*Achieving Salinity-Tolerance in Cereal Crops: Major Insights into Genomics-Assisted Breeding… DOI: http://dx.doi.org/10.5772/intechopen.112570*

To grow and reproduce in high salt stress conditions plants had developed several mechanisms (**Figure 2**) these mechanisms can be categorized into three steps [25]; (i) ions exclusion, it regulates Na<sup>+</sup> and Cl− uptake. it prevent the accumulation of toxic ions into leaves (ii) osmotic stress tolerance, limits the growth of stems which is controlled by long-distance signals and is activated prior shoot Na+ accumulation; and (iii) tissue tolerance, tolerance power of tissues against accumulated Na<sup>+</sup> or Cl− . When Na+ or Cl− ions get succeeded to enter inside the plant tissue it grouped in leaf vacuole to prevent it from salt injury of thylakoid membrane.

### *2.1.2.1 Ion exclusion*

Intercellular compartmentalization of Na<sup>+</sup> ion is a main tolerance mechanism, which provides the ability to leaves to bear with high Na<sup>+</sup> concentration (**Figure 3**). Salt stress imbalance the ion ratio by altering the pathway of sodium intake in place of potassium acquisition. There are four mechanisms for Na<sup>+</sup> exclusion [26]; (i) Selective permeability of ions in cortex and stele by root cells, (ii) Stocking of xylem in root by xylem parenchyma cells, (iii) Elimination of salt from the stem by xylem parenchyma cells and, (iv) phloem stacking. Salt entering through root system can be excluded, or inside the plant system salt can be barred from entering sensitive organs. Most of the plant species grown under salt stress, Na<sup>+</sup> ion competes with Cl<sup>−</sup> to reach a concentration of toxic level. Therefore, the main focus of the researcher is on Na<sup>+</sup> exclusion and transport within plant. Na<sup>+</sup> exclusion by roots regulates toxicity level of Na<sup>+</sup> within leaf blade; however, fails to do that induce Na<sup>+</sup> toxicity after a short or long period, and cause death of older leaves. Exclusion of Na<sup>+</sup> at root level occurs as a result of ion selectivity. A mechanism that operates for uptake of Na<sup>+</sup> and K<sup>+</sup> ions reported earlier by Schachtman and Schroeder [27]. Salt tolerance in many species is associated with a high concentration of K<sup>+</sup> in young expanding leaf tissues. So it clearly shows the possibility of association of Na<sup>+</sup> /K<sup>+</sup>

with salt tolerance. The control of Na<sup>+</sup> uptake and better maintenance of the K+/ Na<sup>+</sup> ratio can be considered as key cellular mechanism to maintain osmotic potential for optimum cell activities, which contributes provides better adaptation capacity in plant under stress conditions [28, 29].

### *2.1.2.2 Tolerance to osmotic stress*

Osmotic tolerance is reducing the osmotic potential due to solute accumulation in response to water stress, plays key role in plant adaptation to dehydration by maintaining turgor pressure, relative water content, and high stomatal conductance [30]. The osmotic effect is measured by growth rate and stomatal conductance of plants. Proline is a widely distributed osmolyte which protects the plant cells against salt stress. Proline acts as osmolyte that protects subcellular structure and biomolecules and chelate metal ions under osmotic stress [31]. In drought conditions, the delayed stomatal closure caused mainly by higher osmotic adjustment led to elevated assimilation rate and assimilates production [32]. In addition, increase in polyphenol content, glycine betaine and antioxidant enzymes activities have been reported associated with stress tolerance in plants providing protection against reactive oxygen species (ROS) [33]. All the possible mechanisms of salt tolerance operate at cellular levels in plants are depicted in **Figure 3**.

### *2.1.2.3 Tissue tolerance*

Third mechanism, as tissue tolerance is increases the shelf-life of older leaves, where it distributes Na<sup>+</sup> and Cl− at intercellular level to remove toxic levels of ions from cytoplasm and leaf mesophyll cells and to avoid detrimental effect on cellular process [25]. It synthesizes compatible solutes and controls transport and biochemical processes in cytoplasm and thus has dual function as osmoprotectant and osmotic adjustment (lowering of osmotic potential) [34]. These compatible solutes regulate osmotic tolerance in plants through different pathways such as it protects enzyme to get denature, stabilize plasma membrane and regulate macromolecules by osmotic adjustment [35].
