REVIEW OF LITERATURE
2.5 Major constrains imposed by salinity stress
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(Chan, 2001). For instance, in India wild regions of former forests become extremely saline in a few years after the removal of the trees (Pessarakli, 1994).
2.4.4 Contamination with Chemicals
Although the quantity of chemical fertilizers, which are used in agriculture, is low compared with the amount of salt in some soils, they have also been considered year after year as a major source of salinity which occurs due to intensive agricultural production, especially in greenhouses where chemical fertilizers are often heavily used. Sewage sludge and industrial emissions can increase the accumulation of some ions causing saline soil which leads to a reduction in the productivity of soil (Pessarakli, 1994; Chhabra, 1996; Bond, 1998).
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the physiological mechanisms for the salinity tolerance of plants (Munns and Tester, 2008). Moreover, Physiological and biochemical processes can be impacted by both osmotic and ionic stresses (Alamgir et al., 2007; Azooz, 2009). In the root zone excess soluble salts shown adverse effects on plant development and yield through nutritional disproportion, osmotic effects and particular ion toxicities (Munns, 2005;
Tahir et al., 2006). Basant et al. (2018), found that salt stress conditions (0, 4 and 8 dSm-1) have a significant decrease in plant height, leaf area, number of effective tillers per plant, number of spikes per plant.
2.5.1 Osmotic stress
The osmotic stress helps to water uptake of plants can be limited by salinity due to a reduction in the osmotic potential of the growth medium (Dixit and Chen, 2010). The osmotic pressure of soil increases by the increase of soluble salts which affect the ability of plants to absorb sufficient amounts of water (Epstein, 1980). Tavili et al.
(2011) observed that the accumulation of salt concentration in the soil results in a reduction in moisture potential, which impacts moisture accessible. Water shortage or osmotic effects are probably the main physiological mechanisms for growth reduction as salinity stress reduces the soil water potential. When salts are accumulated around the root zone, the osmotic pressure will be increased to the threshold level and plants will be affected straight and leaf and shoot development ratio drastically decline (Epstein and Bloom, 2005). The emergence rate of new leaves will be slower than usual and the development of lateral buds will also be slow or will stay dormant. These effects are a result of the osmotic impact of the accumulation of ions in the rooting zone. Subsequently, the reduced rate of photosynthesis (caused by stomata closure) increases the formation of ROS, and increases the activity of enzymes that detoxify these species.
In the osmotic effect, immediately shrink cell expansion in root tips and juvenile leaves, and causes stomatal closure (Munns and Tester, 2008). Normally the most rapid response by plants to osmotic stress, which is presented due to salinity or increased drought, is to reduce the consumption of water by closing stomata or decreasing the leaf surface area. However, these mechanisms may affect the exchange of gases and the ability of the leaf to reduce the temperature of the plant caused by transpiration processes. Also long periods of osmotic stress lead to the extension of the roots to attain deeper soil moisture and transfer it to the plant
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(Epstein and Bloom, 2005). The number of tillers in the whole leaf area is heavily influenced by the increase of salinity in cereal crops (Munns and Tester, 2008). The reduction in water uptake by the plant from the soil is due to the decrease in leaf production, which leads to the retention of moisture in the soil, preventing salt uptake. In addition, the evapotranspiration process can also be affected by the increase of salinity (Chhabra, 1996). This effect is due to the decrease in the availability of water as a result of the decrease in osmotic potential, reduction in leaf area and higher maintenance of water in the plant to reduce the absorbed salts (Chhabra, 1996).
2.5.2 Ionic Toxicity
Na+ appears for the most species, to be the vital toxic ion as it could reach a toxic level premature than Cl- does (Munns and Tester, 2008). When the concentration of salts in the old leaves rise to toxic levels this causes the death of the old leaves.
Moud and Maghsoudi (2008) reported that when the rate of transpiration is high, the salt will be concentrated in the leaf causing it to die. Moreover, Neumann (1997) reported that when ions are accumulated in the transpiring leaves (old leaves), leaf senescence and necrosis are accelerated. Consequently, the provision of carbohydrates and hormones are reduced. Munns and Tester (2008) mentioned that in low and moderate concentrations of salinity, the ionic effect has less effect on the growth than the osmotic effect. Ion toxicity may affect the cell membrane. As the regulation of the exchange of materials between the cell and the surrounded environment is an important function of the cell membrane, the accumulation of salts leads to the destruction of the membrane structure and the replacement of Ca2+ by Na+ at the binding sites (Jacoby, 1994). This effect on membrane structure produces enhanced membrane permeability and ion leakage from the cell (Bewley and Black, 1994). Limitation of Na+ uptake, segregation of Na+ into vacuoles active Na+ exclusion back to the soil solution and to maintain cellular homeostasis, control of xylem Na+ loading and its extraction from xylem are the main implementation for salt tolerance at the cellular level (Munns and Tester, 2008; Shabala and Cuin, 2008;
Maathuis, 2014). A high accumulation of ions in the growth medium or in the plant itself may cause toxicity to the plant. Therefore, the normal growth of plants will be affected (Chhabra, 1996). It has been reported that Ca2+ decreases the effect of salinity on membrane integrity (Easterwood, 2002; Iqbal, 2005; Afzal et al., 2008)
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by maintaining the selectivity of K+ : Na+ ratio (Royo and Abio, 2003; Gobinathan et al., 2009).
Active transport of Na+ out of cytoplasm across the plasma membrane is mostly mediated by the Na+ /H+ antiporters (Barrett-Lennard and Shabala, 2013; Maathuis, 2014). The salt overly sensitive (SOS1) gene can affect the long-distance Na+ transport by controlling Na+ concentrations in xylem sap by unloading Na+ from or loading Na+ into xylem vessels depending on the severity of salinity stress (Guo et al., 2009). Transporters NHXs mediate both Na+ /H+ and K+ /H+ exchange and therefore affect both salinity tolerance and K+ nutrition (Leidi et al., 2010). The high- affinity K+ transporters (HKTs) were first isolated from wheat (Schachtman and Schroeder, 1994). High-affinity K+ transporters can also play an important role in Na+ translocation to the shoots and Na+ accumulation in leaves by contributing to Na+ unloading from the arising xylem sap and favouring Na+ recirculation from leaves surface to roots (Flower and Yeo, 1986; Mian et al., 2011). The up-regulation of the high-affinity K+ transport activity is highly correlated with a decrease in leaf Na+ content (Plett et al., 2010).
2.5.3 Cytosolic K+ homeostasis
Nowadays, the third major obstruction of salt induces stress, namely its distraction to cytosolic K+ homeostasis was uncover (Shabala and Cuin, 2008). As a crucial macronutrient K+ is essential for a varying cellular activity like osmotic regulation, maintenance of membrane potential, enzyme activity, synthesis of protein and starch, respiration and photosynthesis (Schachtman and Schroeder, 1994). Ion transport across the plasma and intra-organelle membranes K+ is essential as a counter ion for the charge balance (Anschutz et al., 2014; Dreyer and Uozumi, 2011). High cytosolic K+ levels are also essential to suppress the activity of caspase-like proteases and endonucleases, thus reduce the cell risk in transition to PCD (Bortner et al., 1997). A strong positive correlation between the ability of plant roots to retain K+ and salinity tolerance was revealed in a wide range of crop species in barley (Chen et al., 2007), wheat (Cuin et al., 2008). In wheat, these K+ retention capabilities give out to 60% of genetic variance in salinity stress tolerance (Anschutz et al., 201; Cuin et al., 2012).
Since excessive Na+ ions repress various important cellular processes many of which are directly correlated with K+ transport and essential functions of K+, it is well recognized that K+ alleviates the toxic effects of Na+, and that a high K+ / Na+ ratio in
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shoots, especially in leaves, is important in glycophytes for enhanced salinity tolerance (Hauser and Horie, 2010). It was also shown that the supply of K+ fertilizers has a beneficial impact on plant performance under salinity stress (Siringam et al., 2013; Umar et al., 2011). Massive K+ leak from cytosol has been observed under saline conditions. It is suggested that outward-rectifying depolarization activated K+ channels (GORK in Arabidopsis) are the main pathway for the salinity induced K+ efflux from the cytosol (Shabala and Cuin, 2008). An important factor contributing to a massive K+ leak is ROS. In salt-stressed plants ROS levels are known to be much higher (Jacoby et al., 2011). Several types of K+ permeable channels, including non-selective ion channels (NSCC) (Demidchik, 2014) and GORK (Demidchik et al., 2010) are activated by ROS, providing an additional (independent of membrane depolarization) pathway for K+ leakage from the cytosol and even the vacuoles (Demidchik, 2014). If the K+ leakage process takes too long, the vascular K+ pool is depleted and the cell collapses. Restricting the Na accumulation or avert the K+ loss from the cell can be maintained by the optimal Na+ /K+ ratio (Shabala and Cuin, 2008), and Na+ /K+ ratio has been considered as a coherent indicative index for salt induce stress tolerance.
2.5.4 Pernicious effects of ROS
Plant’s have the ability to repair ROS-influence damage to essential cellular structures has also been contemplated as a lead attribute of salt stress tolerance principally in halophytes, which are converted to elevated saline soils (Maksimovic et al., 2013; You and Chan, 2015). A comparison of photosynthetic response between the halophyte Thellungiella salsuginea and the glycophyte Arabidopsis thalina during salt stress revealed that electron transport through PSII and the activity of plastid terminal oxidase protein (diverting up to 30% of total PSII electron flow to O2) is substantially increased whereas they are inhibited in Arabidopsis (Stepien and Johnson, 2009), suggesting that alternative electron sinks have the potential to decrease salt-induced ROS production in halophytes (Bose et al., 2014).
Additionally, the production of ROS is critically dependent on K+ accessibility and Increases in the severity of K+ deficiency were also associated with enhanced activity of enzymes involved in the detoxification of H2O2 and utilization of H2O2 in oxidative processes, and K+ deficiency also caused an increase in NADPH-dependent O2•– generation in root cells (Cakmak, 2005). Thus, increasing the K+ nutritional
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supply can reduce the detrimental build-up of ROS either by enhancing photosynthetic electron transport or by inhibiting the membrane-bound NADPH oxidases and lead to enhanced salinity tolerance (Shabala and Pottosin, 2014).