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Effect of foliar application of potassium fertilizers on soybean plants under salinity stress
Journal of the Saudi Society of Agricultural Sciences xxx (xxxx) xxx
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Effect of foliar application of potassium fertilizers on soybean plants under salinity stress Bishnu Adhikari a, Sanjeev Kumar Dhungana a, Il-Doo Kim b, Dong-Hyun Shin a,⇑ a b
School of Applied Biosciences, Kyungpook National University, Daegu 41566, Republic of Korea International Institute of Agriculture Research & Development, Kyungpook National University, Daegu 41566, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 July 2018 Revised 10 September 2018 Accepted 27 February 2019 Available online xxxx Keywords: Foliar application Salinity Soybean Stress Vegetative growth
a b s t r a c t Soybean, one of the major food and fodder crops, yield is significantly reduced by different abiotic and biotic stresses, and salinity is one of the major abiotic stresses. The research objective of this study was to investigate the effect of foliar application of potassium chloride and potassium sulfate fertilizers at the early growth stage of soybean under medium (6 dS/m) and high (12 dS/m) salinity stresses. The effect of fertilizer application was measured on the bases of plant growth, levels of antioxidant activities, total polyphenol, flavonoid, chlorophyll, and carotenoid contents. Potassium sulfate showed better positive effect on the antioxidant activities, polyphenol, flavonoid, carotenoid, and chlorophyll contents compared to those of potassium chloride although the contribution was not noteworthy in comparison to the fertilizer unsprayed plants. The results of this study implied that foliar applications of 2.5% potassium fertilizers could not help reduce the negative effect of salinity stress at the early stage of soybean growth. This study suggests further researches using different concentrations of fertilizers at different plant growth and development stages. Ó 2019 Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Soybean, one of the most economically important crops, provides vegetable protein and oil for millions of people worldwide (Capriotti et al., 2014). Yield of soybean could significantly be improved by managing different biotic and abiotic stresses (Suzuki et al., 2014), including soil salinity (Zhu, 2001). About 30% of the total cultivated lands are affected by salt deposition (Kronzucker et al., 2008) which pose an increasing threat on world’s arable land availability (Nia et al., 2012). Overcoming salt stress is one of the major issues in these regions to ensure sufficient food production in a sustainable way (Heuer, 2003). Soil salinity has negative effects on crop productivity by inducing ion toxicity and osmotic stress in the plants (Zhu, 2001; Zhu et al., 1997). In most of the saline and sodic soils, sodium is one of the major toxic cations which mainly harms plants by disrupting the ⇑ Correspondence author. E-mail address: [email protected] (D.-H. Shin). Peer review under responsibility of King Saud University.
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uptake of potassium ion (K+) (Tester and Davenport, 2003). In addition, deposition of high concentration of sodium in the cultivated land could alter soil texture by decreasing soil aeration, porosity and water conductance (Mahajan and Tuteja, 2005). The response of plants to salinity differs greatly (Bonilla and Bolaños, 2004), and almost all the legume crops are sensitive to salinity (Noble and Roger, 1992). Normally, soybean is sensitive to salinity as it has threshold of 5 dS/m (Kamkar et al., 2014). Soybean growth, under high salinity conditions, is negatively affected resulting in drastically reduced yields (Khan et al., 2016). In order to minimize the damaging effects of salinity, various strategies, including plant nutrient management, are adopted. Foliar application of nutrients is beneficial in reducing the harmful effect of salinity stress (Babar et al., 2014; Fayez and Bazaid, 2014) by enhancing water uptake (Hu et al., 2008). The foliar applications of micro and macro nutrients are practiced whenever roots are restricted to uptake nutrients due to salt stress (Singh et al., 2013). If the nutrient supply via roots is restricted under saline soils, foliar application is highly recommended over the soil applications (Machado and Serralheiro, 2017). Potassium is one of the three essential macro nutrients and a key factor controlling crop productivity (Liu and Zhu, 1997). The salt stressed plants are better benefitted with the application of potassium sulfate and chloride fertilizers than the non-stressed
https://doi.org/10.1016/j.jssas.2019.02.001 1658-077X/Ó 2019 Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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ones (Trolldenier, 1985). Potassium contributes for tolerance against salinity as it has competing nature to sodium for binding and maintaining plant water status (Capula-Rodríguez et al., 2016). At the early plant growth stages, the root system may not get well developed to uptake sufficient nutrients from soil and in such a condition the foliar application of fertilizers could be a good option to supply the essential nutrients like potassium (K+) and phosphorous (P+) to the plants (Mallarino et al., 2001). High NaCl concentration in soil induces the P+ and K+ deficiencies in tomato (Adams, 1991) and in cucumber (Sonneveld and De Kreij, 1999). Fertilizing plants with K+ in order to raise the K+/Na+ ratio is an effective way for increasing the plants tolerance against salinity stress (Elhindi et al., 2016). Foliar application of potassium is carried out to reduce the salinity stress in sunflower (Arshadullah et al., 2014), sugar beet (Zaki et al., 2014), eggplant (Elwan, 2010), ryegrass (Tabatabaei and Fakhrzad, 2008), tomato (Amjad et al., 2014; Kaya et al., 2001), sunflower and safflower (Jabeen and Ahmad, 2011), endives (Tzortzakis, 2010), and wheat (Bybordi, 2015). Although a foliar application of potassium fertilizer has been reported beneficial against salt stress (Kaya et al., 2001; Akram et al., 2009), comparative studies on investigating the effect of different kinds of potassium fertilizer at early soybean growth have not been reported. Considering the economic importance of soybean crop (Capriotti et al., 2014) and ever increasing areas of saline soil (Shrivastava and Kumar, 2015; Zewdu et al., 2017) due to different reasons, including the sea-level rise, this study aimed to find out the comparative effectiveness of foliar applications of potassium chloride and sulfate fertilizers at early soybean growth under salinity stress. 2. Materials and methods 2.1. Plant material and growing condition Soybean [Glycine max (L.) Merrill cv. William 82] plants were used to determine the effect of foliar applications of potassium fertilizers under medium (6 dSm1) and high (12 dSm1) salinity stresses. The plants were grown in nursery-soil-filled 0.5-L plastic pots with drainage holes at bottom (Hester et al., 1996) at greenhouse with the average day and night temperatures of 23 and 15 °C, respectively, 75% relative humidity, and natural light conditions (Quebedeaux and Chollet, 1975). 2.2. Salinity stress Soybean plants were grown until V2 (second trifoliate leaf) stage and exposed to salt stress following an earlier method (Lee et al., 2008) which has reliable and simple screening procedures for salinity stress. In order to prevent soil leaking from the pots, a single layer of paper towel was placed at the bottom of pot. The pots were kept in plastic racks and salt stress was imposed by dipping bottom one-third (3.8 cm) portion of the pots into the medium or high concentration of salt solutions for 2 weeks. For inducing salinity stress common table salt was used. 2.3. Foliar application of fertilizer Potassium chloride and potassium sulfate were selected for foliar application as the two fertilizers have shown different effects in plants (Zehler et al., 1981). In order to maintain a medium level of potassium content, a 2.5% solution of potassium sulfate or potassium chloride was applied. The foliar application of potassium fertilizers was started after a week of salinity stress imposition. Two foliar applications of the fertilizers were carried out at 3 d interval
as more than one application is suggested for good absorption of the nutrients applied (Fageria et al., 2009). The fertilizers were applied to the both, upper and lower surfaces of the leaves. 2.4. Sample preparation and extraction The plants along with roots were collected after 2 weeks of onset of the salinity stress. The external materials and soil particles adhering to the roots were removed from plant and then allowed to surface drying at room temperature, followed by deep freezing (80 °C) before freeze drying. The freeze-dried plants were separated into shoots and roots and then pulverized into fine powders using a commercial grinder. Finely powdered samples were extracted with methanol (100%) using a shaking incubator (150 rpm) for 6 h and centrifuged (15,000g) at room temperature (22–25 °C) for 15 min. Through a syringe filter the supernatants were filtered and stored at 20 °C, and used within a week for measuring antioxidant activities and total polyphenol and flavonoid contents (Adhikari et al., 2018b). 2.5. Antioxidant activities 2.5.1. DPPH free radical scavenging activities DPPH free radical scavenging activities of roots and shoots were assayed following an earlier method (Dhungana et al., 2016). An equal volume of the freshly prepared methanolic solution of DPPH (0.1% w/v) and sample extracts was mixed and kept for 30 min at room temperature in dark. Similarly, equal proportions of methanol and the DPPH solution were mixed to take the absorbance of control. The absorbance values were measured at 517 nm using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific, Vantaa, Finland). The DPPH radical scavenging activity was determined using the following equation (S ß engül et al., 2014).
A 100 DPPH radical scavenging activityð%Þ ¼ 1 B
where A is the absorbance of sample and DPPH mixture and B is the absorbance of the methanol and DPPH mixture. 2.5.2. ABTS radical scavenging activities ABTS radical scavenging activities were assayed following the procedure of Ali et al. (2017). The ABTS radical was generated by mixing 2.5 mM potassium persulfate and 7 mM ABTS and keeping the mixture at dark for 16 h. With double distilled water the ABTS solution was diluted to obtain 0.7 absorbance value at 734 nm. A 20-uL of the sample extract solution was mixed with ABTS solution (180 mL) and the absorbance was measured at 734 nm using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific). The ABTS radical scavenging capacity was calculated using the following formula (Wan et al., 2011).
ABTS radical scavenging activityð%Þ ¼
Ac As 100 Ac
where AC is the absorbance of ABTS radical solution and AS is the absorbance of a mixture of ABTS radical and sample solution. 2.5.3. Superoxide dismutase (SOD)-like activity SOD-like activity was studied on the basis of 50% reduction of nitro blue tetrazolium by following the procedure of Dubey et al. (2015) and Adhikari et al. (2018a). At first, freeze dried leaf powder (0.5 g) was homogenized in a phosphate extraction buffer (5 mL) using an ice-cooled mortar and pestle. The homogenized solution was centrifuged (15,000g) for 20 min. After centrifugation, a reaction mixture consisting of NBT (2.25 mM), 100 mM phosphate buffer (pH 7.8), methionine (200 mM), sodium carbonate (1.5 M) and
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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EDTA (3 mM) was prepared and an enzyme extract of the sample (0.1 mL) was added. The reaction was initiated by adding 0.4 mL (2 mM) of riboflavin and placing under fluorescent light (15 W) for 15 min. A reaction mixture without the samples was considered as a control. The reaction was terminated by turning the light off and keeping the reaction mixtures in dark. Using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific), an optical density of the reaction solution was measured at 560 nm and the SOD-like activity was calculated on the basis of the following formula.
Superoxide dismutase like activityð%Þ ¼
Ao A 100 Ao
where Ao is the changed absorbance value of complete reaction mixture without a sample and A is the changed absorbance value of the complete reaction mixture with sample. 2.5.4. Reducing power potential The reducing power potential of sample extract was measured following the protocol mentioned by Ali et al. (2018). A solution of phosphate buffer (0.2 M, pH 6.6) was mixed with 100 mL of the sample extract and potassium ferric cyanide (1%, 900 mL) followed by incubation for 20 min at room temperature. A 900-mL of 10% trichloroacetic acid was mixed to the reaction solution and centrifuged (3000g) for 15 min. Nine hundred microliters of the supernatant was taken and mixed with an equal volume of double distilled water (900 mL) and 0.1% of ferric chloride each. Absorbance of the reaction mixture was measured at 700 nm using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific). The reducing power potential was expressed in microgram butylated hydroxytoluene equivalents per gram dry weight sample (mg BhtE/g).
extracted with 7 mL of dimethyl sulfoxide at room temperature for 24 h. The extracted solution was filtered through a filter paper (Whatman No. 4) and dimethyl sulfoxide was added to make the final volume 10 mL. For determining the carotenoid content, the absorbance of reaction mixture was measured at 470 nm and for the chlorophyll was measured at 644 nm and 662 nm using a microplate spectrophotometer (Multiskan GO). The chlorophyll a, chlorophyll b, and the total chlorophyll contents (ab) of the sample plants were calculated using the following equations (Arnon, 1949; Ibrahim et al., 2014).
Chlorophyll a ðCaÞ ¼ 12:7 ðOD662Þ 2:69 ðOD644Þ mg=L Chlorophyll b ðCbÞ ¼ 22:9 ðOD644Þ 2:69 ðOD662Þ mg=L Chlorophyll total ðCabÞ ¼ 20:2 ðOD644Þ þ 8:02 ðOD662Þ mg=L The carotenoid content was calculated according to the following equation (Villanueva et al., 1985).
Carotenoid ¼ ½OD470 1:28ðCaÞ þ 5:67ðCbÞ=ð256 0:906Þ mg=L where OD = optical wavelengths.
density
measured
at
the
respective
2.9. Statistical analysis Analysis of variance (ANOVA) was carried out using R-statistical software version 3.3.2 and least significant difference (LSD) was used to measure significant differences among treatment means at p < 0.05. Results are mentioned as means ± SE of triplicate experiments.
2.6. Total polyphenol content Folin-Ciocalteu reagent was employed to assay the total polyphenol content of sample extract following the procedure described earlier (Dhungana et al., 2015). Fifty microliters of sample extract was mixed with 1000 mL of 2% Na2CO3 using a vortexer. After 3 min, 50 mL of 1 N Folin-Ciocalteu reagent was mixed to the solution and allowed to stand at room temperature for 30 min in dark. Reaction mixtures’ absorbance value was measured at 750 nm using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific). The total polyphenol content was expressed in microgram gallic acid equivalents (GAE) per gram dry weight of sample (mg GAE/g) (Zeitoun et al., 2017). 2.7. Total flavonoid content The total flavonoid content of sample extracts was measured by following an earlier report (Zhu et al., 2010). Sample extract (300 mL) was mixed with an equal volume of double distilled water and 30 mL of 5% NaNO2. The mixture was allowed to stand for 5 min at room temperature and 60 mL of AlCl3 (10%) was added to the mixture. The mixture was again allowed to stand for 5 min and 200 mL of 1 M NaOH was added into it. At 500 nm the absorbance reading was taken using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific). The total flavonoid content was expressed in microgram quercetin equivalents (QE) per gram dry sample (mg QE/g) (de Souza et al., 2018). 2.8. Carotenoid and chlorophyll contents The carotenoid and chlorophyll contents of the soybean leaves were determined by following the procedure described by Hiscox and Israelstam (1979). A 100-mg of the ground leaves was
3. Results 3.1. Plant height The height of soybean plants under unstressed control (USC) and medium salinity (MS) conditions significantly differed with the foliar application of potassium fertilizers (Fig. 1). Under the USC, potassium sulfate significantly reduced the plant height compared to the potassium chloride; however under MS, the sulfate fertilizer showed positive effect. The effect of fertilizer application was not significantly different under high salinity (HS) condition. ANOVA shows that the stress on the plant height was highly significant (F = 46.43, p = 0.017) but the fertilizer (F = 2.0, p = 0.2494) and the interaction between fertilizer and stress (F = 1.5, p = 0.353) was not significant (Supplementary Table S1). 3.2. Fresh and dry weights of root and shoot The shoot and root fresh and dry weights were significantly reduced with the fertilizers application except for root fresh weight under USC (Fig. 2). The effect was more severe under HS than under MS. Overall effect of potassium chloride on fresh and dry weights root and shoot was more favorable than potassium sulfate application. Stress had significant effect on both shoot and root fresh (Shoot fresh weight: F = 170.16, p = 0.0001, Root fresh weight: F = 58.98, p = 0.0011) and dry weights (Shoot dry weight: F = 632.82, p < 0.0001, Root dry weight: F = 11.11, p = 0.0233) but the fertilizer (F = 43.27, p = 0.002) and the interaction between stress and fertilizer (F = 13.14, p = 0.0143) were significant only for root fresh weight (Supplementary Table S1).
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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Fig. 1. Soybean plant height under salinity stress with potassium foliar applications. In the same stress conditions different letters above bar diagram are significantly different (p < 0.05).
Fig. 2. Fresh and dry weights of shoot and root of soybean plants under salinity stresses. A: fertilizer unsprayed, B: potassium chloride (KCl) application, and C: potassium sulphate (K2SO4) application. Different letters above each bar diagram within the same measurement are significantly different (p < 0.05).
3.3. DPPH radical scavenging activities The DPPH radical scavenging activity in root was not significantly affected with the fertilizer application under USC, however the activity was reduced in shoot (35.51%) with potassium chloride application (Table 1). Similarly, under MS, potassium chloride reduced the DPPH scavenging activity of root (75.59%) compared to that of fertilizer unsprayed and potassium sulfate treatment. However, under HS condition, the foliar spray of fertilizer enhanced the DPPH radical scavenging activity of roots (18.36– 18.79%) compared to that of the USC (11.52%). DPPH radical scavenging activities on shoot were significantly affected by the stress (F = 33.34, p = 0.0032) and fertilizer (F = 7.6, p = 0.0434) whereas
fertilizer and stress interaction (F = 0.76, p = 0.6009) did not have any significant effect (Supplementary Table S1).
3.4. ABTS radical scavenging activities Potassium sulfate application under MS conditions had the highest ABTS radical scavenging activities in shoot (84.9%) and root (82.59%) as shown in Table 2. Under USC, fertilizer unsprayed (78.03%) and potassium chloride application (74.31%) had significantly higher radical scavenging activities in shoot than the potassium sulfate application whereas the effect was not significant in root for either treatment. Similarly, under HS there were not any
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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B. Adhikari et al. / Journal of the Saudi Society of Agricultural Sciences xxx (xxxx) xxx Table 1 Effect of foliar application of potassium fertilizers on the DPPH radical scavenging activities of soybean plants under different salinity stresses. Treatment
Plant part
Fertilizer unsprayed
Shoot Root Shoot Root Shoot Root
2.5% KCl 2.5% K2SO4
DPPH radical scavenging potential (%) Unstressed control
Medium salinity
High salinity
48.88 ± 4.14aA 27.54 ± 4.47aAB 35.51 ± 3.38bA 19.91 ± 3.40aA 44.93 ± 0.89abA 23.52 ± 3.20aB
38.18 ± 7.06aAB 33.79 ± 2.03abA 31.79 ± 3.68aA 19.97 ± 7.53bA 41.14 ± 0.90aB 39.39 ± 1.32aA
22.33 ± 1.75aB 11.52 ± 2.19bB 19.56 ± 2.75aB 18.36 ± 0.32aA 21.78 ± 0.47aC 18.79 ± 0.93aB
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A– C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05). Table 2 Effect of foliar application of potassium fertilizers on the ABTS radical scavenging activities of soybean plant under different salinity stresses. Treatment
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
Plant part
Shoot Root Shoot Root Shoot Root
ABTS radical scavenging potential (%) Unstressed control
Medium salinity
High salinity
78.03 ± 0.69abAB 81.34 ± 1.36aA 83.72 ± 0.23aA 74.31 ± 1.36bA 75.56 ± 3.81bB 80.58 ± 1.14aA
83.41 ± 0.32aA 78.97 ± 1.47abA 76.94 ± 1.11bB 75.59 ± 1.09bA 84.98 ± 0.59aA 82.59 ± 0.91aA
72.59 ± 2.32aB 73.36 ± 1.21aB 66.69 ± 1.55aC 78.21 ± 1.35aA 68.13 ± 1.25aB 76.68 ± 3.91aA
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A– C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05).
significant differences among the treatments in both the shoot and root parts. 3.5. Total polyphenol content Under USC, in the root of fertilizer unsprayed (503.83 mg GAE/g) and the shoot of potassium sulfate applied (489.78%) plants had significantly high polyphenol content compared to other treatments (Table 3). Under MS, fertilizer unsprayed had the significantly highest total polyphenol content in both shoot (519.94 mg GAE/g) and root (445.53 mg GAE/g) compared to the fertilizer applications. Under HS, in the shoots fertilizer application caused significant increase in the total polyphenol content (540.03–548.81 mg GAE/g) compared to the fertilizer unsprayed (329.53 mg GAE/g). Stress had significant impact on the total polyphenol content in the shoot (F = 47.66, p = 0.0016) and root (F = 41.74, p = 0.0021) but the fertilizer application had only significant effect in the shoot (F = 67.01, p = 0.0008) whereas interaction between the fertilizer and the stress (Shoot: F = 66.65, p = 0.0006, Root: F = 31.03, p = 0.0029) had significant effect on both shoot and root (Supplementary Table S1).
MS, in the shoot and root of fertilizer unsprayed (2053.1 mg QE/g and 551.62 mg QE/g) and root of potassium sulfate (556.71 mg QE/ g) applied plants had significantly high amount of flavonoid content compared to the respective parts of other treatments. Potassium chloride application, under HS, showed significantly high amount of flavonoid content in the shoot compared to the other treatments, however the total flavonoid content in root (699.24 mg QE/g) was not significantly affected by the treatments. 3.7. Super oxide dismutase (SOD)-like activities The root of salinity stressed plants showed comparatively higher SOD-like activities than the shoot (Table 5). Under USC, the shoot of either treatment did not show significant difference whereas the root of potassium sulfate (25.49%) sprayed plant had significantly low SOD-like activities compared to that of fertilizer unsprayed (30.07%) and potassium chloride (29.40%) treatments. The shoot of fertilizer unsprayed (28.36%) plants had significantly high SODlike activities compared to that of potassium fertilizers application (18.11–19.59%). Under HS, the fertilizer application did not cause significant variation in the SOD-like activities of shoot part.
3.6. Total flavonoid content 3.8. Reducing power potential Potassium sulfate application significantly increased the shoot flavonoid content under USC whereas potassium chloride caused significant reduction in shoot as well as in root (Table 4). Under
Application of potassium sulfate fertilizer had significant positive effect on increasing the reducing power potential in the shoot
Table 3 Effect of foliar application of potassium fertilizers on the total polyphenol content of soybean plant under different salinity stresses. Treatment
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
Plant part
Shoot Root Shoot Root Shoot Root
Total polyphenol content (mg GAE/g) Unstressed control
Medium salinity
High salinity
409.31 ± 9.25bB 503.83 ± 8.97aA 368.03 ± 18.67bB 267.17 ± 2.48cB 489.78 ± 5.08aB 352.08 ± 5.61bA
519.94 ± 5.48aA 445.53 ± 8.59aB 394.33 ± 4.12cB 262.33 ± 2.86cB 491.31 ± 2.18bB 358.89 ± 3.39bA
494.56 ± 10.28bA 329.53 ± 5.51bC 548.81 ± 9.01aA 368.06 ± 1.90aB 540.03 ± 4.76aA 267.67 ± 1.0cB
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05). GAE = gallic acid equivalent.
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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Table 4 Effect of foliar application of potassium fertilizers on the total flavonoid content soybean plant under different salinity stresses. Treatment
Plant part
Fertilizer unsprayed
Shoot Root Shoot Root Shoot Root
2.5% KCl 2.5% K2SO4
Total flavonoid content (mg QE//g) Unstressed control
Medium salinity
High salinity
152.61 ± 42.03bB 586.24 ± 45.58aA 1446.1 ± 25.86bA 338.76 ± 14.89bB 1966.43 ± 45.34aA 500.9 ± 19.15aA
2053.1 ± 72.42aA 551.62 ± 20.92aA 1484.57 ± 31.91bA 443.1 ± 2.48bAB 1257.52 ± 61.44cB 556.71 ± 12.72aA
562.81 ± 20.2bC 536.95 ± 54.45aA 699.24 ± 15.23aB 507.81 ± 56.31aA 612.52 ± 62.44bC 472.81 ± 32.88aA
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05). QE = quercetin equivalent.
Table 5 Effect of foliar application of potassium fertilizers on the SOD-like activities of soybean plant under different salinity stresses. Treatments
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
Plant part
Shoot Root Shoot Root Shoot Root
SOD-like activities (%) Unstressed control
Medium salinity
High salinity
18.03 ± 1.53aB 30.07 ± 1.61aB 16.7 ± 1.34aA 29.4 ± 0.96aC 13.32 ± 1.43aA 25.49 ± 0.25bC
28.36 ± 0.88aA 29.05 ± 1.69bB 19.59 ± 2.64bA 41.62 ± 1.74aB 18.11 ± 2.50bA 35.29 ± 3.11abB
17.31 ± 3.14aB 47.78 ± 1.13bA 16.86 ± 3.96aA 55.18 ± 2.18aA 19.74 ± 2.17aA 55.24 ± 1.53aA
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05).
compared to other treatments under USC, however fertilizer application had negative effect on the root reducing power potential (Table 6). Under MS, the fertilizer application (1.6–1.8 mg BhtE/g) significantly reduced the reducing power potential in shoot compared to the fertilizer unsprayed (2.19 mg BhtE/g). Under HS, the reducing power potential of shoot and root of potassium chloride (1.51 and 1.77 mg BhtE/g) sprayed plants was significantly high compared to that of the potassium sulfate (1.3 and 1.55 mg BhtE/g, respectively).
Table 7 Effect of foliar application of potassium fertilizers on the carotenoid and chlorophyll a, b and total contents of soybean plant under different salinity stresses. Treatments
aA
Medium salinity
0.57 ± 0.004cC 0.63 ± 0.007bC 0.72 ± 0.016aC
Chlorophyll a content (mg/L) Fertilizer unsprayed 13.11 ± 0.24aA 2.5% KCl 4.13 ± 0.02bA 2.5% K2SO4 12.36 ± 0.3aA
6.65 ± 0.07aB 3.88 ± 0.02cB 4.75 ± 0.15bB
1.27 ± 0.03bC 1.49 ± 0.05aC 1.55 ± 0.03aC
Chlorophyll b content (mg/L) Fertilizer unsprayed 4.64 ± 0.06aA 2.5% KCl 2.15 ± 0.01bB 2.5% K2SO4 4.69 ± 0.05aA
3.01 ± 0.07aB 2.29 ± 0.03cA 2.59 ± 0.09bB
1.32 ± 0.01bC 1.37 ± 0.01bC 1.69 ± 0.06aC
Total chlorophyll (ab) Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
8.54 ± 0.02aB 5.5 ± 0.05cA 6.53 ± 0.12bB
2.36 ± 0.02cC 2.59 ± 0.03bB 2.96 ± 0.07aC
3.73 ± 0.062 1.34 ± 0.001bA 3.6 ± 0.067aA
content (mg/L) 15.55 ± 0.26aA 5.57 ± 0.01bA 14.97 ± 0.28aA
aB
High salinity
2.05 ± 0.004 1.33 ± 0.012cB 1.57 ± 0.028bB
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
3.9. Carotenoid and chlorophyll contents Potassium chloride had significantly reduced the carotenoid and total chlorophyll contents (1.34 mg/L and 5.5 mg/L) compared to fertilizer unsprayed and potassium sulfate under USC (Table 7). Under MS, both the fertilizers significantly reduced the carotenoid and chlorophyll contents compared to fertilizer unsprayed plants. Under HS, potassium sulfate application had significantly increased the carotenoid and chlorophyll contents (0.72 mg/L and 2.96 mg/L) as compared to other treatments. Stress, fertilizer and their interaction have highly significant effect on the total carotenoid (Stress: F = 839.3, p < 0.001, Fertilizer: F = 114.84, p = 0.0003, Interaction: F = 70.92, p = 0.0006) and chlorophyll (Stress:
Carotenoid content (mg/L) Unstressed control
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05).
Table 6 Effect of foliar application of potassium fertilizers on the reducing power potential of soybean plant under different salinity stresses. Treatments
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
Plant Part
Shoot Root Shoot Root Shoot Root
Reducing power potential (mg BhtE/g) Unstressed control
Medium salinity
High salinity
1.83 ± 0.01bB 2.38 ± 0.008aA 1.82 ± 0.009bA 1.93 ± 0.015cA 2.4 ± 0.018aA 2.03 ± 0.009bB
2.19 ± 0.014aA 2.2 ± 0.007bB 1.6 ± 0.002cB 1.84 ± 0.007cB 1.8 ± 0.015bB 2.3 ± 0.011aA
1.48 ± 0.012aC 1.55 ± 0.008bC 1.51 ± 0.005aC 1.77 ± 0.011aC 1.3 ± 0.014bC 1.55 ± 0.062bC
Results are expressed as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05). BhtE = Butylated hydroxytoluene equivalent.
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
B. Adhikari et al. / Journal of the Saudi Society of Agricultural Sciences xxx (xxxx) xxx
F = 3532.26, p < 0.0001, Fertilizer: F = 839.62, p < 0.0001, Interaction: =481.72, p < 0.0001) content (Supplementary Table S1).
4. Discussion The reduced plant height for fertilizer application might be due to the short-term chemical stress of fertilizer (Hu et al., 2008). Osmotic adjustment of the plant under medium salinity increased the length of the plant as compared to high salinity (Qados, 2011). Early plant growth stages are highly sensitive to the salt stress due to which medium and high salinity stress significantly decreased fresh and dry weights of the shoot and root (Acosta-Motos et al., 2017). The reduced shoot and root fresh and dry weights under medium and high salinity stresses are associated with elevated levels of cell death and lipid peroxidation along with the addition of reactive oxygen species (Klein, 2012). Potassium chloride application caused lower plant height and shoots and root fresh and dry weights than potassium sulfate application. Potassium chloride tends to more strongly salinize than potassium sulfate (Marques et al., 2014) and had detrimental effects on plant growth and development (Teixeira et al., 2011). The responses of plants under medium salinity stress are related to antioxidant potentials. Polyphenol and flavonoid contents are directly related to antioxidant potential which is associated to the free radical scavenging activities (Ksouri et al., 2007). Antioxidant activities under salt stress could vary among the plant species and even among the cultivars of the same plant species. The level of salt stress response depends on duration and intensity of the stress and the plant species and their development and metabolic state (Klein, 2012). Under medium salinity stress, potassium sulfate foliar application had high level of DPPH and ABTS radical scavenging activities. Potassium foliar application could trigger the enzymatic antioxidant mechanisms. These mechanisms protect against the secondary oxidative stress induced by medium salinity (Marques et al., 2014). Increased antioxidant activities have been considered as an adaptation mechanism to salt stress (Bargaz et al., 2015). These activities are positively associated with decreasing oxidative damage and improving salinity tolerance (Kibria et al., 2017). High salinity stress has a negative effect on the physiological activities of plants (Corrêa et al., 2013). Potassium chloride application increased the flavonoid content under the medium salinity which is also corroborated by the findings of Chen et al. (2013). The high polyphenol and flavonoid contents under medium salinity are lined up with results of Valifard et al. (2014). The SOD-like activities were found high in the roots of soybean plant under salinity stress. These results are confirmed by the findings of AbdElgawad et al. (2016) in maize plants. A significant increase in the SOD-like activities in the roots under medium and high salinity stress was due to the increment on reactive oxygen species and defensive mechanism of plants (Gao et al., 2008). With the foliar application of potassium fertilizers, shoot showed a significant decrease in SOD-like activities under medium salinity stress. Similar results are found by Soleimanzadeh et al. (2010) in sunflower under drought stress. Potassium fertilizers application under medium salinity stress elevated the reducing power potential. Foliar application of potassium supplements the potassium content in the plant due to which the plants, under stress, also express potassium ions selectivity and membrane potential which help to overcome the increased sodium ion concentrations (Duarte et al., 2014). The reducing power potential is correlated with the total phenolic content which is a major contributor to the antioxidant activities of the plants (Sharma and Ramawat, 2013). The carotenoid and chlorophyll contents in the shoots and roots were notably decreased under high salinity stress. The results of present study were in agreement with the finding of
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Akcin and Yalcin (2016) and Salicornia and Qados (2011). The reduction in carotenoid and chlorophyll contents might be due to degradation of b-carotene and the negative effect of the accumulated salt ions (Ali et al., 2004; Gomes et al., 2011). Compared to the potassium chloride applications, potassium sulfate had the highest carotenoid and chlorophyll contents. Similar results were obtained by Hussein et al. (2014) in Jojoba plants. Application of potassium fertilizer under salt stress, enhanced the contents of photosynthetic pigment with the increase of potassium ion and decrease of sodium ion (Fayez and Bazaid, 2014). The findings of the present study were in agreement with the results of previous studies. Hu et al. (2008) found that foliar application of potassium could not contribute to lessen the effect of salinity stress in the early growth stage of maize plant. In citrus seedlings, Navarro et al. (2015) observed that the potassium foliar application did not contribute to mitigate the salinity stress. Hussein et al. (2012) found that a 200 ppm potassium foliar application did not mitigate the salinity stress in the early growth stages of pepper plants. With the 2% potassium foliar application on the sunflower seedlings (<10 days), the salinity stress was not notably overcome (Arshadullah et al., 2014). 5. Conclusion Soybean growth attributes like height, shoot and root fresh and dry weights were notably decreased under medium and high salinity stresses. Medium salinity stress induced a significant level of antioxidant potentials and secondary metabolites, indicating soybean shows greater tolerance level in the medium salinity than in the high salinity stress. Foliar applications of potassium sulfate had a positive effect on the antioxidant activities, polyphenol, flavonoid, carotenoid, and chlorophyll contents compared to those of potassium chloride, but the contribution of potassium sulfate application was not noteworthy in comparison to the fertilizer unsprayed treatment. Further studies on long-term effect of foliar application of various concentrations of potassium fertilizers under different levels of salinity stresses are suggested, based on the results of present study. Acknowledgment This research is supported by Kyungpook National University research fund 2017. Conflict of interests Authors declare no conflict of interest. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jssas.2019.02.001.
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Effect of foliar application of potassium fertilizers on soybean plants under salinity stress Bishnu Adhikari a, Sanjeev Kumar Dhungana a, Il-Doo Kim b, Dong-Hyun Shin a,⇑ a b
School of Applied Biosciences, Kyungpook National University, Daegu 41566, Republic of Korea International Institute of Agriculture Research & Development, Kyungpook National University, Daegu 41566, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 July 2018 Revised 10 September 2018 Accepted 27 February 2019 Available online xxxx Keywords: Foliar application Salinity Soybean Stress Vegetative growth
a b s t r a c t Soybean, one of the major food and fodder crops, yield is significantly reduced by different abiotic and biotic stresses, and salinity is one of the major abiotic stresses. The research objective of this study was to investigate the effect of foliar application of potassium chloride and potassium sulfate fertilizers at the early growth stage of soybean under medium (6 dS/m) and high (12 dS/m) salinity stresses. The effect of fertilizer application was measured on the bases of plant growth, levels of antioxidant activities, total polyphenol, flavonoid, chlorophyll, and carotenoid contents. Potassium sulfate showed better positive effect on the antioxidant activities, polyphenol, flavonoid, carotenoid, and chlorophyll contents compared to those of potassium chloride although the contribution was not noteworthy in comparison to the fertilizer unsprayed plants. The results of this study implied that foliar applications of 2.5% potassium fertilizers could not help reduce the negative effect of salinity stress at the early stage of soybean growth. This study suggests further researches using different concentrations of fertilizers at different plant growth and development stages. Ó 2019 Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Soybean, one of the most economically important crops, provides vegetable protein and oil for millions of people worldwide (Capriotti et al., 2014). Yield of soybean could significantly be improved by managing different biotic and abiotic stresses (Suzuki et al., 2014), including soil salinity (Zhu, 2001). About 30% of the total cultivated lands are affected by salt deposition (Kronzucker et al., 2008) which pose an increasing threat on world’s arable land availability (Nia et al., 2012). Overcoming salt stress is one of the major issues in these regions to ensure sufficient food production in a sustainable way (Heuer, 2003). Soil salinity has negative effects on crop productivity by inducing ion toxicity and osmotic stress in the plants (Zhu, 2001; Zhu et al., 1997). In most of the saline and sodic soils, sodium is one of the major toxic cations which mainly harms plants by disrupting the ⇑ Correspondence author. E-mail address: [email protected] (D.-H. Shin). Peer review under responsibility of King Saud University.
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uptake of potassium ion (K+) (Tester and Davenport, 2003). In addition, deposition of high concentration of sodium in the cultivated land could alter soil texture by decreasing soil aeration, porosity and water conductance (Mahajan and Tuteja, 2005). The response of plants to salinity differs greatly (Bonilla and Bolaños, 2004), and almost all the legume crops are sensitive to salinity (Noble and Roger, 1992). Normally, soybean is sensitive to salinity as it has threshold of 5 dS/m (Kamkar et al., 2014). Soybean growth, under high salinity conditions, is negatively affected resulting in drastically reduced yields (Khan et al., 2016). In order to minimize the damaging effects of salinity, various strategies, including plant nutrient management, are adopted. Foliar application of nutrients is beneficial in reducing the harmful effect of salinity stress (Babar et al., 2014; Fayez and Bazaid, 2014) by enhancing water uptake (Hu et al., 2008). The foliar applications of micro and macro nutrients are practiced whenever roots are restricted to uptake nutrients due to salt stress (Singh et al., 2013). If the nutrient supply via roots is restricted under saline soils, foliar application is highly recommended over the soil applications (Machado and Serralheiro, 2017). Potassium is one of the three essential macro nutrients and a key factor controlling crop productivity (Liu and Zhu, 1997). The salt stressed plants are better benefitted with the application of potassium sulfate and chloride fertilizers than the non-stressed
https://doi.org/10.1016/j.jssas.2019.02.001 1658-077X/Ó 2019 Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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ones (Trolldenier, 1985). Potassium contributes for tolerance against salinity as it has competing nature to sodium for binding and maintaining plant water status (Capula-Rodríguez et al., 2016). At the early plant growth stages, the root system may not get well developed to uptake sufficient nutrients from soil and in such a condition the foliar application of fertilizers could be a good option to supply the essential nutrients like potassium (K+) and phosphorous (P+) to the plants (Mallarino et al., 2001). High NaCl concentration in soil induces the P+ and K+ deficiencies in tomato (Adams, 1991) and in cucumber (Sonneveld and De Kreij, 1999). Fertilizing plants with K+ in order to raise the K+/Na+ ratio is an effective way for increasing the plants tolerance against salinity stress (Elhindi et al., 2016). Foliar application of potassium is carried out to reduce the salinity stress in sunflower (Arshadullah et al., 2014), sugar beet (Zaki et al., 2014), eggplant (Elwan, 2010), ryegrass (Tabatabaei and Fakhrzad, 2008), tomato (Amjad et al., 2014; Kaya et al., 2001), sunflower and safflower (Jabeen and Ahmad, 2011), endives (Tzortzakis, 2010), and wheat (Bybordi, 2015). Although a foliar application of potassium fertilizer has been reported beneficial against salt stress (Kaya et al., 2001; Akram et al., 2009), comparative studies on investigating the effect of different kinds of potassium fertilizer at early soybean growth have not been reported. Considering the economic importance of soybean crop (Capriotti et al., 2014) and ever increasing areas of saline soil (Shrivastava and Kumar, 2015; Zewdu et al., 2017) due to different reasons, including the sea-level rise, this study aimed to find out the comparative effectiveness of foliar applications of potassium chloride and sulfate fertilizers at early soybean growth under salinity stress. 2. Materials and methods 2.1. Plant material and growing condition Soybean [Glycine max (L.) Merrill cv. William 82] plants were used to determine the effect of foliar applications of potassium fertilizers under medium (6 dSm1) and high (12 dSm1) salinity stresses. The plants were grown in nursery-soil-filled 0.5-L plastic pots with drainage holes at bottom (Hester et al., 1996) at greenhouse with the average day and night temperatures of 23 and 15 °C, respectively, 75% relative humidity, and natural light conditions (Quebedeaux and Chollet, 1975). 2.2. Salinity stress Soybean plants were grown until V2 (second trifoliate leaf) stage and exposed to salt stress following an earlier method (Lee et al., 2008) which has reliable and simple screening procedures for salinity stress. In order to prevent soil leaking from the pots, a single layer of paper towel was placed at the bottom of pot. The pots were kept in plastic racks and salt stress was imposed by dipping bottom one-third (3.8 cm) portion of the pots into the medium or high concentration of salt solutions for 2 weeks. For inducing salinity stress common table salt was used. 2.3. Foliar application of fertilizer Potassium chloride and potassium sulfate were selected for foliar application as the two fertilizers have shown different effects in plants (Zehler et al., 1981). In order to maintain a medium level of potassium content, a 2.5% solution of potassium sulfate or potassium chloride was applied. The foliar application of potassium fertilizers was started after a week of salinity stress imposition. Two foliar applications of the fertilizers were carried out at 3 d interval
as more than one application is suggested for good absorption of the nutrients applied (Fageria et al., 2009). The fertilizers were applied to the both, upper and lower surfaces of the leaves. 2.4. Sample preparation and extraction The plants along with roots were collected after 2 weeks of onset of the salinity stress. The external materials and soil particles adhering to the roots were removed from plant and then allowed to surface drying at room temperature, followed by deep freezing (80 °C) before freeze drying. The freeze-dried plants were separated into shoots and roots and then pulverized into fine powders using a commercial grinder. Finely powdered samples were extracted with methanol (100%) using a shaking incubator (150 rpm) for 6 h and centrifuged (15,000g) at room temperature (22–25 °C) for 15 min. Through a syringe filter the supernatants were filtered and stored at 20 °C, and used within a week for measuring antioxidant activities and total polyphenol and flavonoid contents (Adhikari et al., 2018b). 2.5. Antioxidant activities 2.5.1. DPPH free radical scavenging activities DPPH free radical scavenging activities of roots and shoots were assayed following an earlier method (Dhungana et al., 2016). An equal volume of the freshly prepared methanolic solution of DPPH (0.1% w/v) and sample extracts was mixed and kept for 30 min at room temperature in dark. Similarly, equal proportions of methanol and the DPPH solution were mixed to take the absorbance of control. The absorbance values were measured at 517 nm using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific, Vantaa, Finland). The DPPH radical scavenging activity was determined using the following equation (S ß engül et al., 2014).
A 100 DPPH radical scavenging activityð%Þ ¼ 1 B
where A is the absorbance of sample and DPPH mixture and B is the absorbance of the methanol and DPPH mixture. 2.5.2. ABTS radical scavenging activities ABTS radical scavenging activities were assayed following the procedure of Ali et al. (2017). The ABTS radical was generated by mixing 2.5 mM potassium persulfate and 7 mM ABTS and keeping the mixture at dark for 16 h. With double distilled water the ABTS solution was diluted to obtain 0.7 absorbance value at 734 nm. A 20-uL of the sample extract solution was mixed with ABTS solution (180 mL) and the absorbance was measured at 734 nm using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific). The ABTS radical scavenging capacity was calculated using the following formula (Wan et al., 2011).
ABTS radical scavenging activityð%Þ ¼
Ac As 100 Ac
where AC is the absorbance of ABTS radical solution and AS is the absorbance of a mixture of ABTS radical and sample solution. 2.5.3. Superoxide dismutase (SOD)-like activity SOD-like activity was studied on the basis of 50% reduction of nitro blue tetrazolium by following the procedure of Dubey et al. (2015) and Adhikari et al. (2018a). At first, freeze dried leaf powder (0.5 g) was homogenized in a phosphate extraction buffer (5 mL) using an ice-cooled mortar and pestle. The homogenized solution was centrifuged (15,000g) for 20 min. After centrifugation, a reaction mixture consisting of NBT (2.25 mM), 100 mM phosphate buffer (pH 7.8), methionine (200 mM), sodium carbonate (1.5 M) and
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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EDTA (3 mM) was prepared and an enzyme extract of the sample (0.1 mL) was added. The reaction was initiated by adding 0.4 mL (2 mM) of riboflavin and placing under fluorescent light (15 W) for 15 min. A reaction mixture without the samples was considered as a control. The reaction was terminated by turning the light off and keeping the reaction mixtures in dark. Using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific), an optical density of the reaction solution was measured at 560 nm and the SOD-like activity was calculated on the basis of the following formula.
Superoxide dismutase like activityð%Þ ¼
Ao A 100 Ao
where Ao is the changed absorbance value of complete reaction mixture without a sample and A is the changed absorbance value of the complete reaction mixture with sample. 2.5.4. Reducing power potential The reducing power potential of sample extract was measured following the protocol mentioned by Ali et al. (2018). A solution of phosphate buffer (0.2 M, pH 6.6) was mixed with 100 mL of the sample extract and potassium ferric cyanide (1%, 900 mL) followed by incubation for 20 min at room temperature. A 900-mL of 10% trichloroacetic acid was mixed to the reaction solution and centrifuged (3000g) for 15 min. Nine hundred microliters of the supernatant was taken and mixed with an equal volume of double distilled water (900 mL) and 0.1% of ferric chloride each. Absorbance of the reaction mixture was measured at 700 nm using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific). The reducing power potential was expressed in microgram butylated hydroxytoluene equivalents per gram dry weight sample (mg BhtE/g).
extracted with 7 mL of dimethyl sulfoxide at room temperature for 24 h. The extracted solution was filtered through a filter paper (Whatman No. 4) and dimethyl sulfoxide was added to make the final volume 10 mL. For determining the carotenoid content, the absorbance of reaction mixture was measured at 470 nm and for the chlorophyll was measured at 644 nm and 662 nm using a microplate spectrophotometer (Multiskan GO). The chlorophyll a, chlorophyll b, and the total chlorophyll contents (ab) of the sample plants were calculated using the following equations (Arnon, 1949; Ibrahim et al., 2014).
Chlorophyll a ðCaÞ ¼ 12:7 ðOD662Þ 2:69 ðOD644Þ mg=L Chlorophyll b ðCbÞ ¼ 22:9 ðOD644Þ 2:69 ðOD662Þ mg=L Chlorophyll total ðCabÞ ¼ 20:2 ðOD644Þ þ 8:02 ðOD662Þ mg=L The carotenoid content was calculated according to the following equation (Villanueva et al., 1985).
Carotenoid ¼ ½OD470 1:28ðCaÞ þ 5:67ðCbÞ=ð256 0:906Þ mg=L where OD = optical wavelengths.
density
measured
at
the
respective
2.9. Statistical analysis Analysis of variance (ANOVA) was carried out using R-statistical software version 3.3.2 and least significant difference (LSD) was used to measure significant differences among treatment means at p < 0.05. Results are mentioned as means ± SE of triplicate experiments.
2.6. Total polyphenol content Folin-Ciocalteu reagent was employed to assay the total polyphenol content of sample extract following the procedure described earlier (Dhungana et al., 2015). Fifty microliters of sample extract was mixed with 1000 mL of 2% Na2CO3 using a vortexer. After 3 min, 50 mL of 1 N Folin-Ciocalteu reagent was mixed to the solution and allowed to stand at room temperature for 30 min in dark. Reaction mixtures’ absorbance value was measured at 750 nm using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific). The total polyphenol content was expressed in microgram gallic acid equivalents (GAE) per gram dry weight of sample (mg GAE/g) (Zeitoun et al., 2017). 2.7. Total flavonoid content The total flavonoid content of sample extracts was measured by following an earlier report (Zhu et al., 2010). Sample extract (300 mL) was mixed with an equal volume of double distilled water and 30 mL of 5% NaNO2. The mixture was allowed to stand for 5 min at room temperature and 60 mL of AlCl3 (10%) was added to the mixture. The mixture was again allowed to stand for 5 min and 200 mL of 1 M NaOH was added into it. At 500 nm the absorbance reading was taken using a microplate spectrophotometer (Multiskan GO, Thermo Fischer Scientific). The total flavonoid content was expressed in microgram quercetin equivalents (QE) per gram dry sample (mg QE/g) (de Souza et al., 2018). 2.8. Carotenoid and chlorophyll contents The carotenoid and chlorophyll contents of the soybean leaves were determined by following the procedure described by Hiscox and Israelstam (1979). A 100-mg of the ground leaves was
3. Results 3.1. Plant height The height of soybean plants under unstressed control (USC) and medium salinity (MS) conditions significantly differed with the foliar application of potassium fertilizers (Fig. 1). Under the USC, potassium sulfate significantly reduced the plant height compared to the potassium chloride; however under MS, the sulfate fertilizer showed positive effect. The effect of fertilizer application was not significantly different under high salinity (HS) condition. ANOVA shows that the stress on the plant height was highly significant (F = 46.43, p = 0.017) but the fertilizer (F = 2.0, p = 0.2494) and the interaction between fertilizer and stress (F = 1.5, p = 0.353) was not significant (Supplementary Table S1). 3.2. Fresh and dry weights of root and shoot The shoot and root fresh and dry weights were significantly reduced with the fertilizers application except for root fresh weight under USC (Fig. 2). The effect was more severe under HS than under MS. Overall effect of potassium chloride on fresh and dry weights root and shoot was more favorable than potassium sulfate application. Stress had significant effect on both shoot and root fresh (Shoot fresh weight: F = 170.16, p = 0.0001, Root fresh weight: F = 58.98, p = 0.0011) and dry weights (Shoot dry weight: F = 632.82, p < 0.0001, Root dry weight: F = 11.11, p = 0.0233) but the fertilizer (F = 43.27, p = 0.002) and the interaction between stress and fertilizer (F = 13.14, p = 0.0143) were significant only for root fresh weight (Supplementary Table S1).
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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Fig. 1. Soybean plant height under salinity stress with potassium foliar applications. In the same stress conditions different letters above bar diagram are significantly different (p < 0.05).
Fig. 2. Fresh and dry weights of shoot and root of soybean plants under salinity stresses. A: fertilizer unsprayed, B: potassium chloride (KCl) application, and C: potassium sulphate (K2SO4) application. Different letters above each bar diagram within the same measurement are significantly different (p < 0.05).
3.3. DPPH radical scavenging activities The DPPH radical scavenging activity in root was not significantly affected with the fertilizer application under USC, however the activity was reduced in shoot (35.51%) with potassium chloride application (Table 1). Similarly, under MS, potassium chloride reduced the DPPH scavenging activity of root (75.59%) compared to that of fertilizer unsprayed and potassium sulfate treatment. However, under HS condition, the foliar spray of fertilizer enhanced the DPPH radical scavenging activity of roots (18.36– 18.79%) compared to that of the USC (11.52%). DPPH radical scavenging activities on shoot were significantly affected by the stress (F = 33.34, p = 0.0032) and fertilizer (F = 7.6, p = 0.0434) whereas
fertilizer and stress interaction (F = 0.76, p = 0.6009) did not have any significant effect (Supplementary Table S1).
3.4. ABTS radical scavenging activities Potassium sulfate application under MS conditions had the highest ABTS radical scavenging activities in shoot (84.9%) and root (82.59%) as shown in Table 2. Under USC, fertilizer unsprayed (78.03%) and potassium chloride application (74.31%) had significantly higher radical scavenging activities in shoot than the potassium sulfate application whereas the effect was not significant in root for either treatment. Similarly, under HS there were not any
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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B. Adhikari et al. / Journal of the Saudi Society of Agricultural Sciences xxx (xxxx) xxx Table 1 Effect of foliar application of potassium fertilizers on the DPPH radical scavenging activities of soybean plants under different salinity stresses. Treatment
Plant part
Fertilizer unsprayed
Shoot Root Shoot Root Shoot Root
2.5% KCl 2.5% K2SO4
DPPH radical scavenging potential (%) Unstressed control
Medium salinity
High salinity
48.88 ± 4.14aA 27.54 ± 4.47aAB 35.51 ± 3.38bA 19.91 ± 3.40aA 44.93 ± 0.89abA 23.52 ± 3.20aB
38.18 ± 7.06aAB 33.79 ± 2.03abA 31.79 ± 3.68aA 19.97 ± 7.53bA 41.14 ± 0.90aB 39.39 ± 1.32aA
22.33 ± 1.75aB 11.52 ± 2.19bB 19.56 ± 2.75aB 18.36 ± 0.32aA 21.78 ± 0.47aC 18.79 ± 0.93aB
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A– C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05). Table 2 Effect of foliar application of potassium fertilizers on the ABTS radical scavenging activities of soybean plant under different salinity stresses. Treatment
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
Plant part
Shoot Root Shoot Root Shoot Root
ABTS radical scavenging potential (%) Unstressed control
Medium salinity
High salinity
78.03 ± 0.69abAB 81.34 ± 1.36aA 83.72 ± 0.23aA 74.31 ± 1.36bA 75.56 ± 3.81bB 80.58 ± 1.14aA
83.41 ± 0.32aA 78.97 ± 1.47abA 76.94 ± 1.11bB 75.59 ± 1.09bA 84.98 ± 0.59aA 82.59 ± 0.91aA
72.59 ± 2.32aB 73.36 ± 1.21aB 66.69 ± 1.55aC 78.21 ± 1.35aA 68.13 ± 1.25aB 76.68 ± 3.91aA
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A– C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05).
significant differences among the treatments in both the shoot and root parts. 3.5. Total polyphenol content Under USC, in the root of fertilizer unsprayed (503.83 mg GAE/g) and the shoot of potassium sulfate applied (489.78%) plants had significantly high polyphenol content compared to other treatments (Table 3). Under MS, fertilizer unsprayed had the significantly highest total polyphenol content in both shoot (519.94 mg GAE/g) and root (445.53 mg GAE/g) compared to the fertilizer applications. Under HS, in the shoots fertilizer application caused significant increase in the total polyphenol content (540.03–548.81 mg GAE/g) compared to the fertilizer unsprayed (329.53 mg GAE/g). Stress had significant impact on the total polyphenol content in the shoot (F = 47.66, p = 0.0016) and root (F = 41.74, p = 0.0021) but the fertilizer application had only significant effect in the shoot (F = 67.01, p = 0.0008) whereas interaction between the fertilizer and the stress (Shoot: F = 66.65, p = 0.0006, Root: F = 31.03, p = 0.0029) had significant effect on both shoot and root (Supplementary Table S1).
MS, in the shoot and root of fertilizer unsprayed (2053.1 mg QE/g and 551.62 mg QE/g) and root of potassium sulfate (556.71 mg QE/ g) applied plants had significantly high amount of flavonoid content compared to the respective parts of other treatments. Potassium chloride application, under HS, showed significantly high amount of flavonoid content in the shoot compared to the other treatments, however the total flavonoid content in root (699.24 mg QE/g) was not significantly affected by the treatments. 3.7. Super oxide dismutase (SOD)-like activities The root of salinity stressed plants showed comparatively higher SOD-like activities than the shoot (Table 5). Under USC, the shoot of either treatment did not show significant difference whereas the root of potassium sulfate (25.49%) sprayed plant had significantly low SOD-like activities compared to that of fertilizer unsprayed (30.07%) and potassium chloride (29.40%) treatments. The shoot of fertilizer unsprayed (28.36%) plants had significantly high SODlike activities compared to that of potassium fertilizers application (18.11–19.59%). Under HS, the fertilizer application did not cause significant variation in the SOD-like activities of shoot part.
3.6. Total flavonoid content 3.8. Reducing power potential Potassium sulfate application significantly increased the shoot flavonoid content under USC whereas potassium chloride caused significant reduction in shoot as well as in root (Table 4). Under
Application of potassium sulfate fertilizer had significant positive effect on increasing the reducing power potential in the shoot
Table 3 Effect of foliar application of potassium fertilizers on the total polyphenol content of soybean plant under different salinity stresses. Treatment
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
Plant part
Shoot Root Shoot Root Shoot Root
Total polyphenol content (mg GAE/g) Unstressed control
Medium salinity
High salinity
409.31 ± 9.25bB 503.83 ± 8.97aA 368.03 ± 18.67bB 267.17 ± 2.48cB 489.78 ± 5.08aB 352.08 ± 5.61bA
519.94 ± 5.48aA 445.53 ± 8.59aB 394.33 ± 4.12cB 262.33 ± 2.86cB 491.31 ± 2.18bB 358.89 ± 3.39bA
494.56 ± 10.28bA 329.53 ± 5.51bC 548.81 ± 9.01aA 368.06 ± 1.90aB 540.03 ± 4.76aA 267.67 ± 1.0cB
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05). GAE = gallic acid equivalent.
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
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Table 4 Effect of foliar application of potassium fertilizers on the total flavonoid content soybean plant under different salinity stresses. Treatment
Plant part
Fertilizer unsprayed
Shoot Root Shoot Root Shoot Root
2.5% KCl 2.5% K2SO4
Total flavonoid content (mg QE//g) Unstressed control
Medium salinity
High salinity
152.61 ± 42.03bB 586.24 ± 45.58aA 1446.1 ± 25.86bA 338.76 ± 14.89bB 1966.43 ± 45.34aA 500.9 ± 19.15aA
2053.1 ± 72.42aA 551.62 ± 20.92aA 1484.57 ± 31.91bA 443.1 ± 2.48bAB 1257.52 ± 61.44cB 556.71 ± 12.72aA
562.81 ± 20.2bC 536.95 ± 54.45aA 699.24 ± 15.23aB 507.81 ± 56.31aA 612.52 ± 62.44bC 472.81 ± 32.88aA
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05). QE = quercetin equivalent.
Table 5 Effect of foliar application of potassium fertilizers on the SOD-like activities of soybean plant under different salinity stresses. Treatments
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
Plant part
Shoot Root Shoot Root Shoot Root
SOD-like activities (%) Unstressed control
Medium salinity
High salinity
18.03 ± 1.53aB 30.07 ± 1.61aB 16.7 ± 1.34aA 29.4 ± 0.96aC 13.32 ± 1.43aA 25.49 ± 0.25bC
28.36 ± 0.88aA 29.05 ± 1.69bB 19.59 ± 2.64bA 41.62 ± 1.74aB 18.11 ± 2.50bA 35.29 ± 3.11abB
17.31 ± 3.14aB 47.78 ± 1.13bA 16.86 ± 3.96aA 55.18 ± 2.18aA 19.74 ± 2.17aA 55.24 ± 1.53aA
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05).
compared to other treatments under USC, however fertilizer application had negative effect on the root reducing power potential (Table 6). Under MS, the fertilizer application (1.6–1.8 mg BhtE/g) significantly reduced the reducing power potential in shoot compared to the fertilizer unsprayed (2.19 mg BhtE/g). Under HS, the reducing power potential of shoot and root of potassium chloride (1.51 and 1.77 mg BhtE/g) sprayed plants was significantly high compared to that of the potassium sulfate (1.3 and 1.55 mg BhtE/g, respectively).
Table 7 Effect of foliar application of potassium fertilizers on the carotenoid and chlorophyll a, b and total contents of soybean plant under different salinity stresses. Treatments
aA
Medium salinity
0.57 ± 0.004cC 0.63 ± 0.007bC 0.72 ± 0.016aC
Chlorophyll a content (mg/L) Fertilizer unsprayed 13.11 ± 0.24aA 2.5% KCl 4.13 ± 0.02bA 2.5% K2SO4 12.36 ± 0.3aA
6.65 ± 0.07aB 3.88 ± 0.02cB 4.75 ± 0.15bB
1.27 ± 0.03bC 1.49 ± 0.05aC 1.55 ± 0.03aC
Chlorophyll b content (mg/L) Fertilizer unsprayed 4.64 ± 0.06aA 2.5% KCl 2.15 ± 0.01bB 2.5% K2SO4 4.69 ± 0.05aA
3.01 ± 0.07aB 2.29 ± 0.03cA 2.59 ± 0.09bB
1.32 ± 0.01bC 1.37 ± 0.01bC 1.69 ± 0.06aC
Total chlorophyll (ab) Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
8.54 ± 0.02aB 5.5 ± 0.05cA 6.53 ± 0.12bB
2.36 ± 0.02cC 2.59 ± 0.03bB 2.96 ± 0.07aC
3.73 ± 0.062 1.34 ± 0.001bA 3.6 ± 0.067aA
content (mg/L) 15.55 ± 0.26aA 5.57 ± 0.01bA 14.97 ± 0.28aA
aB
High salinity
2.05 ± 0.004 1.33 ± 0.012cB 1.57 ± 0.028bB
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
3.9. Carotenoid and chlorophyll contents Potassium chloride had significantly reduced the carotenoid and total chlorophyll contents (1.34 mg/L and 5.5 mg/L) compared to fertilizer unsprayed and potassium sulfate under USC (Table 7). Under MS, both the fertilizers significantly reduced the carotenoid and chlorophyll contents compared to fertilizer unsprayed plants. Under HS, potassium sulfate application had significantly increased the carotenoid and chlorophyll contents (0.72 mg/L and 2.96 mg/L) as compared to other treatments. Stress, fertilizer and their interaction have highly significant effect on the total carotenoid (Stress: F = 839.3, p < 0.001, Fertilizer: F = 114.84, p = 0.0003, Interaction: F = 70.92, p = 0.0006) and chlorophyll (Stress:
Carotenoid content (mg/L) Unstressed control
Results are mentioned as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05).
Table 6 Effect of foliar application of potassium fertilizers on the reducing power potential of soybean plant under different salinity stresses. Treatments
Fertilizer unsprayed 2.5% KCl 2.5% K2SO4
Plant Part
Shoot Root Shoot Root Shoot Root
Reducing power potential (mg BhtE/g) Unstressed control
Medium salinity
High salinity
1.83 ± 0.01bB 2.38 ± 0.008aA 1.82 ± 0.009bA 1.93 ± 0.015cA 2.4 ± 0.018aA 2.03 ± 0.009bB
2.19 ± 0.014aA 2.2 ± 0.007bB 1.6 ± 0.002cB 1.84 ± 0.007cB 1.8 ± 0.015bB 2.3 ± 0.011aA
1.48 ± 0.012aC 1.55 ± 0.008bC 1.51 ± 0.005aC 1.77 ± 0.011aC 1.3 ± 0.014bC 1.55 ± 0.062bC
Results are expressed as mean ± SE (n = 3). Different lowercase superscripts (a–b) followed by the mean values in the same column and different uppercase superscripts (A–C) followed by the mean values in the same row for shoot and root are significantly different (p < 0.05). BhtE = Butylated hydroxytoluene equivalent.
Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001
B. Adhikari et al. / Journal of the Saudi Society of Agricultural Sciences xxx (xxxx) xxx
F = 3532.26, p < 0.0001, Fertilizer: F = 839.62, p < 0.0001, Interaction: =481.72, p < 0.0001) content (Supplementary Table S1).
4. Discussion The reduced plant height for fertilizer application might be due to the short-term chemical stress of fertilizer (Hu et al., 2008). Osmotic adjustment of the plant under medium salinity increased the length of the plant as compared to high salinity (Qados, 2011). Early plant growth stages are highly sensitive to the salt stress due to which medium and high salinity stress significantly decreased fresh and dry weights of the shoot and root (Acosta-Motos et al., 2017). The reduced shoot and root fresh and dry weights under medium and high salinity stresses are associated with elevated levels of cell death and lipid peroxidation along with the addition of reactive oxygen species (Klein, 2012). Potassium chloride application caused lower plant height and shoots and root fresh and dry weights than potassium sulfate application. Potassium chloride tends to more strongly salinize than potassium sulfate (Marques et al., 2014) and had detrimental effects on plant growth and development (Teixeira et al., 2011). The responses of plants under medium salinity stress are related to antioxidant potentials. Polyphenol and flavonoid contents are directly related to antioxidant potential which is associated to the free radical scavenging activities (Ksouri et al., 2007). Antioxidant activities under salt stress could vary among the plant species and even among the cultivars of the same plant species. The level of salt stress response depends on duration and intensity of the stress and the plant species and their development and metabolic state (Klein, 2012). Under medium salinity stress, potassium sulfate foliar application had high level of DPPH and ABTS radical scavenging activities. Potassium foliar application could trigger the enzymatic antioxidant mechanisms. These mechanisms protect against the secondary oxidative stress induced by medium salinity (Marques et al., 2014). Increased antioxidant activities have been considered as an adaptation mechanism to salt stress (Bargaz et al., 2015). These activities are positively associated with decreasing oxidative damage and improving salinity tolerance (Kibria et al., 2017). High salinity stress has a negative effect on the physiological activities of plants (Corrêa et al., 2013). Potassium chloride application increased the flavonoid content under the medium salinity which is also corroborated by the findings of Chen et al. (2013). The high polyphenol and flavonoid contents under medium salinity are lined up with results of Valifard et al. (2014). The SOD-like activities were found high in the roots of soybean plant under salinity stress. These results are confirmed by the findings of AbdElgawad et al. (2016) in maize plants. A significant increase in the SOD-like activities in the roots under medium and high salinity stress was due to the increment on reactive oxygen species and defensive mechanism of plants (Gao et al., 2008). With the foliar application of potassium fertilizers, shoot showed a significant decrease in SOD-like activities under medium salinity stress. Similar results are found by Soleimanzadeh et al. (2010) in sunflower under drought stress. Potassium fertilizers application under medium salinity stress elevated the reducing power potential. Foliar application of potassium supplements the potassium content in the plant due to which the plants, under stress, also express potassium ions selectivity and membrane potential which help to overcome the increased sodium ion concentrations (Duarte et al., 2014). The reducing power potential is correlated with the total phenolic content which is a major contributor to the antioxidant activities of the plants (Sharma and Ramawat, 2013). The carotenoid and chlorophyll contents in the shoots and roots were notably decreased under high salinity stress. The results of present study were in agreement with the finding of
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Akcin and Yalcin (2016) and Salicornia and Qados (2011). The reduction in carotenoid and chlorophyll contents might be due to degradation of b-carotene and the negative effect of the accumulated salt ions (Ali et al., 2004; Gomes et al., 2011). Compared to the potassium chloride applications, potassium sulfate had the highest carotenoid and chlorophyll contents. Similar results were obtained by Hussein et al. (2014) in Jojoba plants. Application of potassium fertilizer under salt stress, enhanced the contents of photosynthetic pigment with the increase of potassium ion and decrease of sodium ion (Fayez and Bazaid, 2014). The findings of the present study were in agreement with the results of previous studies. Hu et al. (2008) found that foliar application of potassium could not contribute to lessen the effect of salinity stress in the early growth stage of maize plant. In citrus seedlings, Navarro et al. (2015) observed that the potassium foliar application did not contribute to mitigate the salinity stress. Hussein et al. (2012) found that a 200 ppm potassium foliar application did not mitigate the salinity stress in the early growth stages of pepper plants. With the 2% potassium foliar application on the sunflower seedlings (<10 days), the salinity stress was not notably overcome (Arshadullah et al., 2014). 5. Conclusion Soybean growth attributes like height, shoot and root fresh and dry weights were notably decreased under medium and high salinity stresses. Medium salinity stress induced a significant level of antioxidant potentials and secondary metabolites, indicating soybean shows greater tolerance level in the medium salinity than in the high salinity stress. Foliar applications of potassium sulfate had a positive effect on the antioxidant activities, polyphenol, flavonoid, carotenoid, and chlorophyll contents compared to those of potassium chloride, but the contribution of potassium sulfate application was not noteworthy in comparison to the fertilizer unsprayed treatment. Further studies on long-term effect of foliar application of various concentrations of potassium fertilizers under different levels of salinity stresses are suggested, based on the results of present study. Acknowledgment This research is supported by Kyungpook National University research fund 2017. Conflict of interests Authors declare no conflict of interest. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jssas.2019.02.001.
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Please cite this article as: B. Adhikari, S. K. Dhungana, I. D. Kim et al., Effect of foliar application of potassium fertilizers on soybean plants under salinity stress, Journal of the Saudi Society of Agricultural Sciences, https://doi.org/10.1016/j.jssas.2019.02.001