In many areas of the globe, plants may be subjected to periods of soil and atmospheric water deficits during their life. Plants often encounter adverse growth conditions. Climate can be affected by many factors such as drought, salinity, chilling, freezing, and high temperatures. These different adverse but not always lethal conditions are usually known as stress, which can delay growth and development, reduce productivity, and, in extreme cases, cause plant death, leading to starvation, depletion of energy and fiber production among other extreme events.

Plant drought response will depend on the species inherent genetics but also on the period and severity of the drought. If prolonged over to a certain extent drought stress will undoubtedly result in oxidative injury due to the over production of reactive oxygen species (ROS) [1,2]. ROS damage membranes and macromolecules affect cellular metabolism and play a crucial role in causing cell injury under drought stress. Moreover, plant stress responses are intense and involve interconnected cross-talk between different regulatory levels, including arrangement of metabolism, gene expression for morphological and physiological adaptation and alteration in enzyme activity [3,4]. The effects of drought and salt stresses on photosynthesis are either direct such as the diffusion limitations through the stomata and the mesophyll and the alterations in photosynthetic metabolism or secondary, such as the oxidative stress rising from the superimposition of several stresses [5]. Oxidative stress is one of the major causes of plant injury under drought conditions. As drought stress proceeds, stomatal closure induces the limitation of photosynthesis by carbon, and the use of energy for photosynthesis then becomes lower than the absorbed light energy [6,7].

Oxidative damage occurring in plant cells under drought condition stress is a known cause of diminished plant primary production. Reducing oxidative damage through oxidative stress tolerance is expected to confer drought stress tolerance. Beyond that, oxidative stress may accompany many abiotic stresses as high temperature, salinity, or drought stress, causes a serious secondary effect on cells. The effects can be direct, as the decreased CO₂ availability [8], alterations on photosynthetic metabolism [9], or they can arise as secondary effects, such as oxidative stress and the associated synthesis of ROS as well as changes in enzyme and antioxidant metabolism. The former are mostly present under multiple stress conditions and can seriously affect leaf photosynthetic machinery [8,10].

Drought stress, in plants, is characterized by reduction of water content, decreased leaf water potential and turgor loss, closure of stomata, diminish in cell expansion and growth and alteration in enzyme activity, among other metabolic events. Severe water stress can result in the arrest of photosynthesis, outbreak of disorder of metabolism until death of plant [11]. Oxidative damage, which often to be associated to several abiotic stresses as salinity, high temperature, or drought stress, causes a serious secondary effect on cells. Oxidative stress is accompanied by the formation of ROS such as O₂^-, ¹O₂^-, H₂O, OH^-. Reactive oxygen species are produced as a normal product of plant cellular metabolism. Many environmental stresses induce to extreme production of ROS leading progressive oxidative damage and ultimately cell death. In spite of their destroyed activity, they are represented second messengers in a diversity of cellular processes, including conferment of resistance to several environmental factor stresses. Whether ROS would act as signaling molecules or might cause oxidative stress to the tissues depend on the refined balance between ROS production, and their scavenging. Satisfactory scavenging of ROS produced at the same time in environmental stresses depends on the action of several nonenzymatic as well as enzymatic antioxidants in the tissues [12]. Drought can produce disequilibrium between light capture and its utilization, which decreases the rate of the photosynthesis in leaves. In this process imbalance between the generation and utilization of electrons is created. Excess light energy dissipated in photosynthetic system results in generation of reactive oxygen species. Denaturation of functional and structural macromolecules is the well-known results of ROS production in cells. DNA nicking, amino acids, protein and photosynthetic pigments oxidation, and lipid peroxidation are the reported effects of ROS. As a consequence, cells activate some responses such as an increase in the expression of genes for antioxidant functions and production of stress proteins, up-regulation of antioxidant systems, including antioxidant enzymes and accumulation of compatible solutes. All these responses increase scavenging capacity against ROS [13].

Drought

Drought may be considered a major environmental factor for limiting plant growth and yield worldwide, peculiarly in arid and semiarid regions [6,11]. It leads physiological, biochemical, and molecular responses in which photosynthesis is one of the primary physiological targets [8,14]. Drought stress usually causes a decrease in crop production, inhibits the photosynthesis of plants, induces alterations in chlorophyll contents and components, causes damage of photosynthetic apparatus [15] and alter the activity of key enzymes involved in antioxidant processes and carbon metabolism [4,16]. It reduces the photochemical activities and diminishes the activities of enzymes in the Calvin cycle [17]. One of the significant causes that environmental stress reduces growth and photosynthetic capabilities of plants is the breakdown of the balance between the production of ROS and the antioxidant defense, causing accumulation of ROS which induces oxidative stress to proteins, membrane lipids and other cellular components [18].

The Solanaceae family comprises many horticultural crops of major economic importance, such as tomato, potato, tobacco, and pepper. Although large tolerance levels to abiotic stresses may be found in their wild relative species, only moderate tolerance is conserved between their cultured varieties [19]. In the case of tomato, most cultivars show negative effects under drought and salinity, resulting in growth inhibition, decreased seed germination, and reduction of fruit quality and production [20]. Nouri-Ganbalani et al., [21] estimated the average yield loss of 17 to 70% in grain yield due to drought stress. Abiotic stresses induce alterations in the expression of a wide number of genes, leading to physiological and biochemical alterations, at the molecular level. Drought and salinity significantly affect photosynthesis, which impacts on the function of other important metabolic pathways such as nitrogen assimilation [8]. Additionally, respiration is increased to provide energy to conserve plant growth and development. Drought and salt stress affect other protection systems, such as the antioxidant and osmoregulation pathways that support plant cells by the biosynthesis of compatible solutes and reactive oxygen species (ROS) scavengers [22]. These molecules may control a series of different processes in plants. However, being toxic molecules, they are also capable of damaging cells [23]. In plant cells, organelles with a highly oxidizing metabolic activity or with a high rate of electron flow, such as chloroplasts, mitochondria and microbodies, are an important source of ROS production [12,24]. The ROS production depends on the Mehler reaction and the antenna pigments, in chloroplasts. Production of ROS by these sources is rising in plants by conditions limiting CO₂ fixation, such as drought, salt and temperature stress, as well as by the association of these conditions with high-light stress. In C3 plants, limiting CO₂ conditions can also activate the photorespiratory pathway [25]. Photosynthesis and plant growth are among the primary processes affected by drought and salinity [8].

Enzymatic role in drought tolerance: Changes in the activity of antioxidant enzymes can be used to predict drought tolerance in different commodities. Devi et al., [4] evaluated the behavior of five different wheat (Triticum aestivum L.) genotypes under normal and water deficit conditions induced by 6% mannitol. When comparing stressed and non-stressed seedlings, catalase activity was upregulated by more than 50% in roots of water-stressed seedlings in drought-tolerant genotypes. Ascorbate peroxidase in the endosperms and glutathione reductase, catalase and peroxidase in the shoots of stressed seedlings were upregulated by water stress deficit. On the other hand, superoxide dismutase activity was very low in roots and shoots and showed nonsignificant increase under water-stress in tolerant genotypes. It was verified that out of five specified enzyme activities, if any three are upregulated in the specified tissues under water deficit conditions, the wheat genotype is likely to be drought-tolerant. The study concluded the status of antioxidant enzymes could provide a meaningful tool for predicting drought tolerance of a given wheat genotype.

Kaur et al., [16] studied the effects of water deficit on enzymes of the carbohydrate metabolism (alpha and beta amylases, sucrose phosphate synthase, sucrose synthase, acid and alkaline invertases) in seedlings of drought-sensitive and drought-tolerant cultivars of wheat (Triticum aestivum L.). Water deficit was induced by adding 6% mannitol (water potential -0.815 Mpa) in the growth medium. Water deficit decreased alpha-amylase activity in the endosperm of both cultivars, but increased alpha and beta amylase activities in the shoots of tolerant ones. Sucrose phosphate synthase activity showed a significant increase at 6 days of seedling growth in the shoots of stressed seedlings of tolerant cultivar. However, enzyme activity in the roots of stressed seedlings of sensitive cultivar was very low at 4 days of seedling growth. Sucrose synthase activity was lower in the shoots and roots of stressed seedlings of tolerant cultivar when compared to sensitive ones at early stages of seedling growth. Enzyme associated with carbon metabolism are believed to be correlated with drought resistance in different wheat genotypes.

Although photosynthetic response to drought and salinity stress is highly complex, this subject will be reported in the present review. Photosynthesis is a well-established source of reaction oxygen species in plants. Regulatory systems are required to reduce reactive oxygen species because the photosynthetic electron transport chain (PET) takes place in an aerobic environment [26]. Photosynthesis can be affected by water stress and salinity directly or indirectly, by decreases in CO₂ availability caused by diffusion limitations, alterations in photosynthetic metabolism or restrictions in the photochemical systems extreme stress conditions [9]. Concomitantly to impairment of photosynthesis, salinity and water stress lead strong changes of leaf water associations and osmotic homeostasis. It is widely accept that after short term (days) exposure salinity causes osmotic effects, while under long term exposure it causes ionic damages to the plant cells. It involves the interaction of restrictions occurring at different sites of the cell/leaf and at different time scales in relation to plant development [27]. In order to maintain an effective process ROS and intracellular ROS pools at low levels is needed to have a capable antioxidant network. An efficient antioxidant network is important to produce elevated rates of photosynthesis and it has been demonstrated in some studies using molecular genetics approaches to either incapable antioxidant enzymes or increasing their activities or alters the abundance of low- Mr molecular antioxidants such as ascorbate and reduced glutathione. Nevertheless, such researches have often failed to take into balance the photosynthetic production of reductants and oxidants within the context of cellular redox homeostasis and redox signaling. Studies of the interactions of redox signaling pathways with photosynthesis has been increased more and more recently, and this provides new opportunities for increasing photosynthesis specifically under conditions that favor cellular oxidation. Here, we consider recent concepts of oxidative stress and oxidative indicating in association to photosynthesis [28].

Antioxidants

A highly efficient antioxidative defense system, present in plant cells, is possible to antagonize the toxicity of active oxygen species. A relevant defense system, including both nonenzymic and enzymic constituents, is represented by glutathione, which defends several cellular components and the thiol status of proteins against oxidative stress [29]. When antioxidants lose or receive electrons, in the presence of reactive species, become unstable and reactive. In these conditions antioxidants exhibit pro-oxidant effects and can be harmful. There are so many definitions about antioxidants but Damiani et al., [30] defined a good antioxidant as one that provides low oxidant reactivity with a low ability to produce peroxidation. To control the level of reactive oxygen species under stress conditions, plant tissues contain a series of enzymes scavengers of ROS. An arrangement of enzymes is needed for the regeneration of the active forms of the antioxidants [31]. Mechanisms for the generation of ROS in biological systems are represented by both non-enzymatic and enzymatic reactions [32]. Nonenzymatic antioxidants include low-molecular-weight compounds, such as vitamins C and E, beta-carotene, uric acid, and GSH, a tripeptide that comprise a thiol (sulfhydryl) group [33].

Ascorbic acid: Ascorbic acid (AA) is one of the most studied and powerful antioxidants [34,35]. It has been observed in the majority of plant cell types, organelles and in the apoplast. In physiological conditions, vitamin C predominantly exists in its reduced form, ascorbic acid (AA); it also exists in trace quantities in its oxidized form, dehydroascorbic acid (DHA), in leaves and chloroplasts [36,37]; and its intracellular concentration can build up to millimolar range - e.g. 20 mM in the cytosol and 20-300 mM in the chloroplast stroma [38]. The capacity to transfer electrons in a wide range of enzymatic and non-enzymatic reactions makes AA the principal ROS detoxifying compound in the aqueous phase. AA can directly scavenge superoxide, hydroxyl radicals and singlet oxygen and diminish H₂O₂ to water via ascorbate peroxidase reaction [39]. In chloroplasts, AA acts as a cofactor of violaxantin de-epoxidase thus sustaining dissipation of excess excitation energy. AA regenerates tocopherol from tocopheroxyl radical giving membrane protection [12]. Additionally, AA carries out a number of non-antioxidant functions in the cell. It has been involved in the regulation of the cell division, cell cycle progression from G1 to S phase and cell elongation. Very recently dehydroascorbic acid has emerged as a signaling molecule regulating stomatal closure [33]. In photosynthesis, ascorbic acid has a role in the defense against oxidative stress. The recent studies, using the transgenic plants and mutants, confirmed the role of ascorbic acid and the glutathione cycle of ascorbic acid in oxidative stress, but in the plants under stress, the amount of ascorbic acid was increased [40].

Tocopherols: Tocopherols (α, β, γ, and δ) belong to the group of lipophilic antioxidants involved in scavenging of oxygen free radicals, lipid peroxy radicals, and ¹O₂ [41]. Relative antioxidant activity of the tocopherol isomers in vivo is α > β > γ > δ which is due to the methylation model and the amount of methyl groups attached to the phenolic ring of the polar head structure. Hence, α-tocopherol with its three methyl substituents has the highest antioxidant activity of tocopherols [42]. The only pathway capable of synthesizing tocopherol is by photosynthetic organisms and they are showed in green parts of plants. Homogentisic acid (HGA) and phytyl diphosphate (PDP) are used as precursors of the tocopherol biosynthetic pathway. Furthermore, 5 enzymes, at least, are involved in the biosynthesis of tocopherols: 4-hydroxyphenylpyruvate dioxygenase (HPPD), homogentisate phytyl transferases (VTE2), 2-methyl-6-phytylbenzoquinol methyltransferase (VTE3), tocopherol cyclase (VTE1), γ-tocopherol methyltransferase (VTE4), excluding the bypass pathway of phytyl-tail synthesis [43]. Tocopherols protect lipids and other membrane components by physically quenching and chemically reacting with O₂ in chloroplasts and can prevent the chain propagation step in lipid autoxidation, which makes it an effective free radical trap [44]. Fully substituted benzoquinone ring and totally reduced phytyl chain of tocopherol act as antioxidants in redox interactions with ¹O₂. ¹O₂ oxygen quenching by tocopherols is highly competent, and it is estimated that a single α-tocopherol molecule can neutralize up to 220 ¹O₂ molecules in vitro before being degraded [45]. Regeneration of the oxidized tocopherol return to its reduced form can be reached by AsA, GSH or coenzyme Q [46]. Accumulation of α-tocopherol has been shown to stimulate tolerance to chilling, water deficit, and salinity in different plant species [47]. In leaves, metabolic engineering of tocopherol biosynthetic pathway affected endogenous ascorbate and glutathione pools. Further study suggested that expression levels of genes encoding enzymes of Halliwell-Asada cycle were up-regulated, such as APX, DHAR and MDHAR [48]. Mutants of Arabidopsis thaliana with T-DNA insertions in tocopherol biosynthesis genes, tocopherol cyclase (vte1) and γ-tocopherol methyltransferase (vte4) showed higher concentration of protein carbonyl groups and GSSG compared to the wild type, indicating the development of oxidative stress [49]. Transgenic rice plants with Os- VTE1 RNA interference (OsVTE1-RNAi) were more sensitive to salt stress whereas, in contrast, transgenic plants overexpressing OsVTE1 (OsVTE1-OX) revealed higher tolerance to salt stress. OsVTE1-OX plants also accumulated less H₂O₂ than control plants [50]. A series of studies showed that stress-tolerant plants usually enhance tocopherol levels, but, under stress, the most sensitive ones reveal chain tocopherol loss, which leads to oxidative injury and cell destruction [51]. Several observations support this state, e.g., α-tocopherol increases notably by water deficit conditions in spinach and pea leaves, in wheat and other grasses, in Mediterranean shrubs such as rosemary and lavender, and in European beech seedlings. However an interesting observation can be also made; the changes in α-tocopherol level during plant responses to environmental stress are characterized by two phases. In the first phase, there is a rise in tocopherol synthesis, which is followed by a second phase of chain tocopherol loss. For this reason, rice seedlings cultured hydroponically and subjected to water stress in 30% polyethylene glycol showed a loss of α-tocopherol in chloroplasts. These studies showed that although α-tocopherol may provide a certain degree of protection against UV-B radiation, this protection is limited by the amount of antioxidants present in membranes and/or by the molecular species of reactive oxygen [48].

Carotenoids: Carotenoids represent a group of lipophilic antioxidants and are able to detoxify various forms of ROS [52]. Carotenoids are found in plants as well as microorganisms. In plants, carotenoids usually absorb light in the region between 400 and 550 nm of the visible spectrum and pass the captured energy to the Chl [53]. As an antioxidant, they scavenge ¹O₂ to inhibit oxidative damage and quench triplet sensitizer (3Chl*) and excited chlorophyll (Chl*) molecule to prevent the formation of ¹O₂ to protect the photosynthetic apparatus. Carotenoids act as precursors to signaling molecules that influence plant development and biotic/abiotic stress responses. The capacity of carotenoids to scavenge, prevent or reduce the production of triplet chlorophyll can be considered for by their chemical specificity [54]. Among the antioxidants present in the chloroplasts, carotenoids are considered to be the first line of defense of plants against ¹O₂ toxicity and, therefore, products resulting from their direct oxidation by ¹O₂ are potential candidates for this function [55]. Carotenoids have additional roles and help plants to withstand adversaries of drought. Although the antioxidant defense system is impaired under stressful conditions, plants are able to release of excessive energy by thermal dissipation associated with an increase in the carotenoid concentration in water stressed plants. This can be attributed to the activation of the xanthophyll cycle. Thus, presumed that the role of antioxidants and carotenoid pigments in regulating photosynthetic electron transport is crucial [56].

Phenolic Compounds

Phenolics compounds are diverse secondary metabolites as flavonoids, tannins, hydroxycinnamate esters, and lignin, which possess antioxidant properties. They are plentiful present in plant tissues [52]. Phenolic or usually known as polyphenols contain an aromatic ring with -OH or -OCH₃ substituents which together contribute to their biological activity, including antioxidant action. They have been shown to overcome well-known antioxidants, AA and α-tocopherol, in vitro antioxidant assays because of their strong capacity to donate electrons or hydrogen atoms. Polyphenols can chelate transition metal ions, can directly scavenge molecular species of active oxygen, and may quench lipid peroxidation by trapping the lipid alkoxyl radical. They can also modify lipid packing order and decrease fluidity of the membranes [57]. These changes could exclusively hinder diffusion of free radicals and restrict peroxidative reactions. Furthermore, it has been shown that, particularly, flavonoids and phenylpropanoids are oxidized by peroxidase, and act in H₂O₂-scavenging, phenolic/AsA/POD system. There is some evidence of induction of phenolic metabolism in plants as a response to numerous stresses [58]. In recent study, authors observed that ROS could serve as a common signal for acclimation to Cu²⁺ stress and could cause accretion of total phenolic compounds in dark-grown lentil roots. A mutant Arabidopsis thaliana L., having a single gene defect which led to a block in the synthesis of a group of flavonoids, revealed a marked increasing in sensitivity to UV-B radiation compared with wild-type plants. Transgenic potato plant with high concentration of flavonoid demonstrated improved antioxidant capacity [59].

Other antioxidants: Superoxide produced by the Mehler reaction may also be directly diminished by ascorbate which is present at elevated concentrations in the chloroplast [60]. Besides ascorbate and glutathione, plant cells possess other non-enzymatic antioxidants that also participate in ROS scavenging under normal and drought stress conditions. Their accumulation under drought stress associates to the drought tolerance of the plant species. Under drought, this potent protector of thylakoidal and chloroplastic membranes has been indicated to accumulate in many plant species. As another point of view, the drought tolerant wild watermelon highly accumulates citrulline and CLMT2, a type 2 metallothionein, both with an extremely efficient hydroxyl radical scavenger activity, efficaciously protecting proteins and DNA from oxidative stress [61].

In recent years, several groups have taken the strong approach of attempting to improve stress tolerance in plants by modifying their ability to scavenge ROS that are generated during stress; it has been made for many abiotic stresses are accompanied by oxidative stress. Increased activity of Halliwell-Asada-Foyer cycle, or the water–water cycle in chloroplasts, can maintain close to normal levels of PSII and PSI activity during stress reducing the inhibition of photosynthesis and decreasing ROS levels [26]. Another strategy that demonstrated beneficial in raising tolerance to drought or salinity is to increase stress tolerance by over-expressing ROS-responsive regulatory genes that regulate a large set of genes involved in acclimation mechanisms, including ROS-scavenging enzymes. Over-expression of transcription factors such as Zat10, Zat12 or JERF3 increased the expression of ROSscavenging genes and tolerance to salt, drought or osmotic stresses [62]. Conversely, deficiency in MKK1 resulted in enhanced ROS production and enhanced stress sensitivity [63]. Recently, ROS-scavenging enzymes were displayed to be involved in signalling in addition to their more traditional function in cellular protection. Cytosolic APX1-deficient Arabidopsis plants had constitutively higher levels of H₂O₂ than wildtype plants, and induced the expression of several stress-responsive genes when subjected to a moderate level of light stress. Knockout APX1 plants were shown to grow better than wild-type plants under hyperosmotic or salinity condition. These results were surprising because APX1 plants show increased sensitivity to photo-oxidative [2]. Similarly, reduced expression of tylAPX in Arabidopsis led to increased tolerance to both osmotic and salt stresses but did not affect growth under oxidative stress conditions. Therefore, in the double mutant APX1/tylAPX, deficiency in both genes caused an increased sensitivity to sorbitol treatment while maintaining salt tolerance [64].

Oxidative stress is induced by a wide range of environmental factors including UV stress, pathogen invasion (hypersensitive reaction), herbicide action and oxygen shortage. Oxygen deprivation stress in plant cells is distinguished by three physiologically different states: transient hypoxia, anoxia and reoxygenation. Generation of reactive oxygen species (ROS) is characteristic for hypoxia and especially for reoxygenation. Of the ROS, hydrogen peroxide (H₂O₂) and superoxide (O2 ·–) are both produced in a number of cellular reactions, including the iron-catalysed Fenton reaction, and by various enzymes such as lipoxygenases, peroxidases, NADPH oxidase and xanthine oxidase. The main cellular components susceptible to damage by free radicals are lipids (peroxidation of unsaturated fatty acids in membranes), proteins (denaturation), carbohydrates and nucleic acids. Oxidative stress is induced by a wide range of environmental factors including UV stress, pathogen invasion (hypersensitive reaction), herbicide action and oxygen shortage. Oxygen deprivation stress in plant cells is distinguished by three physiologically different states: transient hypoxia, anoxia and reoxygenation. Generation of reactive oxygen species (ROS) is characteristic for hypoxia and especially for reoxygenation. Of the ROS, hydrogen peroxide (H₂O₂) and superoxide (O₂^·–) are both produced in a number of cellular reactions, including the iron-catalysed Fenton reaction, and by various enzymes such as lipoxygenases, peroxidases, NADPH oxidase and xanthine oxidase. The main cellular components susceptible to damage by free radicals are lipids (peroxidation of unsaturated fatty acids in membranes), proteins (denaturation), carbohydrates and nucleic acids. Oxidative stress is induced by a wide range of environmental factors including UV stress, pathogen invasion (hypersensitive reaction), herbicide action and oxygen shortage. Oxygen deprivation stress in plant cells is distinguished by three physiologically different states: transient hypoxia, anoxia and reoxygenation. Generation of reactive oxygen species (ROS) is characteristic for hypoxia and especially for reoxygenation. Of the ROS, hydrogen peroxide (H₂O₂) and superoxide (O₂^·–) are both produced in a number of cellular reactions, including the iron-catalysed Fenton reaction, and by various enzymes such as lipoxygenases, peroxidases, NADPH oxidase and xanthine oxidase. The main cellular components susceptible to damage by free radicals are lipids (peroxidation of unsaturated fatty acids in membranes), proteins (denaturation), carbohydrates and nucleic acids. Oxidative stress is induced by a wide range of environmental factors including UV stress, pathogen invasion (hypersensitive reaction), herbicide action and oxygen shortage. Oxygen deprivation stress in plant cells is distinguished by three physiologically different states: transient hypoxia, anoxia and reoxygenation. Generation of reactive oxygen species (ROS) is characteristic for hypoxia and especially for reoxygenation. Of the ROS, hydrogen peroxide (H₂O₂) and superoxide (O₂^·–) are both produced in a number of cellular reactions, including the iron-catalysed Fenton reaction, and by various enzymes such as lipoxygenases, peroxidases, NADPH oxidase and xanthine oxidase. The main cellular components susceptible to damage by free radicals are lipids (peroxidation of unsaturated fatty acids in membranes), proteins (denaturation), carbohydrates and nucleic acids [2].

Future Perspectives

The accumulated knowledge acquired on physiological, cellular and molecular responses of plants to drought and salinity, including the signaling events occurring under both stresses, has already been permitting great advancement in crop management and breeding. Some improvement in plant stress tolerance has been reached using stressinducible genes into some model plants. Utilization of this technology can make it possible to modify the regulation of key genes that will carry improved stress tolerance while maintaining yield. Further researches with new molecular approaches, including the identification of gene variants related to the significant agronomic traits, may facilitate the molecular engineering of plants with enhanced tolerance to severe environmental stresses.