**2. Abiotic stresses**

Multiple abiotic (including; drought, heat, salinity, flood, harmful radiation, heavy metals, gaseous pollutants) and biotic stresses that are attacks of different microbial pathogens (e.g., fungi, bacteria, viruses, viroids, oomycetes, nematodes, and phtyotoplasma) causing adverse effects on morphology, physiology and metabolism leads to impaired plant growth and yield potential. Abiotic stress is progressively predominant in the wake of climate change and global warming affecting overall plants growth at different developmental stages such as germination, vegetative, and reproductive phase [15]. Total yield of crop is immensely affected by various factors like climatic fluctuations, insect incidents, agronomic factors, and nutrient availability in the soil. Stress is the any adverse environmental condition that hampers optimal growth and development of the plants [16]. Crop productivity and adaptability is affected by mainly physiological heat, drought, salinity and cold oxidative stresses. According to statistical estimates, approximately 20% of agricultural land is under salt stress, which negatively affects plant physiology and, ultimately, yield and nutritional value. The role of identified germplasm has been emphasized for drought breeding as the measured performance under drought stress is largely a result of adaptation to stress conditions. Hybridization of adapted landraces with selected elite genetic material has been testified to amalgamate adaptation and productivity. Abiotic stress is governed by quantitative trait hence genes linked to these traits have been identified and used to select desirable alleles responsible for stress tolerance in plant. Abiotic stress reduce water availability to plant roots by increasing water soluble salts in soil and plants suffer from increased osmotic pressure outside the root. Physiological changes include lowering of leaf osmotic potential, water potential and relative water content, creation of nutritional imbalance, enhancing relative stress injury or one or more combination of these factors. Plants operate a number of molecular, cellular and physiological modifications to overcome abiotic stresses [13]. Morphological and biochemical changes include changes in root and shoot length, number of leaves, secondary metabolite (glycine betaine, proline, malondialdehyde (MDA), abscisic acid) accumulation in plant, source and sink ratio.

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

Plants have developed various mechanisms in order to overcome these threats of biotic and abiotic stresses. They sense the external stress environment, get stimulated and then generate appropriate cellular responses. They perform this by stimuli received from the sensors located on the cell surface or cytoplasm and transmitted to transcriptional machinery situated in nucleus through various signal transduction pathways. This leads to differential transcriptional changes making plant tolerant against stress. Signaling pathways act as a connecting link and play an important role between sensing stress environment and generating an appropriate biochemical and physiological response.

#### **2.1 Salinity stress**

Soil salinity or salt stress refers to concentrations of salts in soils that affect physiological, biochemical process, growth and productivity of crops [17]. Salinity is one of the most important challenges which induce various physiological, molecular and cellular responses in plants [12]. Salt stress leads to loss of production by aggregation of soluble salts in the soil of root zone (18) which finally cause about 10 mha annual losses of arable land. A large number of sodium ions (Na+ ), carbonate and bicarbonate anions present in exchange sites affecting pH ranges [18]. Approximately 900 million hectares of agricultural land are affected by salinity globally, with India contributing 7 million hectares [19]. Salt stress affects approx 32 million hectares out of 1500 million hectares of dry agriculture land and 45 million hectares of irrigated land [20]. Therefore, yield reduction because of the increase in salt stress will have a disproportionately large effect.

Based on the salinity stress, three types of the soils are found in different geographical areas [21], which includes; (i) Saline soils which contains high level of water-soluble salt and electro-conductivity (EC) exceeded up to 4ds/m. By definition, saline soil has EC ≥ of 4 dSm−1 (equal to approximately 40 mM NaCl), whereas soils are considered strongly saline if the EC ≥ of 15 dSm−1. (ii) Sodic soils contain high contents of exchangeable sodium on the cation-exchange sites and usually have pH values ranges from 7.0 to 8.5. There are high accumulation of Na<sup>+</sup> which cause soil collides to disperse cations (Ca+2 and Mg+2) insufficiently. Distributed collides clog the soil's pore and reduce the ability to transport water and air. Sodic soils have the tendency to extreme swelling and shrinking during moist and dry conditions respectively. (iii) Saline-sodic soil which is known for high producing soil at the arid and semi-arid area. This soil shows dual nature that associates with high EC (>4 dSm−1) and low pH (below 8.5), hence both excess salts and Na<sup>+</sup> affects plant growth under in saline-sodic soil conditions. Under salinity, plant growth is affected by two-phase salt stress.

#### *2.1.1 Phases of salt stress*

#### *2.1.1.1 Osmotic phase*

In osmotic phase, the excess soil salt concentration reduced water potential in root zone causing water scarcity results in impaired plant growth. Growth reduction in osmotic phase rely only on the outer surface of salt concentration not in inside the plant tissues. A primary cause of reduced growth is that plant has to expense major portion of energy to acquire water from salty soil for routine metabolic process (**Figure 2**). Due to osmotic stress low water potential occurs in salty soil for plant

#### **Figure 2.**

*Diagram displaying plant responses and mechanism of salt tolerance showing ion exclusion, osmotic tolerance and tissue tolerance in crops.*

uptake even if the volumetric soil water content is higher as field capacity. Osmotic stress initially causes numerous physiological changes, like membranes disruption, nutrient imbalance, decreased photosynthetic activity, and reduction of the stomatal aperture [12].

### *2.1.1.2 Ion phase*

Reduction of plant growth in the ionic phase is mainly due to internal tissue injury caused due to high accumulation of toxic Na<sup>+</sup> . In addition, salt stress leads to reduced plant growth due to higher intake of certain ions (Na+ and Cl− ) which is known as ion toxicity [22]. Accumulation of Na+ and Cl− in higher concentration delays the growth and negatively affects the metabolic processes in plants. The Na+ ions inhibits K<sup>+</sup> ion uptake in plants and disturbs stomatal regulation which is the reason for water loss and necrosis. The Cl− ions stimulates chloride toxicity due to defective production of chlorophyll.
