*3.1.1 Ion exclusion, inclusion and compartmentation*

Ion exclusion from salt sensitive organs, inclusion in less sensitive locals such as the root and old leaves and organ compartmentation strategies used for decreasing the damaging ion-specific effects of Na and nutrient deficiencies. If the cytoplasm contain excess ions, it will transported across the tonoplast by the Na/H antiporter and compartmentalized in vacuoles which is useful for protect the plant from salinity [47]. Through the exclusion strategies the salts are removed from aerial part by saline vesicle glands in the epidermis which prevent accumulation and transported to roots which is mostly occur in halophytes and glycophytes. Salt sequestering in old leaves which will give salt-induced senescence [48]. When compared to synthesis of organic osmolytes, the ion compartmentation is low cost effective [49].

#### *3.1.2 Antioxidant responses*

The oxidative stress occur when plants under the salinity [50]. Under salinity stress, to induce increase the reactive oxygen species such as superoxide radicals and hydrogen peroxide [51]. The antioxidative system is response to the salinity exposure [50]. The chloroplasts and mitochondria are play role in the salt tolerance with the increased antioxidant defenses. The salt sensitive plants express in decreased antioxidant level [52]. Nitric oxide (NO) response to salinity tolerance with the NO donor [53].

#### *3.1.3 Hormonal regulation*

The plant hormones such as ABA (Abscisic Acid) and cytokines are increased when plant under the salt stress [54]. The negative effects of salinity such as plant growth, photosynthesis and translocation of assimilates can be reduced by the highly accumulation of hormones. The ABA also involved in compatible solutes and nutritional cations K<sup>+</sup> and Ca2+ in vacuoles of roots for salt tolerance [55].

#### **3.2 Response of rice under salt stress**

Based on the salinity responses, the plants were classified into halophytes and glycophytes. The halophytes plants tolerate to high concentration of NaCl (400 mM) when compared to glycophytes [56]. In rice, the salinity tolerance is controlled by multiple gene [57] and therefore understanding the plant responses for salinity is very much important for developing tolerant cultivars. Under salt stress condition, the rice plants exposure to different responses such as morphological, biochemical, physiological, molecular response.

#### *3.2.1 Morphological response*

Due to salinity stress, the occurrence of morphological changes such as stunted plant growth, leaf burning, chlorosis, low tillering, leaf rolling and poor root growth [58]. Decreased leaf area and changes in leaf anatomy under *invitro* condition was observed by Bahaji et al. [59]. For example, comparable levels of osmolality, the reduction in root and leaf growth were similar for both saline

and osmotically-generated stress. Most of the variations in leaf anatomy features caused by the treatments could be ascribed to osmotic stress [59]. The beginning of salt stress, there is no symptoms are observed but shoot and root growth reduction occur. When plants continues exposure to salt condition, leaf senescence occur. After 3-4 days of exposure under salinity, the plants began to develop leaf symptoms such as yellowing and necrotic lesions of old leaf tips. The senescence of older leaves was observed after two weeks of stress. In Nipponbare, root and shoot growth was affected by salinity [60].

#### *3.2.2 Biochemical response*

Based on the biochemical response, the effect of salinity in plants which leads into two parts such as initial osmotic effect and later ionic stress (where accumulation salt at toxic level) [5]. The plants express some of the biochemical responses such as oxidative stress, altered metabolism, high Na+ transport to shoot, lower K+ uptake and low P and Zn uptake [61]. In initial osmotic effect, water potential is decreased and increased the osmotic potential due to the increased concentration of salts. The salt stressed plants contains the larger amount of proline in higher plants [62] and it act as a osmotic adjustment, shielding the enzymes, membranes and give the energy and nitrogen during salinity [63]. Soluble sugars and starch also responses to salinity as an osmoticum in plants [64]. When rice plants exposure to salinity, sugar content increased in shoot [65] and starch content increased in root, which act as a reservoir for the primary metabolism [66]. Where plants exposure to the salinity, the proteins are synthesized and accumulate as a storage food which is used as a reservoir during salt stress condition and reutilized when absence of stress [67]. The increased protein content is positively correlated to rice seedling tolerance than the sensitive one [68].

#### *3.2.3 Physiological response*

When plants under salinity, they express some of the physiological responses such as inhibition of photosynthesis, stomatal closure, decreased water content, higher amount of osmolytes and low osmotic potential. The response of rice to salinity, to study the physiological mechanism and it was associated with the plant defense mechanism activated during stress. During salinity, chloroplast and mitochondria are mostly affected compared to other organs [69]. In chloroplast, some of the potential indicators show the effects in the photosynthesis efficiency such as changes in chlorophyll fluorescence and membrane permeability [70]. Salinity affects the mesophyll tissue which leads to affect the vascular bundles. The more accumulation of sodium salts is excited by salt exclusion [71], selective ion uptake [72] and regulation of K+/ Na + ratio [73]. Estimating the different plant parameters such as tiller number, leaf area, panicle length, root length, biomass, dry weight, RGR (Relative Growth Rate) and RWC (Relative Water Content) Zeng et al. [89] from different cultivars, leaf RWC is increased in paddy under salinity and suggested the role of osmo-protectants in preventing cell injury from salt stress-induced dehydration [74].

#### *3.2.4 Molecular response*

In molecular response, the main aim is to breed and to develop the salinity tolerance lines. Genetic diversity is a primary work which is used to screen the lines with the various molecular markers such as RFLP, SSLP, RAPD and SSR markers. Salinity is controlled by several genes and inheritance of salinity trait is difficult in rice. These difficulties are overcome by using the positional cloning [75] and insertional mutagenesis [76]. Many genes are identified for the salinity [77]. In rice under stress *Understanding the Responses, Mechanism and Development of Salinity Stress Tolerant Cultivars… DOI: http://dx.doi.org/10.5772/intechopen.99233*

condition some of the genes were identified such as catalase and several denovo genes. The salinity tolerance controlled by major Quantitative Trait Loci (QTL) is *Saltol* which is mapped in the chromosome 1 of the FL478 Recombinant inbred Line (RIL) line. The *saltol* linked with the flanking markers RM1287 and RM6711 and these QTL region contain 15 SSR markers. The FL478 obtained from crossing between Pokkali and IR29. The *saltol* QTL responsible for the maintain the low Na+, high K+ and Na+/K+ homeostasis in shoots of rice. The *saltol* QTL can transfer into superior cultivar and these transformation confirmed by the candidate gene approaches or Marker Assisted Selection (MAS) [78]. Among the molecular marker analysis, the SSR marker is effective for salt tolerance identification in rice [79]. Several QTLs were identified for sodium uptake, potassium uptake, and sodium: potassium selectivity [80]. The molecular markers are used to identified the QTLs and it gives new platform for salinity study.
