2 Effects of Salt Stress on Physiology of Crop Plants: At Cellular Level

       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

2.1 Soil Salinity and Plants

      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 (Ca²⁺), 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 Ca²⁺ 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