Physiological and Biochemical Basis of Stress Tolerance in Soybean

Abstract

Soybean is considered as a species sensitive to several abiotic stresses, such as drought, salinity, and waterlogging, when compared with other legumes, and these abiotic stresses have a negative effect on soybean plants’ growth and crop productivity. Clearing the conception on the physiological and biochemical responses to drought is essential for an overall understanding of the mechanism of plant resistance to water-restricted conditions and for developing drought resistance screening techniques that can be used for plant breeding. Plants can adapt in response to water scarcity situations by altering cell metabolism and activating various defense mechanisms. Higher salt tolerance in resistant soybean genotypes was associated with better water relation, salt dilution by juiciness, and better osmotic adaptation with an accumulation of more amino acids, sugars, and proline. In addition, less damaging chlorophylls, higher photosynthetic efficiency and cell membrane stability, and higher calcium content contributed to the higher salt tolerance of soybean genotypes. Plants adapted to flooded conditions have mechanisms to cope with this stress. Aerenchyma formation increased availability of soluble carbohydrates, greater activity of glycolytic pathways and fermenting enzymes, and involvement of antioxidant defense mechanisms to cope with post-hypoxic/post-anoxic oxidative stress. Ethylene, a gaseous plant hormone, plays an important role in altering a plant’s response to oxygen deficiency.

1. Introduction

Soybean (Glycine max L.) belongs to the family Leguminosae, was domesticated in China, which has the first records of soybean [1]. It is known as “golden bean” and “miracle crop” of the 20th century for its versatile and fascinating crop characteristics. It is an important grain legume because of its high protein (40–42%), oil content (18–22%) as well as Fe, Cu, Mn, Ca, Mg, Zn, Co, P, K, Vitamin B₁ and B₂ [2]. Besides oil, soybean is used as soy milk, soy flour, etc., and as ingredients of animal and poultry feed. In addition to being a source of macronutrients and minerals, soybeans contain secondary metabolites, such as isoflavones [3], saponins, phytic acid, oligosaccharides, goitrogens [4], and phytoestrogens [5]. Soybean is also viewed as an attractive crop for the production of biodiesel [6]. It also has the ability to fix atmospheric nitrogen (17–127 kg N ha⁻¹ year⁻¹) [7] and therefore requires minimal input of nitrogen fertilizer that often accounts for the single largest energy input in agriculture.

Soybean is inherently more stress tolerant [8] than other legume crops, but it still suffers considerable damage due to different abiotic stress. Drought effects negatively on soybean growth, physiology, and yield and yield reduction was observed 40% or even more due to drought. Drought-tolerant soybeans maintain higher proline and other osmoticums as well as higher chlorophyll content and water status in their leaves. Osmotic adjustment in plants subjected to salt stress can occur with the accumulation of high concentrations of either inorganic ions or low-molecular-weight organic solutes. Compatible solute accumulation in the cytoplasm is considered a mechanism to impart salt tolerance [9, 10]. The osmolytes generally found in higher plants are of low-molecular-weight sugars, organic acids, amino acids, proteins, and quaternary ammonium compounds. According to Cram [11], among the various organic osmotica, sugars contribute up to 50% of the total osmotic potential in glycophytes subjected to exposure to salt stress. Higher content of soluble proteins was observed in salt-tolerant than in salt-sensitive cultivars of barley, sunflower [12], and rice [13, 14]. It has also been reported that amino acids (alanine, arginine, glycine, serine, leucine, valine, etc.) and amides (glutamine, asparagine, etc.) accumulate in plants exposed to salt stress [15]. Total free amino acids in the leaves have been reported to be higher in salt-tolerant than in salt-sensitive lines of sunflower [12], safflower [16], Eruca sativa [17], and Lens culinaris [18]. Salt tolerance of plants related to ion transport through cation channel and governed by membrane protein plays a significant role in the redistribution of Na⁺ and K⁺ ions. Proline is widely distributed in higher plants, accumulates more than other amino acids [19], and regulates the accumulation of available N. Proline accumulation usually occurs in the cytosol and contributes significantly to cytoplasmic osmoregulation [20]. It is osmotically very active, contributes to membrane stability, and reduces the effect of NaCl on cell membrane destruction [21].

Waterlogging occurs whenever the soil is so wet that there is insufficient oxygen in the pore space for plant roots to be able to adequately respire. Lack of oxygen in the rhizosphere of plants causes their root tissues to rot. This usually happens from the root tips, making the roots look like they have been trimmed. As a result, plant growth and development are blocked. If the anaerobic condition persists, the plant will eventually die. Floods and inundation are abiotic and hierarchical stresses that, together with water scarcity, salinity, and extreme temperatures, are among the major determinants of the worldwide distribution of plant species. During waterlogging or submersion, plants are exposed to a reduced oxygen supply due to the low rate of oxygen diffusion in water and its limited solubility [22]. Turbid floodwaters can become anaerobic, especially overnight. Growth is greatly inhibited in the deficiency (hypoxia) or complete absence (anoxia) of oxygen. The mechanisms for different stress tolerance are complex and depend upon anatomical, biochemical, and physiological changes occurring in the whole plant rather than in a single cell. This chapter mostly discusses physiological and biochemical parameters that are related to stress tolerance in soybean.

1.1 Physiological and biochemical basis of drought tolerance in soybean

Water deficit or drought is one of the major abiotic stresses that negatively affect crop production worldwide. Intensification of the global water cycle [23] will be extreme events of drought and humidity increasing its frequency of occurrence in different areas of the globe, including the tropical and subtropical areas [24]. Soybean is a legume of great economic importance, but its production is highly dependent on optimal rainfall or abundant irrigation. In addition, during dry periods, additional irrigation may be required for drought-sensitive soybean varieties. Effects of water stress on soybeans, including osmotic regulation, reduce leaf surface area, plant height, decrease branching, reduce chlorophylls, low stomatal conductance and transpiration; fresh and dry matter reduction and finally yield loss have been well documented. Water stress, as a key abiotic limiting factor for soybean production, can cause soybean yield reduction up to 40% or even more [25]. The mechanisms of water stress tolerance, especially at low water stress levels, involve processes at the cellular level, most importantly osmotic regulation and protection of the membrane system. Osmotic regulation is a decrease in osmotic potential due to the active accumulation of organic and inorganic solutes in the cell. High concentrations of inorganic ions become detrimental to cell metabolism and must be sequestered in the vacuole. To maintain osmotic balance, specific types of organic molecules (such as soluble sugars, betaines, and proline) accumulate in the cytoplasm. Those compounds protect plants against stresses by cellular adjustment through the protection of membranes integrity and enzymes stability [26] are termed as compatible solutes, because they can be accumulated in high concentrations without impairing normal physiological function. Water-deficit stress adversely affects many physiological processes related to water use efficiency in soybean, thus leading to a decrease in plant productivity [27]. Relative water content (RWC) is used extensively to determine the water status of plants related to their fully turgid condition. According to Beltrano et al. [28] plants that are able to maintain high levels of RWC under water-deficit conditions are less affected by stress and are able to maintain normal growth and yield. Leaf water potential is considered to be a reliable parameter for quantifying plant water stress response. The effects of water stress on photosynthetic rates of soybean leaves are readily detectable at leaf water potentials about −1.0 to −1.2 MPa [29]. Siddique et al. [30] reported that changes in plant water potential might be attributed to a change in osmotic pressure—the osmotic component of water potential. Water stress significantly reduced the leaf water potential of soybean plants and the potentials fell from −0.88 MPa in unstressed leaves to −1.18 MPa in drought-stressed leaves [31]. Such observation was also reported by Ohashi et al. [32] in soybean. Leaf water potential in all the genotypes was higher under control conditions than that in stress conditions. Raper et al. [29] also reported that the effect of drought stress on photosynthetic rates of soybean declined rapidly with further reductions in leaf water potential to about −1.8 MPa, and then continue to decline gradually with decreasing water potential.

Drought-tolerant soybean cultivars have been investigated for revealing the mechanisms of tolerance and survival. Drought-tolerant soybeans try to adapt in water-deficit conditions through an increase in total sugar, proline, betains, sugar alcohols, and organic acids in their cell. The proline accumulation is a metabolic response characteristic of plants under abiotic stresses, it being showed the increase in the drought-tolerant soybean genotypes [33] because the free proline work as an osmotic adjustor that reduces the negative effects provoked in the plants under adverse conditions [34], besides promoting higher resistance in cells under these circumstances [35]. The proline is synthesized from glutamate and ornitine, in which the production of this organic solute, under conditions of the water shortage, occurs at the major part from glutamate [36]. The relative melondialdehyde (MDA) content was significantly higher (111%) in water stress conditions than control in drought-tolerant soybean variety Bina soybean1 [33], the lower relative values of MDA in Bina soybean1 indicate that at the cellular level this genotype is better equipped with efficient free-radical quenching system that offers protection against oxidative stress. The soybean variety Bina1 showed relatively higher tolerance to water stress in terms of yield compared with other genotypes. Higher water content, leaf proline and sugar accumulation, and lower MDA accumulation contributed to the higher drought tolerance of Bina soybean1 compared with other genotypes [33]. Rima et al. [37] found that higher water-deficit stress tolerance in soybean genotype G00081 was associated with higher water content in leaf, higher accumulation of proline, and less reduction of leaf chlorophyll.

1.2 Physiological and biochemical basis of salinity tolerance in soybean

Soil salt is one of the major problems of crop production in the arid and semi-arid regions of the world. Salt affects plant growth and development by causing a lack of water, reduced uptake and accumulation of essential nutrients, and increased accumulation of toxic ions, such as Na⁺ and Cl⁻ in plant cells. All of these factors cause changes in various physiological and biochemical processes, such as photosynthesis, protein synthesis, and nucleic acid metabolism [38, 39]. Even in well-hydrated soil, salt causes water scarcity by reducing the osmotic potential of dissolved soil material, making it more difficult for roots to extract water from the surrounding medium [10]. Excess sodium inhibits the growth of many salt-sensitive plants, which includes most of the crop plants. The osmotic adjustment is considered as one of the important mechanisms of water-deficit tolerance of plants [40], which promotes the protection of the plant cell structures including membranes and chloroplasts [41].

Ashraf and Harris [42] reported a considerable variation in the accumulation of soluble sugars in response to salt stress between tolerant and susceptible plants of both inter-specific and/or intra-specific genotypes. Regulation of ion transport is one of the important factors responsible for the salt tolerance of plants. Membrane proteins play a significant role in the selective distribution of ions within the plant or cell. According to DuPont [43], membrane proteins are involved in cation selectivity and redistribution of Na⁺ and K⁺. It is well established that Na⁺ moves passively through a general cation channel from the saline growth medium into the cytoplasm of plant cells [44, 45], and the active transport of Na⁺ is also occurred through Na⁺/H⁺ antiports in plant cells [46]. Salt tolerance in the plant is generally associated with low uptake and accumulation of Na⁺, which is mediated through the control of influx and/or by active efflux from the cytoplasm to the vacuoles and also back to the growth medium [44].

Some basic structural components of the membrane are affected by salinity. Vascularization of the plasma membrane is reported to be associated with salt tolerance in halophytes [47]. There can also be a significant increase in the endoplasmic reticulum. The increase in vesicles and endoplasmic reticulum may be a mechanism of compartmentalizing or exporting Na⁺ ions [48]. Under saline conditions, plasma membrane leakage increase in glycophytes, and there is a linear relationship between external salinity and membrane leakage [49].

The mechanisms for salt tolerance are complex and depend upon anatomical, biochemical, and physiological changes occurring in the whole plant rather than in a single cell. Mannan et al. [50] opined that the relatively high salt tolerance of AGS 313 was associated with the limited accumulation of sodium and high accumulation of different mineral ions in different plant parts, as well as the maintenance of better water relations under salinity than in the case of susceptible variety Shohag. Such variation in the response of both genotypes to salt-induced water deficit was attributed to the genetic ability of the resistant trait to undergo certain modifications in their metabolic pathway, thus declining their osmotic and water potentials with a concomitant preliminary decrease in their RWC. White and Izquierdo [51] reported that under severe stress conditions plant cells accumulate metabolites and make the osmotic potential of the cell more negative to maintain turgor pressure. The osmotic potential may be regulated through shifts in concentration of some osmoprotectants, such as proline and sugar. This mechanism is considered to be an important adaptation of plants to stress conditions. Relative protein content was higher in tolerant genotype AGS 313 than that of the susceptible genotype Shohag [52]. The decreased soluble protein due to salinity as found in their study is in agreement with the results of Subbarao et al. [53]. The enhanced protease activity with increasing concentration of NaCl supports the results of Sheoran et al. [54]. Ashraf and Tufail [12] observed a higher content of soluble protein in salt-tolerant cultivars than in salt-sensitive ones of barley and sunflower. Mansour [15] advocated that under salt stress conditions proteins might play a role in osmotic adjustment and thus it is accumulated in response to heat, cold, drought, waterlogging, and high and low mineral nutrient stress. Proteins that are accumulated in plants grown under saline conditions might provide a storage form of nitrogen that would be neutralized when stress is over [55]. In the salinity stress period, proline accumulation was twice in tolerant genotype AGS 313 than that of susceptible one Shohag [52]. Accumulation of proline in response to salinity was also observed by Weimberg et al. and Reddy and Vora [56, 57]. Enhanced proline accumulation with increased salinity levels was also observed by Khawale et al. [58] in different grape cultivars. Accumulation of proline content in plants grown in saline conditions might be due to hydrolysis of storage proteins needed for osmoregulation [59] and determining resistant capacity.

1.3 Physiological and biochemical basis of waterlogging tolerance in soybean

Waterlogging is defined as ponding of water over an area of crop land [60]. Waterlogging occurs whenever the soil is so wet that there is insufficient oxygen in the pore space for plant roots to be able to adequately respire. A lack of oxygen in the root area of a plant causes the root tissue to decompose. This usually comes from the tip of the root, so it looks like the root has been cut. The result of this is to stop the growth and development of the plant. In most cases, submersion does not last long enough for the plant to die. After a period of submersion, the plant begins to respire again. As long as the soil is moist, old roots close to the surface allow the plant to survive. However, additional root cuttings due to submersion and/or dry conditions can weaken plants to the point where they can become very unproductive and eventually die. Waterlogging is a widespread phenomenon drastically reducing the growth and production of soybean in many regions of the world [61], mostly due to the occurrence of flat topography [62], high water tables, and poor drainage of clay-like soils [63]. Effect of waterlogging in soybean plants may include leaf yellowing, reduced root growth, reduced nodulation, stunted growth, defoliation, reduced yields, and plant death [64]. Waterlogging treatment caused a reduction in plant growth in terms of leaf area and growth rate in all the genotypes, and the level of reduction was more pronounced in sensitive genotypes. Solaiman et al. [65, 66, 67] stated that waterlogging induced several physiological disturbances, including a reduction in growth, dry matter, photosynthesis, and pod formation that resulted in a low yield of soybean similar to that in other beans. Submersion causes energy starvation in plants as a result of root respiration difficulties due to O₂ deficiency [68]. Flooding and ultimately anaerobic metabolic energy limitation, accumulation of toxic products (e.g., lactic acid), and carbon loss (due to ethanol loss from the roots) can result in severe stunting and death in most crops [69]. Soybeans accumulate alanine [70], an amino acid produced by the enzyme alanine aminotransferase (AlaAT) under hypoxic conditions. Alanine synthesis plays an important role in the regulation of glycolysis, preventing excessive accumulation of pyruvic acid while maintaining intracellular carbon and nitrogen resources [71]. The production of lactic acid and ethanol, the accumulation of alanine has no harmful side effects on the cells.

2. Conclusions

Water-deficit stress exerted inhibitory consequences on plant morphology, physiological and biochemical parameters that sooner or later decreased the yield of soybean. Drought additionally decreased water content and chlorophyll. On the alternative hand, water-deficit stress improved proline and malondialdehyde content material in soybean leaves. Drought tolerance of soybean became located related to better water content, higher proline and much less malondialdehyde content material, and much less degradation of chlorophyll in the leaf. Salt tolerance in soybean became related to higher water relations, higher osmotic adjustment maintained with the aid of using collecting extra amino acid, sugar, and proline, much less chlorophylls degradation and better photosynthetic efficiency. Waterlogging tolerant soybean plant life capin a position to build up the better quantity of soluble sugars, boom fermentation enzymes and antioxidant protection mechanism under oxygen deficiency.

Conflict of interest

There is no conflict of interest.