Plants Responses and Their Physiological and Biochemical Defense Mechanisms Against Salinity: A Review
Plants Responses and Their Physiological and Biochemical Defense Mechanisms Against Salinity: A Review
Plants Responses and Their Physiological and Biochemical Defense Mechanisms Against Salinity: A Review
Review article
Abstract: Plants confront an extent of abiotic stresses due to environmental hardship, among
which salinity is one of the major abiotic stresses that seizes plant growth and development
resulting in a massive yield loss worldwide. Plants respond to salinity in two distinct phases: a
quick osmotic phase and a sluggish ionic phase also known as hyper osmotic phase. Plants
adjustment and/or tolerance to salinity stress comprise several complex physiological, biochemical
and molecular networks. A widespread understanding of how plants response to salinity stress at
different phases, and a cohesive physiological and biochemical approaches are crucial for the
development of salt adapted and/or tolerant varieties for salt-affected areas. Researchers have
identified several adaptive responses to salinity stress at cellular, biochemical and physiological
levels, even though mechanisms triggering salt stress adaptation and/or tolerance are far from
being entirely understood. This article bestows a spacious review of foremost research advances
on physiological and biochemical mechanisms governing plant adaptation and/or tolerance to
salinity stress relevant to environmental sustainability and as well as food production.
Keywords: Salinity - Osmotic stress - Ionic stress - Photosynthesis - Reactive oxygen species -
Ion homeostasis.
[Cite as: Polash MAS, Sakil MA & Hossain MA (2019) Plants responses and their physiological and
biochemical defense mechanisms against salinity: A review. Tropical Plant Research 6(2): 250–274]
INTRODUCTION
Salinity has gained a global concern due to its fierce environmental stresses that inversely influence the
growth and development of plants with regulation of metabolic changes (Munns 2002a, Vaidyanathan et al.
2003, Munns & Tester 2008). It is categorized by an excessive concentration of soluble salts in growing media,
causes significant crop damage globally (Munns & Tester 2008). Today, it is an ascending challenge towards
global agriculture to produce 70% more food crop for feeding an addition 2.3 billion souls by 2050 throughout
the world (FAO 2009) but this formidable abiotic stress inhibits the agricultural productivity worldwide (Munns
& Tester 2008). The problem is constantly rising because of accretion of salt-affected soil day by day which is
triggered by various environmental and anthropogenic influences (Boesch et al. 1994, Rogers & McCarty 2000).
Accumulation of salts over prolonged periods (Rengasamy 2002) due to weathering of parental rocks (Szabolcs
1998) has arisen the maximum salt-affected land naturally. Another reason is the deposition of marine salts
transported in wind and rain. Munns & Tester (2008) demonstrated that rain with 10 mg kg-1 of NaCl would
deposit 10 kg ha-1 of salt for every 100 mm of precipitation for each year. Aloof from natural causes,
anthropogenic influences are similarly accountable for soil salinization. Poor quality water in irrigation and
global warming with subsequent elevation in sea level and tidal surges, especially in coastal areas are one of the
key factors for soil salinization.
Salinity comprises changes in several metabolic and physiological routes, depending on sternness and extent
of the stress (Munns 2005). It exerts a devastating effect on plants into two phases. One is the rapid osmotic
phase and another is a slower ion toxicity phase. Osmotic phase suppresses the plant/young leaves growth and
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Received: 30 April 2019 Published online: 31 August 2019
https://doi.org/10.22271/tpr.2019.v6.i2.035
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development which is then followed by ionic toxicity due to high accumulation of salt in leaves that speeds
senescence of mature leaves (Munns 2005, Rahnama et al. 2010).
Munns & Tester (2008) suggested plants quench the salt stress challenge via. three tolerance mechanisms i.e.
tolerance to osmotic stress, Na+ exclusion from blades and tissue tolerance whereas McCue & Hanson (1990)
suggested four tolerance mechanisms. First is developmental traits, second is structural traits, third is the
physiological mechanism and the forth is metabolic responses, such as modification in photosynthetic
metabolism (Cushman et al. 1990, Cushman 1992) coupled with biosynthesis of compatible osmolytes and
antioxidant enzymes.
An affluent amount of research has been done in demand to understand the mechanism of salinity tolerance
in plants (Zhang & Shi 2013) in the previous eras. This current flurry of action may also mirrored that the
existing enthusiasms in plant science for building practical support to food production, research progresses on
the complex physiological and biochemical mechanisms against salt stress.
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or hyperosmotic stress phase. Ionic toxicity causing from distorted K+/Na+ ratio and deposition of Na+ and Cl-
ion in leaves over an extended period of time after transpiration, results in injury and/or death of leaves and
decrease the total photosynthetic leaf area which lower the supply of photosynthate in plants and finally alter the
productivity. Leaf injury and/or death are documented to the elevated salt load in the leaf that exceeds the
capability of salt compartmentalization in the vacuoles, that results in the cytoplasm toxic (Munns 2002a, 2005,
Munns et al. 2006). Beneath such condition, a plant eventually may die (Blaylock 1994).
Figure 1. An outline of two phase growth response against salt stress. (Modification of Munns & Tester 2008)
Accumulation of Na+ ions
During salinity, Na+ accumulation is a common phenomenon in leaves rather than in the roots after being
deposited from the transpiration steam (Amtmann & Sanders 1998, Munns 2002a). In standard physiological
circumstances, plants maintain a high K+/Na+ ratio in their cytosol (Binzel et al. 1988) but an elevation in
extracellular Na+ concentrations occurs due to the negative electrical membrane potential at the plasma
membrane (-140 mV) (Higinbotham 1973) that favors the passive transportation of Na+ ions into cytosol from
the environment and deposits into leaf cell after transpiration (Fig. 2). The extreme Na+ in the cytosol has been
exhibited poor survival of plants and eventually death as well (Krishnamurthy et al. 2009). Na+ ions restrict the
function of potassium which performances as a cofactor in several reactions and hence exhibits direct toxicity
on the plant. In addition Na+, however, seems to be detrimental to the structural and functional integrity of
membranes (Iraki et al. 1989).
Figure 2. Accumulation of Na+ ions. Where, a- Passive transportation of Na+ due to the negative electrical membrane
potential; b- Water loss from leaf by transpiration; c- Deposition and/or accumulation of Na+ in leaf cell.
Stomatal closure
A further response of plants to salinity is demonstrated by a reduction in stomatal aperture which is believed
to induce by the osmotic effect. Salinity disturbs stomatal conductance rapidly and transiently due to interrupt in
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water relations and sharply the local synthesis of short-lived ABA in roots (Fricke 2004) and immediately
relocate into the leaves through xylem. ABA then fixes with plasma membrane receptor molecule of guard cells
and this fixation trigger activation of Ca2+ channel proteins which inflows Ca2+ into the cytosol from outside.
Simultaneously activation Ca2+ channels present on tonoplast starts to efflux of Ca2+ in cytosol from the vacuole,
leads to further rise in Ca2+ in the cytosol. High Ca2+ concentration inhibits K+ channel proteins activity though it
keeps normal Cl- channel proteins activity. Consequently, no K+ is influxed and efflux of Cl- from cytosol
initiates to enhance cytosolic pH cause depolarization of plasma membrane. At existing circumstances, K +
(known as water buoy) is effluxed through Guard Cell Outward Rectifying K+ (GORK) channel triggering lose
in turgidity in guard cell and cause stomatal closure (Blatt & Armstrong 1993) (Fig. 3).
Figure 3. ABA mediated stomatal closure. Where, a- ABA binds with PM receptor molecule; b- Boost Ca2+ channel protein
to influx Ca2+ in cytosol; c- Simultaneous Ca2+ efflux in cytosol from vacuole leads further raise in Ca2+; d- Increased Ca2+
inhibit the activity of K+ inward channel while keeps normal the Cl- channel activity causing depolarization of plasma
membrane; e- This situation facilities removal of K+ from guard cell through GORK channel causing stomatal close.
(Modification of outline of Blatt & Armstrong 1993)
Inhibition of Photosynthesis
Figure 4. General reactions of photosynthesis and inhibition of photosynthesis during salt stress.
Salt stress is believed to responsible for lower photosynthesis which is triggered by ABA mediated stomatal
closure. The diminution in stomatal conductance inhibits the accessibility of CO2 for carboxylation reactions in
leaves that decreases photosynthesis under stress (Brugnoli & Björkman 1992) (Fig. 4). Besides, one of the most
noted effects of salinity that reduces the photosynthesis is the variation in biosynthesis of photosynthetic
pigment (Maxwell & Johnson 2000). The reduction in Chlorophyll content under salt stress is a normally stated
phenomenon (Chutipaijit et al. 2011). Chutipaijit et al. (2011) demonstrated that subjected to 100 mM NaCl
showed 30, 45 and 36% reduction in Chlorophyll a (Chl a), Chlorophyll b (Chl b) and carotenoids (Car)
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contents respectively as compared to control in rice. Photosynthesis is also obstructed when excessive
concentrations of Na+ and/or Cl– are amassed in chloroplasts.
Oxidative stress
Salinity invites oxidative stress through a series of actions. It triggers stomatal closure, leading decreases
CO2 availability for carbon fixation in the leaves, unmasking chloroplasts to extreme excitation energy which
in turn rise the generation of reactive oxygen species (ROS) such as superoxide (O2 •– ), hydrogen
peroxide (H2O2), hydroxyl radical (OH•) and singlet oxygen (1O2) (Apel & Hirt 2004, Foyer & Noctor 2005a,
Parida & Das 2005, Ahmad & Sharma 2008, Ahmad et al. 2010a, 2011) that initiate programmed cell death
(Jacobson et al. 1997, Jabs, 1999, Gunawardena et al. 2004) (Fig. 5). On the other hand, physiological water
deficit because of osmotic effect alters a wide range of metabolic activities (Greenway & Munns 1980,
Cheeseman 1988) leads to the generation of ROS (Halliwell & Gutteridge 1985, Elstner 1987). ROS are
extremely reactive and may reason cellular damage through lipid peroxidation as well as proteins and nucleic
acids oxidation (Hasegawa et al. 2000, Pastori & Foyer 2002, Apel & Hirt 2004, Ahmad et al. 2010a, 2010b)
demonstrated that generation of ROS is enhanced under saline conditions and ROS-mediated membrane
destruction has been revealed to be a foremost reason of the cellular toxicity in several crop plants such as rice,
tomato, citrus, pea and mustard (Gueta-Dahan et al. 1997, Dionisio-Sese & Tobita 1998, Mittova et al. 2004,
Ahmad et al. 2009, 2010b).
Figure 5. An overview of oxidative stress during salinity stress. Where, a- No CO2 fixation due to stomatal closure; b-
Initiation of ROS generation via. mehlar reaction.
Nutrient imbalance
High salt concentration due to salinity is believed to cause nutrient imbalance. A number of reports showed
that salinity decreases nutrient uptake and accumulation of nutrients into the plants (Rogers et al. 2003, Hu &
Schmidhalter 2005). Rozeff (1995) demonstrated that salinity lower N accumulation in plants due to the
interaction between Na+ and NH4+ and/or between Cl– and NO3– that finally lessen the growth and yield of the
crop. Plants face phosphorus (P) deficiency in saline soils due to ionic strength effects that decreased the activity
of PO43– and low solubility of Ca-P minerals. Elevated level of Na+ ion concentrations in the soil decreases the
quantity of available K+, Mg2+ and Ca2+ (Epstein 1983) hence, directing to nutrient imbalance. The solubility of
micronutrients, pH of soil solution, redox potential of the soil solution and the nature of binding sites on the
organic and inorganic particle surfaces are the principal factors for the availability of micronutrients in saline
soils. Zhu et al. (2004) reported that micronutrient deficiencies are common in salt stress because of high pH.
Plant yield
The above-stated responses against salt stress lead to the diminution of crop yield which is the most
noticeable effect in agriculture. Salinity causes great crops reduction and yields almost all plant species except
some halophytes. Nahar & Hasanuzzaman (2009) showed an application of 250 mM NaCl decreased 77, 73 and
66% yield in BARI mung-2, BARI mung-5 and BARI mung-6, respectively over control. Later on
Hasanuzzaman et al. (2009) demonstrated that at 150 mM salinity BR11, BRRI dhan41, BRRI dhan44 and
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BRRI dhan46 showed loss of grain yield at 50, 38, 44 and 36% respectively over control. Greenway & Munns
(1980) observed that at 200 mM NaCl, sugar beet (a salt-tolerant species) might have a reduction of only 20% in
dry weight, cotton (a moderately tolerant) might have a 60% reduction, and as a sensitive species soybean might
be dead. In contrast, a halophyte such as Suaeda maritima (L.) might be growing at its optimum rate under
salinity (Flowers et al. 1986). This reduction of yield and yield components under salt stress may also be
assigned to low cell expansion, less photosynthetic rate, senescence and production (Seemann & Critchley 1985,
Wahid et al. 1997).
Figure 6. Model of SOS pathway for ion homeostasis and compartmentalization during salt stress. (Modification of outline
of Gupta & Huang 2014)
Salt Overly Sensitive (SOS) stress signaling pathway is also responsible for ion homeostasis and salt
tolerance (Hasegawa et al. 2000, Sanders, 2000). SOS consists of three major proteins: a) SOS1 protein that
encodes a plasma membrane Na+/H+ antiporter, is crucial in controlling Na+ efflux at cellular level. Besides,
long distance transport of Na+ from root to shoot is assisted by SOS1. Overexpression of this SOS1 protein
bestows salt tolerance in plants (Shi et al. 2000, Shi et al. 2002); b) SOS2 protein that encodes serine/threonine
kinase and consists of a well-developed N-terminal catalytic domain and a C-terminal regulatory domain (Liu et
al. 2000). SOS2 is activated by the action of both SOS3 protein and salt stress elicited Ca2+ signals; c) Another
protein in SOS signaling pathway is the SOS3 protein which is a myristoylated Ca2+ binding protein along with
a myristoylation site at its N-terminus. This myristoylation site shows a crucial role in salt tolerance (Ishitani et
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al. 2000). C-terminal regulatory domain of SOS2 protein performs as a site of interaction for Ca2+ binding SOS3
protein resulting in the initiation of the kinase (Guo et al. 2004). The activated kinase then phosphorylates SOS1
protein thus escalating its transport activity via. Na+/H+ antiporter (Quintero et al. 2002). This result increase
Na+ efflux and thus ease Na+ toxicity (Martinez-Atienza et al. 2007) (Fig. 6).
Compatible solute accumulation and osmotic protection
Biosynthesis and/or accumulation of compatible solutes are inhabitable in stress condition. They are
uncharged, polar, and soluble in nature and do not interfere with the cellular metabolism even at high
concentration. The well documented compatible solutes found in are proline (Pro) (Ashraf & Foolad 2007,
Hoque et al. 2007, Ahmad et al. 2010a, Nounjan et al. 2012, Tahir et al. 2012), glycinebetaine (GB) (Khan et al.
2000, Wang & Nii 2000, Ashraf & Foolad 2007), sugar (Bohnert et al. 1995, Kerepesi & Galiba 2000), and
polyols (Ford 1984, Ashraf & Foolad 2007, Saxena et al. 2013) As their biosynthesis and/or accumulation is
associated to the external osmolarity, the major functions of these osmolytes is to shield the structure of cells
and to maintain osmotic balance thru continuous water influx (Hasegawa 2013). Besides, an inorganic osmolyte
recognized as K+ plays an important role in osmoregulation thus salinity mitigation (Shabala 2003, Polash et al.
2018)
Proline: Proline (Pro) biosynthesis and/or accumulation are a well-known phenomenon for decreasing salinity
stress (Matysik et al. 2002, Ben-Ahmed et al. 2010, Saxena et al. 2013). In osmotically stressed cell Pro is
synthesised either from glutamate or ornithine (Fig. 7). The biosynthetic pathway includes two major
enzymes; a) pyrroline carboxylic acid synthetase and b) pyrroline carboxylic acid reductase, which are
responsible for overproduction of Pro in plants under stress (Sairam & Tyagi 2004). Nounjan et al. (2012)
observed that salt stress resulted in growth reduction, increase in the Na+/K+ ratio, increase in Pro level and
up-regulation of proline synthesis gene as well as accumulation of H2O2, increased activity of
antioxidative enzymes (SOD, POX, APX, CAT) of rice seedlings. Intracellular Pro provides tolerance
toward stress and also behaves as an organic nitrogen reserve during stress recovery. Pro assists in
stimulating the expression of salt-stress-responsive proteins (Khedr et al. 2003) acts as an antioxidant
feature, suggesting ROS scavenging activity and 1O2 quencher, protects the photosynthetic apparatus (Ashraf
et al. 2008) thus develop the plant adaptation against salt stress (Smirnoff & Cumbes 1989, Matysik et al.
2002). Deivanai et al. (2011) demonstrated that pretreatment with 1 mM Pro exhibited advance in growth
during salt stress in rice seedlings. It has been demonstrated by a study that Pro increases salt tolerance in
tobacco by intensifying the activity of enzymes participating in antioxidant protection system (Hoque et al.
2008). Antioxidant enzyme activity such as superoxide dismutase (SOD), catalase (CAT) and peroxidase
(POD) is significantly inhibited by salt which is upregulated by Pro supplements. Ahmad et al. (2010b)
observed in olive trees, that Pro supplements appeared to improve salt stress tolerance by regulating
antioxidant enzymatic activities, enhancing the photosynthetic activity, and thus preserved well plant growth
and water influx. Besides the exogenous application of Pro significantly mitigate the reduction of
photosynthesis (Pn), flurescence (Fv/Fm), and chlorophyll (Chl) content under saline conditions. Nounjan et
al. (2012) reported that exogenous supplementation of Pro repressed the Na+ induced apoplastic flow thus
reduce Na+ uptake in rice. They also demonstrated that application of Pro to the salt stress environment
repressed Na-induced trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid uptake and Na+ accumulation,
whereas the K+ content was fairly increased, leading to a high K+/Na+ ratio under salt stress.
Figure 7. Biosynthesis of Pro from glutamate during salinity. (Modification of Hossain et al. outline 2011a)
Glycinebetaine: Glycinebetaine (GB) is an amphoteric quaternary ammonium compound and non-toxic even at
higher concentrations in cell which plays a defensive role to salt stress (Ashraf & Foolad 2007, Chen &
Murata 2008). The most common pathway of GB synthesis from choline is a two-step reaction, first choline
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is oxidized to betaine aldehyde catalyzed by choline monooxygenase (CMO) which is further undergoes
oxidation to form glycine betaine by the activity of betaine aldehyde dehydrogenase (BADH) (Ahmad et al.
2013) (Fig. 8). Another pathway of synthesis involves three successive N-methylation which are catalyzed
by, glycine sarcosine N-methyl transferase (GSMT), and sarcosine dimethylglycine N-methyl transferase
(SDMT) (Ahmad et al. 2013). The foremost roles of glycinebetaine are shields the cell by stabilizing protein
(Mäkelä et al. 2000), osmotic adjustment (Gadallah 1999), defends the photosynthetic apparatus from stress
injuries (Cha-Um & Kirdmanee 2010) and reduction of ROS (Ashraf & Foolad 2007, Saxena et al. 2013).
Rahman et al. (2002) demonstrated the positive effect of GB on rice seedlings when uncovered to salt stress.
The affirmative effect of exogenous application of GB is related with reduced Na+ accumulation alone with
the maintenance of higher K+ concentration within all parts of salt-stressed plants. This effect might be due
to the creation of numerous vacuoles in the root cells in which Na+ is stored and prevent its accumulation in
the shoots. Cha-Um & Kirdmanee (2010) applied GB on salt-sensitive rice plants bared to 150 mM of NaCl
stress. The results revealed that GB treated plants exhibited higher water use efficiency (WUE) and pigment
stabilization, leading to high CO2 assimilation, photosynthetic performance as well as plant height under
salinized environment.
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inorganic osmoticum involves in retaining water content as well as cell turgidity by maintaining ionic
balance (Shabala 2003).
salt-induced drop in leaf Chl a and b concentrations in chili was enhanced with AsA pre-treatment (Khafagy
et al. 2009). A study by Azzedine et al. (2011) described that the exogenous application of AsA was helpful
to alleviate the negative effect of salt stress on plant growth by increasing leaf area, enriching Chlorophyll
and Carotene contents, boosting Pro accumulation and declining H2O2 content.
Glutathione: Glutathione (GSH) is another strong antioxidant documented in plants which counteracts the
damage of principal cellular components due to ROS generation (Pompella et al. 2003). GSH also defends
proteins from denaturation and function as a substrate for glutathione peroxidase (GPX) and glutathione-S-
transferases (GST), which regulates in the removal of ROS (Noctor et al. 2002). It also contributes in
regeneration of ascorbate via. ascorbate-glutathione cycle (Foyer et al. 1997). A study by Aly-Salama & Al-
Mutawa (2009) reported that exogenous application of glutathione helped to maintain plasma membrane
permeability and cell viability under salt stress in onion. Combind application of glutathione and ascorbate
assists in increasing morphological parameters, antioxidant activity and mineral ion content while exposed to
salinized environment (Rawia et al. 2011).
Tocopherols: Tocopherols are amphiphilic antioxidants belong to the family of vitamin E also found in plants
in stress mitigation. Tocopherols are known to reduce the ROS levels in photosynthetic membranes and
restricts the extent of lipid peroxidation thru decreasing lipid peroxyl radicals (LOO•) (Maeda et al. 2005,
Munné-Bosch 2005). Rady et al. (2011) demonstrated that exogenous α-tochopherol application enhanced
total soluble sugars content and the activities of CAT, POX, PPO and PAL under salt stress. Pre-treatment
with α-tochopherol also enriched the mineral nutrient content in the plant with simultaneous increase
in Pro, total phenols and free amino acids. Application of α-tocopherol helps to reduce salt-induced leaf
senescence by decreasing the Na+ and Cl– content and increasing the K+, Ca2+ and Mg2+ contents (Farouk
2011). In addition, tocopherols involve in ROS, antioxidants, and phytohormones mediated signaling
network thereby maintain cellular signaling in plants (Munné-Bosch 2007).
Gibberellic acids: Gibberellic acids (GAs) play an important role in seed germination, leaf expansion, stem
elongation and flowering (Magome et al. 2004, Kim & Park 2008) thus in growth and development of
plants. Besides their crucial use in several physiological and biochemical processes, they are well-recognized
phytohormones in alleviating salinity (Kaya et al. 2009). Salt stress impairs the seed germination processes
and reduces the growth and grain yield of wheat which is progressed by application of GA3 (Kumar & Singh
1996). Exogenous application of GA3 is reported to decrease the inhibitory effect of salt stress on growth
traits, increase in photosynthetic pigments, RWC and enzymatic activity (Ali et al. 2011). Hamayun et al.
(2010) observed the positive effect of exogenous GA3 on salt-stressed soybean plant by boosting up the level
of phytohormones, growth and development. Recent studies have shown exogenous application of GA3
lessened the negative effects of NaCl-induced salinity by enhancing RWC, Chl content and counteracting the
electrolyte leakage (Ahmad et al. 2009), regulating the ions uptake, ion partitioning and hormones
homeostasis (Iqbal & Ashraf 2013), lowering stomatal resistance and increasing plant water relationships
(Maggio et al. 2010). Lipid peroxidation is indispensible in salt stress counteracting by application of GA3
thus shows improve resistance to salinity (Ahmad et al. 2009).
Jasmonic acid: Jasmonic acid (JA) and its methyl esters are crucial cellular regulators included in varied
physiological and developmental processes, like germination, root growth, fertility, stomatal regulation, fruit
ripening and senescence (Wasternack & Hause 2002, Cheong & Choi 2003, Hossain et al. 2011b). Rohwer
& Erwin (2008) stated the positive role of JA in plant responses to abiotic stresses; however, the role of most
of the derivatives of JA is still unclear. There are little reports on the function of exogenous JA in plant
response to NaCl salt stress. JA pre-treatments assist in the synthesis of abundant proteins (Known as JIPs)
in response to abiotic stress alleviation and/or tolerance (Sembdner & Parthier 1993). MeJA (methylated
ester of JA) supports protection in stress by osmoregulation and enhanced Pro accumulation (Fedina &
Tsonev 1997). Exogenous application of JA on salt treated plants regulates the balance of endogenous
hormones such as ABA (Kang et al. 2005), GAs (Seo et al. 2005) which grant significant protection
mechanisms under salinized environment.
Salicylic acid: Salicylic acid (SA) is a plant-derived phenolic compound that performs a significant role in plant
growth and development alone with the response to abiotic stresses (El-Tayeb 2005, Ahmad et al. 2011,
Fragnière et al. 2011, Tahjib-Ul-Arif et al. 2018). El-Tayeb (2005) found increased synthesis of Chl and
carotene (Car), and maintained membrane integrity to barley with SA pre-treatment leading to the
development of plant growth. Several studies have demonstrated that SA improves salinity tolerance by
restoring membrane potential and checking salt-induced K+ loss (Jayakannan et al. 2013), accumulating of
K+, and soluble sugars in roots (El-Tayeb 2005). SA treatment showed improved growth, lessened lipid
peroxidation and membrane permeability in maize (Gunes et al. 2007), minimized leaf Na+, Cl−, and H2O2
content with increased photosynthesis in mungbean (Nazar et al. 2011) and lntil (Stevens et al. 2006, Poór et
al. 2011), enhanced grain yield in wheat (Arfan et al. 2007) under salinized condition. SA application
triggers the accumulation of ABA and IAA, assists in the development of anti-stress programs in wheat
seedlings thus, accelerates growth and developmental processes (Sakhabutdinova et al. 2003). Gémes et al.
(2011) reported SA-induced generation of H2O2 and NO are believed to assist in cross-tolerance to various
stressors. Exogenous application of SA decreased the NaCl-induced electrolyte leakage and showed adaptive
responses in alfaalfa plant under salt stress (Torabian 2011). Moreover, Yusuf et al. (2012) observed SA
induced antioxidant activities (SOD, CAT and POX) in mustard which might be accountable for improved
tolerance of mustard to NaCl stress.
Brassinosteroids: Brassinosteroids (BRs) is one of the most recent groups of phytohormones act as a strong
growth inducer and stress response aid (Anuradha & Rao 2001, Krishna 2003, Ashraf et al. 2010, El-
Mashad & Mohamed 2012). Anuradha & Rao (2001) reported that BRs plays an important role in activation
of seedling growth and development (Clouse & Sasse 1998) under salt stress which was related with
increased levels of nucleic acids and soluble proteins. Several studies have revealed the prospective
application of BRs in agriculture to improve yield and regulate crop growth under stress (Houimli et al.
2010, Hayat & Ahmad 2011, El-Mashad & Mohamed 2012). Exogenous BR application increases the fresh
and dry weight of plant (Houimli et al. 2010), enhances plant biomass in wheat (Shahbaz & Ashraf 2007),
and alleviates the injurious effect on nuclei and chloroplasts (Krishna 2003). Foliar application of BRs assist
to overcome the adverse effect of salinity on photosynthetic pigments, crop productivity thus increased yield
attributes in wheat (Eleiwa et al. 2011), increase the concentration and total uptake of nutrients (N, P, K,
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Fe, Mn, Zn and Cu) in straw and grains (Eleiwa & Ibrahim 2011). Later on, El-Mashad & Mohamed (2012)
demonstrated that foliar spray of BR (0.05 ppm) alleviated salt stress by b activities of enzymatic and non-
enzymatic antioxidants.
CONCLUSIONS
Based on a profuse research findings, it is obvious that salt stress has detrimental effects on the physiological
and biochemical processes associated with growth, development, the yield of plants. Counteracting the negative
effect of salinity involves a complex of responses at the cellular, molecular, metabolic, physiological as well as
whole plant levels. Plentiful research on cellular, metabolic and physiological strategies regulation demonstrated
the positive role against salt stress tolerance and/or adaptation by controlling ion uptake, transport and balance,
improving osmotic regulation, hormone metabolism, antioxidant enzymatic activity, and stress signaling.
Further experiments are needed to launch for better understanding of the underlying mechanisms in salt stress
mitigation.
ACKNOWLEDGEMENT
The authors are thankful to the Head, Department of Crop Botany for immense support.
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