Salt Stress on Plant Growth: Comparison
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Please note this is a comparison between Version 2 by Vivi Li and Version 1 by Francisco Garcia-Sanchez.

Climate change is causing soil salinization, resulting in crop losses throughout the world. The ability of plants to tolerate salt stress is determined by multiple biochemical and molecular pathways. Here we discuss physiological, biochemical, and cellular modulations in plants in response to salt stress. Knowledge of these modulations can assist in assessing salt tolerance potential and the mechanisms underlying salinity tolerance in plants. Salinity-induced cellular damage is highly correlated with generation of reactive oxygen species, ionic imbalance, osmotic damage, and reduced relative water content. Accelerated antioxidant activities and osmotic adjustment by the formation of organic and inorganic osmolytes are significant and effective salinity tolerance mechanisms for crop plants. In addition, polyamines improve salt tolerance by regulating various physiological mechanisms, including rhizogenesis, somatic embryogenesis, maintenance of cell pH, and ionic homeostasis. 

  • abiotic stresses
  • cell membrane stability
  • climate change
  • osmolytes
  • polyamines

1. Introduction

1.1. Overview of Salinity

Abiotic stresses like salinity, drought, and high temperature have undesirable effects on crop productivity and quality, and negative trends in sustainable agriculture [1]. Salinity in particular is an important limiting factor, causing low yield with inferior quality. Climate change is considered one of the major contributing factors to soil salinization, leading to land degradation and desertification [2]. According to Flowers et al. [3], high salt concentration is responsible for negative impacts on 7% of total land surface, and 5% of cultivated land. Poor irrigation water quality is another important factor contributing to soil salinization [4]. For these reasons, soil salinization is a reported major cause of reductions in the productivity of irrigated and rainfed lands of the world [5,6][5][6].
The adverse effects of salinization on plants are evident from negative growth trends from alteration or inhibition of biochemical and physiological processes. Plants can be classified as glycophytes or halophytes by their ability to survive under high salt concentrations [7]. Glycophytes are plants that are severely affected by saline conditions both at the cellular and whole-plant level. Under saline conditions, these plants exhibit greater accumulation of solutes, and ionic and osmotic stresses confer nutritional imbalances, which limit the productivity of these plants. The majority of terrestrial plants are glycophytes, including crop plants [1,8][1][8].
Conversely, halophytes regulate their biochemical and physiological processes through ionic compartmentalization, production of osmolytes and compatible solutes, enzymatic changes, and absorption of selective ions. These adaptations promote seed germination, succulence, and salt exclusion for these plants in a saline environment [9,10][9][10]. Halophyte succulence keeps the proportion of ions to water in balance under high salt conditions by maintaining high water contents. Succulence is expressed as large cell size, reduced growth and surface area per tissue volume, and increased water constituents. Interestingly, halophytes also have a greater number of mitochondria, indicating that more energy is required to survive under saline conditions [11,12][11][12]. Halophytes also have less sodium and chloride ion accumulation in their cytoplasms, allowing their chloroplasts to survive even while the plant experiences salinity shocks [13,14][13][14]. Halophytes also have a specialized system for salt excretion from the plant tissues via specific glands. These glands are characteristic of halophytic leaves. The leaves will remove the salts onto the leaf surface before the salts can reach the shoots of the plant. The presence of halophytes is limited to habitats with plentiful water (e.g., salt marshes, etc.). “Salt hairs,” which regulate water loss, replace “secretary glands”, if a plant is adapted to a relatively drier climate as compared to marshes [15,16][15][16]. Hydathodes are another adaptation by plants to remove excessive salts, with less stomatal conductance and transpiration water loss [17,18][17][18].
Various undesirable effects appear because of high salt concentration. Ion imbalance is one of the major consequences. A high concentration of Na and Cl ions, as an example, can lead to biochemical processes which can prove to be fatal for the plants [19,20,21][19][20][21]. Sodium and chloride toxicity not only induce nutritional disorders but also cause physiological drought by lowering the osmotic potential of the soil solutions [22]. Soil salinity prevents the plant from taking up water from the soil, resulting in a decline in cellular water, thus affecting cell turgor. Soil salinity also adversely affects photosynthetic activity in the plant and encourages the production of reactive oxygen species (ROS), thus reducing plant growth [23,24][23][24].
The identification of salt stress by plant species and their subsequent response is controlled by signals—signals which are generated by ions, osmotic differential, hormones, or ROS [25]. These signals bind to their respective receptors and initiate the physiological mechanisms which enable a plant to adapt to stress conditions (Figure 1). Under abiotic stress conditions, three types of signal transduction have been categorized, i.e., the ionic signaling pathway, the osmolyte regulation pathway, and the gene regulation pathway [26]. For signal transduction under salinity stress, the ionic stress signaling pathway has been elucidated. Calcium (Ca) occupies a central position in this regard. It induces signal transduction in plants to adapt to stress conditions [27]. High cytosolic Ca concentration initiates many processes involving enzymatic activity regulation, ion channel performance, and gene expression [28]. Exogenously applied calcium regulates K+/Na+ selectivity, and thus confers salt adaptation by improving signal transduction. Glycinebetaine is reported to maintain signal transduction and ion homeostasis under salt-stressed conditions [29,30][29][30].
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Figure 1. Salt stress signals that bind to their respective receptors and initiate the physiological and molecular mechanisms to enable a plant to survive under stressed conditions.
Glycophytes and halophytes mediate serious effects of salinity at the cellular level by inducing changes in the plasma membrane and cytoplasm. As a tolerance mechanism, the plant alters the structure and composition of their plasma membrane, especially lipid and protein contents. The cell membrane is usually the foremost target of any stress [31]. Salt stress also alters the cell cytoplasmic viscosity and composition [32,33][32][33].
It is essential to understand each tolerance mechanism at the cellular level in order to understand each tolerance mechanism at the plant level. The protoplasmic features studied at the cellular level include plasma membrane permeability, cytoplasmic viscosity, cytoplasmic streaming, and cell solute potential. Cytoplasmic viscosity describes the water contents of the cytoplasm in conjunction with its inter-macromolecular interactions. Cell solute potential represents the solute contents of the cell. Cell membrane permeability significantly increases with the increase in salinity [34,35][34][35]. The ability of the plasma membrane to repair, regenerate, and maintain its integrity stabilizes the cell structure and function under stress conditions. It is mainly dependent on the composition of the plasma membrane (i.e., mainly lipid contents). Saline conditions result in enhanced lipid peroxidation [36].
The cell membrane stability technique is widely utilized to judge the behavior of various plant genotypes in response to salt stress [37]. Thus, it can contribute to assessing the salt tolerance of plant genotypes. Cell membrane stability is also reported to correlate with potassium (K) ions, osmotic potential, osmotic adjustment, and relative water contents [38,39][38][39]. The differing patterns of cell membrane permeability help in characterizing genotypes as tolerant or sensitive. In a saline environment, salt-sensitive genotypes show marked alterations, whereas salt-tolerant genotypes show minor changes. Salinity also alters the degree of saturation of membrane fatty acids and membrane fluidity [40].
The salt-tolerant genotypes have high cytoplasmic viscosity due to augmentation in hydrophilic cytoplasmic proteins and other macromolecules [41]. In more sensitive genotypes, a saline environment results in a high concentration of salts in plant cells, lowering the solute potential [42]. Salinity inflicts serious irregularities during cell division, one of the various metabolic processes which face severe alterations in a saline environment. A saline environment especially alters the leaf anatomy by affecting mitochondria and vacuoles [43,44][43][44], plant leaf area, and stomatal thickness [45]. One way plants exhibit tolerance to saline environments is portioning or compartmentalization of toxic ions. This mechanism enables salt-tolerant plant species to retain toxic levels of harmful ions in vacuoles and inhibit their interference with cytoplasmic metabolic activities [46]. The Na+ and Cl− partitioning in the vacuole stimulates higher concentrations of K+ and organic osmolytes in the cytoplasm in order to adjust osmotic pressure of the ions in the vacuole [47].

1.2. Salinity and Morphological Attributes

Halophytes have the unique feature of succulence, a feature which keeps the ionic uptake in proper proportion with water, by maintaining high water contents (Figure 2). Succulence results in large cell size, reduced growth [48[48][49],49], reduced surface area per tissue volume, and increased water constituents. The salt tolerance is evident as maintenance of vegetative growth and yield and lower necrotic percentage [50,51][50][51]. Moreover, halophytes also have a greater number of mitochondria, indicating that more energy is required to survive under saline conditions [11,21][11][21]. Sodium (Na) accumulation causes necrosis in old leaves, initiating from tips and then extending towards the leaf base. It also decreases leaf life span, net productivity, and crop yield [52]. Biomass reduction and foliar damage become more prominent with time and at higher salinity levels. High salt concentration caused a reduction in fresh fruit yield in various vegetables [31,53,54,55][31][53][54][55]. However, salinity treatments did not prove harmful for vegetative growth and the number of flowers [53,55][53][55]. In contrast to the above report, Chartzoulakis and Klapaki, [56], Kaya et al. [57], and Giuffrida et al. [58], noted a reduction in fruit numbers and fruit weight under salt stress. Saline conditions were reported as producing non-significant results on some growth attributes, water status, and tissue concentration of major nutrients [59]. Salt treatments caused a significant reduction in plant height, root length, and dry weight [60,61,62][60][61][62]. Salt stress caused reduction in fresh and dry weight of cotton seedlings [63,64][63][64] and seed germination percentage in wheat varieties [35,65][35][65]. Tomato (Solanum lycopersicum) and pepper plants grown under salt stress experienced a reduction in dry weight, plant height [57[57][66],66], fruit weight, and relative water contents [67]. Broad bean, which is a green vegetable, experienced a significant reduction in plant height, leaf area, pod weight, number of pods per plant, seed yield, number of seeds, and product quality due to salt stress. However, a significant positive trend was observed between dry leaf matter, specific leaf weight, and salinity [68]. Suppression in seedling growth and dry matter accumulation was observed in Indian mustard (Brassica juncea L.) because of salt stress, which was ameliorated by putrescine application [69]. Exogenously applied sugar beet extract and shikimic acid on salt stressed eggplant and tomato crops respectively displayed a marked influence on fresh fruit weight, number of fruits, and shoot and root fresh and dry weight [54,70][54][70].
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Figure 2. Regulation of physiological and biochemical process in halophytes through ionic compartmentalization, osmotic adjustment, enzymatic activities, polyamines, and stress signaling regulation.
According to Caines and Shennan [71], root growth is more susceptible to saline conditions than shoot growth, but both are affected, making them reasonable indicators of salinity damage. A similar suppression in the shoot and root growth was also observed by Evers et al. [72] and Gao et al. [73] in the case of Solanum tuberosum L. Under high salt concentrations, potato root and shoot development was hindered [74,75][74][75]. High salt concentrations reduced the leaf area and increased the root:shoot ratio in wild type (Ailsa Craig) and ABA-deficit mutant (notabilis) tomato genotypes [76]. Suppression of fresh and dry weight of tomato plants because of salt stress can be alleviated by promoting the growth of Achromobacter piechaudii in the tomato growth media. A. piechaudii also caused a reduction in ethylene production by tomato seedlings, the opposite effect of salt stress on tomato seedlings [77]. High ethylene levels proved to be harmful to the growth of the plant [78]. Interestingly, Hu et al. [79] reported that potato root growth could be improved through brassinosteroid application. Salt stress caused a reduction in marketable yield of pepper plants grown hydroponically. Saline conditions also imparted negative features to the fruit quality in terms of fruit pulp thickness and firmness. It also resulted in increased fructose, glucose, and myo-inositol fruit concentrations [4,42][4][42]. The salt-sensitive pepper genotypes showed maximum damage and experienced severe chlorosis and necrosis, whereas tolerant genotypes were slightly less affected. Sodium exclusion can be regarded as a criterion to allocate salt stress tolerance status to pepper genotypes [80]. Korkmaz et al. [81] have reported that the exogenous application of glycine betaine can reduce the effect of salinity in pepper plants.
High salt concentration has been observed to cause detrimental effects on leguminous crops. In fact, saline conditions induced smaller sized nodules, reduced nodule volume per plant, less nodulation, and inferior plant growth. At the cellular level, saline conditions caused drastic alteration in the mechanism of nodule formation. It reduced the turgor of the peripheral cells of the nodule, altered its zonation, enhanced the infection thread enlargement, reduced the release of bacteria from infection threads, and stimulated the electron-dense material (phenolics) and its accumulation in vacuoles. Salinity caused a reduction in nitrogen-fixing ability of the nodules, which has the outcome of reduced respiration rate and protein synthesis [82,83,84][82][83][84]. Salinity tolerance patterns vary considerably among leguminous crops. It is distinctly apparent in Vicia faba, Glycine max, Pisum sativum, and Casuarina glauca, which can be categorized as salt-sensitive [85,86][85][86]. In addition, germination of seeds faces serious limitations upon exposure to salt stress [87,88][87][88]. Halophytes and glycophytes differ significantly in their germination behavior. Halophytes have the ability to maintain their germination mechanism to an extent with the advent of salinity. However, a sharp decline in germination occurs for glycophytes under salt stress. Imbibition is affected by the lower solute potential of the soil solution. It results in enzymatic deregulations and imbalances in source-sink relationships and ratios of different plant growth regulators present in the seed and required for efficient seed germination [89,90][89][90].

1.3. Salinity and Physiological Attributes

Photosynthesis is of prime importance in the production of adenosine triphosphate (ATP), which provides the energy required for CO2 fixation to sugars. Various abiotic stresses alter photosynthetic mechanisms [91] by disrupting thylakoid membranes, modifying the electron transport chain, altering enzymatic activity and protein synthesis, and changing Calvin cycle patterns. All these abnormalities cause deregulation in ATP synthesis [92], which can lead to the deficiency of certain ions due to ion degradation and synthesis inhibition [93,94,95,96][93][94][95][96].
Stepien and Klobus [97] reported a decline in the relative water content of cucumber (Cucumis sativus) leaves after exposure to saline solutions. They attributed their results to higher Na and reduced K content, a situation which reduces photosynthesis because of the antagonistic competition of Na for ion uptake. Gas exchange is inversely related to the concentration of Na and chloride ions [98,99][98][99], and parameters like photosynthesis, stomatal conductance, and transpiration tend to be negatively influenced by saline treatments [100,101][100][101]. Photosynthesis of pepper plants has been reported to be lower when the entire root system is affected by salt stress compared to partial root exposure, and factors like stomatal conductance and transpiration are similarly affected by complete or partial salt stress [99,102][99][102]. A low stomatal conductance was reported in wild type (Ailsa Craig) and ABA-deficit mutant (notabilis) tomato genotypes, which was negatively correlated with increasing xylem ABA for both genotypes [76]. Stomatal conductance is often correlated with photosynthetic efficiency, which is a prerequisite for higher biomass production and yield [10]. In addition, salinity is reported to reduce the maximum quantum efficiency of photosystem II (PSII) [48,94,103,104,105,106][48][94][103][104][105][106]. A distinct correlation has been found between Na ion contents and chlorophyll fluorescence, which is often used to estimate salt tolerance of plants [48]. Higher salt levels are also known to alter photosynthesis via non-stomatal limitations, including variations in photosynthetic enzyme activity and changes in the concentration of chlorophyll and carotenoids [107,108][107][108]. Pepper leaves have exhibited a significant reduction in chlorophyll pigment under saline conditions [109]. A similar reduction in chlorophyll a and b contents has been reported in melon by Kaya et al. [57]. This chlorophyll degradation under salt stress can be attributed to an enzyme called chlorophyllase [110,111,112,113][110][111][112][113] and to the absolute concentration of chloride and Na in the leaves [98,114,115,116,117,118][98][114][115][116][117][118]. Although carotenoid content has been reported to decline in response to salinity, anthocyanin pigments typically increase as a result of salinity.
Plant antioxidants include secondary metabolites (phenolic compounds), which are generated in response to stress conditions. These secondary metabolites may include tocopherol, which serves to stabilize membrane integrity [119[119][120],120], ascorbic acid, carotenoids, flavonoids, and glutathione [90,121][90][121]. Tocopherol plays a key role as a signaling molecule between cells [93]. Ascorbic acid is a significant antioxidant involved in plant adaptation [122] and occurs abundantly in cell organelles and apoplasts [99]. It has the ability to scavenge superoxide, hydroxyl, and singlet oxygen. Carotenoids are found in chloroplasts and reported to aid in light reception for photosynthesis. Moreover, they are also protective compounds which scavenge ROS [95,123][95][123]. Putrescine is reported to increase the level of carotenoids and glutathione in Indian mustard (B. juncea L.) against salt stress. Putrescine supplementation inhibits ROS generation by accelerating antioxygenic enzymes and therefore aiding in the maintenance of chloroplastic membranes and the NADP+/NADPH ratio [124,125,126][124][125][126].
C4 plants are reported to be more resistant against salinity stress than C3 plants by having a better capacity to preserve the photosynthetic apparatus against oxidative stress [127,128][127][128]. Other physiological responses that are indirectly related to salinity stress include changes in water use efficiency and evapotranspiration, which can benefit from the use of beneficial bacteria [77,129,130,131][77][129][130][131]. In addition, higher salt concentration can stimulate the accumulation of spermine and spermidine, which contribute to the induction of salt tolerance in plants and lead to maintenance of fruit quality (Figure 3) [132,133,134,135][132][133][134][135].
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Figure 3. Bunches, grapevine cultivar Shiraz. Control bunch (A); Salt-stressed bunch (B); showing symptoms of coulure and millerandage. Salt treatment was applied from budburst until veraison via fertigation with 35 mM NaCl added to control nutrient solution [136].

2. Salinity and Biochemical Attributes

Plants have the ability to sustain their life under a saline environment through synthesis and accumulation of compatible solutes in the cytosol. These are soluble compounds with low molecular mass. These chemical compounds can maintain physiological and biochemical processes, without having interference in these processes. The chief components of compatible solutes are sugar alcohols (mannitol, sorbitol, ononitol), quaternary ammonia compounds (glycine betaine, proline betaine), proline, and tertiary sulfonium compounds. They act to scavenge the reactive oxygen species (ROS) and inhibit lipid peroxidation, hence preventing damage at the cellular level. These compatible solutes act in favor of osmotic adjustment and prevent ROS damage at the cellular level [146,147][137][138]. These maintain macromolecular conformation in the cytosol, which may be changed due to the accumulation of charged ions, under saline conditions [8,97][8][97]. These organic compounds are termed as compatible due to their consistency with the cell’s metabolism [148][139], and their lowering of the water potential without altering cell water contents. These organic compounds are hydrophilic in nature, having the ability to replace water present on protein surfaces [149[140][141],150], without interfering with their structure and function. These solutes play a key role in preventing the drastic effects of high ion concentration on enzymatic activities [24,151,152,153][24][142][143][144]. Such important roles of compatible solutes lead to osmoregulation of plant cells under osmotic stress. In addition to osmoregulation, these organic compounds have a distinct role in protein stabilization, maintenance of membrane integrity, protection of OEC of PSII from dissociation [154][145], and scavenging of reactive oxygen species (ROS). Mannitol, sorbitol, glycerol, proline, ononitol, and pinitol have been reported to scavenge ROS species [155][146].
Mannitol (sugar alcohol) metabolism in higher plants is a superior attribute, contributing to salt and osmotic stress tolerance while playing a significant role as a compatible solute. It also improves plant responses under biotic stress as well, like under pathogen infestation [156][147]. Mannitol is reported to be synthesized at the same time with either sucrose or raffinose saccharide. In salt-tolerant species, mannitol accumulation increases, indicating that high mannitol levels contribute to salt tolerance. Mannitol acts to scavenge the reactive oxygen species, thus protecting protein molecules [157,158][148][149]. Pinitol and ononitol have been reported to accumulate under various stresses, predominantly drought and salt stress [159,160][150][151]. Interestingly, polyols can be used as a potential biochemical marker for genetically engineered stress resistance plant genotypes [161,162][152][153].

2.1. Salinity and Proline

Proline is an osmolyte, an amino acid, which is thought to play a significant role in inducing tolerance in plants against stressed conditions [163][154]. Salt stress can result in elevation in proline levels [74]. Ethephon, when used with sodium chloride in spinach, also increased proline levels [164][155]. The importance of proline is highlighted by its existence in bacteria with a relationship to plants experiencing water or salinity stress. High proline levels can serve as a nitrogen source for plants during recovery [165][156]. The precursor of proline synthesis is glutamate, involving pyrroline carboxylic acid synthetase and pyrroline carboxylic reductase [166][157]. An increase was noted in the activity of pyrroline-5-carboxylate synthetase (P5CS) and a decline was recorded in proline dehydrogenase activity in potato seedlings under salt stress. These changes of enzymatic activity were more pronounced in salt-sensitive cultivars [74,167][74][158]. However, an increase in proline contents of potato clones was recorded upon salt exposure [50,168][50][159]. It serves to stabilize ultra-structural changes in cells, scavenge ROS (reactive oxygen species), and maintain cellular redox potential. Under stress conditions, a higher accumulation of proline is reported in cell cytosol, strengthening the ability of the cell to make ionic adjustments. Its accumulation is linearly related to stress tolerance in plants [169][160]. Proline biosynthesis is reported to be mediated by Ca [170,171,172][161][162][163] and abscisic acid [8]. Previously, contrasting views about proline accumulation were also reported in plants under stress [19[19][149],158], where it appeared as a salt stress injury symptom, e.g., rice [173][164] and sorghum [174][165].
Some plant genotypes do not respond to proline accumulation, but their salt tolerance potential can be enhanced through the exogenous application of proline [31,175][31][166]. It may be helpful in counteracting the harmful effects through osmo-protection, resulting in a higher growth rate. Proline also increases the activities of antioxidant enzymes like SOD (superoxide dismutase) and POD (peroxidase) [176][167]. Proline is not reported to scavenge ROS directly, but through enhanced antioxidant enzyme activity. It is reported to be more effective in mitigating the drastic effects of salinity than glycine betaine [177][168]. Proline used at higher concentrations may prove to be lethal for the plant, causing ultra-structural damages leading to ROS generation [178][169]. The effective dose of proline varies with genotype and plant developmental stage [179,180,181,182][170][171][172][173]. Proline accumulation has been reported for drought sensitive and tolerant barley genotypes grown under saline conditions. Under salt stress, a considerable amount of proline was present, with relatively lower quantities in root tissues. Proline accumulation is reported to be more prominent in tolerant genotypes [183][174].

2.2. Salinity and Polyamines

Polyamines are multivalent compounds consisting of two or more amino groups. In higher plants, these are identified as putrescine, spermidine, and spermine [147,184][138][175]. These are involved in various physiological mechanisms including rhizogenesis, somatic embryogenesis, maintenance of cell pH and ionic balance [29], pollen and flower formation, abscission, senescence, and dormancy. Endogenous polyamine synthesis can be stimulated by cytokinin [185,186][176][177]. These compounds act to stabilize macromolecules like DNA and RNA. Moreover, polyamines have a significant role in numerous abiotic and biotic stresses [151,187][142][178]. At the cellular level, polyamines contribute to regulating the plasma membrane potential, ionic homeostasis, and tolerance against salinity [188,189][179][180]. Exogenously applied polyamine or ornithine caused a reduction in proline accumulation in plant tissues under salt stress. However, an alternate trend was observed in the case of non-stressed beans [99,190][99][181]. Putrescine is characterized as a de-stressor agent and a nitrogen source under stressed conditions [191][182]. Putrescine has been reported to reverse the biomass reduction in Indian mustard [68,192][68][183]. Its production in plant cells follows two alternative pathways: conversion from ornithine or arginine. Putrescine is then converted to spermidine and subsequently to spermine by addition of an aminopropyl group. Spermine deficiency caused Ca ion imbalance in Arabidopsis thaliana, thus indicating spermine as a maintainer of plant cell ionic homeostasis under salt stress [193][184].

2.3. Salinity and Glycine-Betaine

Glycine-betaine (GB) is present in a wide range of organisms, from bacteria to higher plants and animals. In addition to being involved in osmoregulation, it maintains and regulates the performance of PSII protein complexes by protecting extrinsic regulatory protein against denaturation. It also stabilizes macromolecules, due to its ability to form strong bonds with water [136]. It protects these macromolecules during drought and thermal stress, which is why it is sometimes called an “osmoprotectant” [129,194][129][185]. Glycine-betaine accumulates in some crops under stress, like members of family Poaceae and Chenopodiaceae [195][186], and is absent entirely from other plants, like rice and tobacco. This directed the scientists to develop transgenic plants that have the ability to produce GB. In transgenic plants, the reproductive organs are capable of tolerating abiotic stresses if they can accumulate GB [195][186]. The precursor for GB is choline, and the conversion is managed by enzymes like choline monooxygenase and betaine-aldehyde dehydrogenase [151,196][142][187]. Choline supplementation to the growth media of the salt-stressed plant can act to restore the suppressed growth [197][188]. GB is water soluble, is not harmful at higher concentrations, and accumulates mainly in plastids and chloroplasts. Exogenous application of GB promotes salinity tolerance in plant species which do not naturally produce GB. A plant can utilize exogenously applied GB via leaves [198][189], as well as roots [199][190]. After absorption, GB is translocated in phloem [99,200][99][191]. GB is not directly involved in scavenging ROS species, but it alleviates the damaging effects of ROS by promoting enzymes responsible for the destruction or production suppression of ROS [201][192].
The reproductive stage of any plant during GB application is considered critical to ensure maximum yield. In various studies, it was reported that the plant reproductive organs acquire higher levels of GB than the vegetative parts under stressed conditions. This indicates that high GB accumulation is more necessary for protecting the reproductive organs than it is for protecting the vegetative tissues from abiotic stresses, indicating that application timing is key [181,202][172][193]. The natural GB accumulating species include sugar beets, spinach, wheat, barley, and sorghum. High GB concentration is linearly linked with increased tolerance. Osmotic adjustment is the major mechanism involved in increased tolerance to abiotic stresses, especially salt stress. GB is responsible for turgor maintenance through osmotic adjustment [54,182,203][54][173][194]. However, this relationship is not satisfactory in some cases like Triticum spp. and Agropyron spp. [204][195]. Thus, this relationship varies with genotype [158][149]. The plant species which do not produce GB naturally can give a satisfactory yield and survival rate under salt stress conditions through the exogenous application of GB [42,205][42][196]. Exogenous GB, once applied, is transported rapidly throughout the plant. Exogenous application of GB has been reported in many plant species, including tobacco, rice, soybean, barley, and wheat. In barley, GB application improved stress tolerance by lowering water potential, which improved survivability. GB plays a role in osmotic adjustment and ionic homeostasis by maintaining high K+ concentration compared to Na+ ions. Exogenous application of GB also increased the K+/Na+ ratio [206,207,208][197][198][199]. GB also protects the photosynthetic apparatus. It enhances photosynthetic activity through increased stomatal conductance and reduced photorespiration [42,209,210][42][200][201].
In contrast to its positive influence, some researchers have also suggested neutral or somewhat negative responses to exogenously applied GB in some plant genotypes. For example, it appeared to have a neutral influence on growth in cotton [211][202], turnip, rapeseed, and tomato [212][203]. For the commercial application of GB, the rate, duration, timing, and frequency should be considered [158,213][149][204]. It can be used for seed treatment as well as foliar application. The application method is dependent on the plant material on which it will be applied, the timing of the application relative to plant developmental stage, and environmental conditions during the time of application [214][205].
Exogenously applied GB improved salt tolerance in rice by improving relative water contents in the leaves and increasing antioxidant levels, including superoxide dismutase, ascorbate peroxidase, catalase, and glutathione reductase (GR) [213][204]. Reduction in peroxidase activity was reported in a salt-tolerant rice genotype under salt stress. GB is also reported to reduce lipid peroxidation [215,216,217][206][207][208]. GB can prevent membrane adulterations due to osmotic stress more efficiently than proline [218][209]. Proline accumulation in leaves of salt-stressed plants is not reported to be correlated with exogenously applied glycine betaine [54,180][54][171]. Sugar beet is identified as the foremost source of GB [158,219][149][210]. It is appreciated as a valuable source of GB along with other beneficial compounds and is useful in inducing tolerance against salt stress in eggplant (Solanum melongena L.) as compared to pure GB. It has a marked influence on the morphological (growth and yield) as well as physiological and biochemical (gas exchange, photosynthetic rate, transpiration, GB accumulation) attributes [186,220][177][211].

3. Salinity and Phytohormones

Plant hormones are signaling molecules with the ability to alter the physiological mechanisms of the plant, even if present in very minute quantities [248][212]. Common plant hormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene [249][213]. Plants growing under salt stress experience imbalances in hormonal homeostasis. Stressed conditions drastically alter physiological mechanisms of the plant, creating massive changes in endogenous hormonal contents. Higher concentrations of toxic ions are negatively correlated with the levels of plant hormones like gibberellins, auxins, and cytokinin and are positively associated with the abscisic acid level [250,251][214][215]. Exogenous plant hormone application on salt stressed plants was found to alleviate the negative effects of salinity on the morphological (leaf area, dry mass), physiological (chlorophyll content, stomatal conductance, photosynthetic rate), and yield characteristics of crops [252,253][216][217]. Abscisic acid (ABA) and ethylene are involved in signaling under stress conditions. An increase in ABA concentration in plant cells has been reported under saline conditions [254][218]. Carotenoids are the precursor for ABA synthesis, with roots and leaves the sites of synthesis [255][219]. Water deficit in the root zone causes ABA generation in roots, and ABA transport to shoots is xylem mediated. The increase in pH of xylem sap increases the transport of ABA to the guard cells, where it regulates the stomatal opening and closing through the involvement of Ca ions [256][220]. Alterations in ABA levels indirectly affect photosynthesis through disruption in stomatal opening and closing. The photosynthetic efficiency of the plant cell declines along with deregulation of translocation and assimilate partitioning of photosynthates [8,257][8][221]. Exogenously applied plant growth regulators have been widely reported to enhance stress tolerance in numerous plant species [258][222]. ABA is reported to be useful in alleviating plant salt stress under low water potential. ABA production in the plant cell is also related to ethylene synthesis under salt stress. The interaction between these two stress hormones is apparent from vegetative growth and seed germination under salt stress. During root inhibition by salt stress, ethylene regulates the ABA concentration. However, the reverse has been reported in the case of seed germination [259][223]. Salicylic acid has displayed a defensive role in plants experiencing stress, signaling the plant to adapt to the stressful environment [260][224].

Salinity and Growth Regulation

Brassinosteroids (BRs) are growth regulators which mitigate adverse growth patterns caused by salinity. It improves the germination of seeds experiencing salt stress. The improved germination rate has been reported for rice [261][225] and tobacco. Application of BRs as a seed treatment enhanced the growth of rice seedlings under salt stress [262][226]. It helps the plant to retain its green pigments and enhances nitrate reductase activity [263,264,265][227][228][229] and nitrogen-fixing capability. Brassinosteroids (28-homoBL) increased dry matter accumulation and seed yield [266][230]. Foliar application of brassinosteroids (24-epibrassinolide) on pepper plants grown with saline water greatly affected shoot growth parameters and leaf water contents as compared to roots. However, its effect on chlorophyll fluorescence was non-significant [267,268][231][232]. Similar patterns of brassinosteroidal effects were observed in wheat grown under salt stress. 24-epibrassinolide application on salt-stressed wheat seedlings exhibited non-significant results in terms of plant biomass, chlorophyll content, photosynthesis rate, substomatal CO2 concentration, and water use efficiency. The incremented water use efficiency can be related to higher transpiration rate shown by salt-stressed wheat seedlings, as a result of 24-epibrassinolide application [269,270][233][234]. The efficiency of exogenously applied brassinosteroids to mitigate salinity effects varies with plant species, appropriate growth stage, dose, frequency, and method of brassinosteroidal application [271,272,273][235][236][237]. The results of BR application also vary with climatic conditions—mainly temperature, light duration, and applied fertilizers [274][238]. Brassinolide application to salt stressed Vigna radiata caused enhancement in growth, photosynthetic rate, and maximum quantum yield of PSII. Generally, brassinolide has the potential to protect the photosynthetic apparatus under salt stress. It also contributed to improving the membrane stability index and leaf water potential. However, no significant results were recorded in the case of lipid peroxidation and electrolyte leakage. Brassinolide increases antioxidant enzyme and proline contents [275,276,277,278][239][240][241][242]. Increases in proline level create a protective shield when the plant is under stress by acting as a source of carbon and nitrogen, a stabilizer of the plasma membrane, and an oxygen radical scavenger [279][243]. Brassinosteroids also increase pigment levels in the plant [280][244]. BRs also improve the nitrate and nitrite reductase activity in Vigna radiata under salt stress. This effect can be attributed to the ability of BRs to modulate transcription and translation at the gene level, and to increase cell nitrate uptake. Increased stress tolerance caused by brassinolide application is observable as improved growth parameters such as shoot length, root length, and plant biomass [106,281][106][245].

References

  1. Shrivastava, P.; Kumar, R. Soil salinity: A serious environmental issue and plant growth promoting bacteria as one of the tools for its alleviation. Saudi J. Biol. Sci. 2015, 22, 123–131.
  2. Rogel, J.A.; Ariza, F.A.; Silla, R.O. Soil salinity and moisture gradients and plant zonation in Mediterranean salt marshes of Southeast Spain. Wetlands 2000, 20, 357–372.
  3. Flowers, T.J.; Garcia, A.; Koyama, M.; Yeo, A.R. Breeding for salt tolerance in crop plants—The role of molecular biology. Acta Physiol. Plant. 1997, 19, 427–433.
  4. Rubio, J.S.; Garcia-Sanchez, F.; Rubio, F.; Martinez, V. Yield, blossom-end rot incidence and fruit quality in pepper plants under moderate salinity are affected by K+ and Ca+2 fertilization. Sci. Hortic. 2009, 119, 79–87.
  5. Ghassemi, F.; Jakeman, A.J.; Nix, H.A. Salinization of Land and Water Resources; University of New south Wales Press Ltd.: Canberra, Australia, 1995.
  6. Sonowal, H.; Pal, P.B.; Shukla, K.; Ramana, K.V. Ramana, Aspalatone Prevents VEGF-Induced Lipid Peroxidation, Migration, Tube Formation, and Dysfunction of Human Aortic Endothelial Cells. Oxid. Med. Cell. Longev. 2017, 11, 2017.
  7. Wang, H.; Tang, X.; Shao, C.; Shao, H.; Wang, L. Molecular cloning and bioinformatics analysis of a new plasma membrane Na+/H+ antiporter gene from the halophyte Kosteletzkya virginica. Sci. World J. 2014, 2014, 141675.
  8. Xiong, L.; Zhu, J.K. Molecular and genetic aspects of plant responses to osmotic stress. Plant Cell Environ. 2002, 25, 131–139.
  9. Meng, X.; Zhou, J.; Sui, N. Mechanisms of salt tolerance in halophytes: Current understanding and recent advances. Open Llife Sci. 2018, 13, 149–154.
  10. Ashraf, M.H.P.J.C.; Harris, P.J. Photosynthesis under stressful environments: An overview. Photosynthetica 2013, 51, 163–190.
  11. Siew, R.; Klein, C.R. The effect of NaCl on some metabolic and fine structural changes during the greening of etiolated leaves. J. Cell Biol. 1969, 37, 590–596.
  12. Slama, I.; Abdelly, C.; Bouchereau, A.; Flowers, T.; Savoure, A. Diversity, distribution and roles of osmoprotective compounds accumulated in halophytes under abiotic stress. Ann. Bot. 2015, 115, 433–447.
  13. Cushman, J.C. Molecular cloning and expression of chloroplast NADP-malate dehydrogenase during crassulacean acid metabolism induction by salt stress. Photosynth. Res. 1990, 35, 15–27.
  14. Sévin, D.C.; Stählin, J.N.; Pollak, G.R.; Kuehne, A.; Sauer, U. Global Metabolic Responses to Salt Stress in Fifteen Species. PLoS ONE 2016, 11, e0148888.
  15. Dschida, W.J.; Platt-Aloia, K.A.; Thomson, W.W. Epidermal peels of Avicennia germinans (L.) Stearn: A useful system to study the function of salt glands. Ann. Bot. Lond. 1992, 70, 501–509.
  16. Robert, I.L.; Frank, W.J.; Summy, K.R.; Hudson, D.Y.; Richard, S. The Biological Flora of Coastal Dunes and Wetlands. Avicennia Germinans J. Coast. Res. 2017, 33, 191–207.
  17. Tester, M.; Davenport, R. Na+ tolerance and Na+ transport in higher plants. Ann. Bot. 2003, 91, 503–527.
  18. Shahid, M.A.; Balal, R.M.; Khan, N.; Rossi, L.; Rathinasabapathi, B.; Liu, G.; Khan, J.; Cámara-Zapata, J.M.; Martínez-Nicolas, J.J.; Garcia-Sanchez, F. Polyamines provide new insights into the biochemical basis of Cr-tolerance in Kinnow mandarin grafted on diploid and double-diploid rootstocks. Environ. Exp. Bot. 2018, 1, 248–260.
  19. Zhu, J.; Hasegawa, P.M.; Bressan, R.A. Molecular aspects of osmotic stress in plants. Citri. Rev. Plant Sci. 1997, 16, 253–277.
  20. Serrano, R.; Mulet, J.M.; Rios, G.; Marquez, J.A.; de Larrinoa, I.F.; Leube, M.P.; Mendizabal, I.; Pascual-Ahuir, A.; Proft, M.; Ros, R.; et al. A glimpse of the mechanism of ion homeostasis during salt stress. J. Exp. Bot. 1999, 50, 1023–1036.
  21. Flowers, T.J.; Munns, R.; Colmer, T.D. Sodium chloride toxicity and the cellular basis of salt tolerance in halophytes. Ann. Bot. 2015, 115, 419–431.
  22. Khan, N.; Bano, A.; Babar, M.A. The stimulatory effects of plant growth promoting rhizobacteria and plant growth regulators on wheat physiology grown in sandy soil. Arch. Microbiol. 2019, 201, 769–785.
  23. Price, A.; Hendry, G. Iron catalysed oxygen radical formation and its possible contribution to drought damage in nine native grasses and three cereals. Plant Cell Environ. 1991, 14, 477–484.
  24. Khan, N.; Bano, A. Effects of exogenously applied salicylic acid and putrescine alone and in combination with rhizobacteria on the phytoremediation of heavy metals and chickpea growth in sandy soil. Int. J. Phytoremed. 2018, 16, 405–414.
  25. Stevens, R.M.; Pech, J.M.; Grigson, G.J. A Short Non-Saline Sprinkling Increases the Tuber Weights of Saline Sprinkler Irrigated Potatoes. Agronomy 2017, 7, 4.
  26. Nxele, X.; Klein, A.; Ndimba, B.K. Drought and salinity stress alters ROS accumulation, water retention, and osmolyte content in sorghum plants. Afr. J. Bot. 2017, 1, 261–266.
  27. Golldack, D.; Li, C.; Mohan, H.; Probst, N. Tolerance to drought and salt stress in plants: Unraveling the signaling networks. Front. Plant Sci. 2014, 5, 151.
  28. Naseem, H.; Ahsan, M.; Shahid, M.A.; Khan, N. Exopolysaccharides producing rhizobacteria and their role in plant growth and drought tolerance. J. Basic Microbiol. 2018, 58, 1009–1022.
  29. Zekri, M.; Parsons, L.R. Calcium influnces growth and leaf mineral concentration of citrus under saline conditions. Hort. Sci. 1990, 25, 784–786.
  30. Kumari, A.; Das, P.; Parida, A.K.; Agarwal, P.K. Proteomics, metabolomics, and ionomics perspectives of salinity tolerance in halophytes. Front. Plant Sci. 2015, 6, 537.
  31. Saha, J.; Brauer, E.K.; Sengupta, A.; Popescu, S.C.; Gupta, K.; Gupta, B. Polyamines as redox homeostasis regulators during salt stress in plants. Front. Environ. Sci. 2015, 3, 21.
  32. Mansour, M.M.F. Cell permeability under salt stress. In Strategies for Improving Salt Tolerance in Higher Plants; Jaiwal, P.K., Singh, R.P., Gulati, A., Eds.; Oxford and IBH: New Delhi, India, 1997; pp. 87–110.
  33. Ali, S.; Xie, L. Plant Growth Promoting and Stress mitigating abilities of Soil Born Microorganisms. Recent Pat. Food Nutr. Agric. 2019, 15.
  34. Mansour, M.M.F.; Salama, K.H.A. Cellular basis of salinity tolerance in plants. Environ. Exp. Bot. 2004, 52, 113–122.
  35. Le Gall, H.; Philippe, F.; Domon, J.M.; Gillet, F.; Pelloux, J.; Rayon, C. Cell wall metabolism in response to abiotic stress. Plants 2015, 4, 112–166.
  36. Al-Amri, S.M. Improved growth, productivity and quality of tomato (Solanum lycopersicum L.) plants through application of shikimic acid. Saudi J. Biol. Sci. 2013, 1, 339–345.
  37. Mahlooji, M.; Seyed Sharifi, R.; Razmjoo, J.; Sabzalian, M.R.; Sedghi, M. Effect of salt stress on photosynthesis and physiological parameters of three contrasting barley genotypes. Photosynthetica 2017, 56, 549–556.
  38. Shekari, F.; Abbasi, A.; Mustafavi, S.H. Effect of silicon and selenium on enzymatic changes and productivity of dill in saline condition. J. Saudi Soc. Agric. Sci. 2017, 1, 367–374.
  39. Dolatabadian, A.; Sanavy, S.A.M.M.; Gholamhoseini, M.; Joghan, A.K.; Majdi, M.; Kashkooli, A. The role of calcium in improving photosynthesis and related physiological and biochemical attributes of spring wheat subjected to simulated acid rain. Physiol. Mol. Biol. Plants. 2013, 19, 189–198.
  40. Farooq, S.; Azam, F. The use of cell membrane stability (CMS) technique to screen for salt tolerant wheat varieties. J. Plant Physiol. 2006, 163, 629–637.
  41. Munns, R. Comparative physiology of salt and water stress. Plant Cell Environ. 2002, 25, 239–250.
  42. Dolatabadi, N.; Toorchi, M. Rapeseed (Brassica napus L.) genotypes response to NaCl salinity. J. Biodiv. Environ. Sci. 2017, 10, 265–270.
  43. Wu, J.; Seliskar, D.M.; Gallagher, J.L. Stress tolerance in the march plant Spartina patens: Impact of NaCl on growth and root plasma membrane lipid composition. Physiol. Plant 1998, 102, 307–317.
  44. Balal, R.M.; Shahid, M.A.; Vincent, C.; Zotarelli, L.; Liu, G.; Mattson, N.S.; Garcia-Sanchez, F. (Kinnow mandarin plants grafted on tetraploid rootstocks are more tolerant to Cr-toxicity than those grafted on its diploids one. Environ. Exp. Bot. 2017, 140, 8–18.
  45. Plant, A.L.; Bray, E.A. Regulation of Gene Expression by Abscissic Acid During Environmental Stresses; Marcel Dekker: New York, NY, USA, 1999; pp. 303–331.
  46. Delphine, S.; Alvino, A.; Zacchini, M.; Loreb, F. Consequences of salt stress on conductance to CO2 diffusion, Rubisco characteristics and anatomy of spinach leaves. Aust. J. Plant Physiol. 1998, 25, 395–402.
  47. Mehmood, A.; Khan, N.; Irshad, M.; Hamayun, M. IAA Producing Endopytic Fungus Fusariun oxysporum wlw Colonize Maize Roots and Promoted Maize Growth Under Hydroponic Condition. Eur. Exp. Biol. 2018, 8, 24.
  48. Romero-Aranda, R.; Moya, J.L.; Tadeo, F.R.; Legaz, F.; Primo-Millo, E.; Talón, M. Physiological and anatomical disturbances induced by chloride salts in sensitive and tolerant citrus: Beneficial and detrimental effects of cations. Plant Cell Environ. 1998, 21, 1243–1253.
  49. Hasegawa, P.M.; Bressan, R.A.; Zhu, J.K.; Bohnert, H.J. Plant cellular and molecular responses to high salinity. Annu. Rev. Plant Physiol. 2000, 51, 463–499.
  50. Blumwald, E.; Aharon, G.S.; Apse, M.P. Sodium transport in plant cells. Biochim. Biophys. Acta 2000, 1465, 140–151.
  51. Maser, P.; Gierth, M.; Schroeder, J.I. Molecular mechanisms of potassium and sodium uptake in plants. Plant Soil. 2002, 247, 43–54.
  52. Khan, N.; Bano, A. Modulation of phytoremediation and plant growth by the treatment with PGPR, Ag nanoparticle and untreated municipal wastewater. Int. J. Phytoremed. 2016, 18, 1258–1269.
  53. Zribi, L.; Fatma, G.; Fatma, R.; Salwa, R.; Hassan, N.; Nejib, R.M. Application of chlorophyll fluorescence for the diagnosis of salt stress in tomato “Solanum lycopersicum (variety Rio Grande)”. Sci. Hortic. 2009, 120, 367–372.
  54. Orlovsky, N.; Japakova, U.; Zhang, H.; Volis, S. Effect of salinity on seed germination, growth and ion content in dimorphic seeds of Salicornia europaea L. (Chenopodiaceae). Plant Divers. 2016, 1, 183–189.
  55. Shaterian, J.; Waterer, D.; De Jong, H.; Tanino, K.K. Differential stress responses to NaCl salt application in early and late maturing diploid potato (Solanum sp.) clones. Environ. Exp. Bot 2005, 54, 202–212.
  56. Chartzoulakis, K.; Klapaki, G. Response of two greenhouse pepper hybrids to NaCl salinity during different growth stages. Sci. Hortic. 2000, 86, 247–260.
  57. Kaya, C.; Tuna, A.L.; Ashrafa, M.; Altunlu, H. Improved salt tolerance of melon (Cucumis melo L.) by the addition of proline and potassium nitrate. Environ. Exp. Bot 2007, 60, 397–403.
  58. Giuffrida, F.; Graziani, G.; Fogliano, V.; Scuderi, D.; Romano, D.; Leonardi, C. Effects of nutrient and NaCl salinity on growth, yield, quality and composition of pepper grown in soilless closed system. J. Plant Nutr. 2014, 37, 1455–1474.
  59. Jiang, C.; Zu, C.; Lu, D.; Zheng, Q.; Shen, J.; Wang, H.; Li, D. Effect of exogenous selenium supply on photosynthesis, Na+ accumulation and antioxidative capacity of maize (Zea mays L.) under salinity stress. Sci. Rep. 2007, 7, 42039.
  60. Savvas, D.; Lenz, F. Effect of NaCl or nutrient-induced salinity on growth, yield and composition of egg plants grown in rockwool. Sci. Hortic. 2000, 84, 37–47.
  61. Abbas, W.; Ashraf, M.; Akram, N.A. Alleviation of salt-induced adverse effects in egg plant (Solanum melongena L.) by glycinebetaine and sugar beet extracts. Sci. Hortic. 2010, 125, 188–195.
  62. Mahjoor, F.; Ghaemi, A.A.; Golabi, M.H. Interaction effects of water salinity and hydroponic growth medium on eggplant yield, water-use efficiency, and evapotranspiration. Int. Soil Water Conserv. 2016, 1, 99–107.
  63. Balal, R.M.; Shahid, M.A.; Javaid, M.M.; Iqbal, Z.; Liu, G.D.; Zotarelli, L.; Khan, N. Chitosan alleviates phytotoxicity caused by boron through augmented polyamine metabolism and antioxidant activities and reduced boron concentration in Cucumis sativus L. Acta Physiol. Plant. 2017, 1, 31.
  64. Tabatabaie, S.J.; Nazari, J. Influence of nutrient concentrations and NaCl salinity on the growth, photosynthesis, and essential oil content of peppermint and lemon verbena. Turk. J. Agric. For. 2007, 31, 245–253.
  65. Pardossi, A.; Bagnoli, G.; Malorgio, F.C.; Campiotti, A.; Tognoni, F. NaCl effects on celery (Apium graveolens L.) grown in NFT. Sci. Hortic 1999, 81, 229–242.
  66. Zheng, Y.; Jia, A.; Ning, T.; Xu, J.; Li, Z.; Jiang, G. Potassium nitrate application alleviates sodium chloride stress in winter wheat cultivars differing in salt tolerance. J. Plant Physiol. 2008, 165, 1455–1465.
  67. Saini, S.; Sharma, I.; Pati, P.K. Versatile roles of brassinosteroid in plants in the context of its homoeostasis, signaling and crosstalks. Front. Plant Sci. 2015, 6, 950.
  68. Kalhoro, N.A.; Rajpar, I.; Kalhoro, S.A.; Ali, A.; Raza, S.; Ahmed, M.; Kalhoro, F.A.; Ramzan, M.; Wahid, F. Effect of salts stress on the growth and yield of wheat (Triticum aestivum L.). Am. J. Plant Sci. 2016, 7, 2257.
  69. Xie, Z.; Duan, L.; Tian, X.; Wang, B.; Eneji, A.E.; Li, Z. Coronatine alleviates salinity stress in cotton by improving the antioxidation defence system and radical-scavenging activity. J. Plant Physiol. 2008, 165, 375–384.
  70. Sun, C.; Feng, D.; Mi, Z.; Li, C.; Zhang, J.; Gao, Y.; Sun, J. Impacts of Ridge-Furrow Planting on Salt Stress and Cotton Yield under Drip Irrigation. Water 2017, 9, 49.
  71. Caines, A.M.; Shennan, C. Interactive effects of Ca+2 and NaCl salinity on the growth of two tomato genotypes differing in Ca+2 use efficiency. Plant Physiol. Biochem. 1999, 37, 569–576.
  72. Evers, D.; Hemmer, K.; Hausman, J.F. Salt stress induced biometric and physiological changes in Solanum tuberosum L. cv. Bintje grown in vitro. Acta Physiol. Plant 1998, 20, 3–7.
  73. Gao, Y.; Lu, Y.; Wu, M.; Liang, E.; Li, Y.; Zhang, D.; Yin, Z.; Ren, X.; Dai, Y.; Deng, D.; et al. Ability to Remove Na+ and Retain K+ Correlates with Salt Tolerance in Two Maize Inbred Lines Seedlings. Front. Plant Sci. 2016, 7, 1716.
  74. Hu, Y.; Xia, S.; Su, Y.; Wang, H.; Luo, W.; Su, S.; Xiao, L. Brassinolide Increases Potato Root Growth In Vitro in a Dose-Dependent Way and Alleviates Salinity Stress. Biomed. Res. Int. 2016, 8231873.
  75. Alom, R.; Hasan, M.A.; Islam, M.R.; Wang, Q.F. Germination characters and early seedling growth of wheat (Triticum aestivum L.) genotypes under salt stress conditions. JCSB 2016, 1, 383–392.
  76. Tuna, A.L.; Kaya, C.; Ashraf, M.; Altunlu, H.; Yokas, I.; Yagmur, B. The effect of calcium sulphate on growth, membrane stability and nutrient uptake of tomato plants grown under salt stress. Environ. Exp. Bot. 2007, 59, 173–178.
  77. De Pascale, S.; Barbieri, G. Effects of soil salinity and top removal on growth and yield of broad bean as a green vegetable. Sci. Hortic 1997, 71, 147–165.
  78. Verma, S.; Mishra, S.N. Putrescine alleviation of growth in salt stressed Brassica juncea by inducing antioxidative defense system. J. Plant Physiol. 2005, 162, 669–677.
  79. Hu, Y.; Schmidhalter, U. Drought and Salinity: A comparison of their effects on mineral nutrition of plants. J. Plant Nutr. Soil. Sci. 2005, 168, 541–549.
  80. Rahnama, H.; Ebrahimzadeh, H. The effect of NaCl on antioxidant activities in potato seedlings. Biol. Plant. 2005, 49, 93–97.
  81. Korkmaz, A.; Şirikçi, R.; Kocaçınar, F.; Değer, Ö.; Demirkırıan, A.R. Alleviation of salt-induced adverse effects in pepper seedlings by seed application of glycinebetaine. Sci. Hortic. 2012, 4, 197–205.
  82. Roy, T.S.; Chakraborty, R.; Parvez, M.N.; Mostofa, M.; Ferdous, J.; Ahmed, S. Yield, dry matter and specific gravity of exportable potato: Response to salt. Univ. J. Agric. Res. 2017, 5, 98–103.
  83. Mulholland, B.J.; Taylor, I.B.; Jackson, A.C.; Thornpson, A.J. Can ABA mediate responses of salinity have stressed tomato. Environ. Exp. Bot. 2003, 50, 17–28.
  84. Mayak, S.; Tirosh, T.; Glick, B.R. Plant growth promoting bacteria confer resistance in tomato plants to salt stress. Plant Sci. Plant Sci. 2004, 42, 565–572.
  85. Smalle, J.; Van Der Straeten, D. Ethylene and vegetative development. Physiol. Plant. 1997, 100, 593–605.
  86. Akhtar, S.S.; AkhtarAktas, H.; Abak, K.; Cakmak, I. Genotypic variation in the response of pepper to salinity. Sci. Hortic. 2006, 110, 260–266.
  87. Delgado, M.J.; Garrido, J.M.; Ligero, F.; Lluch, C. Nitrogen fixation and carbon metabolism by nodules and bacteroids of pea plants under sodium chloride stress. Physiol. Plant. 1993, 89, 824–829.
  88. Borucki, W.; Sujkowska, M. The effects of sodium chloride-salinity upon growth, nodulation, and root nodule structure of pea (Pisum sativum L.) plants. Acta Physiol. Plant. 2008, 30, 293–301.
  89. Andrzej, T.; Philip, P. Role of root microbiota in plant productivity. J. Exp. Bot. 2015, 66, 2167–2175.
  90. Zahran, H.H. Rhizobium-Legume symbiosis and nitrogen fixation under severe conditions and in an arid climate. Microbiol. Mol. Biol. R. 1999, 63, 968–989.
  91. Batista-Santos, P.; Duro, N.; Rodrigues, A.P.; Semedo, J.N.; Alves, P.; da Costa, M.; Graça, I.; Pais, I.P.; Scotti-Campos, P.; Lidon, F.C.; et al. Is salt stress tolerance in Casuarina glauca Sieb. ex Spreng. associated with its nitrogen-fixing root-nodule symbiosis? An analysis at the photosynthetic level. Plant Physiol. Biochem. 2015, 1, 97–109.
  92. Othman, Y.; Al-karaki, G.; Al-Tawaha, A.R.; Al-Horani, A. Variation in germination and ion uptake in barley genotypes under salinity conditions. World J. Agric. Res. 2006, 2, 11–15.
  93. Kandil, A.A.; Shareif, E.; Gad, M.A. Effect of Salinity on Germination and Seeding Parameters of Forage Cowpea Seed. Res. J. Seed Sci. 2017, 10, 17–26.
  94. Khan, M.A.; Rizvi, Y. Effect of salinity, temperature and growth regulators on the germination and early seedling growth of Atriplex griffithi Var. Stocksii. Can J. Bot. 1994, 72, 475–479.
  95. Rajjou, L.; Duval, M.; Gallardo, K.; Catusse, J.; Bally, J.; Job, C.; Job, D. Seed germination and vigor. Annu. Rev. Plant Biol. 2012, 2, 507–533.
  96. Meena, K.K.; Sorty, A.M.; Bitla, U.M.; Choudhary, K.; Gupta, P.; Pareek, A.; Minhas, P.S. Abiotic Stress Responses and Microbe-Mediated Mitigation in Plants: The Omics Strategies. Front. Plant Sci. 2017, 8, 172.
  97. Stepien, P.; Klobus, G. Water relations and photosynthesis in Cucumus sativus L. leaves under salt stress. Biol. Plant. 2006, 50, 610–616.
  98. Dobrota, C. Energy dependant plant stress acclimation. Rev. Environ. Sci. 2006, 5, 243–251.
  99. Yeo, A.R.; Capron, S.J.M.; Flowers, T.J. The effect of salinity upon photosynthesis in rice (Oryza sativa L.) Gas exchange by individual leaves relation to their salt content. J. Exp. Bot. 1985, 36, 1240–1248.
  100. Muradoglu, F.; Gundogdu, M.; Ercisli, S.; Encu, T.; Balta, F.; Jaafar, H.Z.; Zia-Ul-Haq, M. Cadmium toxicity affects chlorophyll a and b content, antioxidant enzyme activities and mineral nutrient accumulation in strawberry. Biol. Res. 2015, 48, 1–7.
  101. El-Shintinawy, F.; El-Ansary, A. Differential Effect of Cd2+ and Ni2+ on Amino Acid Metabolism in Soybean Seedlings. Biol. Plant. 2000, 43, 79–84.
  102. Puniran-Hartley, N.; Hartley, J.; Shabala, L.; Shabala, S. Salinity-induced accumulation of organic osmolytes in barley and wheat leaves correlates with increased oxidative stress tolerance: In planta evidence for cross-tolerance. Plant Physiol. Biochem. 2014, 1, 32–39.
  103. Garcia-Legaz, M.F.; Ortiz, J.M.; Garcia-Lindon, A.; Cerda, A. Effect of salinity on growth, ion content and CO2 assimilation rate in lemon varieties on different rootstocks. Physiol. Plant. 1993, 89, 427–432.
  104. Acosta-Motos, J.R.; Ortuño, M.F.; Bernal-Vicente, A.; Diaz-Vivancos, P.; Sanchez-Blanco, M.J.; Hernandez, J.A. Plant responses to salt stress: Adaptive mechanisms. Agronomy 2017, 7, 18.
  105. Hanachi, S.; Van Labeke, M.C.; Mehouachi, T. Application of chlorophyll fluorescence to screen eggplant (Solanum melongena L.) cultivars for salt tolerance. Photosynthetica 2014, 52, 57–62.
  106. Tiwari, J.K.; Munshi, A.D.; Kumar, R.; Pandey, R.N.; Arora, A.; Bhat, J.S.; Sureja, A.K. Effect of salt stress on cucumber: Na+–K+ ratio, osmolyte concentration, phenols and chlorophyll content. Acta Physiol. Plant. 2010, 1, 103–114.
  107. Lycoskoufis, I.H.; Savvas, D.; Mavrogianopoulos, G. Growth, gas exchange, and nutrient status in pepper (Capsicum annuum L.) grown in recirculating nutrient solution as affected by salinity imposed to half of the root system. Sci. Hortic. 2005, 106, 147–161.
  108. Zhang, T.; Gong, H.; Wen, X.; Lu, C. Salt stress induces a decrease in oxidation energy transfer from phycobilisomes to photosystem II but an increase to photosystem I in the cyanobacterium Spirulina platensis. J. Plant Physiol. 2010, 12, 20.
  109. Wu, X.; He, J.; Chen, J.; Yang, S.; Zha, D. Alleviation of exogenous 6-benzyladenine on two genotypes of eggplant (Solanum melongena Mill.) growth under salt stress. Protoplasma 2014, 251, 169–176.
  110. Yan, K.; Shao, H.; Shao, C.; Chen, P.; Zhao, S.; Brestic, M.; Chen, X. Physiological adaptive mechanisms of plants grown in saline soil and implications for sustainable saline agriculture in coastal zone. Acta Physiol. Plant. 2013, 1, 2867–2878.
  111. Hayat, S.; Hasan, S.A.; Yusuf, M.; Hayat, Q.; Ahmad, A. Effect of 28- homobrassinolide on photosynthesis, fluorescence and antioxidant system in the presence or absence of salinity and temperature in Vigna radiata. Environ. Exp. Bot. 2010, 69, 105–112.
  112. Misra, A.N.; Sahu, S.M.; Misra, M.; Singh, P.; Meera, I.; Das, N.; Kar, M.; Shau, P. Sodium chloride induced changes in leaf growth, and pigment and protein contents in two rice cultivars. Biol. Plant. 1997, 39, 257–262.
  113. Neelesh, K.; Veena, P. Effect of Salt Stress on Growth Parameters, Moisture Content, Relative Water Content and Photosynthetic Pigments of Fenugreek Variety RMt-1. J. Plant Sci. 2015, 10, 210–221.
  114. Bethke, P.C.; Drew, M.C. Stomatal and non-stomatal components to inhibition of photosynthesis in leaves of Capsicum annum during progressive exposure to NaCl salinity. Plant Physiol. 1992, 99, 219–226.
  115. Reddy, M.P.; Vora, A.B. Changes in pigment composition, Hill reaction activity and saccharides metabolism in Bajra (Pennisetum typhoides) leaves under NaCl salinity. Photosynthetica 1986, 20, 50–55.
  116. Gul, B.; Ansari, R.; Flowers, T.J.; Khan, M.A. Germination strategies of halophyte seeds under salinity. Environ. Exp. Bot. 2013, 1, 4–18.
  117. Parida, A.K.; Das, A.B. Salt tolerance and salinity affects on plant. Ecotoxicol. Environ. 2005, 60, 324–349.
  118. Cao, Y.Y.; Duan, H.; Yang, L.N.; Wang, Z.Q.; Zhou, S.C.; Yang, J.C. Effect of heat stress during meiosis on grain yield of rice cultivars differing in heat tolerance and its physiological mechanism. Acta Agron. Sin. 2008, 34, 2134–2142.
  119. Elhamid, E.; Sadak, M.; Tawfik, M. Alleviation of Adverse Effects of Salt Stress in Wheat Cultivars by Foliar Treatment with Antioxidant 2—Changes in Some Biochemical Aspects, Lipid Peroxidation, Antioxidant Enzymes and Amino Acid Contents. Agric. Sci. 2014, 5, 1269–1280.
  120. Khan, N.; Zandi, P.; Ali, S.; Mehmood, A.; Adnan Shahid, M.; Yang, J. Impact of salicylic acid and PGPR on the drought tolerance and phytoremediation potential of Helianthus annus. Front. Microbiol. 2018, 9, 2507.
  121. Kartashov, A.V.; Radyukina, N.L.; Ivanov, Y.V.; Pashkovskii, P.P.; Shevyakova, N.I.; Kuznetsov, V.V. Role of antioxidants systems in wild plant adaptation to salt stress. Russ. J. Plant Physiol. 2008, 55, 463–468.
  122. Katsumi, M. Physiological modes of brassinolide action in cucumber hypocotyl growth. In Brassinosteriods: Chemistry, Bioactivity and Applications; Cutler, H.G., Yokota, T., Adam, G., Eds.; American Chemical Society: Washington, DC, USA, 1991; pp. 246–254.
  123. Faried, H.N.; Ayyub, C.M.; Amjad, M.; Ahmed, R. Salinity impairs ionic, physiological and biochemical attributes in potato. Pak. J. Agric. Sci. 2016, 53, 17–25.
  124. Sofo, A.; Scopa, A.; Nuzzaci, M.; Vitti, A. Ascorbate Peroxidase and Catalase Activities and Their Genetic Regulation in Plants Subjected to Drought and Salinity Stresses. Int. J. Mol. Sci. 2015, 16, 13561–13578.
  125. Gupta, B.; Huang, B. Mechanism of salinity tolerance in plants: Physiological, biochemical, and molecular characterization. Int. J. Genom. 2014, 2014.
  126. Bhattacharyya, A.; Chattopadhyay, R.; Mitra, S.; Crowe, S.E. Oxidative Stress: An Essential Factor in the Pathogenesis of Gastrointestinal Mucosal Diseases. Physiol. Rev. 2014, 94, 329–354.
  127. Ashraf, M.; Bhatti, B. Physiological responses of some salt tolerance and salt sensitive lines of wheat (Crthamus tinctorius L.). Acta Physiol. Plant. 1999, 117, 23–28.
  128. Adnan, A.; Erkan, Y. Effect of salinity stress on chlorophyll, carotenoid content, and proline in Salicornia prostrata Pall. and Suaeda prostrata Pall. subsp. prostrata (Amaranthaceae). Braz. J. Bot. 2016, 39, 101–106.
  129. Kamal, A.; Qureshi, M.S.; Ashraf, M.Y.; Hussain, M. Salinity induced some changes in growth and physio-chemical aspects of two soybean [Glycine max (L.) merr] genotype. Pak. J. Bot. 2003, 35, 93–97.
  130. Parvin, K.; Haque, M.N. Protective Role of Salicylic Acid on Salt Affected Broccoli Plants. J. Agric. Ecol. 2017, 10, 1–10.
  131. Stepien, P.; Klobus, G. Antioxidant defense in the leaves of C3 and C4 plants under salinity stress. Physiol. Plant. 2005, 125, 31–40.
  132. Chandrasekaran, M.; Kim, K.; Krishnamoorthy, R.; Walitang, D.; Sundaram, S.; Joe, M.M.; Selvakumar, G.; Hu, S.; Oh, S.H.; Sa, T. Mycorrhizal symbiotic efficiency on C3 and C4 plants under salinity stress—A meta-analysis. Front. Microbiol. 2016, 11, 1246.
  133. Akbar, S.; Sultan, S. Soil bacteria showing a potential of chlorpyrifos degradation and plant growth enhancement. Braz. J. Microbiol. 2016, 47, 563–570.
  134. Bhantana, P.; Lazarovitch, N. Evapotranspiration, Crop coefficient and growth of two young pomegranate (Punica granatum L.) varieties under salt stress. Agric. Water Manag. 2010, 97, 715–722.
  135. Gorai, M.; Ennajeh, M.; Khemira, H.; Neffati, M. Combined effect of NaCl-salinity and hypoxia on growth, photosynthesis, water relations and solute accumulation in Phragmite saustralis plants. Flora 2010, 205, 462–470.
  136. Baby, T.; Collins, C.; Tyerman, S.D.; Gilliham, M. Salinity negatively affects pollen tube growth and fruit set in grapevines and is not mitigated by silicon. Am. J. Enol. Vitic. 2016, 67, 218–228.
  137. Hernandez, J.A.; Almansa, M.S. Short term effects of salt stress on antioxidant systems and leaf water relations of pea leaves. Physiol. Plant. 2002, 115, 251–257.
  138. Khan, N.; Ali, S.; Zandi, P.; Mehmood, A.; Ullah, S.; Ikram, M.; Ismail, M.; Babar, M.A. Role of sugars, amino acids and organic acids in improving plant abiotic stress tolerance. Pak. J. Bot. 1997, 52, 2.
  139. Hong, B.; Barg, R.; Ho, T.D. Developmental and organ specific expression of an ABA and stress induced protein in barley. Plant Mol. Biol. 1992, 18, 663–674.
  140. Hare, P.D.; Cress, W.A.; Staden, J.V. Dissecting the roles of osmolyte accumulation during stress. Plant Cell Environ. 1998, 21, 535–554.
  141. Galinski, E.A. Compatible solutes of halophilic eubacteria-eubacteria-molecular principles, water-solute interactions, stress protection. Cell Mol. Life Sci. 1993, 49, 487–496.
  142. Lutts, S.; Lefèvre, I. How can we take advantage of halophyte properties to cope with heavy metal toxicity in salt-affected areas? Ann. Bot. 2015, 1, 509–528.
  143. Brown, A.D. Microbial Water Stress Physiology, Principles and Perspectives; Wiley: Hoboken, NJ, USA, 1990.
  144. Solomon, A.; Beer, S.; Waisel, Y.; Jones, G.P.; Paleg, L.G. Effects of NaCl on the Carboxylating activity of Rubisco from Tamarix jordanis in the presence and absence of proline-related compatible solutes. Plant Physiol. 1994, 90, 198–204.
  145. Papageorgiou, G.; Murata, N. The unusually strong stability effects of glycine betaine on the structure and function of the oxygen evolving photosystem II complex. Photosynth. Res. 1995, 44, 243–252.
  146. Bose, J.; Rodrigo-Moreno, A.; Shabala, S. ROS homeostasis in halophytes in the context of salinity stress tolerance. J. Exp. Bot. 2014, 1, 1241–1257.
  147. Stoop, J.M.H.; Williamson, J.D.; Pharr, D.M. Mannitol metabolism in plants: A method for coping with stress. Trends Plant Sci. 1996, 1, 139–144.
  148. Moran, J.F.; Becana, M.; Iturbe-Ormaetxe, I.; Frechilla, S.; Klucas, R.V.; Aparicio-Tejo, P. Drought induces oxidative stress in pea plants. Planta 1994, 94, 346–352.
  149. Managbanag, J.R.; Torzilli, A.P. An analysis of trehalose, glycerol, and mannitol accumulation during heat and salt stress in a salt marsh isolate of Aureobasidium pullulans. Mycologia 2002, 1, 384–391.
  150. Khan, N.; Bano, A.; Zandi, P. Effects of exogenously applied plant growth regulators in combination with PGPR on the physiology and root growth of chickpea (Cicer arietinum) and their role in drought tolerance. J. Plant Interact. 2018, 1, 239–247.
  151. Tipirdamaz, R.; Gagneul, D.; Duhazé, C.; Aïnouche, A.; Monnier, C.; Özkum, D.; Larher, F. Clustering of halophytes from an inland salt marsh in Turkey according to their ability to accumulate sodium and nitrogenous osmolytes. Environ. Exp. Bot. 2006, 57, 139–153.
  152. Ashraf, M.; Harris, P.J.C. Potential biochemical indicators of salinity tolerance in plants. Plant Sci. 2004, 166, 3–16.
  153. Sandhu, D.; Cornacchione, M.V.; Ferreira, J.F.; Suarez, D.L. Variable salinity responses of 12 alfalfa genotypes and comparative expression analyses of salt-response genes. Sci. Rep. 2017, 22, 42958.
  154. Liang, X.; Zhang, L.; Natarajan, S.K.; Becker, D.F. Proline Mechanisms of Stress Survival. Antioxid. Redox Signal. 2013, 19, 998–1011.
  155. Dar, M.I.; Naikoo, M.I.; Rehman, F.; Naushin, F.; Khan, F.A. Proline accumulation in plants: Roles in stress tolerance and plant development. In Osmolytes and Plants Acclimation to Changing Environment: Emerging Omics Technologies; Springer: New Delhi, India, 2016; pp. 155–166.
  156. Ali, B.; Hayat, S.; Ahmad, A. 28-homobrassinolide ameliorates the saline stress in Cicer arietinum L. Environ. Exp. Bot. 2007, 59, 217–223.
  157. Mantilla, B.S.; Marchese, L.; Casas-Sánchez, A.; Dyer, N.A.; Ejeh, N.; Biran, M.; Silber, A.M. Proline Metabolism is Essential for Trypanosoma brucei brucei Survival in the Tsetse Vector. PLoS Pathog. 2017, 13, e1006158.
  158. Kahlaoui, B.; Hachicha, M.; Misle, E.; Fidalgo, F.; Teixeira, J. Physiological and biochemical responses to the exogenous application of proline of tomato plants irrigated with saline water. J. Saudi Soc. Agric. Sci. 2018, 1, 17–23.
  159. Nemoto, Y.; Sasakuma, T. Differential stress responses of early salt-stress responding genes in common wheat. Phytochemistry 2002, 61, 129–133.
  160. Jaarsma, R.; de Vries, R.S.; de Boer, A.H. Effect of salt stress on growth, Na+ accumulation and proline metabolism in potato (Solanum tuberosum) cultivars. PLoS ONE 2013, 8, e60183.
  161. Joseph, E.A.; Radhakrishnan, V.V.; Mohanan, K.V. A Study on the Accumulation of Proline—An Osmoprotectant Amino Acid under Salt Stress in Some Native Rice Cultivars of North Kerala, India. Univ. J. Agric. Res. 2015, 3, 15–22.
  162. Knight, H.; Anthony, J.; Knight, M.R. Calcium signaling in Arabidopsis thaliana responding to drought and salinity. Plant J. 1997, 12, 1067–1078.
  163. Misra, N.; Gupta, A.K. Interactive effects of sodium and calcium on proline metabolism in salt tolerant green gram cultivar. Am. J. Plant Physiol. 2006, 1, 1–2.
  164. Ashraf, M.; Foolad, M.R. Roles of glycinebetaine and proline in improving plant abiotic stress resistance. Environ. Exp. Bot. 2007, 59, 206–216.
  165. Huang, Z.; Zhao, L.; Chen, D.; Liang, M.; Liu, Z.; Shao, H.; Long, X. Salt Stress Encourages Proline Accumulation by Regulating Proline Biosynthesis and Degradation in Jerusalem Artichoke Plantlets. PLoS ONE 2013, 8, e62085.
  166. Lutts, S.; Majerus, V.; Kinet, J.M. NaCl effects on proline metabolism in rice (Oryza sativa) seedlings. Int. Rice Res. 1999, 25, 39–40.
  167. Devnarain, N.; Crampton, B.G.; Chikwamba, R.K.; Becker, J.V.W.; O’Kennedy, M.M. Physiological responses of selected African sorghum landraces to progressive water stress and re-watering. S. Afr. J. 2016, 103, 61–69.
  168. Yan, H.; Gang, L.Z.; Zhao, C.Y.; Guo, W.Y. Effects of exogenous proline on the physiology of soybean plantlets regenerated from embryo in vitro and on the ultra-structure of their mitochondria under NaCl stress. Soybean Sci. 2000, 19, 314–319.
  169. Hare, P.D.; Cress, W.A.; Staden, J.V. Disruptive effects of exogenous proline on chloroplast and mitochondrial ultrastructure in Arabidopsis leaves. S. Afr. J. 2002, 68, 393–396.
  170. Zeyner, A.; Romanowski, K.; Vernunft, A.; Harris, P.; Müller, A.M.; Wolf, C.; Kienzle, E. Effects of Different Oral Doses of Sodium Chloride on the Basal Acid-Base and Mineral Status of Exercising Horses Fed Low Amounts of Hay. PLoS ONE 2017, 12, e0168325.
  171. Jimenez-Bremont, J.F.; Becerra-Flora, A.; Hernandez-Lucero, E.; Rodriguez-Kessler, M.; Acosta-Gallegos, J.A.; Ramirez-Pimental, J.G. Proline accumulation in two bean cultivars under salt stress and the effect of polyamines and ornithine. Biol. Plant. 2006, 50, 763–766.
  172. Gill, S.S.; Tuteja, N. Polyamines and abiotic stress tolerance in plants. Plant Signal Behav. 2010, 5, 26–33.
  173. Griffith, S.M.; Banowetz, G.M. Nitrogen nutrition and flowering. In Nitrogen Nutrition in Higher Plants; Srivastava, H.S., Singh, R.P., Eds.; Associated Publ. Co.: New Delhi, India, 1995; pp. 385–400.
  174. Kumar, R.; Khurana, A.; Sharma, A.K. Role of plant hormones and their interplay in development and ripening of fleshy fruits. J. Exp. Bot. 2013, 65, 4561–4575.
  175. Diao, Q.; Song, Y.; Shi, D.; Qi, H. Interaction of Polyamines, Abscisic Acid, Nitric Oxide, and Hydrogen Peroxide under Chilling Stress in Tomato (Lycopersicon esculentum Mill.) Seedlings. Front. Plant Sci. 2017, 8, 203.
  176. Shabala, S.; Cuin, T.A.; Pottosin, I. Polyamines prevent NaCl-induced K+ efflux from pea mesophyll by blocking non-selective cation channels. FEBS Lett. 2007, 581, 1993–1999.
  177. Rodríguez, A.A.; Maiale, S.J.; Menéndez, A.B.; Ruiz, O.A. Polyamine oxidase activity contributes to sustain maize leaf elongation under saline stress. J. Exp. Bot. 2009, 1, 4249–4262.
  178. Mishra, S.N.; Sharma, I. Putrescine as a growth inducer and as a source of nitrogen for mustard seedlings under sodium chloride salinity. Indian J. Exp. Biol. 1994, 32, 916–918.
  179. Lakra, N.; Tomar, P.C.; Mishra, S.N. Growth response modulation by putrescine in Indian mustard Brassica juncea L. under multiple stress. Indian J. Exp. Biol. 2016, 54, 262–270.
  180. Maruri-López, I.; Jiménez-Bremont, J. Hetero- and homodimerization of Arabidopsis thaliana arginine decarboxylase AtADC1 and AtADC2. Biochem. Biophys. Res. Commun. 2017, 11, 508–513.
  181. Wani, S.H.; Singh, N.B.; Haribhushan, A.; Mir, J.I. Compatible solute engineering in plants for abiotic stress tolerance-role of glycine betaine. Curr. Genom. 2013, 14, 157.
  182. Baloda, A.; Madanpotra, S.; Jaiwal, P.K. Transformation of mung bean plants for abiotic stress tolerance by introducing codA gene, for an osmoprotectant glycine betaine. J. Plant Stress Physiol. 2017, 3, 5–11.
  183. Wei, D.; Zhang, W.; Wang, C.; Meng, Q.; Li, G.; Chen, T.H.; Yang, X. Genetic engineering of the biosynthesis of glycinebetaine leads to alleviate salt-induced potassium efflux and enhances salt tolerance in tomato plants. Plant Sci. 2017, 257, 74–83.
  184. Park, E.J.; Jeknic, Z.; Chen, T.H.H. Exogenous application of glycine betaine increases chilling tolerance in tomato plants. Plant Cell Physiol. 2006, 47, 706–714.
  185. Park, H.J.; Kim, W.Y.; Yun, D.J. A New Insight of Salt Stress Signaling in Plant. Mol. Cells 2016, 39, 447–459.
  186. Khan, N.; Bano, A. Rhizobacteria and Abiotic Stress Management. In Plant Growth Promoting Rhizobacteria for Sustainable Stress Management; Springer: Singapore, 2019; pp. 65–80.
  187. Mohammad, N.; Naeem, K.; Zakia, A.; Abdul, G. Genetic diversity and disease response of rust in bread wheat collected from Waziristan Agency, Pakistan. Int. J. Biodivers. Conserv. 2011, 3, 10–18.
  188. Shanker, A.; Venkateswarlu, B. Abiotic Stress in Plants: Mechanisms and Adaptations. BoD–Books on Demand; IntechOpen: Norderstedt, Germany, 2011; p. 22.
  189. Nawaz, K.; Ashraf, M. Exogenous application of glycine betaine modulates activities of antioxidants in maize plants subjected to salt stress. J. Agron. Crop Sci. 2009, 196, 28–37.
  190. Annunziata, M.G.; Ciarmiello, L.F.; Woodrow, P.; Maximova, E.; Fuggi, A.; Carillo, P. Durum Wheat Roots Adapt to Salinity Remodeling the Cellular Content of Nitrogen Metabolites and Sucrose. Front. Plant Sci. 2016, 7, 2035.
  191. Wyn-Jones, R.G.; Gorham, J.; McDonnell, E. Organic and inorganic solute contents as selection criteria for salt tolerance in the Triticeae. In Salinity Tolerance in Plants: Strategies for Crop Improvement; Staples, R., Toennissen, G.H., Eds.; Wiley and Sons: New York, NY, USA, 1984; pp. 189–203.
  192. Woodrow, P.; Ciarmiello, L.F.; Annunziata, M.G.; Pacifico, S.; Iannuzzi, F.; Mirto, A.; Carillo, P. Durum wheat seedling responses to simultaneous high light and salinity involve a fine reconfiguration of amino acids and carbohydrate metabolism. Plant Physiol. 2016.
  193. Majeed, K.; Al-Hamzawi, A. Effect of Calcium Nitrate, Potassium Nitrate and Anfaton on Growth and Storability of Plastic Houses Cucumber (Cucumis sativus L. cv. Al-Hytham). Am. J. Plant Sci. 2010, 5, 278–290.
  194. Kong, X.; Luo, Z.; Dong, H.; Eneji, A.E.; Li, W. Effects of non-uniform root zone salinity on water use, Na+ recirculation, and Na+ and H+ flux in cotton. J. Exp. Bot. 2012, 63, 2105–2116.
  195. Wu, H.; Shabala, L.; Liu, X.; Azzarello, E.; Zhou, M.; Pandolfi, C.; Shabala, S. Linking salinity stress tolerance with tissue-specific Na+ sequestration in wheat roots. Front. Plant Sci. 2015, 6.
  196. Maleka, P.; Kontturi, M.; Pehu, B.; Somersalo, S. Photosynthesis response of drought and salt stressed tomato and turnip plants to foliar applied glycinebetaine. Physiol. Plant. 1999, 105, 45–50.
  197. Athar, H.U.R.; Zafar, Z.U.; Ashraf, M. Glycinebetaine Improved Photosynthesis in Canola under Salt Stress: Evaluation of Chlorophyll Fluorescence Parameters as Potential Indicators. J. Agron. Crop Sci. 2015, 201, 428–442.
  198. Luo, J.Y.; Zhang, S.; Peng, J.; Zhu, X.Z.; Lv, L.M.; Wang, C.Y.; Cui, J.J. Effects of Soil Salinity on the Expression of Bt Toxin (Cry1Ac) and the Control Efficiency of Helicoverpa armigera in Field- Grown Transgenic Bt Cotton. PLoS ONE 2017, 12, e0170379.
  199. Raza, M.A.S.; Shahid, A.M.; Saleem, M.F.; Khan, I.H.; Ahmad, S.; Ali, M.; Iqbal, R. Effects and management strategies to mitigate drought stress in oilseed rape (Brassica napus L.): A review. Zemdirbyste-Agriculture 2017, 104, 85–94.
  200. Akram, R.; Fahad, S.; Hashmi, M.Z.; Wahid, A.; Adnan, M.; Mubeen, M.; Khan, N.; Rehmani, M.I.; Awais, M.; Abbas, M.; et al. Trends of electronic waste pollution and its impact on the global environment and ecosystem. Environ. Sci. Pollut. Res. Int. 2019, 1, 1–6.
  201. Bhuiyan, M.S.; Maynard, G.; Raman, A.; Hodgkins, D.; Mitchell, D.; Nicol, H. Salt effects on proline and glycine betaine levels and photosynthetic performance in Melilotus siculus, Tecticornia pergranulata and Thinopyrum ponticum measured in simulated saline conditions. Funct. Plant Biol. 2016, 4, 254–265.
  202. Demiral, T.; Türkan, I. Does exogenous glycinebetaine affect antioxidative system of rice seedlings under NaCl treatment? J. Plant Physiol. 2004, 18, 1089–1100.
  203. Talaat, N.B.; Shawky, B.T. Influence of arbuscular mycorrhizae on yield, nutrients, organic solutes, and antioxidant enzymes of two wheat cultivars under salt stress. J. Soil Sci. Plant Nutr. 2011, 174, 283–291.
  204. Ruiz-Lozano, J.M.; Porcel, R.; Azcón, C.; Aroca, R. Regulation by arbuscular mycorrhizae of the integrated physiological response to salinity in plants: New challenges in physiological and molecular studies. J. Exp. Bot. 2012, 28, 4033–4044.
  205. Ghosh, B.; Ali, M.N.; Saikat, G. Response of Rice under Salinity Stress: A Review Update. J. Res. Rice 2016, 4, 1–8.
  206. Mack, G.; Hoffmann, C.M.; Marlander, B. Nitrogen compounds in organs of two sugar beet genotypes (Beta vulgaris L.) during the season. Field Crop. Res. 2007, 102, 210–218.
  207. Khan, N.; Bano, A.; Rahman, M.A.; Guo, J.; Kang, Z.; Babar, M.A. Comparative physiological and metabolic analysis reveals a complex mechanism involved in drought tolerance in chickpea (Cicer arietinum L.) induced by PGPR and PGRs. Sci. Rep. 2019, 9, 1–19.
  208. Ashraf, M. Biotechnological approach of improving plant salt tolerance using antioxidants as markers. Biotechnol. Adv. 2009, 27, 84–93.
  209. Turan, S.; Tripathy, B.C. Salt and genotype impact on antioxidative enzymes and lipid peroxidation in two rice cultivars during de-etiolation. Protoplasma 2013, 1, 209–222.
  210. Das, M.K.; Roychoudhury, A. ROS and responses of antioxidant as ROS-scavengers during environmental stress in plants. Front. Environ. Sci. 2014, 2, 1–3.
  211. Elstner, E.F. Mechanisms of oxygen activation in different compartments of plant cell. In Active Oxygen/Oxidative Stress and Plant Metabolism; Pell, E.J., Stefen, K.L., Eds.; American society of Plant Physiol: Rockville, MD, USA, 1991; pp. 13–25.
  212. Vishwakarma, K.; Upadhyay, N.; Kumar, N.; Yadav, G.; Singh, J.; Mishra, R.K.; Sharma, S. Abscisic Acid Signaling and Abiotic Stress Tolerance in Plants: A Review on Current Knowledge and Future Prospects. Front. Plant Sci. 2017, 8, 161.
  213. Mehmood, A.; Hussain, A.; Irshad, M.; Khan, N.; Hamayun, M.; Ismail; Afridi, S.G.; Lee, I.J. IAA and flavonoids modulates the association between maize roots and phytostimulant endophytic Aspergillus fumigatus greenish. J. Plant Interact. 2018, 1, 532–542.
  214. Barnawal, D.; Bharti, N.; Tripathi, A.; Pandey, S.S.; Chanotiya, C.S.; Kalra, A. ACC-deaminase-producing endophyte Brachybacterium paraconglomeratum strain SMR20 ameliorates Chlorophytum salinity stress via altering phytohormone generation. J. Plant Growth Reg. 2016, 35, 553–564.
  215. Beaudoin, N.; Serizet, C.; Gosti, F.; Giraudat, J. Interaction between abscisic acid and ethylene signalling cascades. Plant Cell. 2000, 2, 1103–1115.
  216. Khan, N.; Bano, A.; Rahman, M.A.; Rathinasabapathi, B.; Babar, M.A. UPLC-HRMS-based untargeted metabolic profiling reveals changes in chickpea (Cicer arietinum) metabolome following long-term drought stress. Plant Cell Environ. 2019, 42, 115–132.
  217. Anuradha, S.; Rao, S.S.R. Effect of brassinosteroids on salinity stress induced inhibition of seed germination and seedling growth of rice (Oryza sativa L.). Plant Growth Regul. 2001, 33, 151–153.
  218. Sharma, I.; Ching, E.; Saini, S.; Bhardwaj, R.; Pati, P.K. Exogenous application of brassinosteroid offers tolerance to salinity by altering stress responses in rice variety Pusa Basmati-1. Plant Physiol. Biochem. 2013, 69, 17–26.
  219. Anuradha, S.; Rao, S.S.R. Application of brassinosteroids to rice seeds (Oryza sativa L.) reduced the impact of salt stress on growth, prevented photosynthetic pigments loss and increased nitrate reductase activity. Plant Growth Regul. 2003, 40, 29–32.
  220. Shahid, M.A.; Balal, R.M.; Khan, N.; Simón-Grao, S.; Alfosea-Simón, M.; Cámara-Zapata, J.M.; Mattson, N.S.; Garcia-Sanchez, F. Rootstocks influence the salt tolerance of Kinnow mandarin trees by altering the antioxidant defense system, osmolyte concentration, and toxic ion accumulation. Sci. Hortic. 2019, 10, 1.
  221. Shahid, M.A.; Balal, R.M.; Khan, N.; Zotarelli, L.; Liu, G.D.; Sarkhosh, A.; Fernández-Zapata, J.C.; Nicolás, J.J.; Garcia-Sanchez, F. Selenium impedes cadmium and arsenic toxicity in potato by modulating carbohydrate and nitrogen metabolism. Ecotoxicol. Environ. Saf. 2019, 30, 588–599.
  222. Houimli, S.I.M.; Denden, M.; El-Hadj, S.B. Induction of salt tolerance in pepper (Capsicum annum) by 24-epibrassinolide. Eurasian J. Biosci. 2008, 2, 83–90.
  223. Thussagunpanit, J.; Jutamanee, K.; Sonjaroon, W.; Kaveeta, L.; Chai-Arree, W.; Pankean, P.; Suksamrarn, A. Effects of brassinosteroid and brassinosteroid mimic on photosynthetic efficiency and rice yield under heat stress. Photosynth 2015, 1, 312–320.
  224. Qayyum, B.; Shabbaz, M.; Akram, N.A. Interachive effect of foliar application of 24—Epibrassinolide and root zone salinity on morpho – physiological attributes of wheat (triticum aestivum L.). Int. J. Agric. Biol. 2007, 9, 584–589.
  225. Hairat, S.; Khurana, P. Improving Photosynthetic Responses during Recovery from Heat Treatments with Brassinosteroid and Calcium Chloride in Indian Bread Wheat Cultivars. Am. J. Plant Sci. 2015, 6, 1827–1849.
  226. Amzallag, G.N. Brassinosteroids as metahormones evidences from specific influence during critical period in sorghum development. Plant Biol. 2002, 4, 656–663.
  227. Gruszka, D.; Janeczko, A.; Dziurka, M.; Pociecha, E.; Oklestkova, J.; Szarejko, I. Barley Brassinosteroid Mutants Provide an Insight into Phytohormonal Homeostasis in Plant Reaction to Drought Stress. Front. Plant Sci. 2016, 7, 1824.
  228. Kamuro, Y.; Takatsuto, S. Practical application of brassinosteroids in agricultural fields. In Brassinosteroids: Steroidal Plant Hormones; Sakurai, A., Yokota, T., Clouse, S.D., Eds.; Springer: Tokyo, Japan, 1999; pp. 223–241.
  229. Ali, B.; Hasan, S.A.; Hayat, S.; Hayat, Q.; Yadav, S.; Fariduddin, Q.; Ahmad, A. A role for brassinosteroids in the amelioration of aluminium stress through antioxidant system in mung bean (Vigna radiata L. Wilczek). Environ. Exp. Bot. 2008, 62, 153–159.
  230. Hasan, S.A.; Hayat, S.; Ali, B.B.; Ahmad, A. 28-Homobrassinolide protects chickpea (Cicer arietinum) from cadmium toxicity by stimulating antioxidants. Environ. Pollut. 2008, 151, 60–66.
  231. Rady, M.M.; Osman, A.S. Response of growth and antioxidant system of heavymetal-contaminated tomato plants to 24-epibrassinolide. Afr. J. Agric. Res. 2012, 7, 3249–3254.
  232. Jain, M.; Mathur, G.; Koul, S.; Sarin, N. Ameliorative effects of proline on salt induced lipid peroxidation in Cell lines of groundnut (Arachis hypogea L.). Plant Cell Rep. 2001, 20, 463–468.
  233. Abdullahi, B.A.; Gu, X.; Gan, Q.; Yang, Y. Brassinolide amelioration of aluminum toxicity in mung bean seedling growth. J. Plant Nutr. 2003, 26, 1725–1734.
  234. Martínez-Cuenca, M.R.; Primo-Capella, A.; Forner-Giner, M.A. Influence of Rootstock on Citrus Tree Growth: Effects on Photosynthesis and Carbohydrate Distribution, Plant Size, Yield, Fruit Quality, and Dwarfing Genotypes. Plant Growth 2016, 16, 107.
  235. Khelil, A.; Menu, T.; Ricard, B. Adaptive response to salt involving carbohydrate metabolism in leaves of a salt-sensitive tomato cultivars. Plant Physiol. Biochem. 2007, 45, 551–559.
  236. Bartels, D.; Sunkar, R. Drought and salt tolerance in plants. Crit. Rev. Plant Sci. 2005, 24, 23–58.
  237. Hoekstra, F.A.; Golovina, E.A.; Buitkink, J. Mechanisms of plant desiccation tolerance. Trends Plant Sci. 2001, 6, 431–438.
  238. Pierik, R.; Christa, T. The Art of Being Flexible: How to Escape from Shade, Salt, and Drought. Top. Rev. Plast. 2014, 166, 5–22.
  239. Libal-Wekseler, J.; Fernak, B.; Michael, S.T.; Zhu, J.K. Carbohydrate metabolism is affected in response to salt stress. Horticulture 1997, 20, 1043–1045.
  240. Khan, N.; Bano, A. Role of plant growth promoting rhizobacteria and Ag-nano particle in the bioremediation of heavy metals and maize growth under municipal wastewater irrigation. Int. J. Phytorem. 2016, 18, 211–221.
  241. Schreiber, L. Transport barriers made of cutin, suberin and associated waxes. Trends Plant Sci. 2010, 15, 546–553.
  242. Lux, A.; Šottníková, A.; Opatrná, J.; Greger, M. Differences in structure of adventitious roots in Salix clones with contrasting characteristics of cadmium accumulation and sensitivity. Physiol. Plant. 2004, 120, 537–545.
  243. Shabala, S. Learning from halophytes: Physiological basis and strategies to improve abiotic stress tolerance in crops. Ann. Bot. 2013, 112, 1209–1221.
  244. White, P.J.; Broadley, M.R. Chloride in soil and its uptake and movement within the plant. A review. Ann. Bot. 2001, 88, 967–988.
  245. Grattan, S.R.; Grieve, C.M. Mineral nutrient acquisition and response by plants grown in saline environments. In Hand Book of Plant and Crop Stress; Pessarakli, M., Ed.; Marcel Dekker: New York, NY, USA, 1999; pp. 203–299.
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