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Soil salinity is one of the most severe abiotic stresses, which limits the production of nearly over 6% of the world's land and 20% of the irrigated land (15% of total cultivated areas) and negatively impacts crop production throughout the world. On the other hand, the increased salinity of agricultural land is expected to have destructive global effects, resulting in up to 50% land loss by the middle of the twenty-first century (Mahajan and Tuteja 2005). However, pressure of increasing populations and increasing demand for plant production needs to bring new saline lands into agricultural production. It is a problem of great importance, because many agricultural areas previously fertile became saline due to irrigation with unfit water.
Some of the adverse effects of salinity have been attributed to increase in sodium (Na+) and chloride (Cl-) ions in different plants hence these ions produce the critical conditions for plant survival by intercepting different plant mechanisms. Sodium and chloride which are the major ions produce many physiological disorders in plants but chloride is the most dangerous. Excess of these salts also enhances the osmotic potential of soil matrix as a result of which water intake by plants is restricted. Due to the accumulation of Cl-, relative salt tolerance has been linked to plant growth water use efficiency and transpiration. Inspite of upper plant parts, salinity also effects roots growth and physiology and ultimately their function of nutrient uptake. High salinity causes both hyperionic and hyperosmotic stress and can lead to plant death (Jacoby 1999).
Plant response to salt stress
Plant's responses to salt stress depend on genotypes, developmental stage, as well as the intensity and duration of the stress. It is reported that plants growing under saline conditions are affected in three ways: reduced water potential in root zone causing water deficit, phytotoxicity of ions such as Na+ and Cl- and nutrient imbalance depressing uptake and transport of nutrients. Na+ competes with K+ for binding sites essential for cellular functions. Increased salinity has diverse effects on the physiology of plants grown in the saline conditions. These effects have been recognized due to major factors like osmotic stress, ion-specificity, nutritional and hormonal imbalances and oxidative damage. Outcomes of these effects may cause disorganization of cellular membranes, inhibition of photosynthesis, generation of toxic metabolites and reactive oxygen species (ROS) and declined nutrient absorption, which ultimately lead to plant death (Hasegawa et al., 2000; Ashraf, 2004; Chartzoulakis and Psarras, 2005; Flowers and Flowers, 2005). The primary effect of salinity especially at high salt concentrations is due to osmotic variations. The osmotic effects of salinity lead to slow growth rate and developmental characteristics such as root/shoot ratio and maturity rate. This altered water status leads to initial growth reduction and limitation of plant productivity. Since salt stress involves both osmotic and ionic stress (Hagemann and Erdmann, 1997; Hayashi and Murata, 1998), growth suppression is directly related to total concentration of soluble salts or osmotic potential of soil water (Flowers et al., 1977; Greenway and Munns, 1980). The detrimental effect is observed at the whole-plant level as death of plants or decrease in productivity. Suppression of growth occurs in all plants, but their tolerance levels and rates of growth reduction at lethal concentrations of salt vary widely among different plant species. Although the change in water status is the cause of growth suppression, the contribution of subsequent processes to inhibition of cell division and expansion and acceleration of cell death has not been well documented (Hasegawa et al., 2000). Salt stress affects all the major processes such as growth, photosynthesis, protein synthesis, and energy and lipid metabolism.
Although the effect of salinity on plants greatly varies within the plant species and cultivars, in general, the response of crop plants to salinity is the reduction in growth (Romero-Aranda et al., 2001; Ghoulam et al., 2002). Such growth reductions have been reported in tomato (Romero-Aranda et al., 2001), cotton (Meloni et al., 2003) and sugarbeet (Ghoulam et al., 2002). However, differences are present among different species and cultivars for salinity stress tolerance. Salinity affects the plant growth at various developmental stages including germination and emergence of embryonic tissues (Shannon et al., 1994), vegetative and reproductive growth stages (Abrol et al., 1988). Time course studies show that salinity influences relative growth, net assimilation capacity, leaf expansion rate in sunflower (Rawson and Munns, 1984) and leaf area index in wheat (Zheng et al., 2008). Studies show that sugarcane lines capable of producing stronger and ramified root systems showed relatively better salt tolerance than the sensitive one (Wahid et al., 1997a, b).
Oxidative stress due to salinity
Salt stress can lead to stomatal closure, which reduces CO2 availability in the leaves and inhibits carbon fixation, exposing chloroplasts to excessive excitation energy which in turn could increase the generation of reactive oxygen species (ROS) (Parida and Das, 2005; Parvais and Satyawati, 2008). On the other hand, as salt stress is complex and imposes a water deficit because of osmotic effects on a wide variety of metabolic activities (Greenway and Munns, 1980; Cheeseman, 1988). This water deficit leads to the formation of reactive oxygen species (ROS) such as superoxide (O2-), hydrogen peroxide (H2O2), hydroxyl radical (OH•) (Halliwell and Gutteridge, 1985), and singlet oxygen (1O2) (Elstner, 1987). ROS are highly reactive and may cause cellular damage through oxidation of lipids, proteins and nucleic acids (Pastori and Foyer, 2002; Apel and Hirt, 2004). If there is a serious imbalance in any cell compartment between the production of ROS and antioxidant defense, oxidative stress and damage occurs (Mittler, 2002). Enhanced produc tion of ROS under salinity stress induces phytotoxic reactions such as lipid peroxidation, protein degradation, and DNA mutation (Hefny and Abdel-Kader 2009; Tanou et al. 2009). When a plant faces harsh conditions, ROS production will overcome scavenging systems and oxidative stress will burst. In many plant studies, it was observed that production of ROS is increased under saline conditions (Hasegawa et al. 2000) and ROS-mediated membrane damage has been demonstrated to be a major cause of the cellular toxicity by salinity in different crop plants such as ricee, tomato and citrus (Gueta-Dahan et al., 1997; Dionisio-Sese and Tobita, 1998; Mittova et al., 2004). Long-term salinity treatments (EC 5.4 and 10.6 dS m-1, for 60 days) caused significant increase in H2O2 and lipid peroxidation in wheat seedlings, which were higher in salt-sensitive cultivar than salt-tolerant cultivar (Sairam et al. 2002). In recent study, increased lipid peroxidation and levels of H2O2 was observed with increased salinity in rapeseed and mungbean (Hasanuzzaman et al. 2011) and Vigna radiata (Hossain et al. 2011). .
Antioxidant defense in plants under salinity
In case of salinity, when ROS increases, plants use their scavenging mechanism of ROS categorized as non-enzymatic and enzymatic antioxidants (Reddy et al., 2004; Demiral and Turkan, 2005). The non-enzymatic antioxidant involves ascorbic acid, AsA; glutathione, GSH; phenolic compounds, alkaloids, non-protein amino acids, and α-tocopherols. When ROS increases, chain reactions start, in which superoxide dismutase (SOD) catalyzes the dismutation of O2- radicals to molecular O2 and H2O2 (Meloni et al. 2003). The H2O2 is then detoxified in the AsA-GSHcycle (Asada, 1999; Mittler, 2002), which involves the oxidation and re-reduction of AsA and GSH through the APX, MDHAR, DHAR and GR action (Noctor and Foyer, 1998). Besides this GST, GPX and CAT also play important role in antioxidant defense. An efficient role of antioxidant enzymes and non-enzymatic antioxidant (AsA and GSH) in conferring salt stress tolerance was observed in our recent study (Hasanuzzaman et al. 2011; Hossain et al. 2010, 2011).
Exogenous AsA showed the increased capacity of tomato seedling to recover from salt stress (Shalata and Neumann 2001). They observed that the addition of exogenous AsA to the root medium remarkably increased seedling survival of treatments with 300 mM NaCl for 7, 8 or 9 h. in addition, exogenous AsA partially inhibited the increases of lipid peroxidation. Hamada and Al-Hakimi (2009) found that exogenously applied AsA were generally effective in partially or completely countering the inhibitory effects of salt stress on net photosynthetic rate, pigments biosynthesis and membrane integrity, exerting a stimulatory action on these parameters, especially in plants subjected to moderate and low salinity levels. The leakage of K+ was also reduced by the application of AsA. In contrary, Khan et al (2006) applied AsA as foliar spray (0, 50, 100 mg L-1) on wheat grown on hydroponics and observed that foliar spray with AsA improved the growth of non-stressed plants of both cultivars, but did not alleviate the adverse effects of salt stress on plants. However, salt-induced reduction in leaf chlorophyll 'a' was improved with AsA application. AsA application enhanced the Na+ accumulation in the leaves of salt stressed plants of both cultivars, but it did not change the K+ accumulation in the leaves and roots of the salt stressed plants (Khan et al. 2006). In sunflower seedling, in vitro experiments were performed by Zeid et al. (2008) and observed that AsA concentrations improved tolerance of wheat calli to salinity. These effects might be attributed to the protective role of AsA in plant cells from the oxidative stress induced by salinity. Khafagy et al. (2009) observed that pre-soaking of sweet pepper seeds in AsA partially counteracted the harmful effect of NaCl salinity. Chlorophyll a and b concentrations significantly increased in AsA pre-soaked salt-stressed seedlings compared to the seedlings subjected to salt only. Huang et al. (2005) found that under salt stress, the AsA-deficient Arabidopsi mutant vtc-1, having 30-60% of the AsA content of wild-type plants, accumulated a much higher level of H2O2 than wild type, which coincides with a greater decrease in the ratio of reduced AsA to total AsA and with reduced activity of the AsA-GSH cycle enzymes. Supplying whole tomato seedlings with exogenous AsA enhanced the resistance of seedlings to salt stress and decreased lipid peroxidation (Shalata and Neumann 2001).
Like other stresses, GSH plays a protective role in salt tolerance by maintaining the redox state (May et al. 1998). Ruiz and Blumwald (2002) investigated the enzymatic pathways leading to GSH synthesis during the response to salt stress of wild-type and salt-tolerant Brassica napus L. (Canola) plants overexpressing a vacuolar Na+/H+ antiporter (Zhang et al. 2001). Wild-type plants showed a marked increase in the activity of enzymes associated with cysteine synthesis (the crucial step for assimilation of reduced sulfur into organic compounds such as glutathione) resulting in a significant increase in GSH content. On the other hand, these activities were unchanged in the transgenic salt-tolerant plants and their GSH content did not change with salt stress. These results clearly showed that salt stress induced an increase in the assimilation of sulfur and the biosynthesis of cysteine and GSH in order to mitigate salt-induced oxidative stress (Hussain et al. 2008). Jain et al. (2002) reported the increased total glutathione content by extreme salt treatment in groundnut cell lines. In addition, a comparison of various plant species revealed that salt tolerance was greater for those that had higher GSH content (Tepe and Harms 1995, Hossain et al. 2011; Hasanuzzaman et al. 2011). In addition, maintaining a high ratio of GSH/GSSG that functions as a redox couple was shown to play an important role in salt tolerance as observed in tomato, Myrothamnus flaberllifolia, and wheat (Shalata and Neumann 2001; Kocsy et al. Galiba 2002; Kranner et al. 2002). Studies carried out on several plant species subjected to various abiotic stresses indicate that a high GSH/GSSG ratio, maintained by increased GSH synthesis and/or GSSG reduction, may be necessary for efficient protection of plants against abiotic stress-induced accumulation of ROS. In maize, stress condition resulted in minor changes in the EGSSG/2GSH value, but there were substantial changes in the GSH concentration (Kocsy et al. 2004a; Kellös et al. 2008). The stress-induced changes in GSH levels and the GSH/ GSSG ratio may derive from altered GSH synthesis. After NaCl treatment the GSH, and GST activity was much greater in a tolerant cotton cell line than in a sensitive one, indicating adaptation at the level of GSH synthesis, GSSG reduction, and GS-conjugate formation (Gossett et al. 1996). The GSH/GSSG redox couple is involved in several physiologic processes in plants under both optimal and stress conditions. Sumithra et al. (2006) observed found the higher amount of GSH concentration in the leaves of Pusa Bold than in CO 4, whereas GSSG concentration was found to be higher in the leaves of CO 4 compared to those in Pusa Bold which indicated that Pusa Bold more tolerant that Co 4 as the levels of lipid peroxidation and H2O2 concentration in Pusa Bold was lower than in CO 4 under salt stress conditions. Gossett et al. (1996) reported that the salt tolerant cultivars had a higher AsA/DHA and GSH/GSSG ratio than the salt sensitive lines under saline conditions.
The activities of ROS scavenging enzymes are highly correlated with antioxidant defense and stress tolerance. These activities in fact, vary with the plant cultivars, stress duration and doses. The generation of ROS and increased activity of many antioxidant enzymes during salt stress have been reported in rapeseed (Hasanuzzaman et al. 2011), mungbean (Hossain et al. 2011), cotton (Desingh and Kanagaraj 2007), mulberry (Sudhakar et al. 2001; Harinasut et al. 2003), wheat (Sairam et al. 2002), tomato (Mittova et al. 2002), rice (Vaidyanathan et al., 2003) and sugar beet (Bor et al. 2003). In general, the activities of antioxidant enzymes were increased in the root and shoot under saline stress. But the increase was more significant and consistent in the root (Kim et al., 2005). The activities of antioxidant enzymes were increased in the root and shoot under saline stress in barley seedlings treated with 200 mM NaCl (Kim et al., 2005). Several studies have pointed out that salt-tolerant species increased their antioxidant enzyme activities and antioxidant contents in response to salt treatment, whereas salt-sensitive species failed to do so (Shalata et al. 2001; Demiral and Turkan 2005). The scavenging of ROS by increased activation of antioxidant enzymes can improve salt tolerance (Alscher et al. 2002). A relationship between salt tolerance and increased activation of antioxidant enzymes has been demonstrated in Plantago (Sekmen et al. 2007), pea (Hernandez et al. 2000), rice (Dionisio-Sese and Tobita 1998), tomato, maize (Neto et al. 2005), sorghum (Costa et al. 2005; Heidari 2009), soybean (Cicek and Cakirlar 2008) and mulberry (Harinasut et al. 2003).
The activities of the antioxidative enzymes such as CAT, APX, GPX, GR and SOD increase under salt stress in plants and a correlation of these enzyme levels and salt tolerance exists (Gossett et al., 1994; Hernandez et al., 1995, 2000; Sehmer et al., 1995; Kennedy and De Fillippis, 1999; Sreenivasulu et al., 2000; Benavides et al., 2000; Lee et al., 2001; Mittova et al., 2002, 2003). In soybean root nodules APX, CAT and GR activities decrease under salt stress, while SOD activity increased (Comba et al., 1998). Salt stress preferentially enhances the content of H2O2 and the activities of SOD, APX, and GPX, whereas it decreases CAT activity in rice leaves (Lee et al., 2001). On the other hand, salt stress has little effect on the activity levels of GR (Lee et al., 2001). Lechno et al. (1997) have reported that NaCl treatment increases the activities of the antioxidative enzymes CAT and GR and the content of the antioxidants ASA and GSH but does not affect the activity of SOD in cucumber plants. While studying with wheat seedlings, El-Bastawisy (2010) concluded that in wheat tolerance appeared to be related to the endogenous levels of the enzymatic and the non-enzymatic antioxidants. Among the three wheat cultivars (H 168, Gimmeza 7 and Beni swif 1) under observation, the activities of SOD, CAT, APX and GR as well as the non-enzymatic antioxidants (AsA and GSH) were mostly increased in H 168, but declined in Gimmeza 7 and particularly in Beni swif 1. It was indicated that that H 168 was superior with respect to its antioxidant defense systems and should be more tolerant to NaCl than the other two cultivars due to the higher enzymatic and non-enzymatic antioxidants.
In the leaves of the rice plant, salt stress preferentially enhanced the content of H2O2 as well as the activities of the SOD, APX and GPX, whereas it induced the decrease of CAT activity (Lee et al., 2001). The SOD activity in roots and GR activity in both shoots and roots were decreased significantly under high salinity levels in rice plant (Kumar and Shriram, 2009). It was suggested that the ratio between SOD and H2O2-scavenging enzyme activities could be used as a working hypothesis for a biochemical marker for salt tolerance (Costa et al. 2005). Salt stress was associated with an increase of the antioxidant enzyme response and induction of new CAT isoforms in rice (Srivalli et al. 2003). Salt-tolerant cotton cultivars exhibited significantly greater in CAT, APX and SOD activities as compared to the salt-sensitive ones (Gossett et al., 1994). Nagamiya et al. (2007) introduced katE, a CAT gene of Escherichia coli, into japonica rice cultivar and found that CAT activity in the transgenic rice plants was 1.5- to 2.5- fold higher than non-transgenic rice plants. Dionisio-Sese and Tobita (1998) showed the mechanism of damage by NaCl stress in leaves of four varieties of rice (Oryza sativa L.) exhibiting different sensitivities to NaCl. High salinity treatment in salt-sensitive varieties exhibited increase in lipid peroxidation accumulation in the leaves under salt stress. The salt-tolerant variety however, showed only slight increase is SOD activity, decreased POD activity, and virtually unchanged lipid peroxidation. Higher H2O2 accumulation and lipid peroxidation has been reported in salt stress sensitive rice varieties due to inefficient ROS-scavenging system. The antioxidant enzymes viz. SOD, CAT and GR are reported to increase under salt stress as well as comparatively higher activity has been reported in tolerant cultivars than the susceptible ones, suggesting that higher antioxidant enzymes activity have a role in imparting tolerance to these cultivars against environmental stresses (Sairam et al. 2002). In another study Vaidyanathan et al. (2003) investigated the immediate responses to salinity-induced oxidative stress in two major rice (Oryza sativa L.) cultivars, salt sensitive Pusa Basmati 1 (PB) and salt-tolerant Pokkali (PK). Seedlings of both cultivars were subjected to NaCl stress (100-300 mM) for 42 h. Under NaCl stress, the salt-tolerant cv. PK showed higher activity of the ROS scavenging enzyme, CAT and enhanced levels of antioxidants like AsA and GSH, than the sensitive cv. PB. Although SOD activity was lower in cv. PK, it showed lesser extent of lipid peroxidation and lower levels of H2O2 than cv. PB under stress. The high levels of CAT activity indicate efficient scavenging of H2O2, which is produced more by non-enzymatic means than via SOD in cv. PK. To compare the underlying mechanisms of antioxidant production in salt tolerant and salt sensitive plants, Dionisiosese and Tobita (1998) reported a decline in SOD activity and an increase in POX activity in the salt sensitive rice varieties, Hitomebore and IR28, in response to salt stress. These salt sensitive varieties also showed an increase in lipid peroxidation and electrolyte leakage as well as Na+ accumulation in the leaves under saline conditions. In contrast, two salt tolerant rice varieties, Pokkali and Bankat, showed differing protective mechanisms against activated oxygen species under salt stress. Cv. Pokkali showed only a slight increase in SOD but a slight decrease in peroxidase activity, and almost unchanged lipid peroxidation, electrolyte leakage and Na+ accumulation under saline conditions. In contrast, cv. Bankat showed Na+ accumulation in leaves and symptoms of oxidative damage similar to the salt sensitive cultivars. Hong et al. (2009) analysed the expression patterns of the gene family encoding GR in roots of etiolated rice (Oryza sativa L.) seedlings in response to NaCl stress. They demonstrated that NaCl increased the expression of OsGR2 and OsGR3. The time course analyses of NaCl treatment clearly indicated that the expression of OsGR2 and OsGR3 increased first (1 and 0.5 h, respectively, after NaCl treatment) and then GR activity increased (6 h after NaCl treatment) in rice roots. These results have led to the conclusion that early expression of OsGR2 and OsGR3 during NaCl treatment is associated with an enhancement in its GR activity.
In a study Mandhania et al. (2006) treated salt-tolerant and salt-sensitive wheat found that the salt-sensitive cultivar suffered greater damage to cellular membranes due to lipid peroxidation as indicated by higher accumulation of H2O2, MDA than tolerant cultivars. The activities of CAT, POD, APX and GR increased with increase in salt stress in both the cultivars, however, SOD activity declined. Upon desalanization, partial recovery in the activities of these enzymes was observed in salt-tolerant cultivar and very slow recovery in sensitive ones. In barley leaves, Hafsi et al. (2010) observed that H2O2 concentration was negatively correlated with the activities of antioxidative enzymes and the accumulation of non-enzymatic antioxidants implicated in its detoxification. Results suggest that a cooperative antioxidant defense system plays an important role for the tolerance of H. vulgare to salinity which maintains the H2O2 levels below the toxic limit. Similarly, Tewari et al. (2007) on Morus alba (cv. Kanva-2) observed no significant effect of potassium deprivation on H2O2 accumulation. H2O2, which is still toxic, can be efficiently removed by CAT, POX, APX and enhanced stress tolerance. However, Harinasut et al. (2003) observed that in mulberry plants, catalase activity with increasing salinity was not correlated with H2O2 content. Azooz et al. (2009) reported that the activity of antioxidant enzymes CAT, POD, APX and SOD in the salt-tolerant maize cultivars increased markedly during salinity stress, while they were mostly decreased by salinity stress in the salt sensitive cultivar. Consequently, this led to a marked difference in the behavior of lipid peroxidation in the three maize cultivars. The activities of antioxidant enzymes and lipid peroxidation were associated with the dry mass production and consequently with the salt tolerance of the cultivars. Salt-tolerant and salt-sensitive cultivar of foxtail millet (Setaria italica L.) responded different in terms of antioxidant enzymes activity under different NaCl concentrations was observed by Sreenivasulu et al. (2000). Under conditions of salt stress, the salt-tolerant cultivar exhibited increased total SOD and APX activity, whereas both enzyme activities decreased in acutely salt-stressed seedlings of the sensitive cultivar. These results indicated that salt-induced oxidative tolerance is conferred by an enhanced compartment-specific activity of the antioxidant enzymes. da Costa et al. (2005) hypothesized that the higher efficiency of the antioxidant-enzymatic system of the salt tolerant sorghum genotype could be considered as one of the factors responsible for its tolerance to salt stress. When the two genotypes were subjected to salt stress (75 mM NaCl) no significant change in SOD and CAT activities was noticed. These salt-induced increases were higher in the salt-tolerant genotype. POX activity was differentially affected by salt stress in the two genotypes. The activities POX were decreased by salt stress in the salt-sensitive genotype and increased in the salt-tolerant genotype.
In oilseed rape (Brassica napus L.), the increased activity of APX, MDHAR, DHAR, GR, GPX, GST and CAT was found as an efficient tools to enhance short-term salt-tolerance (Hasanuzzaman et al. 2011). In another study, the activity of POD, SOD and CAT was gradually increased in Brassica napus L. seedlings during 0- 24 h under 200 mmol l-1 NaCl stress, and after 24 h, the activities of these antioxidants were maximum and subsequently decreased (Dai et al. 2009). They indicated that the expressions of POD, SOD and CAT genes were induced by NaCl; the activities of POD, SOD and CAT were increased, which enhanced the tolerance of oilseed oilseed rape plants against NaCl stress. Hussain et al. (2008) reported that transgenic tobacco plants overexpressing both GST and GPX showed improved seed germination and seedling growth under stress (Roxas et al., 1997). Ruiz and Blumwald (2002) investigated the enzymatic pathways leading to glutathione synthesis during the response to salt stress of wild-type and salt-tolerant B. napus L. (Canola) plants overexpressing a vacuolar Na+/H+ antiporter (Zhang et al., 2001). In greengram (Phaseolus aureus), salt-tolerant cultivar showed high activities of ROS scavenging enzymes such as CAT, APX, GPX, GR and GST as well as enhanced levels of AsA and GSH, than the salt-sensitive cultivar. However, SOD, MDHAR and DHAR were lower in salt-tolerant varieties. The higher activities of these enzymes resulted a decrease in salinity-induced damage in tolerant cultivar of green gram. In mungbean seedlings, salt tolerance was closely correlated with the higher activities of AsA-GSH cycle enzymes including CAT and GPX (Hossain et al. 2011).
Shalata and Tal (1998) examined the possible roles of the antioxidant system in the salt tolerance of cultivated tomato and its wild salt tolerant relative Lycopersicon pennellii. and reported that in the latter species the constitutive level of lipid peroxidation and activities of CAT and GR were lower, whereas the activities of SOD, APX, and DHAR were inherently higher than those in the cultivated tomato species. Working with the same two species of tomato, Mittova et al. (2000) concluded that high salt tolerance of the wild salt tolerant species was due to maintenance of high SOD to ascorbate peroxidase activity. In another study, Mittova et al. (2002) found that compared with cultivated tomato (Lycopersicon esculentum), the better protection of wild salt tolerant tomato (L. pennellii) root plastids from salt induced oxidative stress was correlated with increased activities of SOD, APX and GPX. Recently, Chookhampaeng (2011) showed that the activity of CAT and POD in Pepper (Capsicum Annuum L.) Seedling was significantly increased by high concentration of NaCl (100 and 200 mM, for 18 days) compared with the control one. Tewari et al. (2007) on Morus alba (cv. Kanva-2) observed no significant effect of potassium deprivation on H2O2 accumulation. H2O2, which is still toxic, can be efficiently removed by CAT, POX and APX. Benavídes et al. (2000) studied the response of salt stress in sensitive and tolerant potato clone and observed a relationship between salt tolerance and the antioxidant defence system in the two clones. The antioxidant defence system of the sensitive clone responded differently to 100 and 150 mM NaCl. At 100 mM NaCl, growth, DHAR and CAT activities remained unaltered, but chlorophyll and reduced glutathione content decreased (23% and 35%, respectively), while AsA content and SOD activity were increased 34% and 63%, with respect to the control. The SOD increment was higher under 150 mM NaCl treatment, while a general decrease (except for DHAR and CAT activities) in all the antioxidant parameters studied was observed in the sensitive clone.
Although different studies have established that antioxidant defense system play a crucial role in salt-stress tolerance in plants, defining salt tolerance is quite difficult till now because of the complex nature of salt stress and the wide range of plant responses.
Salinization of underground water resource is another major problem affecting the agricultural productivity. It is very important to sustain the soil fertility and quality of water resources to fulfill the food, feed and fiber demand of ever-growing population of the world.
Sodium chloride is the most important constituent of saline environments. The accumulation of NaCl by plant cells for turgor regulation is limited by the toxicity of a high salt concentration. Such cytoplasmic Na+ toxicity is ubiquitous in all eucaryotes and bacteria. Chloride is the prevalent anion accompanying Na+ and K+, hence its concentration in vacuoles, as well as cytoplasm, is usually in the same range as the sum of Na+ and K+. This concurrence of Na+ and Cl- complicates the evaluation of Cl--speci¬c toxicity (Jacoby 1999).
Earth is a salty planet because its water is enriched with 30 g of sodium chloride per liter, so this salt solution affects the land as well as crops. Salt stress causes altered K+/Na+ ratios and Na+ and Cl- ion concentrations that are harmful to plants.
Plant responses to salinity:
The effects of salinity arise from osmotic and ionic toxicity. Plant metabolism should be ¬‚exible to allow plants to cope with environmental stresses.
It is surprisingly difficult to quantify differences in salt tolerance between closely related species, as the growth reduction depends on the period of time over which the plants have grown in saline conditions. During a short time in salinity, there will be a significant decrease in growth rate, but the decrease may be the same for species that have quite different reputations for salt tolerance (Munns 2002).
Salt stress, like other abiotic stresses, can lead to oxidative stress through the increase in reactive oxygen species (ROS), such as superoxide (O2-), hydrogen peroxide (H2O2) and hydroxyl radicals (OH•),
Generally, salt stress is caused by high Na+ and Cl-. Salt stress has threefold effects; viz. it reduces water potential and causes ion imbalance or disturbances in ion homeostasis and toxicity (Parida and Das 2005).
Antioxidant defense under salinity stress in plants:
The higher lipoxygenase and antioxidant enzyme activities such as SOD, catalase, ascorbate peroxidase, glutathione reductase, and GST are observed in tomato under NaCl stress (RodriguezRosales et al., 1999).
According to Hernandez et al. (2000), activities of antioxidative enzymes such as APX, GR, MDHAR, DHAR, and Mn-SOD increase under salt stress in wheat, while Cu/Zn-SOD remains constant and total ascorbate and glutathione content decrease.
Substantive proofs to this argument came from the fact that a recessive deletion mutant of Arabidopsis showing higher activities of SOD and APX was salt tolerant compared with salt sensitive wild-type (Tsugane et al., 1999). In other studies, over-production of glutathione reductase (GSH) and APX improved oxidative stress and salt tolerance of wheat (Sairam et al., 1998).
In Calendula officinalis, both roots and leaves the total ascorbate content decreased significantly under low salinity, while it increased under high salinity (Chaparzadeh et al. 2004). The AsA/DHA ratios showed a different behaviour in leaves and roots. In the leaves it increased at low salinity and decreased at high salinity, while in the roots only an increase at high salinity was observed (Chaparzadeh et al. 2004).
Hernandez et al. (1998) observed that antioxidant enzyme's activities in pea plants (Pisum sativum cv. Puget) varied with the extent of salt stress. In their study, the lowest NaCl concentration (70 mol m−3) had no effect on the activity of these antioxidant enzymes while higher concentrations (110-130 mol m−3) enhanced the activity of cytosolic CuZn-SOD I and chloroplastic CuZn-SOD II as well as that of mitochondrial and}or peroxisomal manganese-containing superoxide dismutase (Mn-SOD). These inductions were matched by increases in the activity of ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR). Glutathione reductase (GR) and dehydroascorbate reductase (DHAR) activities were only induced under severe NaCl stress (130-160 mol m−3) and were accompanied by losses in the ascorbate and glutathione pools, lower ASC/DHA and GSH/GSSG ratios and increases in GSSG.
Chaparzadeh et al. (2004) reported that under high salinity stress (100 mM for 3 weeks), a decrease in total glutathione and an increase in total ascorbate (AsA + DHA), accompanied with enhanced GR and APX activities, were observed in leaves. In addition, salinity induced a decrease in superoxide dismutase SOD and POX activities. The decrease in DHAR and MDHAR activities suggests that other mechanisms play a major role in the regeneration of reduced ascorbate. The changes in CAT activities, both in roots and in leaves, may be important in H2O2 homeostasis.
In cotton seedlings (cv. Arya-Anubam and LRA-5166), the activities of key antioxidative enzymes, superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) was significantly increased due to salt stress (0, 50, 100 and 150 mM salt for a duration of 30 days). However, plants of variety Arya-Anubam exhibited higher adaptive potential under salinity stress as judged by increased activities of photosynthetic and antioxidative enzymes as well as higher accumulation of proline and glycine betaine when compared to variety LRA-5166 (Desingh and Kanagaraj, 2007).
However, The plants were treated with low concentration of NaCl (50 mM , for 18 days), POD activity was significantly decreased in both of shoots and roots.
GapiÅ„ska et al. (2008) observed a clear differences in the activities of antioxidative enzymes under short- and long-term salinity in tomato seedlings. The severe stress caused an increase in GST, GSH-Px and SODs activities from the beginning of the experiment while mild stress induced enhancement of GST activity from the second day of experiment. The maximum increase in SODs after both NaCl solutions were applied and in GST activity after the higher NaCl dose on the second day of the experiment was observed. Moreover, after 1 h of NaCl treatment with both tested NaCl solutions, the highest induction of GSH-Px activity appeared. They also indicated that enhanced activities of tested enzymes indicate their involvement in early and late defence systems under salinity stress. Moreover, the dynamics of the changes in the antioxidant enzymes suggests that the second day following NaCl application is a crucial moment of the experiment with regard to salt mediated oxidative stress.
According to Merata et al. (2008), A. glandulosum (hexaploid species) showed a better protection mechanism against salinity induced oxidative damage than A. sordidum (tetraploid species). The differences in the antioxidant enzyme activities of seedling may, at least in part explain the greater tolerance of A. glandulosum comparing to A. sordidum and A. laxiusculum. A. laxiusculum exhibited a decrease in POX and polyphenol oxidase (PPO) under NaCl stress; while A. glandulosum showed a remarkable increase in POX and PPO between 50 to 200 mM NaCl. In A. sordidum, POX and PPO activities increased at 50 mM NaCl and then decreased at higher salinities.
The plant response to salinity consists of numerous processes that must function in coordination to alleviate both cellular hyperosmolarity and ion disequilibrium (Yokoi et al. 2002). In recent years, the biochemical responses of plants to salt stress have been studied intensively. Information on the tolerance mechanism is useful for developing new cultivars that are adaptable in salinity environments although defining salt tolerance is quite difficult because of the complex nature of salt stress and the wide range of plant responses.