Physiology of Salt Stress in Plants

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Vivekanand Tiwari1, Abhay Kumar2, and Pratibha Singh3

1 Department of Fruit Tree Sciences, Institute of Plant Sciences, Agricultural Research Organization (ARO) ‐ Volcani Institute, Rishon LeZion, Israel

2 Department of Biotechnology, ICAR‐National Research Centre on Litchi, Muzaffarpur, Bihar, India

3 Department of Botany, School of Life Sciences, Mahatma Gandhi Central University, Motihari, Bihar, India

Salt stress to plants occurs due to the accumulation of soluble salt in the plant rhizosphere beyond a threshold level, which can disturb the plants’ optimal metabolic homeostasis. This accumulation of the salt ions may happen either due to natural means such as the weathering of rocks, oceanic salt carried by the rain and wind, flooding of the seawater and leaching of saline water from the sea to the underground water resources of the coastal area, or by uneven irrigations and excess use of chemical fertilizers (Munns and Tester 2008). Natural weathering of parental rocks releases chloride salts of sodium, magnesium, and calcium, of which the most soluble and maximum proportion is sodium chloride (NaCl) (Szabolcs 1989). In general, the soil is defined as saline if its measured electrical conductivity (EC) is equal or higher than 4 dS/m, which is equivalent to 40 mM of NaCl concentration (Munns and Tester 2008). Since most cultivated crops are sensitive to salt stress, at soil salinity higher than 4 dS/m, the reduction in crop productivity due to salt stress accounts 50–80% (Zörb et al. 2019). The land area across the globe affected by the salinity is more than 800 million hectares and facing the problem of moderate to extreme salinity (Munns and Tester 2008). A total of 230 million hectares agriculture lands have a proper irrigation system and are the source of maximum crop productivity. Surprisingly, the salinity analysis of these irrigated agriculture lands revealed that approximately 20% (45 million hectares) is affected by salt stress (Munns and Tester 2008).

The increasing soil salinity affects the plants negatively for their growth, survival, and productivity. In saline soil, accumulation of salt ions triggers osmotic stress to the plant's root cells, which are then being absorbed by the plants to adjust the osmotic balance. The excess entry of the salt ions to the root cells creates the ionic imbalance at the cellular level. However, the halophytic plant species have adapted to thrive even under these adverse conditions of high salinity and can complete their life cycle under the extreme saline soil conditions. The halophytes have a better ability to tolerate the salt stress than a glycophyte (plants sensitive to salt stress) and can survive and grow well in the saline soil with soil salinity equivalent or higher than 200 mM NaCl (Flowers and Colmer 2008).

The effect of salt stress on plant physiology and its productivity depends on the level of soil salinity and how long plants get the stress. Immediately after the exposure to the salt stress, plants induce the signaling cascades to adjust their metabolic pathways. The plant cells exhibit biphasic response under salt stress dealing with osmotic and ionic stress, which overlap at some points. Earlier researchers assumed that the osmotic stress signaling initiates immediately after the salt stress. In contrast, the signaling cascade and response to the ionic imbalance initiate later due to the slow accumulation of sodium ions (Na +) in shoot tissues beyond a threshold level and corresponding inhibition of the photosynthesis (Zörb et al. 2019). Intriguingly, the new findings showed the root growth response specific to the Na +accumulation and rapid signaling cascade mediated by reactive oxygen species (ROS) or calcium ion (Ca 2+), specific to the salt ionic stress (Choi et al. 2014; Galvan‐Ampudia et al. 2013; van Zelm et al. 2020). Apart from the ROS and Ca 2+signals, the phytohormones viz. absiscic acid (ABA), jasmonic acid (JA), salicylic acid (SA), gibberelic acid (GA), and ethylene play crucial role in signal transduction and regulation of expression and function of several proteins during salt stress (reviewed in Zhao et al. 2020). These signals are perceived at the organelle level or at the level of the nucleus and responded by the plant cell in terms of stress‐responsive gene expression, different degrees of mRNA stability, and varied way of translational or post‐translational regulation to change protein abundance and the activity. These responses depend not only on the extent and duration of the stress but also on the plants’ genetic nature. The halophytes are evolutionary adapted to survive in the salt stress with unique genetic makeup, morphological, physiological, and anatomical adaptation (Munns and Tester 2008; van Zelm et al. 2020; Zhao et al. 2020). They are adapted to sequester the excess salt ions in the root or shoot vacuoles and secretion of excess salt through different kinds of salt glands and epidermal bladder cells [EBCs; (Zhao et al. 2020)]. However, salt‐stress tolerance is a complex trait regulated by several genes and pathways; engineering the crops using a single gene is inefficient. Moreover, the pyramiding of several genes is time‐consuming and seems less realistic to improve the salt‐tolerance capacity of conventional crops. Cultivation of halophytes for food, forage, renewable energy, and phytoremediation emerged as an alternative and economic strategy in the salt‐affected areas (Panta et al. 2014). Thus, in 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.

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.