SOIL SALINITY EFFECT ON SOYBEAN PLANT: A COMPREHENSIVE MORPHOPHYSIOLOGICAL STUDY

. Soil salinity is a big barrier to soybean production in many areas of the world. The world’s salinity-affected areas are rising year after year. Therefore, the salinity stress events tend to become more severe in soybean production, thereby, the authors introduced a critical review of the literature to understand the changes in the different traits exposed to salinity stress and their mitigation strategies for the salinity stress of soybean cultivation. Under salinity stress, the soybean plant showed various morphological, physiological, biochemical, and structural changes. Results indicated that salinity stress adversely affects the soybean emergence, nodulation, growth and development, seed quality, oil and protein content of seed as well as yield. Effective salinity management strategies in soybean cultivation are the use of different exogenous components, organic and nano-fertilizers, breeding approaches and application of arbuscular mycorrhizal fungi. Further studies require on the underlying screening of salt-resistant soybean genotypes and their use in molecular breeding.

By considering above mentioned background, it is necessary to know how salinity affects soybean plants and the management of salinity stress to grow soybean potentially in saline-prone areas.This review was carried out to understand the possible changes in different traits of soybean plants exposed to salinity stress and find out the practical solution to the salinity problem in soybean cultivation.

EFFECT OF SALINITY STRESS IN SOYBEAN GERMINATION
Germination is vulnerable to salinity stress, and it is well recognized that seed germination is affected by salinity stress in two ways: one is osmotically through reduced water absorption another is ionically through the accumulation of Na + and Cl -causing an imbalance in nutrient uptake in soybean (Kumar, 2017).It is also documented that salinity stress affects the time of germination and germination percentage as well as the failure of germination completely in soybean with a high concentration of salinity in the soil (Kumar, 2017;Mauromicale and Licandro, 2002).Belaqziz et al. (2009) report that a high percentage of salinity stress affects the germination of soybean by destroying the embryo.Toxic effects of certain ions, the higher concentration of salt reduces the water potential in the medium which hinders water absorption by germinating seeds and thus reduces germination.Jeannetter et al. (2002) reported that the germination rate and the final seed germination decrease with the decrease of the water movement into the seeds during imbibition under salinity stress in crops.Our understanding is that inhibition of seed germination by salt stress could be due to osmotic stress or specific ionic toxicity.Finally, it can be stated that the germination percentage of soybean significantly decreased as the level of salinity increased.

PLANT GROWTH AND DEVELOPMENT
The effect of salinity stress causes character changes in the plant from the point in time of occurrence to maturity (Munns, 2002).Generally, the plant cell gets to shrink and dries immediately when expose to salt stress, and the affected cell tries to recover hours later.However, cell elongation and division could be affected, consequently, reducing root and leaf growth rates.A week after salinity stress occurrence, lateral shoot enlargement is affected and a month later, clear differences in overall growth may clear.Furthermore, salinity stress is also liable for osmotic changes outside the root (osmotic effect).The osmotic effect guided the reduction in the capability of crops to absorb water consequently, affecting crop growth and development (Munns, 2005).Xiangjun et al. (2006) reported that soybean plant responses slow leaf expansion and photosynthesis under salinity stress, resulting in defoliation and reduced plant growth.Shiv et al. (2019) reported that the reduction in growth might be due to a reduction in cell elongation that affects plant height, survival and biomass.Amirjani, (2010) recorded that increasing salinity levels of 50, 100, and 200 mM consequently reduction of plant height by 30, 47, and 76% and a reduction of fresh weight by 32, 54, and 76%, respectively in soybean.Shoot dry weight is also responsible for soybean growth.Essa, (2002) recorded that shoot dry weight was greatly reduced by increasing the salinity level of the soybean plant.Some studies have recorded the significant negative impacts of salinity stress on the growth and development of soybean plants (Shereen et al., 2005;Kazem et al., 2009).Therefore, it is clear that the growth and development of soybean could be adversely affected by salt stress.

VEGETATIVE GROWTH
Soybean plants are more sensitive to salt stress, particularly in seedlings and early vegetative stages in comparison to other stages.Hosseini et al. (2002) reported that the early vegetative stage is highly sensitive to the salinity stage.For obtaining higher yield in soybean optimum vegetative growth is necessary.When plants are subjected to high salinity levels for a week at the early vegetative stage causes retardation in growth by reducing photosynthesis activities in soybean (Shiv et al., 2019).This author also stated salt stress causes a decrease in soybean potential activities for higher production by disrupting photosynthesis.Machado and Serralheiro, (2017) also reported crop vegetative growth is significantly decreased by salinity stress.Since salt stress adversely affects vegetative growth in soybean, therefore, plant height, viability and plant biomass are similarly affected by salinity; resulting, in limiting the productivity of soybean.One question emerges that efficient salinity management in vegetative stages could be a priority topic for sound soybean production.

REPRODUCTIVE STAGES
To prioritize soybean seed yield, the reproductive stage is most important.Zhang et al. (2020) reported that salinity stress delayed soybean flowering time and negatively affect flower sets in the soybean plants.Flowering and pod set in soybean are intrinsically related to each other and subsequent pod set depend on flowering.Based on our knowledge, there is no fruitful data related to the effect of salinity stress on pod development in soybean.However, it is recognized that soybean is less sensitive during flowering and least sensitive during the seed-filling stage (Maas and Poss, 1989).Kazem et al. (2009) documented that grain filling duration decreased with increasing salinity and resulted in decreasing final seed yield.This author also reported that the seed yield of soybean is significantly reduced by affecting seed filling stages.Ney et al. (1993) reported reproductive stages are affected by hampering cell division, dry matter accumulation, and photosynthesis activities under salinity stress.Thus, salinity stress affects greatly reproductive stages.The generalized point is that the seed filling time is the utmost significant time of the soybean plant's life cycle in terms of yield.Salt stress reduces seed number or seed size per pod, accordingly decreasing yield potential.NODULATION Nodulation has a significant impact to settle the atmospheric nitrogen and the number of soybean nodules is closely related to seed yield (Tagoe et al., 2008).Nodule formation and nitrogen fixation of soybean have two prerequisites: (1) root surfaces colonization and rhizobia involvement to roots; (b) infection to the root hairs.Soil salinity has a detrimental effect on these two processes and nodule number reduction through inhibiting nitrogen fixation (Paul and Ben, 1984).It is also well-recognized that the number and weight of root nodules may reduce when exposed to saline stress in soybean (Lakshmi-Kumari et al., 1974;Lauter et al., 1981;Tu, 1981).Tu, (1981) reported that nodule formation is detrimentally affected by the disturbance of rhizobial colonization in soybean roots.Nodule initiation is negatively affected by NaCl concentrations which do not obstruct rhizobial survival or colonization of root surfaces and this may be due to the salt susceptibility of the root-infected site (Paul and Ben, 1984).It is still ambiguous how salinity negatively affects nodules on soybean, therefore, it can be stated that legume-rhizobium symbiosis is associated with a complex interaction between the host root, rhizobial strain, and the environment for nodule initiation under salinity stress.

CHLOROPHYLL CONTENT AND PHOTOSYNTHESIS
Chlorophyll content, which leads the photosynthetic activities, is inversely related to salinity level (Ota and Yasue, 1962).Many researchers documented that chlorophyll content notably decreased under salinity stress (Yasar et al., 2008;Kusvuran, 2010;Nazarbeygi et al., 2011).The inhibition in chlorophyll synthesis results in loss of photosynthetic activity leading to senescence of the leaves by soil salinity (De Michele et al., 2009).Zhou et al. (2018) reported that there is a high correlation between the chlorophyll content of the leaf and rate the of soybean tolerance for salinity stress.Salt-tolerant genotypes have a high chlorophyll content, which could be protected from degradation due to the presence of antioxidant enzyme activity (Shiv et al., 2019).
It is well-recognized that photosynthetic activity decreases when soybean is subjected to saline conditions, resulting in reduced growth and productivity (Netondo et al., 2004).Chlorophyll content and photosynthesis are closely associated with the plant.When soybean is exposed to saline conditions, chlorophyll content and activity of photo-system ΙΙ may decrease and affect photosynthesis (Jamil et al., 2007;Ganivea et al., 1998).These authors further stated Fluorescence of chlorophyll reflected the activities of photochemical in photo-system ΙΙ under salinity stress.Reddy and Vora, (1986) also reported salinity can affect chlorophyll content through inhibition of chlorophyll synthesis or acceleration of its degradation.Few researchers documented that the photochemical efficiency of photo-system ΙΙ is reduced under salinity stress, consequently reducing photosynthesis (Netondo et al., 2004;Jamil et al., 2007).Regarding above mentioned discussion it can be summarized that soybean yield decreases due to the adverse effect of total chlorophyll on the rate of photosynthesis under salt stress.

NUTRIENT UPTAKE
It is well confirmed that ionic imbalance, particularly of Ca 2+ and K + could have occurred when plants meet salinity stress (Lynch and Lauchli, 1985;Cerda et al., 1995).The accumulation of K + , calcium (Ca 2+) and magnesium (Mg 2+ ) could reduce the leaves in soybean under saline stress.Plants accumulate Na+ at the expense of Ca 2+ and K + under saline conditions (Kuiper, 1984).K + concentration could limit growth either by reducing osmotic adjustment and turgor maintenance or by a detrimental effect on metabolic functions (Helal and Mengel, 1979).Greenway and Munns, (1980) documented that salt-tolerant cultivars must maintain relatively high concentrations of Ca 2+ and K + for surviving a saline soil environment.Some cultivars can grow higher growth through gathering fewer toxic ions and maintaining a high tissue Ca 2+ concentration under saline stress (Huang and Redmann, 1995).

OXIDATIVE STRESS
Oxidative stress is an increment of oxidant species or a depletion of antioxidant defences.Oxidative stress phenomena happen when there is an imbalance between the production of reactive oxygen species and antioxidant defence.Reactive oxygen species seemed like a dangerous molecule and their concentrations were to be maintained in minor amounts.Many reports showed the improvement of oxygenase activity leads to counteract oxidative stress generation (Noriega et al., 2004;Balestrasse et al., 2005;Yannarelli et al., 2006).The fact is that oxidative stress is produced by a decrease in antioxidant defences.This antioxidant response is associated with nitrogenase activity diminution.A significant decrease in the classical antioxidant enzyme activities was observed when plants were treated with 100 and 200 mM NaCl (Amirjani, 2010).

CELL STRUCTURE
It is well known that salt stress also affects cell structure.Salinity stress affects both leaf and root cells in soybean (An et al., 2003;Hu and Schmidhalter, 2001;Kiliç et al., 2007).Belda and Ho, (1993) reported that the development of vascular tissue is affected by salinity stress.Dolatabadian et al. (2011) reported that cutin mass and trichome density may increase in epidermal cells when soybean exposes to salinity stress.Furthermore, this report also reported that cortex thickness was decreased under salinity stress in soybean and xylem formation and arrangement also changed.Several reports stated that trichome density may increase under salinity stress (Abernethy et al., 1998;Aguirre-Medina et al., 2002).Therefore, it is crystal clear that anatomical characteristics could be changed like increment of cutin synthesis on epidermal stem cells, trichome density, and xylem structure in soybean under salinity stress.Besides, osmotic stress diminishes outside water capacity and leads to a decreased water take-up potentiality of plants, hence influencing cell development.It moreover leads to stomata closure, decreasing the plant's capacity to assimilate CO2.The ionic push is caused by an abundance take-up of poisonous salt particles (basically Na + and Cl − ) that obstruct normal metabolic activities in plants (Quamruzzaman et al., 2021).YIELD Seed yield is the main concern for soybean producers.Kazem et al. ( 2009) tested three soybean cultivars with NaCl salinity at 3, 6, and 9 dSm -1 and stated that the number of pods, seed per plant, and seed weight significantly decreased with increasing salinity.Another report also noted a significant decrease in shoot length, shoot and root dry weight, chlorophyll content, the number of pods per plant, 100-seed weight, and seed yield decreased with increasing salinity (Hamayun et al., 2010a).Phang et al. (2008) reported that the plant height, leaf size, biomass, number of internodes, number of branches, number of pods, total weight per plant, and 100-seed weight significantly decreased with increasing salinity.Many reports documented that seed yield reduced significantly with increasing salinity (Katerji et (1996) reported that increased proline accumulation is an adaptation to compensate for the energy for the growth and survival of the stressed plants.In the same way, glycine betaine treatment is responsible for increasing proline concentrations.Glycine betaine is helping to increase the proline which can be involved in enhancing the ability of soybean to cope with salinity stress (Liu et al., 2017).When provided exogenously, proline has improved salt stretch resistance in different plant species.Beneath high-salt conditions, proline application upgrades plant development with increments in seed germination, biomass, photosynthesis, gas trade, and grain surrender.These positive impacts are primarily driven by superior supplement procurement, water take-up, and natural nitrogen obsession (El Moukhtari et al., 2020).

OIL AND PROTEIN ACCUMULATION
Soybean seed is a good source of protein and oil which is used for human and animal consumption (Katerji et al., 2001).Soybean oil, a direct item of soybean processing, is greatly vital, particularly in human and animal nourishment, and the oil contents of soybean seeds are influenced by salinity stress (Nakasathien et al., 2000).It is reported that the oil percentage is reduced with increasing salinity levels in soybean (Zadeh and Naeini, 2007).The soybean is financially the foremost important bean in the world for its high content of proteins.Kazem et al. (2009) reported salinity had a great effect on the rate of protein accumulation in soybean seeds and protein yield per plant large reductions in durations of protein accumulation with increasing salinity.Sabagh et al. (2015) found the appearance of a significant decrease in protein content with salt stress.The authors of the previous study only mentioned the negative effect of salinity on oil and protein content, not how it affects them.

MANAGEMENT STRATEGIES FOR SALINITY STRESS Salinity stress can be managed in different ways which are discussed below-EXOGENOUS CHEMICAL APPLICATION
The exogenous application of osmoprotectants including plant growth regulators, or optimized nutrient content could be a good way to overcome salinity.Hamayun et al. (2010b) the negative impacts of NaCl and polyethene glycol on soybean growth can be minimized by the application of silicon at 100 or 200 mg per litre.Thapa et al. (2011) documented that the exogenous application of proline or glycine betaine is a simple and practical strategy to improve salinity tolerance in soybean plants.Ascorbate is one of the major water-soluble antioxidants, securing organically critical macromolecules from oxidative harm caused by hydroxyl radicals, superoxide and singlet oxygen.In expansion to its significance in photoprotection and the control of photosynthesis and it plays a critical part in the control of the cell cycle and a few crucial forms of plant development and advancement, as well as ascorbate, checks the antagonistic impacts of salt push on the development of soybean (Dehghan et al., 2011).Al-Hakimi and Hamada, (2011) reported that the application of ascorbate for most plants diminished the inhibitory impacts of salt and made advancement the net photosynthetic rate and pigments biosynthesis.
Among the micronutrients, zinc (Zn) is detailed to have a noteworthy potential to ease plant salinity stretch and foliar application of Zn improves plant growth (Broadley et al., 2012).Al-Zahrani et al. (2021) reported that zinc application mitigates soil salinity through the improvement of different physiological and photochemical activities, which seem demonstrate to be valuable in nutrient-mediated administration for crop improvement.Selenium, an Essential trace mineral, reduces salt stress in soybeans by enhancing growth, physiology, glutathione homeostasis and antioxidant defences (Alharby et al., 2021).
Plant growth regulators play a noteworthy part in reducing salt stress using a wide extent of physiological modifications (Datta et al., 1997).In wide terms, plant growth regulators expanded the physiological accessibility of water and basic supplements, whereas making a difference in plants diminishes poisonous salt stack (Iqbal and Ashraf, 2007).Plant growth regulators actuate salt resistance by expanding the action of reactive oxygen species rummaging chemicals to preserve the reactive oxygen species at a nontoxic level beneath push conditions.The improving capacity of plant growth regulators depends on environmental variables that influence their retention, the concentration at which they are connected, and the physiological state of the plant (Tognetti et al., 2012).Shu et al. (2017) reported that fluridone (FLUN), an ABA biosynthesis inhibitor, is a potential plant development controller that could advance soybean seed germination beneath salt stress.Thus, distinctive sorts of exogenous compounds may well be the more dependable road for minimizing the effect of salinity stress on soybean plants.

SOIL ORGANIC AMENDMENTS
The natural alteration through organic amendments decreases salinity stress and progresses crop development.It is also documented that organic amendments increased the yield however also can ameliorate soil salinity (Kamal et al., 2021).There are many sorts of organic soil amendments, including compost, farmyard manure, water hyacinth, kitchen scraps, poultry manure and green manuring.The application of compost can restore degraded soils, and increase biological functions, organic carbon, and soil physical fertility, consequently reducing the negative impacts of salinity stress on plants (Diacono and Montemurro, 2010;Hoque et al., 2022).Furthermore, the farmyard manure improved fatty acid content in the saline soil and increase soybean productivity in a salinity-stress environment (Mohammadi, 2015).It is also documented that the physical, chemical and biological characteristics of saline soil could be improved by applying organic manure (Wong et al., 2009).Application of water hyacinth compost and rice husk biochar (pyrolysed organic material) natively mitigates the salinity stress of soybean (Ferdous et al., 2018;Imran et al., 2022).
Although organic amendments approaches are especially advantageous for mitigating soil salinity, they have certain disadvantages, including the need for more labour, time, space, and raw resources, and more experienced and skilled people (Chowdhury et al., 2019).Alternately, Hoque et al. (2022) reported that organic amendments for relieving soil salinity are more accessible and less costly, environmentally safe and sustainable methods compared to inorganic amendments.The author seemed that the advantages of organic amendment outweigh than disadvantages.Therefore, organic amendments could be a sustainable option to increase microbial activities, fertility and productivity as well as reduce soil salinity in soybean.The authors concluded that soybean production in salt-affected areas could be conceivable through suitable soil management through organic amendments.

APPLICATION OF NANO-FERTILIZERS AND FUNGI
The imperative benefits of nano-fertilizers over chemical fertilizers depend on their supplement conveyance framework; they control the accessibility of supplements in crops through moderate slow mechanisms and such a moderate conveyance of supplements is related to the covering of supplements with nanomaterials (Liu and Lal, 2015;Solanki et al., 2016).If nano iron chelates are combined with organic manure that improved soybean seed oil yield and productivity under a salinity stress environment (Mohammadi, 2015).Application of nano-SiO2 enhanced soybean growth and increased germination while a mixture of nano-SiO2 and nano-TiO2 significantly increased the nitrate reductase, superoxide dismutase, and catalase under saline stress.Non-fertiliser improves soybean productivity under salinity (Lu et al., 2002).
Mycorrhizal organisms under saline make a difference to upgrade rates of particle transport, hormonal signalling, upgraded root development, advancement of metabolite generation by the host plant and expanded generation and movement of antioxidants (Miransari, 2017).Besides, arbuscular mycorrhizal fungi increase root hydraulic conductivity by adjusting the osmotic balance and composition of carbohydrates, which are colonizing halophytes that assist the survival of plants under salt stress (Evelin et al., 2009;Meena et al., 2018).When arbuscular mycorrhizal fungi were applied in soybean under saline stress, resulting in improved growth of plants, showing an increase in root and shoot fresh weight, dry weight, root proline, P, K, and Zn content (Sharifi et al., 2007).
Furthermore, potential salt-tolerant varieties can help overcome salt stress; however, these technologies are time-consuming and expensive.Basic, low-cost natural strategies for salt stress management that can be used in the short term must be developed.Non-fertilizers and organisms can play an important role in this perspective.

BREEDING APPROACHES
Salt-tolerant genotype establishment is an effective strategy for sustainable soybean production in salineprone areas.The use of molecular breeding strategies to selectively introduce desired genes may give effective ways to classical plant breeding to achieve salinity tolerance genotypes.These procedures will advantage the advancement of salinity-tolerant cultivars based on particular characteristics.Salt-tolerant genotypes development through the selection under salinity stress and the implementation of suitable management technologies are essential for sustaining soybean productivity under saline conditions (Fita et al., 2015;Nongpiur et al., 2016).Transgenic soybean development is another improved technology to grow soybean under a salinity stress environment (Zhang et al., 2013).In that case, high-throughput advances, counting entire genome sequencing, and genomics or proteomics approaches have driven the distinguishing proof of particular bunches of qualities that are expressed differentially in opportune and facilitated ways to upgrade salinity tolerance (Paul and Roychoudhury, 2018).Therefore, outstanding advancements in salt tolerance of soybean genotypes may be accomplished through the breeding approach and priority is supposed to give the improvement of salinity tolerance soybean genotypes worldwide through breeding.The following genotypes (Table 1) will help to the breeder to enhance the genetic studies to build up salt-tolerant soybean genotypes.

CONCLUSION AND FUTURE PERSPECTIVES
This review article discussed the morphological, physiological, and anatomical changes of soybean under salinity stress as well as stress management.Salinity stress can change the soybean germination, growth, development, nodulation, seed yield, and quality of seed which could lead to loss of seed yield and quality.Salinity stress affects the germination of soybean either osmotically through reduced water absorption or toxic effects of certain ions.Soybean growth and development are mainly affected by the hampering of cell division and elongation in soybean.Additionally, it is reduced by the inhibition of photosynthesis activities by the decrease in chlorophyll content.Salinity stress affects different reproductive stages and altering the flowering time, pod development and seed filling time detrimentally affects seed weight and seed yield.The number of nodulations in soybean could be reduced by the obstruction to rhizobial survival.Ionic imbalance is a serious problem under salinity stress which has a detrimental effect on metabolic functions.oxidative stress is developed by a decrease in antioxidant defences.This antioxidant response is associated with nitrogenase activity diminution in soybean.Anatomical traits could be changed under saline stress for example cutin synthesis on epidermal stem cells, trichome density, and xylem structure in soybean.Seed yield, oil, and protein content significantly decreased with increasing salinity in soybean.Salinity stress can be managed in different ways e.g., by the application of different exogenous components, organic and nano-fertilizers, arbuscular mycorrhizal fungi, and breeding approaches.The results of these results enhanced the knowledge to understand all the possible changes under salinity stress, which is basic for future enhancement of salinity-related investigations.The most impediment to this review is to lack discussion about the salt-tolerant genotypes and their molecular breeding.Future inquiries about assumed to be conducted on the screening of salt-tolerant soybean genotypes and their application in molecular breeding, which can offer assistance to create a potential salt-tolerant genotype to develop in salineinclined zones of the world.