**2. Salinity-impacts in plants**

Three major physiological stresses encountered by salinity-exposed plants include ion (Na+ and Cl− ) toxicity; physiological drought due to low osmotic potential; and nutrient imbalance in plants (**Figure 4**) [15–17]. Salinity-accrued decrease in plant growth and photosynthesis and related variables may also be possible as a result of hyperionic and hyperosmotic stress [18]. Plant tolerance to salinity of the soils of the root zone greatly varies with plant types and their age, and the salinity concentration and exposure duration. However, exhibition of yield reduction was noted in most crop plants at the electrical conductivity (EC) of the saturation extract (Ece) in the root zone >4.0 dS m−1 (≈40 mM NaCl; exchangeable sodium of 15%) at 25°C [19–21]. Taking into account representative studies, the major impacts of salinity on plant growth, photosynthesis, yield, nutrient uptake and metabolism, water status, oxidative stress, and antioxidant metabolism are briefly overviewed hereunder.

#### **2.1 Growth, photosynthesis and yield**

Soil salinization impacts almost every stage of plant growth and development. Salinity impact on germination has been extensively reported in several test plants [22, 23]. Contingent to salinity concentration, type of salt present, type of plant species, age of test plant, and time of exposure were reported to control the extent of salinity impacts. The major growth traits, namely the rate, percentage and index of germination; leaf area; length, fresh and dry weights of root and shoot; and plant dry mass, were varyingly decreased in salinity-exposed plants [23–25]. Salt stress impacts on photosynthesis and involved several mechanisms. Salinity-accrued impediment in photosynthesis is caused by decreased leaf area and stomatal conductance, declined CO2 availability and assimilation; Cl<sup>−</sup> led inactivity of RuBisCO; degraded D1 and D2 proteins of PSII reaction center; diminished activity of enzymes involved in photosynthetic pigment synthesis; low uptake of Mg2+, and destruction of pigment-protein *Introductory Chapter: Making Plant Life Easier and Productive under Salinity – Updates… DOI: http://dx.doi.org/10.5772/intechopen.113407*

#### **Figure 4.**

*Schematic representation of the major targets of salinity impacts in plants.*

complexes [26, 27]. Elevation in the soluble salts in the rhizosphere is bound to result in brutal yield losses in most crops, which was widely argued to involve salinity impact on different yield components [28, 29].

#### **2.2 Nutrients uptake and metabolism, and water status**

Elevated soil salinity significantly impacts the acquisition of most mineral nutrients including Ca, Cu, Fe, N, P, K, S and Zn. Decreased solubility and mobility of Cu and Fe were earlier noted in plants under salinity stress [30, 31]. In fact, salinity conditions tend to immobilize nitrate (NO3 2−; by Cl− ) and ammonium (NH4 + ; by Na+ ) ions, the major plant-absorbed N forms [32]. Unavailability of soil-P to plants, its deficiency therein are caused by salinity [33]. Impaired uptake and metabolism of Ca, K, P, N and S were observed in salinity-treated plants, which mainly involved salinity-accrued changes in soil solution's osmotic potential and/or the activity of K+ -selective ion channels [22, 34]. Salt-affected plants exhibit significantly decreased uptake and use efficiency of B, K and P due mainly to the negative interactions with higher concentrations of cations and anions. Salinity-exposed plants also exhibited inhibited K+ uptake and decreased K+ / Na+ ratio, which were argued to involve salinity-mediated depolarization of the plasma membrane potential, activation of voltage-gated guard cell outward rectifying K+ channels and eventual K+ efflux [35, 36]. Salinity-caused reduced uptake and deficiency of Mn, and decreased solubility and P uptake were also observed in plants under salinity exposure [37, 38]. In several instances, elevated salinity also impacted the assimilation of both N and S via impacting the major N-S assimilatory enzymes [39–41]. Notably, high salinity was reported to affect the major genes involved in the uptake and transport of N, and assimilation can also be affected by high salinity [42]. Salt stress can also impact nitrification and ammonification [43]. The proportion of N-transport amino acids (such as asparagine, glutamate, aspartate and glutamine) decreased in several salinity-exposed test plants [44, 45]. Notably, physiological drought in plants has been reported due to the imposition of high salt concentration in the rhizosphere, and eventual salt-accrued immobilization of water and its unavailability to the plants [46].

#### **Figure 5.**

*Schematic representation of the main reactions performed by selected enzymatic antioxidants involved in the scavenging of varied reactive oxygen species (ROS). Abbreviations: DHA, reduced dehydroascorbate; GSSG, oxidized glutathione/GSH disulfide; MDHA, monodehydroascorbate; NADH, nicotinamide adenine dinucleotide; NAD, nicotinamide adenine dinucleotide; NADPH, nicotinamide adenine dinucleotide phosphate; NADP<sup>+</sup> , oxidized form of nicotinamide adenine dinucleotide phosphate [49–52].*

#### **2.3 Oxidative stress and antioxidant metabolism**

Elevated soil salinity is widely known to accelerate the accumulation of reactive oxygen species (ROS; such as O2 − , H2O2, •OH, and <sup>1</sup> O2) leading to oxidative stress, a physiological condition of imbalance between generation of ROS and their scavenging [22, 47, 48]. Elevated or non-metabolized ROS cause lipid peroxidation and damage macromolecules including DNA and protein. Interestingly, plants possess inherent capacity to activate ROS-scavenging system in order to counteract potential ROSaccrued consequences. Interestingly, plant antioxidant defense system is comprised of enzymatic (superoxide dismutase, SOD; peroxidase, POD; catalase, CAT; ascorbate peroxidase, APX; glutathione peroxidase, GPX; glutathione reductase, GR; monodehydro ascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR); and nonenzymatic (reduced ascorbate, AsA; reduced glutathione, GSH; phenolic, vitamin E, carotenoids and mannitol etc.) components (**Figure 5**) [49–51].

Extensive reports are available on the salinity-mediated elevation in the levels of varied (such as O2 − , H2O2, •OH, and <sup>1</sup> O2), and their impact on membrane lipids and leakage of electrolytes have been reported. Salinity stress was also found to induce different components of antioxidant defense system in salinity-exposed plants. Significant enhancements in the activity of enzymes involved in the dismutation of O2 − (SOD); H2O2-metabolism (CAT; APX) and GSH-regeneration (GR) [22, 47, 48].
