Physiology of Salt Stress in Plants. Группа авторов
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of reactive oxygen species (ROS) can cause potential oxidation injury to the interstitial components like DNA structure, cell protein, and cell walls. The other sets of nondestructive defense mechanisms include elevation of photosynthetic rate, re‐exercising ion and water relation in the vesicular system, etc.
The saline balance of the terrestrial water sources is globally affected due to continual anthropogenic activities, for example, new England Marshes (Williams 2002). Perennial inland sweet water sources are worst affected due to the waste charging throughout the trajectory. Whereas, the salinity of global waters is also sacrificed due to the melting of glaciers. Territorial brackish streams such as arid estuaries and salt‐marshes are as well reasonably affected by the artificial agricultural discharge. But, this sudden change caters to a tremendous threat to the aquatic ecosystem. It incorporates abridged survivability, inhibited fertility, metabolic disorder, and retarded physiology (Velasco et al. 2019). Any varied salinity can mainly disrupt the osmotic balance between the surrounding hydrosphere and organismic, cellular fluid. The severity of impact varies between a minor metabolic malfunction and decease. To counteract the above, marine organisms develop an osmoregulation defense mechanism and eliminate hypo and hyperstress conditions. But, severe salinity or attenuation majorly conquers over the internal defense. Moreover, the consequences of successful defense are also majorly anonymous.
Additionally, investigations were also pursued to recognize the impact of ancillary factors such as temperature, but it seemed to be relatively insignificant. For instance, a study performed on zooplanktons reported metabolic issues upon exposure to altered salinity, but no apparent influence of temperature (Garreta‐Lara et al. 2018). Basic metabolic disfunctionalities elicited due to OS are addressed by spontaneous excretion. But, stress ascertained due to bioaccumulation of metals is far more complex, so the defense is required. Moreover, each metal is highly specific in terms of threat enforced.
An advantageous fact is that the combined stress imposed by the salinity and metal toxicity gets tackled by a common mechanism in case of catadromous and anadromous fishes during the migration, in and out from the oceanic environment. For instance, a study reported the secretion of carbonic anhydrase in sheepshead minnow, an estuarine variety while exposed to the combined stress evolved due to amended salinity and copper toxicity (Velasco et al. 2019). Furthermore, OS plays a protagonist role against metal stress for invertebrates residing in saline waters by incurring the ion transport system. Lack of free radicals and ions in the freshwaters facilitate metal uptake in the inhabitants. Therefore, the vulnerability of the sweet water species against the metal toxication is indeed more severe (Halse et al. 1998; Velasco et al. 2019).
Dry farming also raises intricate contact interaction amid washed off pesticides and water salinity. These halogenated chemicals probably act as a neurotoxin and inhibit the counteract mechanisms of the nervous system. Remarkably, hypersalinity pivotally helps anadromous varieties such as Brown trout to subside the impacts of exposure efficaciously. The effective counter mechanism correlated with an interpretation of malfunctioning of the neural system upon instantaneous exposure to elevated salinity. Furthermore, collateral stress induced due to the combined effect of salinity and dehydration results in protraction of the osmoregulatory responses in some plants and marine bugs, thereby diminishing the moisture loss. It portrays the discordant individual stressors that abolish each other, while severe impacts were observed upon elementary exposure (Williams 1998; Kultz 2015; Cañedo‐Argüelles et al. 2018).
1.7 Role in Sustainable Agriculture
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.
1.8 Unintended Effects of Salt‐Containing Substance Application in Agricultural Land
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 CO2 caused by limiting diffusion and decreased concentration of pigments. For instance,