**1. Introduction**

Rice (*Oryza Sativa)* is a diploid (2n = 12) belongs to the Poaceae family (Umed [1]). It is an important cereal food crop cultivated and consumed across the world population and most of the Asiatic peoples. Rice is a staple food crop and gives 50 to 80% calories to the more than three billion peoples (Umed [1]). The global rice production needs to be increased from 560 million tonnes to 850 million tonnes by 2025 to meet the growing demand of rice [2]. Rice cultivation is continuously threatened by several biotic and abiotic stresses [3]. Salinity is the major soil problem in rice cultivation especially saline prone coastal areas and it is a second most abiotic problem than the drought [4]. Globally, 800 million ha area affected by salt accumulation, which is about 12% of the world land and 20% of the cultivated land [5]. The 1500 million ha of dry land farming and 230 million ha of irrigated farming comes under saline condition and 33% of agricultural land affected with high salinity [6]. In earth surface, the enormous amount of water is available but all the water is not useful for to survival of plants and animals, because 72% of water available as a sea water [7].

Rice is highly is sensitive to salinity at early vegetative and late vegetative stages [8, 9]. Increasing salinity (22 dS m−1) affects the seed germination mainly due to the osmatic stress [10, 11]. It reduces the photosynthetic activity, chlorophyll content, leaf area and stomata where high level of salinity occurs [12]. Yield components such as panicle length, spikelet number per panicle and grain yield were also affected; and panicle emergence and flowering affect the seed set through pollen viability [13].

The present review to show the constrains of the salinity to the rice growth and development and provide the information about the plants responses against the salinity and what are the different approaches are used to develop the salt tolerance rice cultivar.

## **2. Salinity -abiotic constraint devastating agricultural production**

Among abiotic stresses, the salinity which adversely affects the crop growth, development and production [14]. The deposition of salts or salinization is a natural phenomenon with evaporation of saline underground water, sea water infiltration of coastal ground waters, sea water salts in wind and rain and human initiated process such as irrigation with marginal water and poor agronomic practices. Salinity affected almost the 900 M ha of land [15]. In total global cultivated land, 23% contain saline and 37% contain sodic soils and 2.5x108 ha of global irrigated lands affected by salinity and water logging. The salinity mostly occurs in the south Asia and Southeast Asia which contributes the 90% of the world's rice production [16, 17]. The abundant accumulation of dissolved salts in soil and water which affect the plant growth that leads to decrease the agricultural production. In ocean water contain 480 mM of Na<sup>+</sup> and 560 mM of Cl\_ . If saline water contains the above the optimal level of sodium and chlorine for plant growth and development, they require the techniques to maintain the quality of irrigation water [18]. In some of the plants show the wide range salinity tolerance especially against the Na and Cl salinity. Based on the salinity tolerance and sensitivity, the plants are classified into two types such as glycophytes and halophytes. Glycophytes means which plant grown in low tolerance condition. Halophytes means which plant grown in higher salinity condition. Mostly all plants coming under the glycophytes category.

Cereal crops show wide range of response to salinity, for example barley show most tolerance and rice show the most sensitive to salinity [5]. Lutts et al. [19] reported that seedlings of rice are highly sensitive to toxicity of salts. According to Shannon [8] early vegetative and later vegetative stages of rice are highly sensitive to salinity. Khatun and Flowers [20] reported that the salt toxicity affected the panicle length, spikelets per panicle and 1000-grain weight; decreased photosynthesis were observed by Munns and Tester [5] which results in unfilled spikelets in rice. Delay in flowering and ripening, reduced number of tillers and biomass and leaf area also occur due to the salinity [21].

In saline soil contains numerous soluble salts such as Ca2+, Mg2+, Na+ and anions SO4 2−, Cl− , HCO3 − , K+ , CO3 2− and NO3 − and these soil contain the EC is 4 dS/m or more [22]. Based on the electrical conductivity influence in soil salinity and crop responses are mentioned in **Table 1**.

In arid and semi-arid area where low precipitation and high evaporation leads to accumulation of more salts [24] and accumulation of salts in sea shores is a natural phenomenon of sea water flooding. Worldwide fertility status of rice growing areas were mentioned in **Table 2.**

The quality of irrigation water affected by accumulation of salts. If abundance of salt accumulation in the root zone which alter the water relations in the plant. Due

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


**Table 1.**

*Crop response to salinity influenced by electrical conductivity of saturated soil extract [23].*


#### **Table 2.**

*Worldwide fertility status of Rice-growing areas [25].*

to the salt irrigation water induce the some of the effects in plants such as necrosis, chlorosis and interfere the plant physiological activities with depends on the some of the environmental factors such as temperature, humidity, light intensity and soil conditions [26].

#### **2.1 Effects of salt stress in rice**

The millions hectares of lands continuously salinized and also affect the crop production. It leads to contributing to the future biological catastrophe [27]. Rice is a salt sensitive crop [28]. In rice if increase in salt stress (in terms of 5 to 7.5 dS m–1), the seedling growth and fresh weight decreased [29]. The salt stress decrease the seedling biomass production in rice [10, 11]. The increased salt stress significantly reduce the mean root length, mean root numbers per plant, and shoot length [30]. In rice early stage, leaf mortality increased with increased salt stress [31] and also observed the reduction in growth and development in later stages [32]. Salt stress cause the effects in plant cell metabolism and leaf senescence and old leaves were death which indicate the crucial for the survival of a plant [33]. Due to salt stress panicle sterility observed in pollination and fertilization stage which leads to a decline in grain setting [34]. The lack of transformation of carbohydrate to growth of spikelet which leads to decrease in grain yield due to the salt stress (Sajid [35]). The salt stress contain negative linear raltionship with important factors such as number of tillers per plant, number of spikelets per panicle, and percent of sterile florets [10, 11].

In rice, the salt stress show the physiological effects such as decrease in photosynthetically active radiation (PAR), net photosynthesis (*P*n), stomatal conductance (*G*s), transpiration rate (*T*r), degradation of pigment, and relative water content (RWC) [36]. The chlorophyll and carotenoids contents in rice leaves were decreased after the salt stress [37]. The increased salt stress affect the Na+, Ca2+, K+, and Mg2+ concentration in root and shoot in rice plant [38]. The availability of zinc were decreased and increased cadmium (Cd) toxicity observed where high level of NaCl occur [39].
