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Drought is an extended period without water. It can be defined as an extended period when water became available to plants. Plants adapt to environmental conditions by four mechanisms which are, tolerance, resistance, avoidance, and acclimatization. Avoidance is the ability of the plant to adapt to drought due to its internal environment. For example, a plant can have specialized parts such as succulent stem and bulb roots that store water, this includes plants such as cactuses . The reserved water will be utilized during water shortages periods like drought.
Tolerance is the capacity to endure stress. ‘Drought avoidance’ is the ability of plants to maintain higher tissue water content despite reduced water content in the soil. This occurs through the use of traits that deal with water loss minimization (water savers) and optimization of water uptake (water spenders). Water spenders achieve higher tissue water status by maintaining the water uptake through increased rooting, hydraulic conductance under drought stress. Water savers reduced loss of water by reducing transpiration, transpiration area, and radiation absorption.
Drought tolerance is the ability of plants to endure low tissue water content through adaptive traits such as maintenance of cell turgor through osmotic adjustment and cellular elasticity, and increasing protoplasmic resistance. Drought tolerance can also be called drought dormancy refers to a plant’s ability to withstand desiccation without dying. It includes plants that shed their leaves and enter into deep dormancy.Thes plants conserve water in their stems. Plants that do not shed their leaves have resinous coatings that retard water loss for example creosote bus.
Drought tolerant plants have the ability to absorb water from soil that is much drier such that they can photosynthesize with low leaf moisture contents that would prove fatal to most plants. They can survive long periods without water for about 3 years. These plants can be back to life as soon as they get water for example a in a day or two days for example the resurrection plant.
Drought resistance is when plants develop characteristics that enable them to survive in drought periods. Resistance occurs through the use of defense strategies such as escaping. Escape is the ability of a plant species to complete its life cycle before the onset of drought for example plants that has a short life span. In this case, plants will undergo the vegetative and reproductive stages when water is still available. This occurs in two mechanisms called the phonological development and developmental plasticity. Rapid phenological development involves rapid plant growth, producing a minimal number of seeds before the soil water decelerates. These plants have special morphological, physiological, or biochemical adaptations. Plants with mechanisms of developmental plasticity show little growth during the dry season or they will be in dormancy hence no metabolic processes will occur hence no growth will be the result. During these periods, these plants will develop very few flowers and seeds, but in wet seasons they grow faster and produce more seeds. All these will be done to reduce the utilisation of water as to conserve it for them to survive.
Drought avoidance exists in plants that escape unfavorable conditions by not existing for example annual plants. They mature in a single season, and then die after channeling all of their life energy into producing seeds instead of reserving some for continued survival. Plants adapt to drought through morph-physiological alterations which controlled by molecular mechanisms that regulate the expression of genes. This enable plants to cope with drought. (Bray EA: 1997)
Phytohormones play a significant role in regulating processes that enable plants to adapt to drought for example abscisic acid, cytokines, gibberellic acid auxin, and ethylene. Abscisic acid enable plants to close stomata to decrease transpiration and reduce plant growth . They decrease metabolic processes that utilize water and transpiration. Abscisic acid also -induces senescence, especially if the drought occurs at the reproductive stage hence reduce water loss. . There are ABA signaling genes, such as OsNAP, OsNAC5, and DSM2 that promote improved yield under reproductive drought. These ABA-induced non-stomata adaptations of plants under drought stress can be exploited to improve grain yield under reproductive drought.
Furthermore, some plants like deciduous plants developed xeromorphic traits to adapt to drought. They shed their leaves in dry periods for example Brachystegia specific shed in winter. Shedding of leaves reduces the number of leaves of a plant hence reducing transpiration as it will be mostly done through the lenticels of the bark . Some plants have reduced leaf size to reduce the number of pores for example pine trees that have needle-shaped leaves. In dicotyledonous plants the stomata are more at the lower side of the leaf to reduce transpiration as light firstly strike the upper part of the leaf before the upper part. The development of sunken stomata is another adaptation that enables plants to survive in drought. The development of sunken stomata reduces transpiration loss as it will be difficult for the wind to blow water in a hole or a depression.
Apart from the xeromorphic traits of leaves mentioned above, some plants developed small mesophyll leaves that have thick leaves. Sclerophyllous plants have more sclerenchyma cells that store water and maintain the shape of the leaf as they do not suffer permanent damage due to wilting and can be restored to full functionality when conditions became normal. Some have few branches to conserve water by reducing transpiration through lenticels. Decrease in water loses enable plants to be more adaptive to drought and they managed to increase their yields for example in rice. The decrease in suberization and compaction of sclerenchyma layer cells increases retention of water under drought stress in rice which results in high yields. In addition, Plants also developed adaptive features of specialized tissues such as rhizodermis, with a thickened outer cell wall or suberized exodermises and reduction in the number of cortical layers. These features reduce the rate of water lose hence plants sustain in drought periods
The overexpression of the Arabidopsis improved the ratio of biomass produced to water used by enhancing photosynthesis and reducing transpiration. Plants also adapt drought through hydrotropism to counter drought stresses. Furthermore, some plants developed more lateral roots and root hairs for the maximum absorption of water.
Osmotic adjustment (OA) also plays a significant role in the adaptation of plants to drought.OA is a process where solutes accumulate in dividing cells. This maintains the turgidity of the cells as they will have a lower water potential. When cells have high solute potential, water moves from a region of low solute potential to a region of high solutes when the water potential hence maintains cell water. Moreover, the flaccid cells keep the stomata closed which reduce transpiration. The compatible solutes include proline and glycine betaine and the inorganic cations include organic acids, carbohydrates, and free amino acids. These solutes accumulate in response to water stress. OA is common in rice and it results to an increase in yield. Some plants are succulents for example desert succulents. They store water in their fleshy barks and they also have specialized roots called bulbs that are water reservoirs. Some developed very long tape roots, more lateral roots, and root hairs for the maximum absorption of water.
Some plants evolved and developed smaller leaves with fewer stomata, some plant leaves developed to resemble spiky thorns. Some plants may also completely shed their leaves in a drought, to prevent water loss. The basic rule is that fewer leaves mean less water loss through transpiration. These extreme leaf adaptations can also protect the plants from hungry and thirsty birds and animals that feed on them. Plants can also adapt to drought by develop seeds that can stay in dormancy for a long period of time. The seeds produced will not germinate until the environment became conducive. The dormancy seeds will form a seed bank and some may be dispersed by wind and animals to different areas where they will germinate. The seeds survive during the dry spells and quickly germinate, grow and produce more seeds when rains fall.
Apart from the adaptations mentioned above, plants can also alter the way they photosynthesis when water is scarce and this leads to the formation of damaging chemicals called free radicals. Some plants only open their stomata during the cool of the night to take up CO2 which is stored and used in the daytime for photosynthesis. Through this adaptation, plants can lost less water but the production of nutrients will keep on going for growth although it will be slightly decreased. Resurrection plants are able to survive complete loss of water. They accumulate vast quantities of OAs, release free radical scavengers, and produce special protective proteins to survive long and severe droughts. They do all of this while they also fold their leaves away and wait until rainfalls.
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