Physiology of Salt Stress in Plants. Группа авторов
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spinach entirely impedes photosynthesis by reducing the conductivity of CO2 both 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+, Ca2+ responsible for principal nutritive balance get replaced by Na+ while NO3− 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).
1.9 Role of Salt Toxicity in the Operation of Green Revolution
The third agricultural revolution colloquially termed as the Green Revolution was a path‐breaking exercise that occurred between the early 1950s and late 1960s in the twentieth century. Impacts were majorly widespread in developing nations. Beside positive, the revolution also brought specific rampant detrimental impacts on both land and crop health (Pingali 2012). Several newly derived products and technologies such as synthetic fertilizers, herbicides, pesticides, and irrigation were introduced to amplify the crop yield. But, both in terms of dependency and severity, chemical fertilizers surpass the influence of introductions. The scarcity of scientific information deceived to excessive usage of the above without evaluating the requirement resulted in reduced soil buffer. Furthermore, to compensate for the disorder, an additional application of fertilizer becomes a mandate. Constituents such as sodium and potash salt present in abundance in those fertilizers influence pH imbalance and turn the soil alkaline (Shrivastava and Kumar 2015). The assimilation of salt makes the soil susceptible to degradation. It is accounted for globally approximately9 M ha of cultivable land transforms nonfertile every year due to enforced salinity. Moreover, salinity can also be induced due to flooding caused by over‐irrigation and inadequate irrigation. Vegetable crops are more prone to vulnerability upon saline exposure resulting in depleted yield (Pingali 2012; Patel 2013; Sharma and Singh 2015; Eliazer‐Nelson et al. 2019).
1.10 Reaching the Current Status and Conclusion
Subjectivity to the external stressors makes the plant vulnerable and critically impacts productivity. Salinity being a predominant stress factor accounts for a significant chunk of yield‐oriented issues and up to further extent desertification. The usage of inferior quality of irrigation water and unlimited usage of chemical fertilizers often provoke saline intensification in arid and semiarid regions. Saline soil interrupts the unrestricted plant water uptake by hampering the osmotic balance. Sodic soil enforces the accumulation of sodium and chloride ions in the plant tissue. It leads to the scarcity of the essential minerals and inhibition of physiological growth due to retarded cell division rate. Plants exposed to sodicity often subject to ionic inequity and report an acute dearth of vital minerals such as potassium, magnesium, and calcium.
Moreover, excess sodium gets further transported to the xylem tissue of leaves and settle as a residue after evaporation (Baum et al. 2000). The accumulated salt mass disrupts the chloroplast structure and interferes with photosynthesis. As a preventive measure, the inherent water potential of the plant surge below that of the resident soil to withhold the negative turgor and uphold water balance. However, to mitigate a specific situation, plants internally enhance the solute concentration to uplift the osmotica (Carillo et al. 2011). Failing so may trigger the generation of ROS. Inequity between excessive synthesis and destitute scavenging often leaves insistent ROSs in the system. It oxidizes organics and destructs sensitive structures such as cell membranes, DNA and RNA structures, etc. (Gupta and Huang 2014).
Contrarily, the assimilation of metabolic solutes and other soil‐driven solutes in the plant is a distinct scenario, while exposed to stressors (Thomas and Bohnert 1993). These solutes safeguard the vegetation against alien stresses by different mechanisms, including osmoregulation, ROS decontamination, shield the cell membrane against the rupture, and enhancement of enzymatic/protein balance (Hasegawa et al. 2000; Hasanuzzaman et al. 2014; Acosta‐Motos et al. 2017). Furthermore, solutes such as osmoprotectants provide auxiliary protection to cellular components against desiccation. Solutes such as sucrose, glycine betaine, trehalose, and proline are appropriate examples of the above (Hanin et al. 2016).
While salinity is a violent threat steadily evolving across the globe, sustaining food security at this juncture is an enormous task before the agriculturists. Including genetic engineering, several approaches were embraced to produce salt‐tolerant vegetative species mimicking halophytes. Unfortunately, none of the species could be commercialized due to certain limitations.
Reclamation of the deserted lands could be the other way around. Restoration of the sodic soil is more accepted. The conventional approach includes the application of gypsum or pyrite. At the same time, recent researches recommended the usage of biodegradable municipal waste and industrial sludge as a sustainable alternative. Further research needs to be carried out to comprehend the challenges oriented and develop this technique as the alternate disposal model, especially for a small town with no existing waste management mechanism (ICAR 2015).
Acknowledgments
The review work was assisted by Dr. DK Sharma, Emeritus Scientist and Ex. Director, CSSRI, Karnal. We also thank Dr. Prasenjit Mondal, Associate Professor, Chemical Engineering Department, IIT Roorkee, for extending his insights and expertise related to chemical desalination techniques. Authors would also like to acknowledge Dr. RD Tripathi, Chief Scientist, CSIR‐NBRI, for his comments to escalate the technical quality of the manuscript. Ultimately, we would like to extend our gratitude to UGC for providing financial assistance for the literature study.
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