Plant Response to Salt Stress and Role of Exogenous Protectants to Mitigate Salt-Induced Damages

Abstract

Plants are frequently exposed to a plethora of unfavorable or even adverse environmental conditions, termed as abiotic stresses (such as salinity, drought, heat, cold, flooding, heavy metals, ozone, UV radiation, etc.) and thus they pose serious threats to the sustainability of crop yield. Soil salinity, one of the most severe abiotic stresses, limits the production of about 6 % of the world’s total land and 20 % of irrigated land (17 % of total cultivated areas) and negatively affects crop production worldwide. On the other hand, increased salinity of agricultural land is expected to have destructive global effects, resulting in up to 50 % land loss by the next couple of decades. The adverse effects of salinity have been ascribed mainly to an increase in sodium (Na⁺) and chloride (Cl^–) ions and hence these ions produce the critical conditions for plant survival by intercepting different plant mechanisms. Both Na⁺ and Cl^– produce many physiological disorders in plants but Cl^– is the most dangerous. A plant’s response to salt stress depends on the genotype, developmental stage, as well as the intensity and duration of the stress. Increased salinity has diverse effects on the physiology of plants grown in saline conditions and in response to major factors like osmotic stress, ion-specificity, nutritional and hormonal imbalance, and oxidative damage. In addition to upper plant parts, salinity also affects root growth and physiology and their function in nutrient uptake. The outcome of these effects may cause the disorganization of cellular membranes, inhibit photosynthesis, generate toxic metabolites and decline nutrient absorption, ultimately leading to plant death. In recent decades, exogenous protectants such as osmoprotectants, phytohormones, signaling molecules, polyamines, antioxidants and various trace elements have been found effective in plants in mitigating the salt induced damages. These protectants showed the capacity to enhance the plants’ growth, yield as well as stress tolerance under salinity. In this chapter we attempt to summarize differential responses of plants to salinity with special reference to growth, physiology and yield. Further, we have discussed the progress made in using exogenous protectants to mitigate salt-induced damages in plants.