Plant Adaptations to Drought Stress

Physiological and morphological adaptations of plants to drought

Drought stress is more prevalent in the environment and even so as a result of global warming. It poses a serious threat to food security as it limits the yield of food crops tremendously (Abobatta, 2019). Plants unlike animals cannot run away from drought conditions around them and are therefore forced to develop adaptive measures which enable them to survive drought. These adaptations include biochemical adaptations, physiological adaptations and morphological adaptations. The plants which have developed the adaptations which enable them to tolerate drought stress are referred to as drought resistant plants (Basu et al. 2016). Some plants escape drought by completing their life cycle before the start of drought. They achieve this through rapid phonological development where they undergo rapid growth and produce a minimum number of seeds before the water in the soil is depleted. Some show developmental plasticity where they do not grow during drought conditions. Drought stress severely affects the yield of plant crops posing a great challenge on sustainable food production (Abobatta, 2019). Plant physiological processes involve the various functions performed by the different parts of the plant. Plant morphology on the other hand studies the form or anatomy of the plant and reflects how plants are adapted to their environment. It is however important to note that plant morphology affects its physiology to a great extent because form and function normally has a strong relationship (Sahebi et al. 2018).

As a physiological adaptation to drought stress plants reduce the rates of photosynthesis. Plants achieve this by decreasing leaf area and the rate of photosynthesis per unit area of the leaf. They reduce the rate of photosynthesis by closing the stomata (Harwoth et al. 2017). When the light reactions of photosynthesis are carried out under low levels of carbon dioxide concentration in the cell which is the case during drought, the components of the photosynthetic electron transport accumulate in the cell reducing molecular oxygen and eventually there is generation of reactive oxygen species. The reactive oxygen species produced are capable of destroying the photosynthetic apparatus (Chen et al. 2016). To prevent damage to photosynthetic apparatus as a result of drought stress plants have developed adaptive responses which include thermal dissipation of energy from light, dissociation of complexes which harvest light from the reaction centers of photosynthesis, the water cycle and the xanthophyll cycle (Basu et al. 2016). The plants offsets an imbalance of the ratio of photosynthesis and rates of transpiration by maintaining a high concentration of carbon dioxide in the sub stomata chamber thus reducing the levels of stomata opening effectively reducing the transpiration rates (Murtaza et al. 2016). The presence of a C4 pathway rather than a C3 pathway in plants is an adaptive measure to reduce water loss from the plant during drought stress. Reduced photosynthetic levels have been reported to be the cause of reduced yields of herbage in the chamomile and oregano plants during drought stress (Tatrai et al. 2017) Plants are able to modulate their pigments in order to adapt themselves to the drought conditions around them. The pigments involved are chlorophyll a and b and carotenoid. During drought the content of chlorophyll b increases in the plant while chlorophyll a remains constant. This has been seen with okra. Reduced chlorophyll reduces primary production. Much chlorophyll is lost from the mesophyll cells as compared to bundle sheath cells. There is also a decrease in the amount of carotenoids during drought stress (Murtaza et al. 2016).

Plants also regulate their hormones in a bid to adapt to drought stress. When plants are exposed to drought stress the roots synthesize abscisic acid which is translocated to the leaves to cause the closure of the stomata and at the same time reduce the growth of the plant. This closure of stomata mediated by abscisic acid results when the high levels of abscisic acid increases the uptake of potassium ions from guard cells leading to a corresponding loss of turgor pressure. Plants produce more. Cytokinins during drought stress through the expression of isopentenyltransferase ( IPT) gene and they play a critical role in reducing the premature senescence and death of the leaves. A decrease in the levels of indole-3-acetic acid regulates the gene which codes for proteins for late embryogenesis thus creating an adaptation to drought. A reduced level of gibberellic acid inhibits plant growth (Basu et al. 2016). Auxin counters the adaptations of the plant to drought condition. Plant hormones therefore work together to regulate the biosynthesis and responses of each other and for every adaptation created by a hormone there exists a counteracting influence of another hormone thus creating a balance (Killi et al. 2017). Rice shows tillering which is an outcome of the synergy involving three hormones cytokinins strigolactone and auxin where cytokinins promote branching while auxin and strigolactone inhibits the same (Sahebi et al. 2018). Reduction of the rates of transpiration is important during drought stress. In order to reduce the rate of transpiration the plant has to reduce the number of leaves and their sizes (Li et al. 2019). Reducing the number of leaves can be achieved through shedding of the leaves and reduced branching. Thyme plants have shown reduced vegetative development during drought stress and it is postulated that this is in order to conserve growth (Supratim et al. 2016). Some plants use sclerophylly which is the development of hard leaves which could undergo wilting without being damaged and are thus able to be restored to normalcy when the drought stress is over. The size and numbers of the stomata become reduced during drought to enable plants to survive drought stress (Khan et al. 2018). Decreased conductance by the stomata reduces the surface area of the chloroplast which is exposed to intercellular space and thus regulates water loss during drought conditions. Rice has shown an overexpression of Arabidopsis AP2/ERF TF HARDY an adaptation which has been related to enhanced photosynthesis and a reduction in the rates of transpiration thus it is able to survive drought conditions (Sahebi et al. 2018).

Roots are the first part of the plant to feel the impact of drought. They however continue to grow even under drought conditions. The presence of drought reduces the growth of lateral roots of the plant while the growth of primary root is maintained. The Arabidopsis R2R3-type MYB TF MYB96 regulates the activation of the meristems of the lateral roots in a process that involves abscisic acid signaling cascade. This action suppresses the activation of lateral root meristems. Plants form small roots during drought conditions to increase the surface area of water absorption. The plant roots develop specialized tissues such as rhizodermis which has a suberized exodermis to survive drought stress. The roots also reduce to the number of cortical layers to deal with drought. When plants are exposed to drought the amyloplasts in the columella cells of their roots degrade thus increasing hydrotropism (Xiao, Xu and Young, 2008). A combination of hormonal changes involving auxins, giberrellic acid and other hormones are responsible for changing the architecture of the root system thus enabling the structure to be adapted to drought conditions. During the onset of drought the expression of enzymes such as xyloglucan endotransglucosylase which control root morphology are induced, at the same time there is down regulation of other structural proteins responsible for root growth thus increasing surface area for the uptake of water by the roots. Formation of more root hairs is an adaptation of the roots to drought. For example, in rice the roots become adapted to drought by the conversion of the cell walls into corky tissues through suberin infiltration and the root has also shown compaction of cells of the sclerenchyma layer I order to retain more water during drought (Sahebi et al. 2018). There are other changes that occur in the environment of the plant during drought other than limited water supply. There is an increase in the concentration of salts and mechanical impedence. Examples of solutes which accumulate are carbohydrates, inorganic cations, free amino acids and organic acids. Plants in response t this accumulate compatible solutes including betaine, proline and glycine to protect itself by osmotic regulation, protection of the integrity of the membranes, detoxification of reactive oxygen species and enzyme and protein stabilization. In cotton an increased accumulation of proline and sugars which are soluble as a result of the increased expression of ArabidopsisEDT1/HDG11 has been shown to increase drought tolerance and lead to improved yields. Previous studies have shown that drought-resistant wheat varieties, with yield stability under drought stress, have a greater capacity for osmoregulation than less resistant varieties (Basu et al. 2016). Betaine aldehyde dehydrogenase (BADH), ornithine δ-aminotransferase (OAT) and pyrroline-5-carboxylate reductase (P5CR) are enzymes which have been implicated to play important roles in osmotic adjustment. In some plants however the main osmolytes which play important roles in osmotic adjustment are sugars. Examples of such sugars are glucose, fructose,sucrose and trehalose. Examples are the overexpression of the genes sucrose:fructan-6-fructosyltransferase ( 6-SFT) in tobacco and trehalose-6-phosphate phosphatase in rice (Basu et al. 2016).

Reference list

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Jafarnia, S., Akbarinia, M., Hosseinpour, B., Modarres Sanavi, S.A.M. and Salami, S.A., 2018. Effect of drought stress on some growth, morphological, physiological, and biochemical parameters of two different populations of Quercus brantii. iForest-Biogeosciences and Forestry, 11(2), p.212.

Khan, A., Pan, X., Najeeb, U., Tan, D.K.Y., Fahad, S., Zahoor, R. and Luo, H., 2018. Coping with drought: stress and adaptive mechanisms, and management through cultural and molecular alternatives in cotton as vital constituents for plant stress resilience and fitness. Biological research, 51(1), p.47.

Killi, D., Bussotti, F., Raschi, A. and Haworth, M., 2017. Adaptation to high temperature mitigates the impact of water deficit during combined heat and drought stress in C3 sunflower and C4 maize varieties with contrasting drought tolerance. Physiologia plantarum, 159(2), pp.130-147.

Li, Y., Bian, H., Ren, B., Xie, Y., Ding, X., Yao, X. and Zhou, Q., 2019. Morphological responses of two plant species from different elevations in the Dongting Lake wetlands, China, to variation in water levels. Nordic Journal of Botany, 37(1), p.e01987.

Murtaza, G., Rasool, F., Habib, R., Javed, T., Sardar, K., Ayub, M.M., Ayub, M.A. and Rasool, A., 2016. A review of morphological, physiological and biochemical responses of plants under drought stress conditions. Imp J Interdiscip Res, 2, pp.1600-1606.

Sahebi, M., Hanafi, M.M., Rafii, M.Y., Mahmud, T.M.M., Azizi, P., Osman, M., Abiri, R., Taheri, S., Kalhori, N., Shabanimofrad, M. and Miah, G., 2018. Improvement of drought tolerance in rice (Oryza sativa L.): Genetics, genomic tools, and the WRKY gene family. BioMed research international, 2018.

Tátrai, Z.A., Sanoubar, R., Pluhár, Z., Mancarella, S., Orsini, F. and Gianquinto, G., 2016. Morphological and physiological plant responses to drought stress in Thymus citriodorus. International journal of agronomy, 2016.