Physiology of Salt Stress in Plants

The existence and survivability of the global population are mostly dependent on agriculture. It is estimated that about 99% or more consumable fodder sources are scattered across the lithosphere, whereas a hydrosphere contributes a negligible fraction of 0.5% or less. Thus, it is evident that a healthy and sufficient existence of earth crust is mandatory for the sustainable coexistence of the human being. Furthermore, soil erosion drastically impacts the agricultural yield. It is estimated that annually approximately 75 million tonnes of soil loss occurs only from the cultivable topographic regions worldwide. Other prominent effects of salinization include the erosion of the hilly terrains, which is probably less investigated (Aslam et al. 2017).

Saline soil mostly produces superficial seals due to two causes: (i) sodium pressure fragmentizes the soil structure and eliminates clay particles, resulting in clogging of interstitial voids and (ii) lean vegetative cover exposes the saline soil to precipitation compaction (Agassi et al. 1994, Singer and Lindquist 1998). Both the processes mainly decrease percolation and enhance surface runoff. Though the layer beneath gets safeguarded against vigorous erosion, the top layer gets severely imposed due to the disintegration caused by salinization (Agassi et al. 1994). Therefore, it is evident that soil salinity also can indirectly influence soil erosion up to a greater extent.

In this ever‐raising context of fodder demand and versatile challenges, ensuring a hassle‐free supply for the global population is a mammoth task. Amid eyeing for the alternate sources, existing challenges such as unavailability of the fertile land footprint, overconsumed natural resources, water and energy scarcity, and climate variance cannot be overlooked. Sustainability can only be achieved by compensating the need, not greed. Advanced issues need modern solutions, and indeed few are emerging as follows: reparation of sodicity with gypsum dosing, subsurface drainage of water‐stagnant flood‐planes, adaptation of agroforestry, and generating genetically engineered species and switching to them (ICAR 2015). The detailed pathway is delineated below:

1 The satellite‐based remote‐sensing approach with geographic information system (GIS) mapping and real‐time ground truthing can provide an array of escalating salinity footprint (Singh et al. 2010).

2 Gypsum‐dosed alkali reparation techniques for soils affected with sodium toxicity.

3 Reclamation of flooded wetlands through downward drainage– the method is quite useful in addressing multidimensional issues such as water stagnation and salinization.

4 Chemical regeneration of saline soil with ameliorants is also practiced in some parts of the globe. The method is expensive and hence challenging to impose for more giant footprints.

5 Phytoremediation with salt‐tolerant species is contrarily an inexpensive and eco‐friendly mechanism.

6 Multilayer agroforestry is a recent trend in the agricultural industry to mitigate rising demand. Anyhow, the method also assists in reclaiming saline soil by reducing the soil density and thereby causing an elevated percolation rate. Furthermore, the littered biomass improves soil fertility and yield (Kaur et al. 2000; Nosetto et al. 2007).

7 Nonconventional techniques such as inland fishery have also gained limited popularity, majorly in the southern peninsula of the country. Flood‐planes and wetlands near to the coastal regions are effectively serving as the source of alternate revenue generation.

8 Microbialremediation: Desalination through microbial action is indeed rigorous. The inoculants are expensive and seek a suitable environment.

Salinity intervenes with plant nutrition and growth by exerting osmotic and ionic stress. Higher salinity level in soil hinders water absorption ability, referred to as the osmotic effect. The utmost concern is when elevated concentration can deter biomass growth. OS in plants influences metabolic amendments similar to wilting and sometimes depicts genotype changes. Furthermore, factors such as ion toxicity and nutritional inequity ensure impeded plant growth. Thus, it is evident that the impact of salinity on vegetative growth is a timevariant. Therefore, a bi‐phase kinetic model proposed by Munns et al. (1995) is considered as a benchmark for the present work. The primary phase is exceptionally speedy. OS resulting from internal water scarcity leads to growth retardation. Whereas, the secondary phase is relatively much slower and happens because of acute assimilation of salts in the shoot. But, still differentiating amid both the phases is a difficult task due to smooth transition array. High salinity downgrades the photosynthetic rate by reducing the availability of CO 2caused by limiting diffusion and decreased concentration of pigments. For instance, salt assimilation in spinach entirely impedes photosynthesis by reducing the conductivity of CO 2both in mesophyll and stomata. Also, by decreasing the chlorophyll concentration, salt stress can inhibit light absorption, thereby reducing photosynthesis. Furthermore, salinity causing reduced leaf expansion had reported an 80% reduction in growth rate in radish, while reduced conductance only retards body growth by up to 20% (Savci 2012).

Root zone salt assimilation activates OS and interrupts cell ion homeostasis by replacing the uptake of essential salts such as calcium and potassium nitrate with NaCl. Stem and leaf zone assimilation causes reduced photosynthetic rates, damaged chloroplasts, degraded metabolism, enzymatic malfunction, and other organelles. The impacts are predominant in adult leaves due to the longer accumulation tenure. Furthermore, nutrient deficiency and inequality occur in plants due to ionic substitution. Cations such as K +, Ca 2+responsible for principal nutritive balance get replaced by Na +while NO 3−as a major anion gets substituted by Cl −, leading to significant imbalance. In the case of higher soil sodium–calcium ratio, deficiency symptoms appear as a first sign. Though, plants such as tomato minimize the calcium absorbance to lower the rate of transpiration, and sodium competency plays a dormant factor there (Yadav et al. 2011; Zhang et al. 2018).

Primarily, a reduction in vegetative biomass, leaf surface area, and retarded plant growth is encountered chronologically in almost all the vegetative crops due to external salinity issues. The understanding of the interaction between plant root and salt‐imposed stress is still clumsy. Conversely, root biomass found to be nearly unaffected when compared with upper ground organs, except cauliflower, broccoli, and tomato. Biomass reduction is prevalent in cauliflower and broccoli, whereas in tomato, root length and density reduction are observed. The signs of salinity exposure appear gradually in plants. The primary symptoms include the transformation of green leaves, wilting, and hindered growth. Furthermore, advanced symptoms such as chlorosis, leaf burning, scorching, necrosis, etc., start manifesting after two weeks and prolonged exposure. The visuals of the above issues negatively influence sellability and affect economics. Commercial varieties such as roots, fruits, and tubers are the worse affected. Also, rotting of blossom‐end has been detected in tomato, eggplants, etc., due to saline irrigation (Maas 1993; Chandna et al. 2013).

Nonetheless, exposure to limited salinity also exerts some beneficial impacts, especially on vegetative crops. It improves the quality of the edible parts despite impinging certain visual defects. For instance, it reduces water content in fruits, enhances soluble solids and acid concentration in tomato, cucumber, and watermelon. Additionally, salinity can also improve the concentration of antioxidants and carotenoids in tomato and romaine lettuce. Studies also depicted that the beneficial nutritional properties (i.e. polyphenol concentration) of broccoli and spinach also flourish under a controlled saline environment with a dip in oxalic acid and nitrate ion content. All the prior mentioned effects are timedependent and only visible when subjected to the stress at the right moment (Thomas and Bohnert 1993; Chandna et al. 2013).