Beneficial effects of exogenous osmoprotectants in mitigating salt stress-induced damages in rice.
Excess salinity in soil is one of the major environmental factors that limit plant growth and yield of a wide variety of crops including rice. On the basis of tolerance ability toward salinity, rice is considered as salt-sensitive crop, and growth and yield of rice are greatly affected by salinity. In general, rice can tolerate a small amount of saltwater without compromising the growth and yield. However, it greatly depends on the types and species of rice and their growth stage. Salinity-induced ionic and osmotic stresses reduce rate of photosynthesis and consequently cause oxidative stress, which is also responsible for growth reduction. The negative effects of salt stress that mentioned ultimately reduced yield of most crops including rice, except some halophytes. In recent decades, researchers have developed various approaches toward making salt-tolerant rice varieties. Using phytoprotectants is found to be effective in conferring salt tolerance to rice plants. In this chapter, we reviewed the recent reports on different aspects on salt stress tolerance strategies in light of using phytoprotectants.
- Oryza sativa
- abiotic stress
- oxidative stress
In a changing world, plants experience various kinds of environmental stresses (salinity, drought, heat, cold, flooding, heavy metals, ozone, UV radiation, etc.), which affect plant growth, yield and productivity that challenges the food security of ever-growing population all over the world [1, 2]. Of the environmental factors, salinity is one of the most brutal abiotic stresses, because most crop plants are sensitive to salt stress [1, 3, 4]. The condition of soil characterized by high concentrations of soluble salt is called salinity. About 6% of world’s total land area and one-third of world’s irrigated land area are affected by salinity directly or by secondary salinity. Among the cultivable area, 20% of irrigated land and 2% of the dry land area are affected by salinity directly or by secondary salinity . The problem of salinization is increasing day by day, often due to bad agricultural practices and climate changes. It is assumed that 50% of the cultivable land will be salt affected by the middle of the twenty-first century .
Excess salinity in soil is one of the major environmental factors that limit growth and yield of a wide variety of crops including rice [3, 4, 7]. Many studies revealed the negative effect of salinity on plant growth, development and yield [8–13]. The damages by higher salinity in plant start from germination and exist till death of plant . It is evident that salt stress has negative correlation with seed germination and vigor of wide variety of crops . In the seedling stage, salt stress affects plant growth by osmotic and ionic stress. Salinity-induced ionic and osmotic stresses reduce rate of photosynthesis and consequently cause oxidative stress, which is also responsible for growth reduction [3, 4, 14]. The negative effects of salt stress that mentioned ultimately reduced yield of most crops including rice, except some halophytes.
On the basis of tolerance ability toward salinity, rice is considered as salt-sensitive crop, and growth and yield of rice are greatly affected by salinity . Salinity-induced yield reduction of rice is alarming for the food security of ever-growing population of the world, especially in Asia, because 90% of the world’s rice is produced and consumed in Asia and more than 3 billions of Asian intake their 50–80% daily calorie from rice . So, it is an urgent task of plant biologists to develop salt-tolerant rice cultivar to ensure food security of rising population, since expansion of rice-growing areas is limited because of industrialization and various stresses including salinity .
Along with developing tolerant cultivar by breeding and genetic engineering, detoxification of reactive oxygen species (ROS) and methylglyoxal (MG), maintenance of nutrient homeostasis and reduction of salt uptake comprise some stress tolerance mechanisms within the plant under salt stress condition [11, 14, 17, 19]. Considering the factors that discussed, use of exogenous phytoprotectants become one of the important approaches for improving salt stress tolerance by osmoregulation, ROS and MG detoxification and ion homeostasis in rice [11, 14, 17, 18]. Considering the above-mentioned factors, in this chapter, we discuss salinity-induced damages and alleviation of salt stress by using exogenous phytoprotectants in rice plant.
2. Effect of salinity on rice plants
Soil salinity is not a new problem for rice production. In the coastal area, rice has been growing in saline soils since long time. The adjacent rivers, canals, streams and other water bodies are always contaminated with salt to a certain extent. With the initiation of green revolution, most of the rice production is being depending on irrigation water and salt intrusion in coastal areas becomes a common phenomenon recently. In general, rice can tolerate a small amount of saltwater without compromising the growth and yield. However, it greatly depends on the types and species of rice and their growth stage . Lee et al.  reported that the tolerance level of indica is higher than that of japonica at seedlings stage. At early stage of growth, rice is grouped as salinity susceptible cereal and confines its efficiency of production at mature stage [20, 21]. According to IRRI , soil salinity beyond EC ~4 dS m−1 is considered as moderate salinity for rice, while more than 8 dS m−1 is high. However, it is not absolute measure, and it depends on other soil factors because they are interacted with each other. Excess salt caused both ionic toxicity and osmotic stress in rice plants. Under high salinity, rice plants show various morphological, physiological or biochemical alterations and symptoms and even may die when the salt stress becomes very high ( Figure 1 ). Sodium ion itself causes direct cellular injury to plants, and additionally, higher amount of Na+ in root zone inhibits K+ uptake because of their antagonistic effect . This shortage of K+ inside the cell unavoidably leads to decrease in plant growth because K+ has vital role in preserving membrane potential, enzyme activities and cell turgor [21, 23]. Apart from Na+ or other anions, some of the cations like Cl− also show toxicity in rice. Ionic stress causes chlorosis and necrosis which either accelerate senescence or impair growth and development. Due to the excess accumulation of Na+ in the cytoplasm during NaCl salinity, cellular metabolisms such as protein synthesis and enzyme activities are hampered, and therefore, source-sink relationship and photosynthesis are disrupted . Many reports showed that Na+ accumulation in shoots is relatively well correlated with the survival of rice plants under salinity stress , and hence, keeping a lower cytosolic Na+ is considered as one of the vital strategies for salt tolerance in glycophytes . As a result of salinity-induced osmotic stress, water uptake by plant is hampered and plant suffers from physiological drought. This also led to the interruption of nutrient uptake. Under osmotic stress, regulation of water transport becomes a vital adaptive strategy of rice plants because a sufficient amount of water is indispensable for the cells to maintain their growth and vital cellular functions such as photosynthesis and metabolisms. This situation also induces stomatal closure, which results in the reduction of evaporation and water transport. A set of hormonal regulations also associated with these processes is upregulated many-fold under salt stress . In the next subsections, we provided a description of the specific responses of plants under salinity.
2.1. Seed germination
Germination of seed is the starting and one of the most important phases of plant life cycle that determines the better establishment of seedlings as well as growth, development and yield of plant. It is well documented that the presence of salt in germination media hampers germination and seedling establishment by osmotic and ionic stress . Salinity hampers seed germination by affecting major events of germination such as imbibition, metabolism activation, emergence of embryonic tissues and seedling establishment [4, 25]. Salinity-induced osmotic and ionic stresses inhibit and delay seed germination by limiting hydrolysis of seed through water imbalance, cell membrane destruction and enzyme activity reduction. Poor hydrolysis also limits translocation of food reserve from storage tissue to develop embryo that also negatively affects germination and seedling establishment. Salinity inhibits amylase activity, which is the major reason of poor hydrolysis or imbibitions of stored substances . However, the negative effect of salinity on seed germination may vary due to various factors, such as varietal variation, level of salinity and other external factors . Like other crop plants, rate of germination, speed of germination and seedling establishment of rice are greatly affected by salinity [26, 28]. Many studies revealed that salinity reduces germination percentage, germination index, germination speed, germination energy percentage (GE%) and mean germination time along with seedlings establishment in rice. Rajakumar  reported that salinity decreased germination percentage gradually with the increase of salt stress (0–300 mM NaCl) by higher accumulation of proline (Pro) and lower starch and protein content. Higher salinity (100 and 200 mM) delayed germination (3–6 days) and decreased germination percentages (upto 61%) by increasing solute leakage and decreasing α-amylase activity . Islam and Karim  reported that germination percentage and germination index decreased in 17 rice genotypes with increasing the level of salinity in germination media. Germination percentage and mean germination time decreased in six rice genotypes with increasing the concentration of salinity . Kazemi and Eskandari  observed the effect of salinity (0–8 dS m−1) on three rice cultivars (Anbar, LD and Hamar) and reported that salinity decreased germination percentage (up to 96%), germination speed, and plumule and radical growth with increasing the level of salinity. Effect of six levels of salinity (0–20 dS m−1) was studied by Hakim et al.  on 12 rice cultivars and noticed that the level of damage on seed germination and early seedling growth increased with increasing the level of salinity. They found that salinity decreased germination percentage, speed of germination and GE% up to 88, 99 and 100%, respectively, with highest level of salinity (20 dS m−1). Ologundudu et al.  also found that germination percentage, speed of germination and GE% decreased in eight rice cultivars under different levels of salinity (0–15 dS m−1).
2.2. Plant growth
It is well documented that seedling and early vegetative growth stages are most susceptible to salinity during the entire life cycle of plant [3, 4]. On the basis of tolerance ability, rice is salt-sensitive crop and the sensitivity to salinity varies with the growth stages. Among the growth stages, early seedling stages of rice are also considered as most sensitive to salinity compared with other stages . Aref  subjected rice plant to salinity at tillering, panicle initiation, panicle emergence and ripening stage where salt-induced damages were higher at tillering and panicle initiation stages compared with other two stages.
The immediate response of plant seedlings to high concentration of salt around the root is osmotic effect that reduces water uptake capacity. Osmotic stress disturbs the water balance and causes water loss from the cell that gradually reduces cell elongation, cell division, stomatal closure and leaf area as well as photosynthesis and growth [3, 5]. The presence of high concentration of salt (NaCl) around the root zone also reduces plant growth by ionic toxicity through over accumulation of Na+ and Cl−. In addition, Na+ influx causes chlorosis, necrosis and premature senescence of adult leaves and thus limits the photosynthetic area available to support continued growth of salt-affected plants [5, 16, 37]. In later stage, salt-induced ionic and osmotic stresses and lower rate of photosynthesis cumulatively cause oxidative stress by overproduction of ROS, which is also responsible for growth reduction under salt stress condition [3, 12].
However, many studies revealed the salt-induced growth reduction in rice. Exposure of higher salinity (150 mM NaCl) in rice seedlings reduces plant height and biomass by salt-induced osmotic, ionic and oxidative stresses . Rahman et al.  also reported that salt stress (200 mM NaCl) reduced plant growth by creating ionic and water imbalance, and oxidative stress. Salt-induced growth reduction by oxidative stress in rice seedlings was also reported by Özdemir et al. . Under salt stress condition, growth reduction and water loss were higher in susceptible cultivar compared with tolerant cultivar [3, 31]. Kumar and Khare  found that salt stress (100 mM NaCl ≈10 dS m−1) reduced root length, root dry weight, shoot length and shoot dry weight both in sensitive and in tolerant cultivar where growth reduction is higher in sensitive cultivar compared with tolerant. On the other hand, growth of rice seedlings decreased with increasing the level of salinity [31, 33]. Ologundudu et al.  conducted experiment with eight rice cultivars under different level of salinity (0–15 dS m−1) and reported that root and shoot length, root and shoot dry weight, and total dry matter production decreased with increasing the level of salinity.
2.3. Physiological attributes
Among different physiological processes, photosynthesis is a vital physiological attribute related to plant growth and development that is affected by salinity. Photosynthesis is a complex process depending on gas-exchange characteristics, photosynthetic pigments, photosystems, components of electron transport system and activities of different enzymes involved in carbon metabolism. Therefore, damage to any of those components affects photosynthesis negatively [39, 40]. Rate of photosynthesis declines under saline condition primarily due to osmotic stress that results in stomatal closure and secondarily by higher accumulation of Na+ and Cl− that can damage thylakoid membrane in the choloroplast . Salinity-induced limitation of CO2 diffusion causes inactivation of RuBisCo. Moradi et al.  demonstrated a remarkable decrease in photosynthetic CO2 fixation, transpiration and stomatal conductance (gs) in three rice cultivars due to salt stress. Salt-sensitive genotype IR29 showed greater reduction of these attributes than tolerant genotypes IR652 and IR632. In another study, Cha-um et al.  reported that in dark reaction, net photosynthetic rate (NPR), gs and transpiration rate (E) decreased in both varieties, Homjan (HJ) and Pathumthani 1 (PT1) varieties, under salt-stressed condition. Photosynthetic pigments, chl a and chl b, are greatly affected by different abiotic stresses including salinity. Accumulation of toxic Na+ reduces the content of precursor of chl biosynthesis (such as glutamate and 5-aminolevullinic acid) and thus interrupts chl biosynthesis under saline condition . Salinity-induced chlorophyll (chl) reduction is observed in rice like other crops. Amirjani  found a dose-dependent reduction of chl content against salt stress (0, 25, 50, 100 and 200 mM NaCl). At 200 mM NaCl, chl a and chl b reduced by 44 and 27%, respectively, compared to control. Rahman et al.  reported that 12-day-old rice seedlings exposed to 150 mM NaCl for 3 days caused 23 and 19% reduction in chl a and chl b, respectively, compared to control. As the duration of salt stress extended for further 3 days, chl a and chl b reduced by 46 and 48%, compared to control.
Reduction of photosynthesis due to stomatal closure and subsequent CO2 shortage affects source to sink translocation of photosynthates and carbohydrate metabolism in leaves. Photosynthates may contribute to salt stress tolerance by serving as osmolytes. Pattanagul and Thitisaksakul  observed carbohydrate metabolism in salt-tolerant and salt-susceptible rice genotypes. They mentioned increased total soluble sugar and sucrose content in susceptible genotype under salt stress, while starch accumulation increased in salt-tolerant genotype. To explain this result, they assumed that salt-susceptible rice genotype could not use carbohydrate for plant growth, thus growth reduced and total soluble sugar and sucrose content increased.
2.4. Osmotic stress
Salt stress imposes hyperosmotic stress in plants. Osmotic stress is physiological dysfunction resulted from alteration of solute concentration around a cell. Salt imposition in the growing media increases solute concentration around the root zone and lowering of the soil water potential for which plants cannot uptake water as a result plants suffer from osmotic stress. Salt-induced osmotic stress is often termed as physiological drought which is accountable for physiological disorders and injuries within the plants [45, 46]. Several research findings demonstrated osmotic stress and its damage effects induced by salinity. Osmotic potentials in roots and in the oldest and youngest leaves were measured. Salt-resistant cultivars (Nona Bokra and IR 4630) were characterized by lower osmotic potential, in contrast to the salt-sensitive cultivars (I Kong Pao and IR 3 1785) . Redillas et al.  reported that trehalose (Tre) producing transgenic Oryza sativa expressing TPSP (Ubi1:TPSP) accumulated more Tre under salt stress and showed less osmotic stress, in contrast to nontransgenic cultivar. Two rice cultivars [IR651 (salt-tolerant) and IR29 (salt-sensitive)] were grown under salt stress (100 mM NaCl), where total soluble sugars accumulation was higher in shoot of tolerant cultivar, compared to sensitive cultivar. Increased total soluble sugar was suggested to regulate osmotic potential and water uptake capacity under salt stress . Starch degradation and sugar accumulation were also reported in salt-affected rice plant and described as a strategy to improve osmotic status and plant survival. Salt-sensitive rice cultivar (Pathumthani 1) showed lower starch degradation and reduced sugar accumulation, compared to Homjan (salt-tolerant cultivar expressing starch metabolism related genes, AGPL1, AGPS2b and SBEIIb) under salt stress (150 mM NaCl, 7 days). Water use efficiency of Pathumthani 1 significantly reduced by 35%, whereas water use efficiency in salt-tolerant cultivar did not change. Starch metabolism capacity of sensitive cultivar was lower in salt-sensitive cultivar that decreased the water use efficiency . Salt stress due to lowering the soil water potential causes reduction of water uptake and as a secondary effect stomata closes. This stomatal closure reduces CO2 availability and fixation which reduce photosynthesis . Reduction of tissue water content, modulation of osmotic potential and accumulation of compatible solutes are the most common indications of salt-induced osmotic stress as studied in several research findings. Two different levels of salt stresses (150 and 300 mM NaCl, 48 h) were imposed to two different cultivars of rice, namely BRRI dhan49 and BRRI dhan54. Salt stress imposition decreased leaf RWC and increased Pro content in both cultivars. Being a salt-sensitive cultivar, BRRI dhan49 showed decrease in leaf RWC by 19 and 29%, under 150 and 300 mM salt stresses, whereas in salt-tolerant cultivar BRRI dhan54, RWC decreased by 12 and 28% under 150 and 300 mM NaCl stress, respectively, compared to control . Cha-Um and Kirdmanee  observed decreased water use efficiency in salt-sensitive rice cultivar exposed to salt (150 mM NaCl) stress. The KDML105 rice callus was grown in salt (250 mM NaCl) containing media for 2, 4, 6, 8 and 10 days. With the increase of salt stress duration, the water content of rice cells progressively decreased. Salt stress increased accumulation of Pro. The Pro content increased with the increase of duration of stress. Proline level reached to the highest level at the 10th day of salt stress . Rahman et al.  reported reduction of leaf RWC and increase of Pro as stress marker due to exposure of rice (Oryza sativa L. cv. BRRI dhan47) seedlings to 200 mM NaCl stress. Rice cultivars differing in salt tolerance capacity were subjected to different levels of salt stresses (20, 30, 40 or 50 mM NaCl) .
2.5. Ionic toxicity/imbalance
Salt stress generally involves with ionic toxicity/injury, and a high salt content in the growing media is able to break the ion homeostasis, destroy the ionic balance and induce nutritional disorder in plant cells . The nutritional disorders may be associated with the effect of salinity on nutrient availability, competitive uptake, distribution or transport within the plant . Generally, Na+, K+, Ca2+, Mg2+, Cl− and sometimes SO4 2− and CO3 2− ions are responsible for salinization. As Na+ predominates in the soils, the common problem for crop production is soil sodicity. In addition, sodic soils are important because they create very poor soil structure due to disaggregation of soil properties which impair water movement . So, most of the plant researches focuses on the transport mechanism of Na+ ion and its action to the plant cell. The ionic imbalances, which are developed by the accumulation of toxic ions, such as Na+ and Cl−, and the depletion of ions, such as K+ and Ca2+, are directly affected by salinity . The maintenance of Na+ and K+ homeostasis becomes more vital under salt stress. Plant exposed to salt stress is affected by Na+ influx, which causes K+ efflux and triggers K+ and Ca2+ leakage from plant cells because plants uptake higher amount of Na+ and show increasing Na+/K+ ratio [17, 18, 55]. Entrance and accumulation of Na+ to a high level become toxic to many enzymes because it competes with K+ for binding sites of important enzymes . Moreover, Na+ influx causes chlorosis and necrosis in adult leaves and premature senescence of mature leaves by upsetting protein synthesis and interfering with enzyme activity [5, 57]. Micronutrient deficiencies are very common under salt stress because the availability of micronutrients in saline soils is dependent on its solubility, pH and redox potential of the soil solution and the nature of binding sites on the inorganic and organic particle surfaces . Rahman et al.  carried out an experiment with rice plant and found NaCl-induced stress disturbed ion homeostasis through increase of Na+ content and decreased of K+ content in the shoots and roots of rice seedlings, which might be due to entrance of higher amount of Na+ into plant by nonselective cation channel (NSCC) that caused K+ efflux or leakage through NSCC and guard cell outward rectifying potassium channels (GORK). Importantly, they reported that higher Na+ accumulation also resulted in a higher Na+/K+ ratio which disrupted homeostasis by decreasing Mg, Mn and Zn contents. In rice, substantial variation in uptake and accumulation of Na+ between genotypes was repeatedly observed, and tolerant genotypes tended to accumulate less Na+ and maintain higher ratios of K+/Na+ and Ca2+/Na+ in plant tissues under salt stress [5, 59]. Azarin et al.  demonstrated that salt treatment strongly stimulated accumulations of Na+, Na+/K+ ratio, Cl− in root and shoot and reduced K+, NO3 − contents in both organs under 1.2% NaCl stress condition. They also stated that Cl− ions negatively affected the biomass growth and survival of rice plants. So, it can be concluded that salinity initially causes disruption of ion homeostasis and increases ionic toxicity and nutritional disorder in plant by increasing Na+ and Cl− uptake which finally hampers the growth and development of rice plant.
2.6. Oxidative stress
One of the common consequences of salt stresses in plant is the accelerated production of ROS . Exposure of plant to high salinity induces closure of stomata. As a result, CO2 availability as well as fixation in the leaf tissues becomes reduced. At the same time, a decreased reduction of CO2 by Calvin cycle and a state of excessive excitation energy occur, of which chloroplasts become exposed, and sequentially, photosynthetic electron transport system becomes impaired. Thus, salinity causes the excessive synthesis of ROS including superoxide (O2 •−), singlet oxygen (1O2), hydroxyl radicals (OH•) and hydrogen peroxide (H2O2) and results in oxidative stress [3, 4, 61]. A water deficit condition also occurs under salinity which further contributes to the generation of ROS . In plant cells, ROSs are continuously produced as a consequence of aerobic metabolism in all the intracellular organelles, particularly in chloroplast, mitochondria and peroxisomes  ( Figure 2 ). The reduction of the CO2/O2 ratio of chloroplast increases the synthesis of H2O2 by accelerating the photorespiration process . These ROSs are very reactive in nature and interfere with plants normal metabolism by causing the peroxidation of lipid, oxidation of protein, nucleic acid and DNA . However, plants try to detoxify ROS by their well-established antioxidant defense system, which includes both the enzymatic antioxidants, that is, catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione S-transferase (GST) and glutathione reductase (GR), and nonenzymatic antioxidants, that is, glutathione (GSH), ascorbate (AsA), tocopherols and carotenoids . These act coordinately in scavenging ROS and protecting cells from oxidative stress [2, 9].
Vaidyanathan et al.  studied with two rice cultivars, namely salt-sensitive Pusa Basmati 1 (PB) and salt-tolerant Pokkali (PK), and observed that the lipid peroxidation and H2O2 production were lower in PK than those of PB with concomitant enhancement of the activities of ROS-detoxifying enzymes as well as elevated levels of AsA and GSH. According to Rahman et al. , salt stress (150 mM NaCl) increased oxidative damage in time-dependent manner. Lipid peroxidation increased by 80 and 203%, H2O2 content by 74 and 92%, and LOX activity increased by 69 and 95% after 3 and 6 days of treatment, respectively. Accumulation of H2O2 and O2 •− in leaves was much higher than control, especially after 6 days of stress ( Figure 3 ). Results confirmed that the enhanced activities of MDHAR, DHAR, SOD and CAT in salt-stressed rice plant reinforced the salt stress tolerance. Salinity-induced oxidative stress in two rice varieties viz. Swarna (susceptible) and Nonabokra (tolerant) was investigated by Ghosh et al. . The two rice varieties were subjected to 200 mM NaCl, and both varieties showed increased accumulation of ROS and higher lipid peroxidation. Tolerant variety Nonabokra showed reduced damage of biomolecules including lipid, protein, enzymes and membranes with significantly higher accumulation of protective phenolic compounds compared to sensitive variety Swarna. Under high salinity, salt-sensitive varieties, that is, Hitomebore and IR28 showed decreased SOD activity and increased POX activity. The research result of Abdallah et al.  suggested that salt stress increased the level of SOD, CAT and Peroxidase (POX) in the shoot of two studied rice varieties, that is, Giza 178 and Giza 177. They found the increased SOD and POX activities in the NaCl-stressed treatment. Hong et al.  demonstrated that upon NaCl exposure, in rice roots, the OsGR2 and OsGR3 isoforms of GR were expressed and triggered by the increased H2O2 level under salt stress. The two important enzymes those scavenge ROS under salt stress in rice are APX and GR . Although the excess generation of ROS under salt stress is very common, it depends on the types of the plant genotype, dose and duration of stress . Tolerant plants show lower ROS generation rate which is correlated with their higher antioxidant defense capacities. In our study with two rice genotypes grown under salt stress (150 and 300 mM NaCl, 48 h), we observed that compared with the salt-tolerant variety, the salt-sensitive one showed increased lipid peroxidation (76 and 159%) and H2O2 (35 and 69%) generation at 150 and 300 mM NaCl, respectively . These were due to the increased activities of APX, DHAR, MDHAR, GPX, GR, CAT and glyoxalase I (Gly I) in tolerant (BRRI dhan54) variety than the sensitive variety (BRRI dhan49) .
2.7. Yield and grain quality
Salt-induced damages not only affect the vegetative growth but also affect reproductive development and yield of plant. Yield and grain quality of rice greatly influenced by salinity as salinity hampers growth, photosynthesis and net assimilation rate. Along with the vegetative stages, salt stress affects reproductive stage of rice that reduces yield, yield contributing parameters and grain quality. Zheng and Shanon  reported that salt stress affected reproductive stage of rice due to lower growth and lower survival percentage of seedlings under different levels of salinity (19–11.7 dS m−1). They noticed that salinity decreased grain yield by decreasing tiller number, pollen viability, fertility percentage and 1000-grain weight where the level of yield reduction increased with increasing the level of salinity. Kumar and Khare  subjected tolerant and sensitive cultivar to 100 mM NaCl (≈10 dS m−1) and noted that salinity reduced number of grain per panicle, filled grain percentage, 1000-grain weight and grain yield both in sensitive and in tolerant cultivar where yield reduction was higher in sensitive cultivar. They also noted that grain quality of rice deteriorated by salinity through reduction of protein and starch content of grain. Saleethong et al.  reported that salt stress reduced grain yield both in tolerant (32%) and in sensitive cultivars (56%) by affecting yield contributing parameters where yield reduction is higher in sensitive cultivar. They also reported that salt stress decreased grain quality by reducing N, P, K and Mg content in rice grain. Ali et al.  noticed that salinity (8.5 dS m−1) reduced grain yield of rice by reducing photosynthesis, leaf area index (LAI) and productive tiller. Chunthaburee et al.  conducted experiment with four different cultivars and found that salt stress (25 mM NaCl) reduced yield and harvest index by decreasing 1000-grain weight, filled grain percentage and panicle fertility. Arsa et al.  reported that salt stress (2.5% NaCl) along with drought stress reduced grain yield and aroma of rice grain. The level of yield reduction increased with increasing the level of salinity. Yield reduction also depends on stages of plant growth when it is affected by salinity. A green house experiment was conducted by Aref  with different levels of salinity (2–8 dS m−1) where salt stress exposed at different stages (tillering, panicle initiation, panicle emergence and ripening) of plant. He reported that salinity decreased grain yield, biomass yield and harvest index when salt stress was exposed at tillering and panicle initiation stage and yield reduction increased with increasing the level of salinity.
3. Use of phytoprotectants in conferring salt stress tolerance in rice
Plants to cope with salt-induced osmotic stress synthesize and accumulate compatible solutes or osmoprotectants. These are electrically neutral nontoxic and highly soluble molecules. Osmotic balance, stabilization of proteins and membranes are vital functions of osmoprotectants. Plants overexpressing genes for biosynthesis or metabolic genes for osmoprotectants or their precursors showed enhanced salt stress tolerance. Induction of salt stress enhances biosynthesis of osmoprotectants such as Pro, glycine betaine (GB), trehalose (Tre), sorbitol and ectoine, which contribute hyperosmotic stress tolerance generated from salt stress ( Table 1 ) [51, 76]. Several research findings evidenced the role of osmoprotectants to confer salt stress tolerance in rice seedlings ( Table 1 ). Salt stress (150, 200, 250 and 300 mM) was imposed to in vitro rice shoot apices cultures of two Malaysian rice cultivars MR 220 and MR 253. Salt stress decreased plant height, root length, biomass and chl content, and these stress effects were alleviated by exogenous Pro application. Supplementation of 5 mM Pro increased endogenous Pro level and plant height of both cultivars. Application of different levels of Pro (5, 10, 15 and 20 mM) increased fresh weight under 150 mM NaCl stress in both cultivars . Exogenous Pro reduced the Na+/K+ ratio and contributed an improved ionic homeostasis and reduced ion toxicity in salt-affected rice plant . Effect of exogenous Pro (25 and 50 mM) was studied in salt-sensitive (BRRI dhan29) and moderately salt-tolerant (BRRI dhan47) cultivars of rice grown under salt stress (50 and 100 mM NaCl) condition. Exogenous Pro (either doses) reversed salt-induced growth reduction of both cultivars under 25 mM NaCl stress. Proline also increased yield of salt-sensitive rice at same salt stress. Exogenous Pro increased chl and endogenous Pro levels and AsA contents. Increasing K+/Na+ ratio exogenous Pro contributes to reduce ionic toxicity. Increasing AsA content and activity of antioxidant enzyme GPX, and Pro imparted oxidative stress tolerance . Improved germination, growth and photosynthetic pigment levels were featured due to Pro (1, 5 and 10 mM) supplementation under salt stress (NaCl 100, 200, 300 and 400 mM) . Abdallah et al.  demonstrated that application of Tre (25 mM) alleviated salt damage effects resulted from 30 and 60 mM NaCl. They observed the beneficial effects on different physiological parameters. Trehalose supplementation with salt stress increased the content of photosynthetic pigments including chl a, chl b and carotenoid. Decreasing total carbohydrates exogenous Tre increased total soluble sugar and cellular Tre and decreased Pro level. Trehalose application also modulated antioxidant enzymes such as CAT, SOD and POX activities. Increased fresh weight, dry weight and RWC were also attributed by Tre application under salt stress. Exogenous GB sprays improved water use efficiency, structure of chl, CO2 assimilation and rate of photosynthesis and growth under salt stress condition . Increasing activities of GST and SOD and upregulating the levels of AsA and GSH exogenous Pro (5 mM) and GB (5 mM) conferred oxidative stress tolerance in both salt-sensitive (BRRI dhan49) and salt-tolerant (BRRI dhan54) rice cultivars where the performance of BRRI dhan54 was better . Nounjan et al.  also reported exogenous Tre application-modulated antioxidant enzymes activities including SOD, POX and APX. The increase of Gly II activity by exogenous Pro and GB application is an indication of improving MG toxicity tolerance in salt-affected rice seedlings . Sorbitol (5 and 10 mM) and Tre (5 and 10 mM) supplementation improved oxidative stress tolerance of salt (170 mM) stress-affected rice (Oryza sativa L. cv. KDML105) plant by preventing H2O2 generation, lipid peroxidation and membrane electrolyte leakage .
|Species and cultivars||Salinity dose and duration||Protectants||Protective effects||References|
|O. sativa ssp. indica||150 mM NaCl||50 mM GB||Increased chl and carotenoid contents|
Increased water use efficiency (WUE)
Increased seed weight and yield
|Cha-Um and Kirdmanee |
|O. sativa cv. Nipponbare||25 mM NaCl, 12 h||1 and 5 mM Pro and GB||Reduced Na+ uptake|
Increased K+/Na+ ratio
|Sobahan et al. |
|O. sativa cv. KDML 105||100 mM NaCl, 6 days||10 mM Pro||Increased FW and DW|
Reduced Na+/K+ ratio
Increased endogenous Pro and transcript levels of P5CS and P5CR
Upregulated transcription of genes encoding several antioxidant enzymes
|Nounjan et al. |
|O. sativa cv. Nipponbare||150 mM NaCl, 5 days||5 mM GB||Prevented salt-induced swelling of thylakoids, disintegration of grana stacking and intergranal lamellae, and disruption of mitochondria||Rahman et al. |
|O. sativa cv. KDML 105||170 mM NaCl, 24 h||5–10 mM Sorbitol and Tre||Enhanced growth|
Reduced H2O2 and MDA contents and electrolyte leakage
|Theerakulpisut and Gunnula |
|O. sativa L. cv. BRRI dhan29 and BRRI dhan47||50 and 100 mM NaCl||25 and 50 mM Pro||Increased K+/Na+ ratio|
Increased chl, intracellular Pro
Increased AsA content and activity of GPX
Increased growth and yield
|Bhusan et al. |
|O. sativa cv. Giza 177 and Giza 178||30 and 60 mM NaCl||25 mM Tre||Increased photosynthetic pigments and total carbohydrate|
Increased FW, DW and RWC
Decreased total carbohydrates but increased total soluble sugar and Tre, decreased Pro level
Increased CAT activity and decreased SOD and POX activities
|Abdallah et al. |
3.2. Plant hormones
Plant hormones or phytohormones are chemicals produced within the plants at low concentration and function as signaling molecules those regulate growth of plant. Plant hormones are essential endogenous molecules involved in regulation of plant development and tolerance toward various stresses including salinity . Recently, various kinds of plant hormones such as abscisic acid (ABA), auxin, cytokinins (CK), brassinosteroids, jasmonates and gibberellins (GA3) are exogenously applied for alleviating various kind of abiotic stresses including salinity ( Table 2 ). Li et al.  reported that ABA pretreatment conferred salt stress tolerance in salt-treated rice seedlings by abundant energy supply and active anabolism of nitrogen, nucleotide acid and carbohydrate through upregulating protein and energy metabolism ( Table 1 ). Seed treatment with ABA played potential role in alleviating salt stress damages by reducing Na+ and Cl− concentrations, Na+/K+ ratio and increasing K+ and Ca2+ contents .
|Species and cultivars||Salinity dose and duration||Protectants||Protective effects||References|
|O. sativa ssp. Indica||150 mM NaCl, 48 h||5 μM ABA pretreatment, 48 h||Upregulated energy metabolism, primary metabolism, secondary metabolism, protein and defense|
Increased FW and decreased Pro content
|Li et al. |
|O. sativa cv. IR-26||0, 50 and 75 mM NaCl|
Transplanting to harvest
|10 μM ABA priming, 24 h||Reduced Na+ and Cl– concentrations, Na+/K+ ratio|
Increased K+ and Ca2+ contents
Increased grain yield
|Gurmani et al. |
|O. sativa cv. IR 651, IR29||6 dS m−1|
Panicle initiation to harvest
|50 μM IAA, foliar spray||Increased sucrose and glucose contents|
Increased filled grain percentage, 1000-grain weight and grain yield
|Javid et al. |
|O. sativa cv. Hwangyoungbyeo||20 mM NaCl||20 μM IAA|
priming, 12 h
|Increased α-amylase activity|
Increased germination percentage
|Kim et al. |
|O. sativa cv. IR 651, IR29||6 dS m−1|
Panicle initiation to harvest
|50 μM kinetin, foliar spray||Increased sucrose and glucose content|
Increased filled grain percentage, 1000-grain weight and grain yield
|Javid et al. |
|O. sativa cv. Pokkali||50 and 100 mM NaCl||150 ppm GA3 foliar spray||Increased panicle plant⁻1, filled grains plant⁻1 and weight of 1000 grains|
Increased grain yield
|Misratia et al. |
|O. sativa cv. Pusa Basmati-1||NaCl 75–125 mM, 12 days||10−11, 10−9, 10−7 M EBL, priming 8 h||Increased chl a, chl b and total chl content|
Decreased MDA content
Upregulated SOD, CAT, GPX and DHAR activities
|Sharma et al. |
|O. sativa cv. IR-64||150 mM NaCl, 7 days||0.5, 1 and 3 μM EBL and 28-homobrassinolide||Increased germination percentage|
Increased seedlings length, fresh weight and dry weight
Enhanced nucleic acid and soluble protein level
|Anuradha and Rao |
cv. Dongjinchalbyeo and Dongjinbyeo
|20, 40 and 80 mM NaCl, 8 days||30 μM JA||Decreased Na content|
Increased K, Ca and Mg contents
Increased JA and ABA contents
Increased quantum yield of PS II
|Kang et al. |
The plant growth regulator auxin, namely indole-3-acetic acid (IAA), plays potential role in improving stress tolerance and grain quality of rice by regulating physiological and biochemical attributes under salt stress condition. Javid et al.  reported that exogenous application of IAA improved grain yield of rice by improving 1000-grain weight and filled grain percentage through increasing sucrose and glucose content under salt stress condition ( Table 1 ). Seed priming with IAA improves germination percentage by improving α-amylase activity ( Table 1 ) . The phytohormone cytokinin, namely kinetin, improves grain yield and grain quality of rice under salt stress condition by improving sucrose and glucose content in rice grain .
It is well documented that exogenous brassinosteroids play pivotal role in enhancing salt stress tolerance. Seed priming with 24-epibrassinolide (EBL) improved chl content and stress tolerance by activating antioxidant enzymes SOD, CAT, GPX and DHAR and reducing lipid peroxidation in salt-treated rice seedlings . Exogenous application of EBL and 28-homobrassinolide on salt contaminated growing media (150 mM NaCl) reverse inhibitory effect of salt stress and improve germination percentage, seedling growth by enhancing levels of nucleic acid and soluble proteins . Jasmonate is involved in plant developmental processes and the defense response by acting as signaling molecule. Kang et al.  reported that the exogenous application of jasmonic acid (JA) after 24 and 48 h of salt stress (20 and 40 mM NaCl) exposure recovered salt-induced damages in terms of reduction in root and shoot dry weight, especially in salt-sensitive cultivar compared with tolerant. Exogenously applied JA inhibited salt-induced damage by decreasing Na content and increasing K, Ca and Mg levels through upregulating plant hormone such as ABA and JA under salt stress condition. Seed priming with GA3 played positive role in germinating seed and on yield of rice under salt stress condition. Priming with 10 μM GA3 improves growth of seedling by increasing starch and total soluble and reducing sugar contents and α-amylase activity in germinating seed under salt stress condition . Misratia et al.  reported that supplementation with GA3 improved grain yield by increasing panicle plant−1, filled grains plant−1 and weight of 1000 grains under salt stress condition.
3.3. Signaling molecule
Similar to the nervous system of human and other animals, plants also sense different situation through the complex interplay of signal transduction networks and machinery. Accordingly, plants develop behavioral changes or develop cognition and storage of processed information to adapt in rapidly changing or variable environment. In recent years, considerable attempts have been taken to elucidate the signaling network in plants through physiological and molecular approaches. Moreover, exogenous application of some signaling molecules has also established their role in abiotic stress tolerance, and eventfully, these molecules became suitable candidate to be used as exogenous protectants ( Table 3 ). Since last few years, NO has showed multifarious roles in regulating redox-related gene expression, scavenging O2 •− and terminating chain reaction of lipid oxidation. Therefore, role of NO in salt stress tolerance in different crop plants is well documented [91, 92]. Habib et al.  reported that among different doses of NO (0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5 mM SNP as NO donor), 0.1 and 0.2 mM SNP improved germination under 80 mM salt stress in four rice genotypes. Uchida et al.  found that pretreatment with NO (1 mM SNP, 48 h) upregulated the activity of antioxidant enzymes such as SOD, CAT and APX, and thus, NO alleviated injuries caused by salt stress (100 mM NaCl, 8 days) in rice seedlings.
|Species and cultivars||Salinity dose and duration||Protectants||Protective effects||References|
|O. sativa cv. IRRI-6||80 mM NaCl||0.1 mM SNP, spray||Habib and Ashraf |
|O. sativa cv. Shaheen, PB-95, IRRI-6 and KS-282||80 mM NaCl||Seed soaking with 0.05–0.5 mM SNP||Habib et al. |
|O. sativa cv. Nipponbare||100 mM NaCl, 8 days||H2O2 (<10 μM), pretreatment, 48 h||Uchida et al. |
|O. sativa cv. Pokkali and KDML 105||25 mM NaCl||1 mM Spd||Paweena et al. |
|O. sativa cv. Pokkali and KDML 105||150 mM NaCl, 7 days||1 mM Spd||Saleethong et al. |
|O. sativa cv. Nonabokra and Swarna||200 mM NaCl, 7 days||1 and 2 mM Put||Ghosh et al. |
|O. sativa cv. IKP and Pokkali||50 and 100 mM NaCl, 5 and 12 days||1 mM Put||Quinet et al. |
|O. sativa M-1-48 and Pokkali||150 mM NaCl, 6 h||1 mM Spd or Spm||Chattopadhayay et al. |
|O. sativa cv. I Kong Pao||150 and 300 mM||1 and 10 mM Put, Spd and Spm||Ndayiragije and Lutts |
|O. sativa cv. I Kong Pao||50 or 100 mM NaCl, 5, 12 and 19 days||1 mM Put, Spd and Spm||Ndayiragije and Lutts |
|O. sativa cv. M-1-48, Nonabokra and Gobindobhog||200 mM NaCl||1 mM Spd or 1 mM Spm||Roychoudhury et al. |
|O. sativa cv. Niewdam Gs. no. 00621 and KKU-LLR-039||150 mM NaCl, 10 days||1 mM Spd||Chunthaburee et al. |
|Oryza sativa cv. IKP and Pokkali||50 and 100 mM NaCl, 5 days||1 mM Put, 5 days||Quinet et al. |
|O. sativa cv. Pokkali and KDML105||150 mM NaCl, 7 days||1 mM Spd, 24 h||Saleethong et al. |
|O. sativa cv. M-1-48, Nonabokra and Gobindobhog||200 mM NaCl, 15 days||1 mM Spd or 1 mM Spm, 15 days||Roychoudhury et al. |
Unlike NO, H2O2 is a long-lasting ROS, is able to cross organelle membrane and thus contributes to regulate redox-related gene expression. Depending on concentration, H2O2 itself causes damages to cell at higher concentration like other ROS or alleviates damaging effect of different abiotic stresses including salt stress at low concentration in various crop plants [2, 95]. However, triggering programmed cell death (PCD) during biotic stress is a well-known fact regulated by H2O2 signaling . Use of H2O2 (at low concentration) as protectant has been well studied under salt stress. Uchida et al.  found that exogenous application of H2O2 upregulated activities of antioxidant defense enzymes (CAT, SOD, POX, APX and GR), and thus, H2O2 mitigated salt-induced damages in rice. In another study, Wang et al.  investigated potential role of H2O2 in salt stress tolerance in two rice varieties (Zhenghan-2 and Yujing-6). Exogenous H2O2 enhanced APX, CAT, peroxidases (POD), SOD and G6PDH activities in roots of both rice cultivars in a dose-dependent manner. Using a plasma membrane (PM), NADPH oxidase inhibitor, diphenyleneiodonium (DPI) and NaCl-induced H2O2 accumulation were reduced, and therefore, the activity of above enzymes was downregulated. They concluded that PM NADPH oxidase-mediated reduction of H2O2 is responsible for salt stress tolerance in Zhenghan-2 variety of rice by modulating the cellular antioxidant level. In a recent study, Habib and Ashraf  found that exogenous nitric oxide donor (0.1 and 0.2 mM SNP) could enhance salt (80 mM NaCl) stress tolerance in rice by enhancing the activities of antioxidant enzymes (SOD, POD and CAT). However, the effect was dose dependent where 0.1 mM SNP was found to be more effective. They studied four different cultivars and found variable amount of decrease in growth and biomass production which was associated with increase in Pro, AsA, H2O2 and MDA contents. However, spraying with NO donor effectively upregulated the antioxidant defense and improved growth and grain yield .
Polyamines (PAs) are organic cations distributing in broad range of organism perform a range of physiological functions . Modulation of PAs interacting with other biomolecules (hormones/signaling molecule/amino acids) plays pivotal roles during plant developmental and stress adaptation process. PAs being cations bind to negatively charged surfaces of membrane and biomolecules and protect those from damage . PAs stimulate antioxidant system which reduces ROS production and oxidative damages to plants [51, 101]. PAs have some direct action under salt stress which alleviates salt-induced damage in plants. As polycation, PAs can block a variety of cation through different pathways like K+-selective channels, vacuolar-type channels and ammonium channels . Salt-induced Na+ influx, K+ efflux, shoot-to-root K+ recirculation and disruption of K+/Na+ homeostasis are directly modulated by PAs due to cationic nature which have been reported as obvious positive effects of PAs under salt stress [51, 103, 104]. Some previously reported roles of PAs in rice plant under salt stress condition have been discussed here ( Table 3 ).
Ndayiragije and Lutts  reported that 1 mM Put, Spd or Spm application increased K+/Na+ ratio in the shoots of salt-sensitive Oryza sativa L. cv. I Kong Pao. In their further study, they reported that exogenous Put, Spd or Spm (1 and 10 mM) application decreased Na+ and Cl− accumulation, improved cell viability and inhibited the alternative respiratory pathway in salt-sensitive Oryza sativa L. cv. I Kong Pao . Chattopadhayay et al.  described the roles of exogenous PAs to improve cation anion balance and photosynthesis. Exogenous Spd and Spm supplementation prevented salt-induced chl loss, inhibition of photochemical reactions of photosynthesis, improved PS I and PS II activities. Spd and Spm supplementation also decreased leakage of electrolytes and amino acids from roots and shoots caused by salinity. Exogenous Spd and Spm on endogenous decreased Na+ content and increased K+, Ca2+ and Mg2+ contents in salt-affected rice seedlings. Exogenous PAs were more effective to improve the performance of salt-sensitive M-1–48 plants, compared to tolerant Pokkali . Exogenous Put (1 mM) addition increased endogenous Put level in both cultivars under salt stress. Exogenous Put-induced endogenous PAs metabolism was correlated with improved performance of rice cultivars under salt stress. Salt stress modulated the activities of PAs biosynthesis enzymes. Activities of ornithine decarboxylase and arginine decarboxylase (involved in Put synthesis) were higher in salt-treated plants and after exogenous Put addition in salt-resistant Pokkali than in salt-sensitive IKP cultivars of rice. Exogenous Put reduced Na+ content in root of IKP cultivar . Spermidine pretreatment (1 mM Spd, 24 h) improved K+/Na+ homeostasis in salt-tolerant (Pokkali) and salt-sensitive (KDML 105) rice cultivars in exposure with salt stress (150 mM NaCl, 7 days). Increased chl content and improved plant height, fresh and dry weight of both cultivars were confirmed by Spd pretreatment under salt stress . Ghosh et al.  demonstrated that exogenous Put (1 and 2 mM) improved antioxidant defense system that reduced oxidative stress in salt-affected rice seedlings. Salt stress (200 mM NaCl) increased generation of O2 •−, H2O2 in tissues and caused membrane lipid peroxidation and protein carbonylation. Exogenous Put application increased the contents of nonenzymatic antioxidants such as anthocyanin and flavonoid. Activities of antioxidant enzymes GPX, CAT and GR were enhanced by exogenous Put application. Thus, enhanced antioxidant system conferred oxidative stress tolerance and overall salt stress tolerance in rice . Two cultivars of rice differing in their salt tolerance capacity [Pokkali (salt-tolerant) and KDML 105 (salt-sensitive)] were examined to evaluate their performance after Spd pretreatment. Spermidine pretreatment decreased H2O2 accumulation. Exogenous Spd pretreatment also prevented lipid peroxidation and decreased electrolyte leakage by stabilizing membrane. Prevention of chl breakdown was also demonstrated in Spd pretreated salt-affected rice seedlings. These results indicated the roles of exogenous Spd to improve oxidative stress tolerance under salt stress . Paweena et al.  noted the roles of Spd to improve yield attributes which ultimately increased the yield of salt-affected rice cultivars. Pretreatment with Spd increased grain yield per plant of KDML 105 by 62% and that of Pokkali by 16% under salt stress (25 mM NaCl) condition. Seed priming with Spd improved growth, anthocyanin and chl contents and decreased Na+/K+ ratio, Pro and H2O2 contents, particularly in the sensitive cultivar . Alleviation of growth inhibition, cellular damages, decrease of H2O2, MDA, LOX activity, protein oxidation and protease activity, maintenance of K+/Na+ balance, modulation of osmolytes and improvement of activity of antioxidant enzymes (GPX, APX and CAT) were attributed by exogenous Spd and Spm supplementation those conferred salt stress tolerance in different rice cultivars. However, among different rice cultivars [(M-1-48 (salt-sensitive), Nonabokra (salt-tolerant) and Gobindobhog (highly sensitive)], salt-sensitive rice cultivars responded better compared to salt-tolerant cultivars in response to exogenous PAs application under salt stress .
3.5. Trace elements
The effect of trace elements in mitigating the effect of abiotic stress in plants including salinity has become a matter of interest in recent decades. However, the mechanism is still unclear, and their responses are mostly dose dependent [3, 113]. In recent decades, plenty of researches have been carried on the positive effect of trace element on salt stress tolerance in rice ( Table 4 ), while many of them are focused on silicon (Si) as rice is a Si accumulator and its essentiality is well documented in rice.
|Species and cultivars||Salinity dose and duration||Protectants||Protective effects||References|
|O. sativa cv. BRRI dhan47||150 mM NaCl, 3 and 6 days||0.5 mM MnSO4, 3 and 6 days||Rahman et al. |
|O. sativa cv. Khazar and Zayandehrood||100 mM NaCl||3 mM Si as Na2SiO3||Mahdieh et al. |
|O. sativa cv. IR29||10 dS m−1||150 mg kg−1 Na2SiO3||Farooq et al. |
|O. sativa cv. KS-282 and IRRI-6||10 dS m−1||150 mg kg−1 Na2SiO3||Farooq et al. |
|O. sativa cv. Dongjin||100 mM NaCl; 6, 12 and 24 h||0.5, 1 and 2 mM Na2SiO35 H2O; 6, 12 and 24 h||Kim et al. |
|O. sativa cv. GR4, CSR10 and IR36||50 mM NaCl||3 mM Na2SiO3||Shi et al. |
|O. sativa cv. Pokkali and Peta||200 mM NaCl, 6–8 days||0.065 mM GSH, 6–8 days||Wang et al. |
|O. sativa cv. MRQ74||200 mM NaCl, 3 months||0.5 mM AsA, 3 months||Alhasnawi et al. |
|O.sativa cv. MRQ74 and MR269||200 mM NaCl, 14 days||1.0 mM AsA, 14 days||Alhasnawi et al. |
|O. sativa cv. Ciherang, IR 64, Lambur, Banyuasin, IR 42, Inpara 10, Margasari||5.99 dS m−1 (EC), whole life cycle||1500 ppm AsA, applied in 4 times for whole life cycle||Barus et al. |
|O. sativa cv. Pokkali and Peta||200 mM NaCl, 6–8 days||5 mM AsA, 6–8 days||Wang et al. |
|O. sativa cv. Pokkali||120 mM NaCl, 72 h||0.75 and 1.5 mM gallic acid (phenolic compounds), 72 h||Ozfidan-Konakci et al. |
Zinc (Zn)-supplemented rice plants showed enhanced salt tolerance as reported by Iqbal and Aslam . In their study, both salt-sensitive and salt-tolerant genotypes showed less plant height tiller, dry weight and fresh weight under salinity (70 mM NaCl), while these effects were reversed by the application of Zn (1–1000 nM). However, the effect of Zn was dose dependent. Mehmood et al.  observed that boron (B) could ameliorate the negative effect of salt stress in rice grown in the saline and saline sodic soils (9 dS m−1). However, the effect was mostly dose dependent. Boron fertilized at 1.5 kg B ha−1 showed higher yield of rice which was due to reduced shoot Na+ and Cl− concentration and better ratio of K+ and Na+ in shoot. However, B application at 6 kg B ha−1 showed adverse affect on grain and straw production in saline sodic soils. Boron supplementation also improved seed setting of rice . Zayed et al.  studied the effect of manganese (Mn), Zn and iron (Fe) on the performance of rice in saline soil. They observed that the application of the all micronutrients either alone or combined improved rice growth, dry matter production, leaf area index and chl content under saline condition. Moreover, yield attributes like panicle numbers, panicle weight, filled grains panicle−1 and 1000-grain weight were markedly increased when plants received the micronutrient which resulted in higher yield under saline soil . Farooq et al.  investigated the beneficial effects of exogenous Si in rice grown under salinity. Both salt-sensitive (IRRI-6) and salt-tolerant (KS-282) cultivars were used in this study. Their study showed that salt stress (10 dS m−1) reduced plant biomass by damaging the membrane and reducing special products analysis division values and photosynthetic efficiency which was accompanied by higher accumulation of Na+ and lower K+. However, the damages were higher in IRRI-6 than KS-282. Importantly, Si (150 mg kg−1) application counteracted the adverse effects of salt stress by reducing the uptake Na+, lowering transpiration rate and improving water content and photosynthetic efficiency . They also found the Si-induced upregulation of the activities of antioxidant enzymes viz. APX, GPX and CAT, and thus, the oxidative stress was mitigated . Mahdieh et al.  investigated the relative performance of Si-supplemented salt-sensitive and salt-tolerant rice cultivars (25-day-old) under saline condition (100 mM NaCl, 96 h). Investigation revealed that both rice cultivars showed stunted growth, reduced photosynthetic efficiency (Fv/Fm) and increased chlorosis which were associated with increased Na+ accumulation. On the other hand, Si supplementation decreased shoot Na+ concentration, but increased Si uptake which in turn increased the growth, chl content and Fv/Fm ratio in salt-treated plants. In our laboratory, we studied the protective effect of exogenous Mn (0.5 mM MnSO4) in mitigating short-term salt stress (150 mM NaCl, 3 and 6 days) in rice seedlings cv. BRRI dhan47. Any level of salinity not only diminished the ion homeostasis due to Na+ influx and K+ efflux but also caused osmotic stress due to limited amount of water and increased compatible solute content. As a result, salt-treated seedlings showed inhibited growth, chlorosis and excess generation of ROS (H2O2 and O2 •−) and higher amount of lipid peroxidation which was due to the disruption of antioxidant defense system . However, the seedlings supplemented with exogenous Mn showed significant recovery of growth and chlorosis ( Figure 4 ) by improving ionic and osmotic homeostasis through decreasing Na+ influx and increasing water status, respectively. Exogenous Mn also enhanced the activities of antioxidant enzymes (MDHAR, DHAR, SOD and CAT) and maintained higher level of nonenzymatic antioxidants (AsA and GSH) which were able to detoxify ROS. Exogenous Mn also upregulated the glyoxalase system and detoxified the level of toxic MG. Moreover, exogenous Mn effectively maintained the ion homeostasis and improved the growth of seedlings under salinity. The beneficial role of Se in mitigating salt stress has been studied in many plants but hardly found in rice. Recently, we completed a pot experiment with three rice varieties viz. BRRI dhan45, BRRI dhan47 and Nipponbare grown under different concentration of saltwater (50–150 mM). Salt stresses reduced the plant height and tillers hill−1, leaf relative water content and chl content in dose-dependent manner. Salt stress also reduced the effective tillers hill−1, number of filled grains panicle−1, 1000-grain weight, grain yield and straw yield. However, when the plants were supplemented with 0.5 mM Se (sodium selenite, Na2SeO3), these observed parameters were significantly increased compared to salt treatment alone. Importantly, the beneficial effect of Se on salt stress tolerance was prominent up to 100 mM NaCl, while it could not be more beneficial above this level of salt concentration . The response of rice plant to Se and salt stress greatly varied in different cultivars ( Figure 5 ).
Mitigation of abiotic stresses including salinity by using antioxidants as exogenous protectants is becoming familiar for plant biologists in recent time. There are many antioxidants including AsA, GSH, tocopherol, nonprotein amino acid, phenolic compounds and alkaloids that play a significant role in mitigation of excessive cellular reactive oxygen species activities caused by salt stresses [2, 121]. Moreover, these protectants have diverse roles in plant growth and development. Several research results have indicated that exogenous application of antioxidants provided significant protection against salt-induced damages in plants . However, very few researches have been carried out on the positive effect of antioxidant under salt stress in rice plant ( Table 4 ).
Wang et al.  carried out an experiment to identify the effects of exogenous AsA and GSH on two rice cultivars, the salt-tolerant cultivar Pokkali and the salt-sensitive cultivar Peta, under salt stress (200 mM NaCl). They found that salt stress hampered the antioxidant enzyme activities and increased MDA and H2O2 contents of the chloroplasts. But, addition of exogenous AsA and GSH reduced H2O2 and MDA contents and increased SOD, APX, GR activities and endogenous levels of AsA and GSH in the chloroplasts of both cultivars under salt stress, but the effects were significantly more prominent in cv. Pokkali. On the other hand, GSH played more significant role than the AsA on the plastidial reactive oxygen scavenging systems. These results indicated that exogenous AsA and GSH differentially increased salinity tolerance and mitigate the damages of salt-induced stress. Alhasnawi et al.  investigated the effect of exogenous AsA application on embryogenic callus of indica rice cv. MRQ74 cultured under saline conditions (200 mM NaCl). They reported that salt stress reduced relative growth rate (RGR), callus fresh and dry masses, K+ and Ca+2 content, and increased Na+ content and Na+/K+ ratio. Application of AsA (0.5 mM) alleviated these effects of salinity. The activities of SOD, CAT and POX, as well as content of Pro, were increased due to NaCl treatment, and these parameters were mostly further increased by AsA application. Accordingly, AsA was able to enhance callus tolerance to NaCl stress. Similarly, AsA lessened the damaging effect of salt stress in two rice cultivars (MRQ74 and MR269) under NaCl stress . They demonstrated that AsA (1.0 mM) considerably influenced a number of metabolic processes, leading to enhanced ability for seedling survival and growth through balancing or changing in the Na+ and K+ levels as well as the intracellular level of AsA. Exogenous AsA had effects on Na+ and K+ uptake and improving chl which have vital roles in number of metabolic processes. Ascorbic acid decreased Na+ uptake and the Na+/K+ ratio and increased the uptake of Na+ and K+ in contrast to salt stress alone. It also increased the length, FW and DW of shoot and root and chl content. Therefore, it may be concluded that exogenous application of AsA has low external osmotic potential and ion toxicity and might be effective in mitigating the effects of salt stress. Barus et al.  studied the effect of salt stress and its interaction with AsA on some morphological traits of eight varieties of rice (Ciherang, IR 64, Lambur, Batanghari, Banyuasin, IR 42, Inpara 10 and Margasari). AsA (0, 500, 1000 and 1500 ppm) was applied in four times at the age of 15, 35, 55 and 75 days after sowing. All concentrations of ascorbic acid generally had a positive effect on morphological characters. The leaf area, productive tiller and panicle length increased in all cultivars after application of AsA. However, the best response was found at 1500 ppm concentration of AsA on Banyuasin variety. Besides the AsA and GSH, gallic acid (GA) had a remarkable biological effect on plants growth, including an improvement in plants’ tolerance under salinity stress conditions. Ozfidan-Konakci et al.  conducted a study to examine the effect of exogenously applied GA on the tolerance of two rice cultivars (tolerant cultivar Pokkali and sensitive cultivar IR-28) to salt (NaCl) stress. Salt stress reduced the maximum photochemical efficiency (Fv/Fm) ratio, photochemical quenching coefficient (qP) and actual quantum yield (ΦPSII) of two rice cultivars, but the effect was more distinct in IR-28. Also, a significant increase in H2O2 content and lipid peroxidation was observed in NaCl treatment. The salt stress sensitivity of plants was higher in IR-28 than in Pokkali. However, addition of GA in NaCl-stressed Pokkali rice markedly decreased H2O2 and thiobarbituric acid reactive substances (TBARS) contents, and enhanced the activities of SOD, CAT, APX and POX as well as increase of relative growth rate (RGR), osmotic potential (ΨΠ), Fv/Fm ratio and Pro level in comparison with salt stress alone. So, it can be said that GA is able to mitigate NaCl toxicity of rice plant by increasing the level of antioxidants activities and photosynthetic efficiency.
4. Conclusion and perspectives
Salinity is one of the major abiotic stresses and constraints for agriculture worldwide because most of the crop plants are sensitive to salt stress . Salt stress adversely affects physiology and biochemistry of plants primarily by creating water stress, ionic imbalance and toxicity, nutritional disorders, alteration of metabolic processes, oxidative stress, membrane disorganization and reduction of cell division and expansion. However, plants have well-organized antioxidative defense mechanisms at the cellular level to minimize the toxicity. But in severe stress condition plant cannot cope with stress by its own mechanism. So, use of exogenous protectants like osmoprotectants, plant hormones, signaling molecules, polyamines, trace elements, antioxidants, etc., are now popular in research aimed at enhancing abiotic stress tolerance. On the other hand, rice is one of the world’s most important cereal crops with exceptional agricultural and economic importance as being a staple food for more than 50% population worldwide. This crop is highly susceptible to salt stress, and the productivity of rice decreased to a great extent under salinity. In recent time, the biochemical responses of rice plants to salt stress have been studied intensively. The use of exogenous protectants in rice plant under salt stress condition has been found to be very much effective to mitigate salt-induced damages. The mechanisms by which protectants regulate activities of enzymes are enhanced and are undoubtedly interesting and demand insightful studies in rice plant. The clear mechanism of defense and signal transduction pathways is still in dark and should be discovered. Furthermore, the exact dose, duration and proper methods of application of exogenous protectants in rice plant under salt condition should be studied more precisely for complete elucidation of mechanism of protection.