summary, to understand better the effect of salt stress on plants, a comprehensive approach is required to understand the cellular ion transport system in different tissues, major phytohormone, or osmotic stress‐specific signaling pathways not only in the model plant Arabidopsis thaliana but also in the halophytic plant species (van Zelm et al. 2020) in order to understand the advantageous differences in the halophytes.
2.2 Crop Loss Due to Salt Toxicity – An Estimation Worldwide
The soil salinization is one of the three soil degradation processes that pose a threat to human health and crop productivity by affecting more than one billion hectares land across the globe (Ondrasek et al. 2011). The negative effect of salt stress on crop productivity indirectly affects the economy dependent on the agricultural produce, resulting in the loss of billion dollars annually. The economic loss caused by the salt stress can include two components: first, the loss of crop productivity (presented in t/ha) and thus, the loss of income generated from the agricultural production and, second, the cost spent for the restoration of degraded land. Estimating the global loss due to soil salinization can be heterogeneous among the different countries or geographical regions because of factors such as labor costs, the market price of the agricultural produce, fertilizers, seeds, and other operational costs affecting the total input and income differentially. There is the possibility that in some regions of the world (developed countries), even the moderate salinization of the soil could result in higher economic loss due to higher operational and labor costs. In many developing countries, most of the poor farmers depend on agriculture for their livelihood and loss of crop productivity due to salt stress affects their livelihood. In Asia, the Maldives is a low‐lying country, always on risk of submergence due to increasing sealevel and salt deposition. The intrusion of seawater on its land area due to Tsunami and deposition of toxic salts caused degradation of more than 70% of the agricultural land (FAO 2005; Ondrasek et al. 2011). The salt deposition destroyed more than 3 70 000 fruit trees, with an estimated loss of AU $ 6.5 million, which affected around 15 000 farmers economically. In a previous report by Qadir et al. (2014), the total estimated economic loss globally was more than the 27 billion US dollars per year. The loss of productivity among the crop also varied depending upon their genetic makeup, for example, moderate salt stress of 8–10 dS/m results in the yield losses of 15%, 28%, and 55% in cotton, wheat, and corn cultivars, respectively (Satir and Berberoglu 2016; Zörb et al. 2019), showing that cotton performs better at moderate salt stress; however, at the higher salt stress, the cotton also became susceptible, and the yield loss at the 18 dS/m resulted in a 55% loss in cotton productivity (Satir and Berberoglu 2016). The yield loss estimation by salt stress in comparison with the healthy growth conditions in some of the major crops of Indian subcontinent revealed the loss of yield by 45, 39, 63, and 48%, in rice, wheat, cotton, and sugarcane, respectively (Qadir et al. 2014; Tripathi 2009), which again suggest the variation in yield losses could be the result of combined effects of many factors such as the cultivars used for the cultivation, environmental condition of that specific area, and extent and time of exposure to the saline soil conditions.
2.3 Effect of Salt Stress on Target and Nontarget Plants and Microorganisms
The adoption of agricultural practices leads to the colonization of humans. For agriculture, domestication of crop plants began a million years ago, and after that, the crops were selected for numerous beneficial traits. The selection continued along with the development of civilization and till the recent. During the selection process, the plant species with the higher yield and less fruit dehiscent property became target species for the selection and agriculture. In comparison, several nontarget plant species with better stress tolerance but less yield were ignored or not prioritized. Their further improvement for the better yield through conventional breeding approaches narrowed down the plant genetic diversity. Approximately, 30 plant species fulfill 90% of the total food demand of the world human population, but they are highly sensitive to salt stress (Zörb et al. 2019). The horticultural crops, vegetables, or fruits are a nutritious source of minerals, carotenoids, vitamins, and fiber in the human diet and are more sensitive to salt stress than the cereals. The salt‐stress threshold level for the metabolic homeostasis in most horticultural crops is ≤2.5 dS/m (SNAPP et al. 1991). Among the cereal crops, the effect of salinity varies depending on the cultivars’ salt‐tolerance ability. Rice and barley are both the targeted cereal crops used in agriculture for food production but affected differently by salt stress. The rice is the most sensitive to salt stress, whereas the barley is the most tolerant of salt stress among all the cereal crops used for agriculture (Munns and Tester 2008). The salt tolerance ability among the different targeted dicot crop species varies greatly. For example, some legumes are very sensitive to the salt‐stress conditions among the legume species. In contrast, the Medicago is tolerant of the salt stress up to the level of salinity equivalent to the seawater (Munns and Tester 2008).
Recent advances in the field of salinity stress tolerance and research identified close relative of the crop species, which were never targeted for agriculture but are halophytes and survive the salt stress more efficiently. The halophyte Eutrema salsugineum belongs to the same family as the horticultural crop cabbage or the oilseed crop mustard, and the model plant A. thaliana. Under 100 mM of NaCl salt stress which is deleterious for cabbage (Pavlovic et al. 2019) and A. thaliana, the E. salsugineum not only survived but completed its life cycle with a negligible effect on their growth (Kant et al. 2006). The close relative of the wheat, the wheatgrass (Thinopyrum ponticum, syn. Agropyronelongatum) is one of the most salt‐tolerant monocotyledonous plants (Munns and Tester 2008), which can complete its life cycle at the soil salinity equivalent to the seawater salinity. Few halophytic nontarget plant species grow like weeds in the saltmarshes and have their ability to grow and complete the life cycle in the highly saline soil (Flowers and Colmer 2008; van Zelm et al. 2020).
The soil microbial community is an indispensable component of the soil nutrient cycling and supports crop plants growth and immunity. The microbial community helps plants absorb nutrients by participating in chemical modification of mineral, which becomes suitable for the plants’ easy absorption, increasing the soil porosity and organic matter, and soil aggregation (Xu et al. 2016; Zhang et al. 2019). Importance of rhizospheric microbial communities in plant nutrient availability and health suggests that any impact of the soil microbial community diversity or structure would directly affect the crop productivity in those soils. The incident of salt stress in soil has exhibited a reduction in the rhizosphere respiration, enzyme activity, soil microbial community size, and their growth rate, which ultimately affects the biogeochemical cycle negatively (Tripathi et al. 2006; Zhang et al. 2019). The increasing soil salinity has negatively impacted the bacterial community of Planctomyces and Archangium, associated with the soil organic matter input and stabilization (Zhang et al. 2019). The fungal mycorrhizal community helps plant better absorption of the mineral from the soil, and the activity of these mycorrhiza communities affects the soil nutrient availability (Babu and Reddy 2011). Under salt‐stress conditions, the arbuscular mycorrhizae Glomus was suggested to enhance the phosphorus and potassium uptakes by the plants (Porcel et al. 2012), but the increasing salt stress reduces the Glomus availability in the soil (Zhang et al. 2019). The fungus Hydropisphaera, which is involved in degrading the lignin in the salt marsh, brings a negative impact on the plants’ growth and survival during the salt‐stress conditions. In salt stress, the Hydropisphaera got dominated by the rhizosphere. Degradation of the lignin by Hydropisphaera reduces the soil organic matter and soil fertility (Zhang et al. 2019) posing a threat to the survival of the plants under the salt‐stress conditions. The salt stress to the horticultural crop tomato showed the increased incidence of growth of the root‐rot disease‐causing fungal pathogen Phytophthora parasitica (SNAPP et al. 1991), showing the importance of understanding the effect of salt stress on the nontargeted organism in parallel. There are several beneficial halotolerant plant growth‐promoting rhizobacteria (PGPR) that assist plants in their survival under the salt‐stress conditions. The PGPRs and their functional role are described in detail by Kumari et al. (2019), and studying these PGPRs will help us improve the salt‐stress tolerance in plants up to some extent.
2.4