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Drought stress is one of the most threatened environmental stresses to mankind. It is the most serious stress which limits the agricultural growth and its development and further leads to a great threat to world food security. Drought stress mainly occurs in arid and semiarid regions, in the conditions of low rainfall, high and low temperature, salinity, high intensity of light. The dry spell of weather over a certain period of time may leads to the conditions of drought stress in plants which negatively affect the growth and total yield of the plants. Plants have developed different morphological, biochemical and physiological response mechanisms to cope with these environmental stresses but are different in different species of the plants. Plants response to drought stress by various mechanism like escape, avoidance, tolerance, use of growth regulators and some molecules which helps them to survive under high and low temperature. In this review, we study about the effect, mechanism and management of drought stress in plants. Here, we discuss about various morphological, physiological and biochemical mechanisms which helps to tolerate the drought stress in plants and promote them to survive under stress conditions.
Plants are generally exposed to various environmental stresses during varying weather and different climatic conditions under natural and agricultural influences. Various environmental stresses such as heat, cold, drought, high and low temperature, salinity, chilling, freezing, molecular stress affects the plants from seed germination to seed maturity. Water accounts for about 80-95% of the fresh biomass of the plants. It plays a very important role in most aspects of plant growth, development, metabolism, biochemical activities etc. In present scenario, water stress is a main environmental stress for the plants. Drought stress is one of the most important environmental stresses which limits the plant growth, inhibits crop production and distribution in worldwide especially in arid and semiarid areas. In the next decades, as a continuous increase in the dry periods or high temperature is going on, it is predicted that there is drastically great increase in the drought conditions (IPCC, 2007). Drought stress adversely affect the plant growth and limits its total yield potential. It inhibits the growth of the plant, decrease photosynthetic activity, damage organelle structure and function, degrade chlorophyll content, include water loss in the leaves and accelerate aging process in the early stages of the plant. Drought stress induces oxidative stresses by the accumulation of reactive oxygen species (ROS) in the plant cells. The main target of relative oxygen species (ROS) is membrane phospholipids which prevent membrane damage and increases its permeability also influence the peroxidation of lipids (Sharma et al., 2012). Drought stress tolerance is found in almost all the plants but varies from plant to plant and species to species. The drought stress in plants can be mitigated by different strategies. Plants have various mechanism to tolerate the drought stress. They ability to withstand drought up to a certain extent. Certain molecules, proteins and growth regulators serves as osmolytes to protect the structure and functioning of cellular components inside the cell membrane. Molecules like Melatonin regulates different physiological, biochemical and molecular processes in the plants and further help the plant to cope with the drought stress or to survive under drought stress conditions. Drought stress is obtained to be an ordinary loss of water, which causes stomatal closure and limits the gaseous exchange in the plant species. Dehydration is much more which can lead to degradation of metabolism and cell structure. To develop or create a new variety of crops to obtain a good productivity under water stress conditions a better understanding of plant morphological and physiological changes is required. To understand the plant response towards drought is one of the most important and fundamental part to make the plants stress tolerant. Plants can perform various mechanism like drought escape and drought avoidance and drought tolerance in response to drought stress. Drought escape is defined as the ability of the plants to complete their life cycle before the entrance of severe stress. Drought avoidance is described as maintaining high water potential in the tissue instead of soil water deﬁcit. Drought tolerance is the ability of the plant to perform functions in tolerance to drought stress.
Under drought stress, plants face many challenges in the physiochemical and molecular functioning of the plants which can ultimately affects the growth, development and yield quality of the plants. During drought stress, by light harvesting mechanization there is a significant decrease in the photosynthetic activity of the plants which ultimately decreases the functioning of the enzyme Rubisco. Photosynthetic performance also declined by the misfunctioning of the chloroplast structure. Drought condition also influence the carbon dioxide level in the plants which takes part in the photosynthetic electron transport generation. This results in the enhance degeneration of ROS activity which directly affect the photosynthetic apparatus and damage the apparatus so that decreases the level of photosynthesis. Due to disfunctioning of photosynthetic apparatus in drought stress, there is a decline in photosynthetic rate, stomatal conductance, transpiration rate, photochemical eﬃciency of PSII and photosynthetic electron transport rate. During drought or water deficit conditions, closing of stomata is regulated by the increase in the level of abscisic acid (ABA). This act as a molecule of signaling which regulates the functioning of various physiological and molecular processes. Due to drought, there is a decline in water potential and also a decrease in the relative water content of the plants. With the decrease in the water potential there is a reduction in the uptake of the many different macro and microelements, nitrogen transports and its metabolism, reduction in compounds like ammonium transporter, nitrate reductase, nitrite reductase and glutamine synthetase. During water stress at root zone there is a great decline in the growth of the root tissues which affect the nutrient uptake by the roots and their translocation to the target sites.
The drought stress affects the range of plant species from their morphological to molecular levels and inhibits growth and development of the plants. The various effects of drought stress on the plants are described as:
Drought affect the crop growth and its development in great way and limits its quality yield or yield potential. The ﬁrst eﬀect of drought is that it imparts poor germination and poor plant stand establishment. Drought stress severely decreases germination rate and seedling stand of various crops. Cell division, elongation and maturation are the process of overall growth of the plant. It also includes physical, chemical, genetic and molecular growth functions. The water deficit conditions i.e. drought stress affect these growth functions and leads to a great loss in the both quality as well as quantity of the produce. Under severe drought condition, by decrease in the level of water ﬂow from the xylem to the elongating cells, cell elongation of higher plants can be reduced and there is a decline in the growth rate (Nonami, 1998). During grain filling stage, there is the formation of starch and carbohydrates. Due to water deficiency in grain filling stage there is reduction in synthesis of sucrose and carbohydrate and decrease in quality yield.
The plant water relations in the plants is influenced by some factors which are named as: Relative water content, leaf water potential, stomatal resistance, rate of transpiration, leaf temperature and canopy temperature. During initial phase of wheat i.e. leaf development stage, relative water content of leaves is higher and decrease the dry matter accumulated when leaf matured (Siddique et al., 2001). wheat and rice plants having water-stress in initial stages shows lower relative water content than non-stressed plants. When these plants are opted to drought stress, they show subsequent decrease in the relative leaf water potential, relative water content and transpiration rate, and a prominent increase in the leaf temperature. A conservative inﬂuence of decreased stomatal conductance in non-irrigated plants was negated by a leaf-to-air vapor pressure diﬀerence caused by the associated higher leaf temperature. Transpiration rates also increases and occurs at high rate due to high temperature during drought stress.
Drought stress limits the availability of total nutrient uptake in the plants and there is a limited concentration of the mineral and nutrients in the crop plants. Cellular tissues get shrinked or ultimately damaged due to severe water deficit conditions. There is a limitation in the accumulation and absorption of the nutrients in the root zone and their translocation from root to shoot becomes difficult during drought stress. Limited absorption of mineral and nutrients can lead to a decline in other nutrient uptake and can reduce transpiration flow. During water deficit, transpiration occurs at high rate initially and dehydrate the cellular tissues and its components. In severe water stress condition, there is a decline in transpiration rate so that lower absorption of nutrients from root to shoot.
Drought stress mainly reduce the photosynthetic rate due to decrease in the leaf area expansion, improper photosynthesis apparatus, senescence of the leaves prematurely (Wahid and Rasul, 2005). CO₂ uptake can be reduced with the degradation in the opening and closing of stomata. In very severe drought conditions, photosynthetic activity is reduced by a decrease in Rubisco enzyme activity. Cellular tissues shrinkage and decrease in their structure and volume occurs due to high water stress. Drought stress causes changes in the photosynthetic functions which damages the biochemical structure and functioning of calvin cycle enzymes which are directly related with the yield reduction. Rubisco enzyme performs various functions, under water deficit or drought stress conditions it acts as oxygenase and therefore ﬁxation of CO2 is reduced.
When there are any stress conditions to the plants, the activation of relative oxygen species can lead to an increase in the levels of superoxide anion radicles, hydroxyl ions, hydrogen peroxide. Reactive oxygen species (ROS) causes a great damage to the normal functioning of the cells and cellular tissues and causes oxidative damage to the plants by reacting with the proteins, lipids and deoxyribonucleic acid (DNA).
The symptoms of drought in plants vary depending on the plant species, developmental stage, growth conditions, and other environmental factors. Drought severity, drought length, soil physicochemical conditions, and plant vigor are other factors inﬂuencing drought symptoms in plants. Generally, drought symptoms include loss of leaf turgor, drooping, wilting, etiolation, yellowing, and premature leaf downfall. Also, some unusual symptoms include bark and twig crack, branch dieback, thinning tree and shrub canopy, necrosis, and poor and stunted growth. Finally, under extreme conditions, plant death occurs.
Plants are multicellular organisms; therefore, their responses to environmental stresses such as drought are complex. Plant resistance to environmental stress can be divided into two main strategies: avoidance and tolerance. Plant strategies can further divide into three subsections to deal with the drought i.e. escape, avoidance and tolerance.
The plant life cycle is dependent on the plant genotype and the environmental conditions. Because of a shortened life cycle or growing seasonally and allowing plants to reproduce before the environment becomes dry, escape from drought is possible. A short life cycle leads to drought escape due to early flowering, which is considered a form of adaptation to drought by stress avoidance.
The main aim is to preserve the high-water potential in plants. The chief characteristic of this strategy is reducing water loss from plants by stomatal control of transpiration and maintaining water uptake from the soil by an extensive and proliﬁc root system. A deep and thick root system helps the plant to explore water from a considerable soil depth. The plants that use avoidance strategy to maintain relatively high-water potential are generally small in size.
Plants that use tolerance strategy for drought resistance, reduces the number of the leaves from the plants and area of leaves in response to water deficit conditions. Some plants are able to survive droughts because of their unique structures. These structural features include the external membrane of plants that protects them against water loss, as well as tools to help the plants absorb and store water, for example, Desert plants. Hairiness reduces the leaf temperature, while transpiration increases light reﬂectance and minimizes water loss by increasing the boundary layer resistance to water vapor movement away from the leaf surface. Inter- and intracellular changes in leaves are visible.
Drought stress affects the plants physiological, biochemical and molecular functions. It causes reduction in growth and quality yield by affecting flowering and seed filling phase. It is the most serious stress which limits the agricultural growth and its development and further leads to a great threat to world food security. It affects the plant water relation and damage the structure and functioning of various cellular tissues of the root and shoot cells. The plant itself has some molecular or physiological mechanisms by which they can survive under environmental stress conditions. These mechanisms help the plants to tolerate biotic and abiotic stresses and perform their regular functions to enhance the growth and quality yield. Drought stress damages the photosynthetic apparatus and decreases the chlorophyll content so that the plant cannot synthesis its food for better development.
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