Salt Stress: Causes, Types and Responses of Plants

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Abstract

Salt stress is one of the major abiotic stresses limiting crop production especially in arid and semi-arid regions. It is reported that about 7 % of the total earths land and 20 % of the total arable area are affected by high salt contents. Reclamation of salt affected soils is necessary as the salt is engulfing the cultivable land day by day. The earliest response of plants to salt stress is reduction in the rate of leaf surface expansion followed by cessation of expansion as the stress intensifies but growth resumes when the stress is relieved. Metabolic processes like photosynthesis, protein synthesis and lipid metabolisms are affected due to salt stress. Salinity is responsible for different types of stresses like, osmotic stress, ionic stress, oxidative stress and hormonal imbalances. The osmotic stress is caused by the excess of Na+ and Cl ions in the soil that decrease the osmotic potential and hampers the water uptake and nutrients. Low molecular mass compounds known as compatible solutes is accumulated under salt stress. These compatible solutes include proline, glycinebetaine, sugars, proteins, polyols, etc. They do not interfere with the normal biochemical reactions and helps the plants in building the resistance against the stress.

Oxidative stress leads to generation of reactive oxygen species (ROS) such as hydrogen peroxide, superoxide ions, singlet oxygen, peroxides, etc. These ROS are toxic to biomolecules like proteins, lipids, nucleic acids, etc. They disturb the normal functioning of the cell and leads to decreased growth and development of the plant. Plant systems are equipped with enzymatic and non-enzymatic antioxidants such as: superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), ascorbic acid (AsA), glutathione, etc. They minimize the deleterious effects of ROS. Every compartment of the cell contains one or more antioxidants that act on a particular ROS and detoxifies it. Introduction or over-expression of selected genes is the promising way to generate stress tolerant plants. The chapter deals with the types of salt, causes of salt stress, reclamation of salt affected soils. The chapter also highlights the responses of plants to salt stress and the role of different biochemical attributes and key antioxidants to withstand the salt stress.