.

Mechanisms of drought tolerance

Plants responses in various ways for acclimatization and survive under drought conditions. Their responses may be through induction of morphological, biochemical, physiological changes in the plants; also, there is a molecular mechanism in the plant under water deficit conditions. Drought stress affects the water relations of plants at cellular, tissue and organ levels, causing specific as well as unspecific reactions, damage and adaptation reactions.

Morphological mechanisms

Under drought condition plants use various changes to tolerate stress conditions which include changes in whole plant, organ or tissue at morphological, physiological and molecular levels. Appearance of a single or a combination of inherent changes determines the ability of the plant to stand under stress conditions. Plants use various morphological mechanisms that operates under drought conditions like:-

Drought escape: Drought escape is attained through shortening the life cycle or growing season, so that the plants reproduce before the onset of drought stress or the environment becomes dry. Early flowering is an important mechanism that plants use to adapt with drought. Escape from drought occurs when the physiological development of the plant is successfully matched with periods of soil moisture availability, where the growing season is shorter and terminal drought stress predominates. Therefore, it is considered that the short life cycle is a proper technique to escape from climatic stresses.

Drought avoidance: Drought avoidance consists of mechanisms that decreases the water loss from plants due to stomatal control of transpiration and increases the efficiency of water usage. The root system plays a vital role in avoiding drought stress. Root characters such as biomass, length, density and depth are the main drought avoidance traits that help the plants to thrive during drought conditions. The roots become deeper and thicker to absorb water from considerable depths. Plants also reserve water uptake through an extensive and prolific root system.

Phenotypic flexibility: Water deficit affect plant growth and the most affected portions are the root and the shoot. Both root and shoot are the key components of plant adaptation to drought. To minimize the loss of water during drought, plants limit the number and area of leaves to avoid loss of yield. Since the root is the only source for water absorption from soil, therefore improvement of root growth rate, its density, size and spread are the basic reactions of the plants for drought stress tolerance along with reduced number and size of leaves.

Physiological mechanisms

There are various physiological responses for drought tolerance. The most essential responses are osmotic adjustment, osmoprotection, antioxidation and a scavenging defense system. However, the physiological basis of genetic variation is not clear in drought tolerant. Some of the physiological mechanisms are as follows:-

Osmotic adjustment: The capacity of plant cells to accumulate solutes and use them to lower the potential of water during the time of osmotic stress is known as osmotic adjustment. It involves a net increase in solute content per cell and is independent of the change in volume that results from water loss. Osmotic adjustment can take place through vacuole and cytosol. Plants either take up ions from the soil or transport it from other parts of the plants to the root to increase the solute concentration in the roots. But the accumulation of ions during osmotic adjustment is predominantly restricted to the vacuoles, where these are kept out of contact with cytosolic enzymes and organelle.

Compatible solutes or osmolytes are organic compounds that are osmotically active in the cell. Unlike ions, their high concentration does not destabilize the membrane or interfere with enzyme function. Also, cell can tolerate their high concentration without any detrimental effects on metabolism and the synthesis of these solutes is an active metabolic process. Some examples of compatible solutes are proline, sorbitol, glycerol etc. Solutes like proline also act as osmoprotectant where they protect plants during water stress from toxic byproducts and provide a source of carbon and nitrogen to the cell when the conditions return to normal.

Roles of osmotic adjustment: It is an important trait in delaying dehydration damage by maintaining the cell turgor and physiological processes under water shortage conditions.

It enhances the translocation of pre-anthesis carbohydrate partitioning during filling of grain.

It also kept the water balance of the cell with active accumulation of solutes in the cytoplasm, while high turgor maintenance increases photosynthetic rate and growth.

Cell membrane stability: The main component of drought tolerance in plants is to keep intact the integrity and stability of cell membrane as the first target of many abiotic stresses is the cell membrane. The co-existence of heat and drought stresses affects the cell membrane function. The increased permeability and leakage of ions out of the cell has been used as a measure of cell membrane stability. To examine the germplasm for drought tolerance, the membrane stability of leaf segment is an essential trait.

Plant growth regulators: Phytohormones play vital roles in drought tolerance of plants. The production of endogenous auxins is reduced under drought stress, but the production of abscisic acid and ethylene increases. Auxins break root apical dominance induced by cytokinins and helps in new root formation. Auxins have indirect but important role in prolific root system. It has been experimentally seen that the exogenous application of Indole 3-acetic acid (which is a naturally occurring auxin) enhanced net photosynthesis and stomatal conductance.

An adaptive strategy that occurs during progressive drought stress is drought rhizogenesis and it is reported from Brassicaceae and related families by the formation of short, tuberized, hairless root. These roots have the capability to withstand prolonged drought period and give rise to new functional root system upon rehydration. According to some studies, gibberellic acid might participate in the process of drought rhizogenesis.

Abscisic acid, a phytohormone and natural growth inhibitor, is produced under abiotic stress conditions, including drought. By accumulating abscisic acid, plant response to drought and other stresses. Abscisic acid is recognized as a stress hormone as it regulates the expression of gene and acts as a signal for the initiation of processes involved in adaptation to drought and other stresses. It has been proposed that the roles of abscisic acid and cytokinin is opposite in drought stress. Increase in the amount of Abscisic acid and decrease in cytokinin levels favors the closure of stomata and thereby limits the loss of water through transpiration under drought stress. Wilting of plants leads to the rise of Abscisic acid level in plants. High concentration of Abscisic acid leads to many changes in development, physiology and growth. Alteration of the relative growth rate of various plant parts such as increase in the root-to-shoot dry weight ratio, inhibition of leaf area development and production of prolific and deeper root also occur due to Abscisic acid. Expression of various water stress related genes are also induced by Abscisic acid. Experiments have suggested the presence of Abscisic acid dependent as well as independent transduction cascades and pathways that act as a signal of drought stress snd the expression of specific water stress induced genes. Changes are produced in plants by Abscisic acid that confers an ability to maintain turgor to withstand dehydrative forces.

Ethylene is a growth inhibitory hormone. It is involved in environmentally induced growth inhibition and stimulation. In cereals, the response to drought includes loss of leaf function and onset of premature senescence in older leaves. Ethylene may serve to regulate leaf performance as well as to determine the onset of natural senescence and drought mediated induced senescence.

Another phytohormone known as salicyclic acid, have also been found to induce tolerance against several abiotic stresses. It has been found experimentally that salicyclic acid potentiates the generation of reactive oxygen species in photosynthetic tissues of Arabidopsis thaliana during osmotic stress.

Polyamines are cationic in nature and are known to have profound influence on plant growth and development. Polyamines associates with membrane’s anionic components, such as phospholipids, and protects the lipid bilayer from deteriorating effects of stress. Recent studies suggested that polyamines have considerable roles in drought tolerance in rice.

Antioxidant defense: Exposure to various abiotic stresses including drought leads to the generation of Reactive Oxygen Species (ROS). The ROS may react with the biomolecules like proteins, lipids and DNA, causing oxidative damage and impair the normal functioning of cells. In plant cell, the antioxidant defense system includes both enzymatic and non-enzymatic constituents. The enzymatic components include superoxide dismutase, catalase, peroxidase, glutathione reductase etc. Tolerant cells activate their enzymatic antioxidant system upon exposure to abiotic stresses. Activation of enzymatic antioxidant system leads to the quenching of ROS and thus protects the cell.

Molecular mechanisms

Water deficit conditions in plants leads to the changes in gene expression (up-and down regulation). Various genes are induced at transcriptional level in response to drought and these gene products are thought to function in drought tolerance in plants. Expression of genes may be triggered directly by stress conditions or from injury responses.

Stress proteins: Under stress conditions, proteins are produced by plants as a response to stress and to survive under different stresses including drought. Majority of stress proteins are water soluble and play its role by hydrating the cellular structures. Variety of transcription factors and stress proteins are synthesized exclusively for drought tolerance in plants. There are a number of regulatory and functional stress associated proteins that are reported to participate in response to heat and drought stresses. Some of them are protein kinases, phosphatases, dehydrins, osmotins, heat shock proteins etc.

Signaling and drought stress tolerance: General responses to stress involves signaling stress detection via the redox system, checkpoints arresting the cell cycle and DNA repair processes stimulated in response to DNA damage. The association of complex signaling events with sensing of stress and defense activation and acclimation pathways is believed to involve ROS, Calcium, Calcium regulated proteins, Mitogen-activated protein kinase cascades and cross-table between different transcription factors.

Chemical signals (ROS, Calcium, Phytohormones) induce stress tolerance by acting via signal transduction cascades and activates genomic reprogramming. Mitogen-activated protein kinase cascades are involved in various stresses signaling including drought. Also, cytoplasmic calcium level increases in living plant cells during drought. Recently, it has been reported that Calcium can improve tolerance of water stress in Catharanthus roseus by increasing ?- glutamyl kinase and reducing the proline oxidase activities.

Thus, under drought stress, various chemical signal transduction activate an array of genes, leading to the synthesis of proteins and metabolites, conferring drought tolerance in a number of plant species.

Managing of Drought Stress

• Using of classical breeding techniques and production of appropriate drought tolerant genotypes can help in managing drought stress and improve plant response to stress.
• Use of exogenous plant growth regulators has been verified for enhancing growth against a variety of abiotic stresses including drought. During drought stress, treatments by exogenous plant growth regulator increase the water potential inside the cell and improve the chlorophyll content.
• Another way to manage drought stress is proper agricultural practice that includes sowing time, plant density and farm management.
• Use of potassium as fertilizer under drought stress increases drought tolerance by implementing cell membrane stability.

Thus, drought tolerance mechanism involves a number of physiological and biochemical processes at cell, tissue, organ and whole plant levels, when activated at different stages of plant development.