The Biochemical Indices of Drought Resistant Species of Iori Plateau (East Georgia)
Abstract
Existing forecasts of climate change predict significant warming, seasonal precipitation change, and strong and frequent droughts in the coming decades. Drought resistant plant species have more chance to survive. Predictions make the study of the biology of drought-resistant species especially relevant today. Antioxidant system, which plays an important role in plant stress resistance, is of special interest. Moreover, antioxidant substances are characterized by healing properties as well. Mechanisms of drought resistance of plants growing on arid territories of Georgia are practically unexplored. Presented study aimed to investigate the characteristics of antioxidant system of leaves of drought resistant species (Euphorbia falcata L. (sickle spurge), Lycopsis orientalis L. (small bugloss), Cotinus coggygria Scop. (smoke tree), Elaeagnus angustifolia L. (Russian olive) and Amygdalus communis L. (almond)) growing atone of the most arid regions of Georgia-Iori plateau (East Georgia). Analyses were made in two vegetative phases – flowering and fruit-bearing. From the obtained results, it is clear that the studied species have more or less different biochemical stress-adaptive mechanisms, which include certain enzymatic and non-enzymatic components of antioxidant system. In small bugloss in response to stress catalase was activated and synthesis of ascorbate-tocopherol and anthocyanins was enhanced; especially high amount of proline accumulation was noted. Phenols, anthocyanins and proline should be actively involved in stress resistance of sickle spurge. Russian olive was distinguished with high levels of ascorbate-tocopherol and anthocyanins, as well as proline; in addition the enzymatic antioxidants-catalase and peroxidase were activated, and soluble carbohydrates were accumulated. The protective systems of ascorbic acid and tocopherol, as well as phenolic compounds were active in smoke tree. From osmolytes content of proline increased, while the level of soluble carbohydrates was already the highest, compared to all tested species. Catalase was activated in response to stress in almonds; protective systems of ascorbate-tocopherol, phenols, and anthocyanins were active as well; among osmolytes content of carbohydrates was increased. The stability of carotenoids protective system of all studied species under stress conditions presumably indicates that experimental plants are less sensitive to radiation stress, and water deficiency is the main stress factor for them.
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Introduction
Existing forecasts of climate change predict significant warming, seasonal precipitation change, and strong and frequent droughts in the coming decades (IPCC, 2007). It is likely that the combination of drought and unusually high temperatures that accompany global warming will cause vegetation change on Earth (IPCC, 2012; Jentsch et al., 2007; Smith, 2011). The mentioned combination of drought and temperature forms the so-called "Global change type drought", also known as "hot drought". Under such drought conditions, the risk of severe plant stress and death is significantly increased (Allen et al., 2015; McDowell et al., 2008; Overpeck and Udall, 2010). Drought resistant plant species have more chance to survive. Drought resistance is based on evolutionarily established physiological-biochemical mechanisms, many of which are studied today. Resistant plants are characterized by the high content of antioxidants and osmolytes. They possess the ability to alter metabolism in order to strengthen the antioxidant system, which helps to overcome the lack of water, as well as to recover from dehydration. (Aslam et al., 2015; Laxa et al., 2019).
Humans have been using plants for medicinal purposes since time immemorial. According to statistics more than 60% of the world'spopulation today prefers medicinal plants and natural products. Moreover, 25% of the commercial preparations produced in the world are of plant origin. In recent decades, interest in herbal remedies has grown in both Europe and America. This is due to the fact that phytopreparations are more available; furthermore, they are thought to have fewer side effects than synthetic drugs (Benzie and Wachtel-Galor, 2011; Harvey, 2000).
Against the background of expected climate change it may be assumed that the resources of plant raw material for medical use will be significantly reduced and probably replaced by drought-resistant species. All abovementioned predictions make the study of the biology of drought-resistant species especially relevant today. Characteristics of the antioxidant system, which play an important role in plant stress resistance, including drought, are of special interest. Moreover most antioxidant substances are characterized by healing properties as well (Pisoschi and Negulescu, 2011).
The physiological and biochemical indices of drought resistant plants growing on arid territories of Georgia are practically unexplored.
Based on the above, the aim of the study was to investigate the biochemical characteristics of leaves of the drought-resistant plants of one of the most arid regions of Georgia-Iori plateau (east Georgia). Mainly the elements of the antioxidant system have been studied: plastic pigments, ascorbic acid, tocopherol, anthocyanins, soluble phenols, total proteins, soluble carbohydrates, proline; as well as enzymes’- catalase, peroxidase and nitrate reductase activity, and total antioxidant activity, expressed in percents of inhibition.
Such studies give an idea of the adaptive mechanisms that either plant uses under stressful conditions and deepen our knowledge of the biology of drought-resistant plant species. This type of information under variable climate conditions allows to select plant species, that may be used against soil erosion and desertification, on the one hand, and as raw material for the preparation of natural remedies, on the other.
Conclusion
From the obtained results, it is clear that the studied drought-resistant species have more or less different biochemical stress-adaptive mechanisms. This is not surprising, as such differences may exist not only at species but at the genotypic level within a single species as well (Abuelsoud and Papenbrock, 2019).
For each studied species, it may be identified a stress coping strategy that includes certain enzymatic and non-enzymatic components of the antioxidant system.
Catalase was activated in small bugloss in response to stress enhancement, while from low-molecular-weight antioxidants synthesis of ascorbate-tocopherol and anthocyanins was enhanced. An especially large amount of proline accumulation was noted from osmolytes in its leaves.
Phenols, anthocyanins and proline should be actively involved in drought and high temperature stress in sickle spurge. Russian olive was distinguished by the maximal level of almost all studied indices, compared to other species. Apparently, this is its specific feature. Since some of these characteristics were reduced under increased stress, they still retained their maximal value and obviously played an active role in stress protection of the plant. In particular, high levels of ascorbate, tocopherol and anthocyanins, as well as proline, were found in Russian olive leaves. In addition, the enzymatic antioxidants catalase and peroxidase were activated, and carbohydrates were accumulated.
The protective systems of ascorbic acid and tocopherol, as well as phenolic compounds were active in smoke tree. From osmolytes content of proline increased, while the sugars level was already the highest, compared to all other species. Catalase was activated in response to stress in almonds; protective systems of ascorbate-tocopherol, phenols, and anthocyanins were active from the low-molecular compounds; among osmolytes content of carbohydrates was increased.
The stability of carotenoids protective system of all studied species under stress-enhancing conditions suggests that experimental plants were less sensitive to radiation stress, and water deficiency was the main stress factor for them. All tested species can be recommended as drought resistant, which may be cultivated underwater deficiency conditions.
