Physiology of Salt Stress in Plants. Группа авторов

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.

1.5 Salt Toxicity Level: A Worldwide Report

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).

State	Sodic soil (M ha)	Saline soil (M ha)	Total a (M ha)
Gujarat	0.54	1.7	2.24
Uttar Pradesh	1.35	0.02	1.37
Maharashtra	0.42	0.18	0.6
Rajasthan	0.18	0.195	0.375
Tamil Nadu	0.35	0.013	0.363

      ^a 0.35 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).

As per the statistics published by the Food and Agriculture Organization (FAO) in 2007, about 770 000 sq. km of the global land surface area is already salt affected, and approximately 430 000 sq. km is under secondary salinity threat. The study further estimated that around one‐third of the world's irrigated lands are either already affected by higher salinity or will be affected in the recent future.The global share distribution (in percentage) of saline soil across the countries is portrayed in Figure 1.1.

      Multiple studies highlighted that abnormal abstraction of groundwater and waterlogging due to excessive irrigation often lead to desertification of fertile land. Therefore, urban local bodies (ULBs) and concerned government authorities should spread awareness among the farmers against salinization and over usage of water. Also, imposing water cess for misuse/overuse can be a productive measure to combat the typical tendency instantly, especially in the water‐scarce areas.

The overall salinity profile of India was first estimated in 1966 under the accountability of the Ministry of Agriculture. The raising concern developed due to uncontrolled usage of the chemical fertilizer provoked the study. Furthermore, in the period of 1980–2015, a total saline footprint of approximately 2 Mha has been reclaimed under the effort of the ICAR. The retracted area presently yields around 16 MT of fodder grains per annum. The trajectory of the salinity footprint of the nation and foreseen values are portrayed in Figure 1.2.

Figure 1.1 Worst‐affected nations vs. salinity share.

      Source: Adopted from Rasool et al. (2013) and Mandal et al. (2018).

Figure 1.2 Salinity index and prediction for India since the Green Revolution.

      Source: Adopted from Sharma and Singh 2015.

      The figure portrays the sudden escalation in salinity footprint (i.e. up to 16.2 Mha) between 2020 and 2050 as per the prediction made by the ICAR (ICAR 2015). The prediction is highly likely with no scientific intervention. Moreover, the usage of inferior irrigation water and overconsumption, leading to negative pressure, may further accelerate the salinization process.

1.6 Effect of Salt Stress on Flora and Fauna of the Ecosystem

      Salt stress induces a diverse range of metabolic and growth‐oriented detrimental changes in plants. Furthermore, protracted exposure can also inhibit crop yield. Primarily, saline exposure incurs OS, and it ultimately leads to ionic toxicities (Bano and Fatima 2009). Induced OS negatively impacts the root absorption capacity and accelerates the stomatal evaporation loss. Saline exposure elicits hyperosmotic pressure, which causes an adverse situation like the above. Initially, OS provokes several physicochemical amendments, which include membrane disruption and disfunctionality, disproportionate nutritive levels, retarded detoxification mechanism, and impaired photosynthesis rate (Munns and James 2003). Particularly, sodicity incites ionic stress by assimilating excess sodium and chloride ions into the plant tissue. Surplus accumulation of the above ions triggers ionic inequity leading to several growths related to detrimental changes. Elevated cell sodium ion concentration limits the required level of other essential plant nutrients such as potassium, thereby causing reduced yield and, ultimately, senescence (Ashraf 2004; Zhu 2007).

      The inherent countermeasures also