**4. Screening and development of salt tolerant cultivars**

The rice germplasm with diverse and significant varietal differences shows a pool for developing screening techniques. This is important technique for to select varieties for breeding purposes. This techniques are economically feasible, easy and efficient. Rice expresses highly sensitive in early seedling stage and low sensitive in reproductive or late vegetative stage [19]. The continues accumulation of NaCl in older leaves through long exposure [81] which affect the efficiency of photosynthesis and whole plant metabolism [82]. Tolerance in vegetative and reproductive stage of the rice was expressed in seedling stage [83]. So, precise screening method is necessary to identify the salt tolerance line. Seedling stage screening is acceptable and it is rapid screening but it is difficult in vegetative and reproductive stage [4]. From salinity screening, highly tolerant line was identified such as pokkali which is used as a donor for further breeding program of salt tolerance [84].

#### **4.1 Phenotypic and genotypic screening**

Salinity tolerance is made as complex in physiologically and genetically [85]. It requires more time and expensive tissue analysis. In early seedling stage of phenotypic screening done in first, second and growing leaves. The phenotypic screening in early seedling to select first, then second and growing leaves. Where salinity occur leaf elongation and new leaf formation is suppressed [86]. In phenotypic screening, for identification of potential rice lines, first step is to screen the already available germplasms of rice. Field level phenotypic screening is difficult because it contain soil heterogeneity, climatic factors and other environmental factors which affect the physiological functions. But, phenotypic screening under laboratory or green house (hydroponics) is better merits than the field screening because it was not affected by the environment and soil factors such as temperature, relative humidity, and solar radiation. In hydroponic phenotypic screening, the seedlings are grown in salinized nutrient solution and where salinity inversely proportional to photosynthesis and chlorophyll content [87]. The vegetative and reproductive stage screening is difficult because decrease in plant height, root length and biomass. In reproductive stage, sterility of florets occurs due to the effect of reduced panicle length, number of primary branches and spikelets per panicle, fertility and panicle weight thus reducing grain yield [88]. It is a rapid, perfect and easy method. Phenotypic screening for salt tolerance is not easy because of environmental effects which hinders the development of accurate and reliable screening technique. Salt tolerance screening at early stage is not correlate with further stages [89]. The

germplasm are evaluated by using the high-throughput phenotyping saves time and resources compared to traditional phenotyping methods [90].

Among molecular research tools, quantitative traits loci are useful to study the genotypic of salinity tolerance. These QTL techniques useful in the markerassisted selection [85]. Saltol is a major QTL which is accounted more than 70% of the variation in salt uptake and these QTLs are incorporated into high yielding varieties by Marker Assisted Backcrossing [91]. While transfer of salt QTL, some of the unwanted traits of linkage drag transferred which is drawback in MABB and genotypic screening [92]. In genotypic screening, the use of molecular markers such as RFLP (Restriction Fragment Length Polymorphism), RAPD (Random Amplified Polymorphic DNA), ISSR (Inter Simple Sequence Repeats), SSR (Simple Sequence Repeats) and AFLP (Amplified Fragment Length Polymorphism) for screening of germplasm is more reliable than the phenotypic screening [93]. Among the above molecular markers, microsatellite markers to be more effective [79].

#### **4.2 Approaches for the development of salt tolerant cultivars**

The salt tolerant cultivars are developed by using several ways such as, use interspecific hybridization to raise the tolerance of current crops, use the variation already present in existing crops, generate variation within existing crops by using recurrent selection, mutagenesis or tissue culture and breed for yield rather than tolerance [85]. To improve salinity in rice genotypes by the incorporation of salt tolerance genes into high yielding cultivars.

#### *4.2.1 Conventional breeding*

In conventional Plant breeding methods, different approaches are used for development of salt tolerance cultivar such as introduction, selection, hybridization, mutation and shuttle breeding approach. The beginning the salt tolerant rice cultivars (Damodar (CSR 1), Dasal (CSR 2), and Getu (CSR 3)) are developed from pure line selection from the local traditional cultivars. In segregating population, Selection pressure is gradually increased with the generation advancement simultaneously in moderate stress and high stress of sodicity and salinity [94].

In conventional plant breeding, the new and better variety was developed by the combining the different parents genes. In this breeding method, first to generate a breeding population with highly variable and identifying parents for traits of interest. Several conventional breeding programmes are involved in to developing salt tolerance lines such as in vitro selection, pooling physiological traits, interspecific hybridization, using halophytes as alternative crops [85]. Through conventional plant breeding method, some varieties such as CSR10, CSR13, CSR27, Narendra usar 2 and Narendra usar 3 were developed and released as salt tolerant for cultivation [95]. In development of salt tolerance cultivar through conventional breeding is require more time and it depends on the environmental factors. It contains very limited success, due to the complexity of the trait.

#### *4.2.2 Genomics based approaches*

In Marker Assisted Selection (MAS), some important strategies are followed such as markers are tightly linked to loci and specific marker alleles are associated with desired alleles at target loci consistently across the different breeding populations [94].

The conventional back cross breeding method drawbacks are overcome by usage of Marker Assisted Backcrossing Method (MABB). Young and Tanksley [96], demonstrated that large amount of DNA from the donor can remain around the target

*Understanding the Responses, Mechanism and Development of Salinity Stress Tolerant Cultivars… DOI: http://dx.doi.org/10.5772/intechopen.99233*

gene even after many generation of backcrossing. In this method, through three important approaches used to reduce the linkage drag such as foreground selection, recombinant selection and background selection.

By using the genomic approaches, the genetic map and genetic diversity in germplasm which is done by usage of the molecular markers [97]. The most efficient markers to screen the salt tolerant genotypes were SSR (Simple Sequence Repeats) markers RM8094, RM336 and RM8046 which were contain higher polymorphic information content coupled with higher marker index value [98]. Two EST markers such as CP3970 and CP6224 and Five SSR markers such as RM1287, RM8094, RM3412, RM493 and RM140 were linked to saltol QTL on chromosome 1 [99]. In salt tolerance rice, transgenic approaches are attempted. Hoshida et al. [100] have reported the transgenic salt tolerance rice by over expresses chloroplast glutamine synthetase gene. These transgenic plants shoots contain more accumulated K+ , Ca2+, Mg2+ and less Na+ compared with those of non-transformed controls [101, 102]. Xujun Chen and Zejian Guo [103] recorded that tobacco OPBP1 increase the salt tolerance and disease resistance in transgenic rice.
