Physiology of Salt Stress in Plants

The severity of salt stress is most predominant in the case of agricultural crops from a food security perspective; impacts include retarded seed germination, reduced biomass, and small yield. Higher abscisic acid (ABA) concentration results in the formation of specific genes through the plant defense mechanism which leads to counteracting its generation cause (Godoy et al. 1990; Lodeyro and Carrillo 2015). Generally, the acute level of salt toxicity causes instantaneous death in various species, whereas, in selected species, limited stress influences defense mechanisms mimicking halophytes. For instance, conversion of C 3to CAM ,amendment in epidermal bladder cell to withhold excessive NaCl enabling better survivability over the saline condition. Significant parts of the coastal irrigated areas face salination issues majorly due to the seawater intrusion. More than 45 M ha of cultivable land distributed among hundreds of countries covering more than 10% of the global land surface area have already been sacrificed due to saline irrigation. Additionally, approximately 1.5 M ha of fertile land becomes nonproductive every annum due to soil salinity (Munns and Tester 2008). Presently, about 1150 M ha of productive land are under induced stress, while80 M ha are only affected due to the anthropogenic activities (Rasool et al. 2013; Hossain 2019).

Based on the origin and root cause, there are two different categories of salinity, namely, primary and secondary. Primary salinity is a natural phenomenon and mostly occurs due to the former presence of salt lakes, slat clads, tidal swamp, etc., at a particular location. It is majorly a kind of sodicity. At the same time, secondary salinity is imposed due to man‐made activities such as urbanization, saline irrigation, etc. (Shahid and Rahman 2011). Detailed reasons are delineated below.

Primary salinity:

1 Spreading from the saline artesian well.

2 Capillary rise from saline groundwater.

3 Seawater intrusion.

4 Canopy formation due to the movement of fine sea sand by the sea breeze.

5 Waterlogging.

Secondary salinity:

1 Irrigation with impeded drainage

2 Effluent discharge

3 Excess fertilizer dosing

4 Deforestation

5 Saline irrigation

Furthermore, based on the predominance of the type of anions present and the pH value, salt‐affected soils are categorized as saline soil and sodic soil. Sodic soil typically comprises sodium carbonate and or bicarbonate ions with a pH value beyond 8.5, but contrarily, saline soil majorly incorporates chloride and sulphate ions with pH value below 8.5. Certain plant species manage to compensate the imparted stress through its metabolism and survive in the severe salt conditions known as halophytes. Remaining plant species are termed as glycophytes with a higher mortality rate overexposure to 10% or more concentration of saline water (Gorham 1995; Parida and Das 2005; Mane et al. 2011; Gupta and Huang 2014).

Primarily, hydro‐geological activities contribute in escalating soil salinity and sodicity. Moreover, the soil is generated because of the weathering actions on intermediate and basic igneous rocks; sandstones already carry salt as a primary constituent. In the regions with moderate to low rainfall, a greater rate of evapotranspiration induces higher salinity and sodicity. Furthermore, coastal regions with tidal exposure may also develop salinity problems. A study conducted by Sultana et al. (2001) depicted that rice yield in coastal Asia gets often impaired due to the intrusion of saline Indian Ocean water. Inland precipitation also surprisingly elevates the soil sodicity. It is evidenced that rainwater can constitute up to a few milligrams of salt against each kilogram of a downpour with an electrical conductance (EC) value of 0.01 dS/m (Cucci et al. 2016; Corwin and Yemoto 2017; Hossain, 2019).

However, the deteriorating impacts of artificially induced salinity are more predominant. Over‐irrigation or saline water irrigation is cited as one of the prima facie reason for human‐induced salinity. Roughly, it is estimated that globally half of the irrigated lands are anyhow saltaffected. Other than irrigation, probable sources of inland salinity are the following:

1 Salt accumulation: Effluent and waste discharged into the surface water bodies from the industries and effluent treatment plants (ETPs) beyond absolute concentrations can accumulate and form salt films downstream to cause acute saline toxicity (Naidoo and Olaniran 2013).

2 Reduction of greenbelt: Deforestation accelerates the salinization process by facilitating salt movement both through upper and lower soil layers. It further results in depleted annual precipitation and elevated soil temperature. Subsequent heating and cooling promote wear and tear, higher runoff, and substantial sedimentation to cause flooding and salt assimilation.

3 Overdosing fertilizers: Post‐Green Revolution, the usage of chemical fertilizers, herbicides, and pesticides has abruptly increased. Overdosing often results in underutilization and accumulation.

4 Excessive grazing: Areas with scarce soil cover often suffer the root zone saline toxicity due to overgrazing. Surface waterlogging (i.e. either due to over‐irrigation or riverbed sedimentation) in such areas can cause elevation of the water table and thereby facilitating salt migration from the deep aquifers.

Soil salinity and sodicity is a global issue faced by more than 115 nations with annual yield depletion of 7% or more (Yadav 2010). A total of 955 M ha of world surface area is either primarily or secondarily affected by salt pollution. Sodicity is predominant with impact over more than half of the land surface, e.g. Australia. Salinity issue dominates about 21% of comprehensive land footprint, especially arid regions of Asia and Pacific and areas with impeded drainage. Coming to India, the ambiguity of figures (salt‐affected land) is quite a concern in the absence of liable evidence. The reported niche is found to be varying from 7 to 25 M ha (Rasool et al. 2013; Shahid et al. 2018; Isayenkov and Maathuis 2019). A location receiving low to moderate precipitation poses a tremendous threat to native agriculture.

Table 1.1 Soil salinity/sodicity scenario in worst‐affected partsof India.

Source: Data from Mandal et al. (2018).

a0.35 M ha: Threshold limit.

The scenario is quite predominant in the southern region of India. These semiarid zones experience more than 300 sunny days per annum with high solar radiation, causing elevated evaporation rate and thereby moisture loss. The soil resource maps published by the National Bureau of Soil Survey and Land Use Planning, Nagpur (NBBS and LUP) were considered as the baseline data for the present study. The salinity of the soil was subcategorized into six basic classes depending on the EC of the saturated extract. The categories (based on severity) are as follows: very severe, severe, strong, moderately strong, moderate, and slight. The soil extracts portraying EC values between 200 and 400 mS/m were neglected for the above study. Furthermore, the sodicity of the soil was categorized into three major classes, namely, strong, moderate, and high. The above classification was done based on the presence of exchangeable sodium percentage (ESP), and the scale ranges from <5 to >15. The soil samples with sodicity <5 were considered as nonsodic, whereas the black soil samples with sodicity more than five were considered as alkali or sodic (Rasool et al. 2013; Shrivastava and Kumar 2015). The Rann of Kutch, Gujarat, an area with a footprint of 7500 sq. km mostly comprising salt marshes was marked as a separate entity by NBBS and LUP ( Table 1.1).