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Abstract [Objectives] Salt stress is an important abiotic stress affecting soybean yield. This study aimed to screen salt-tolerant soybean varieties to improve the utilization efficiency of saline-alkali land. [Methods] Under three concentrations of salt stress （5， 10 and 15 g/L）， salt tolerance of nine Shandong soybean cultivars was identified at the germination stage. The number of germinated seeds， relative germination rate， plant height， radicle length and number of fibrous roots were recorded to analyze the relative salt damage index and evaluate the salt tolerance of different soybean cultivars. [Results] With the increase of salt concentration， the number of germinated seeds， plant height and number of fibrous roots showed a decreasing trend with significant differences among different soybean cultivars. Under three concentrations of salt stress， Shengdou 10， Lindou 10 and Hedou 28 exhibit relatively strong salt tolerance， indicating that these cultivars are relatively salt-tolerant and highly salt-tolerant cultivars； Hedou 12， Gaofeng 1， Weidou 9 and Qihuang 34 are moderately salt-tolerant cultivars； Qihuang 30 and Qihuang 31 are relatively sensitive to salt solution under 15 g/L NaCl stress. [Conclusions] This study provided high-quality germplasm materials for the improvement of salt tolerance of soybean varieties.
Key words Soybean； Bred varieties； Salt tolerance evaluation
Previous studies have shown that the cultivation of new salt-tolerant crop varieties is one of the most economical and effective ways to improve the utilization efficiency of saline-alkali soils[1-3]. Carrying out research on salt-tolerance in existing germplasm resources and cultivating new salt-tolerant varieties to improve the utilization efficiency of salinized land is an effective way to increase grain output and to develop sustainable agricultural development.
Soybean is the main oil crop in China， which is rich in fat， proteins and a variety of biologically active substances that are beneficial to health such as soy isoflavones， soyasaponin and vitamin E. Soybeans are moderately salt-tolerant crops[4-5]. Salt stress is an important abiotic stress that affects soybean yield. Soil salinization causes inestimable losses to soybean yield and quality[6-8]. Therefore， screening salt-tolerant germplasm resources is important to the cultivation of salt-tolerant soybean varieties.
To accurately evaluate the salt tolerance of soybean varieties， it is necessary to identify the salt tolerance of soybean at various stages. The primary problem that actually exists in production is whether the seeds can germinate from saline-alkali topsoil and whether the seedlings can survive normally. Moreover， plants at the germination stage are most sensitive to salt throughout the whole growth period， and the salt damage is reduced subsequently[9-10]. Zhang et al.[11] analyzed the salt tolerance of soybean at the germination stage and compared the salt tolerance of different soybean cultivars， which provided important theoretical basis for revealing the physiological mechanism of salt tolerance of soybean at the germination stage and cultivating salt-tolerant soybean varieties. Indoor identification of salt tolerance has the advantages of short operation time， strong operability， short cycle， high efficiency， strong repeatability， and low environmental impact， which can be used to identify the salt tolerance of large batches of soybean varieties. Significant progresses have been made in the identification and evaluation of salt tolerance of soybean germplasm resources In China[12-18]. In this study， the salt tolerance of different soybean cultivars at the germination stage under salt stress was evaluated to analyze the differences in salt tolerance among different soybean cultivars and to screen salt-tolerant materials， aiming at providing data reference and parent materials for physiological research and cultivation of salt-tolerant soybean. The salt tolerance of nine soybean cultivars bred in Shandong Province was evaluated at the germination stage， and five highly salt-tolerant materials were initially screened， which provided high-quality germplasm materials for improving the salt tolerance of soybean varieties.
Materials and Methods
Experimental materials
Nine soybean cultivars with large cultivation area in Shandong Province were collected as experimental materials： Qihuang 30， Qihuang 31， Shengdou 10， Hedou 12， Gaofeng 1， Lindou 10， Hedou 28， Weidou 9， Qihuang 34.
Experimental design and implementation
A two-factor experimental design was performed at different levels. Factor 1： treatment solution， including CK （distilled water）， T1 （NaCl solution， 5 g/L）， T2 （NaCl solution， 10 g/L） and T3 （NaCl solution， 15 g/L）； factor 2： nine soybean cultivars. Plump soybean seeds with consistent growth were selected， disinfected with 70% ethanol for 2 minutes， and rinsed with distilled water three times； 20 ml of corresponding treatment solution was added into the germination box with three layers of filter paper， so that the treatment solution was fully absorbed by filter paper and overflowed slightly； 100 soybean seeds were uniformly placed in the germination box， covered with two layers of filter paper that was moistened with the corresponding treatment solution， covered with the lid， and cultured in an incubator at （25±1）℃. The seeds were rinsed with the corresponding solution and filter paper was replaced every 24 h to maintain constant concentration of NaCl solution and good ventilation. Each treatment was repeated three times.
Determination indicators and evaluation methods
Determination of germination indicators
The number of germinated soybean seeds in each treatment was recorded to calculate the germination rate and the relative salt damage index according to the following equations.
Germination rate = Number of germinated seeds within 7 days/Total number of seeds × 100% Relative salt damage index = （Germination rate in control group-Germination rate in treatment group）/Germination rate in control group × 100%
In accordance with Descriptors and Data Standard for Soybean （Glycine spp.）[19]， salt tolerance grades were divided according to the relative salt damage index of different soybean cultivars at the germination stage （Table 1）. Smaller salt damage indexes suggest stronger salt tolerance and higher salt tolerance grades.
Determination of growth indicators
After 7 days of culture， growth indicators of soybean were determined. The plant height and radicle length of soybean seedlings were measured with a ruler. The number of fibrous roots was recorded. Ten seedlings of each cultivar in each treatment were selected as samples. The data were averaged with three replications.
Data processing and statistical analysis
Statistical analysis and differential significance test were performed using DPS 7.05 software package； Microsoft Excel 2007 software was used for mapping.
Results and Analysis
Effect of NaCl stress on seed germination rate of soybean cultivars bred in Shandong Province
As could be seen in Table 2， the germination rate of soybean in CK group varied from 90.0% to 100.0%， with an average of 95.19% and a range of 10.0%， which was relatively improved by 10.0%. The germination rate of Qihuang 34 was 100%. The germination rate of Qihuang 31， Shengdou 10， Hedou 12， and Gaofeng 1 reached 96.67%， which was significantly higher than that of Qihuang 30， Lindou 10， Hedou 28， and Weidou 9.
The germination rate of soybean in T1 treatment varied from 85.00% to 96.67%， with a range of 11.67%， which was relatively improved by 12.07%. The germination rate of Shengdou 10 and Gaofeng 1 was the highest， which was significantly higher than that of other seven cultivars； the germination rate of Qihuang 30， Qihuang 31 ， Hedou 28 and Weidou 9 was 85.00%， indicating that these soybean cultivars were relatively sensitive to NaCl treatment； the differences between soybean cultivars were statistically significant. Compared to CK group， the germination rate of soybean in T1 treatment was reduced by 5.56% on average， which varied from 0.00% to 11.93%， with a range of 11.93%.
The germination rate of soybean in T2 treatment varied from 60.00% to 93.33%， with a range of 33.33%， which was relatively improved by 35.71%. The germination rate of Gaofeng 1 and Lindou 10 was the highest， which was significantly higher than that of other seven cultivars； the germination rate of Qihuang 30， Hedou 28， Weidou 9 and Qihuang 34 was lower than 75.00%， indicating that these soybean cultivars were relatively sensitive to NaCl treatment； the differences between soybean cultivars were statistically significant. Compared to CK group， the germination rate of soybean in T2 treatment was reduced by 16.28% on average， which varied from 1.75% to 33.33%， with a range of 31.58%. The germination rate of soybean in T3 treatment varied from 8.33% to 78.33%， with a range of 70.00%， which was relatively improved by 89.37%. The germination rate of Shengdou 10 and Hedou 28 was the highest， which was significantly higher than that of other seven cultivars； the germination rate of Qihuang 30， Qihuang 31， Weidou 9 and Qihuang 34 was lower than 54.00%， indicating that these soybean cultivars were relatively sensitive to NaCl treatment； the differences between soybean cultivars were statistically significant. Compared to CK group， the germination rate of soybean in T3 treatment was reduced by 46.64% on average， which varied from 17.54% to 90.61%， with a range of 73.07%.
Effect of NaCl stress on seedling growth of soybean cultivars bred in Shandong Province
As could be seen in Table 3， the plant height of soybean in CK group varied from 8.64 to 4.61 cm， with a range of 4.03 cm， which was relatively improved by 46.64%. In T1 treatment， seedling growth of various cultivars was inhibited； the plant height of soybean varied from 5.68 to 2.67 cm， with a range of 3.01 cm， which was relatively improved by 53.00%. In T2 treatment， seedling growth of various cultivars was inhibited； the plant height of soybean varied from 3.92 to 1.73 cm， with a range of 2.19 cm， which was relatively improved by 55.87%. In T3 treatment， seedling growth of various cultivars was inhibited； the plant height of soybean varied from 2.25 to 0.74 cm， with a range of 1.51 cm， which was relatively improved by 67.11%.
Compared to CK group， the plant height of soybean in T1， T2 and T3 treatments was averagely reduced by 35.94%， 56.38% and 73.69%， with a range of 34.44% （25.18%-59.62%）， 40.85% （35.42%-76.27%） and 38.51% （48.00%-86.51%）， respectively.
Effect of NaCl stress on root growth of soybean cultivars bred in Shandong Province
As could be seen in Table 4， the radicle length of soybean in CK group varied from 6.44 to 3.29 cm， with a range of 3.15 cm， which was relatively improved by 48.91%； the radicle length of soybean in T1 treatment varied from 6.10 to 2.71 cm， with a range of 3.39 cm， which was relatively improved by 55.57%； the radicle length of soybean in T2 treatment varied from 1.57 to 0.50 cm， with a range of 1.07 cm， which was relatively improved by 68.15%； the radicle length of soybean in T3 treatment varied from 1.37 to 0.13 cm， with a range of 1.24 cm， which was relatively improved by 90.51%.
Compared to CK group， the radicle length of soybean in T1， T2 and T3 treatments was averagely reduced by 12.29%， 78.02% and 90.57%， with a range of 53.54% （-10.37%-43.17%）， 20.69% （70.13%-90.82%） and 18.7% （77.48%-96.18%）， respectively. As could be seen in Table 5， the number of fibrous roots in CK group varied from 26.78 to 13.22， with a range of 13.56， which was relatively improved by 50.63%； the number of fibrous roots in T1 treatment varied from 19.00 to 4.22， with a range of 14.78， which was relatively improved by 77.79%； the number of fibrous roots in T2 treatment varied from 1.11 to 0， with a range of 1.11， which was relatively improved by 100%.
Compared to CK group， the number of fibrous roots in T1， T2 and T3 treatment was averagely reduced by 43.11%， 98.87% and 100.00%， with a range of 50.78% （13.85%-64.63%）， 4.35% （95.65%-100.00%） and 0.00% （100.00%-100.00%）， respectively.
Classification of salt tolerance of nine soybean cultivars at the germination stage
As shown in Fig. 1， nine soybean cultivars in T1 treatment were highly tolerant to salt stress. In T2 treatment， Qihuang 31， Shengdou 10， Hedou 12， Gaofeng 1 and Lindou 10 were highly tolerant to salt stress； Qihuang 30， Lindou 10， Hedou 28， Weidou 9 and Qihuang 34 were relatively tolerant to salt stress. In T3 treatment， Hedou 28 was highly tolerant to salt stress； Shengdou 10， Gaofeng 1 and Lindou 10 were relatively tolerant to salt stress； Hedou 12， Weidou 9 and Qihuang 34 were moderately tolerant to salt stress； Qihuang 30 and Qihuang 31 were sensitive to salt stress.
According to the average relative salt damage index in three treatments， Shengdou 10， Hedou 12， Gaofeng 1， Lindou 10 and Hedou 28 are highly salt-tolerant cultivars； Qihuang 30， Qihuang. 31，
Weidou 9 and Qihuang 34 are relatively salt-tolerant cultivars. The results indicate that these nine soybean cultivars bred in Shandong Province exhibit high levels of salt tolerance.
Correlation analysis of relative salt damage index and different growth indicators of soybean under different salt stresses
In order to investigate the correlations between relative salt damage index and different growth indicators of soybean under different salt stresses， correlations between relative salt damage index and germination rate， radicle length， plant height， number of fibrous roots were analyzed （Table 6）. In T1， T2 and T3 treatments， relative salt damage index and germination rate exhibited an extremely significantly negative correlation， with a correlation coefficient of -1.00**； relative salt damage index was negatively correlated with the radicle length and plant height； moreover， there was no significant correlation between relative salt damage index and number of fibrous roots. Conclusions and Discussion
The improvement of salt tolerance of crops is an important way to ensure high yield of crops under salt stress. Screening salt-tolerant varieties using existing germplasm resources is a simple and effective breeding method. At present， the salt tolerance of crops is mainly improved by improving cultivation techniques and breeding salt-tolerant varieties. The tolerance of soybean to saline-alkali stress at the germination stage reflects the comprehensive ability of soybean seeds to absorb water and sprout under stress conditions.
Both seed germination and seedling growth are based on embryo growth. The growth of embryos is resulted from the coordination of all physiological and biochemical systems within seeds. Therefore， the germination rate is a comprehensive reflection of seed vitality. Higher seed germination rate under saline-alkali stress indicates stronger comprehensive ability of seeds to absorb water and sprout. Under 5， 10 and 15 g/L NaCl stresses， salt damage index of soybean seeds varied significantly. Although soybean varieties exhibiting salt tolerance at the germination stage may not be salt-tolerant in the whole growth period[12]， the tolerance at seedling stage is more important to production practice.
Based on comprehensive analysis of various growth indicators， under three concentrations of salt stress， Shengdou 10， Lindou 10 and Hedou 28 exhibit relatively strong salt tolerance， indicating that these cultivars are relatively salt-tolerant and highly salt-tolerant cultivars； Hedou 12， Gaofeng 1， Weidou 9 and Qihuang 34 are moderately salt-tolerant cultivars； Qihuang 30 and Qihuang 31 are relatively sensitive to salt solution under 15 g/L NaCl stress.
During the process of seed germination， low concentrations of salt stress have no significant inhibitory effect on the germination rate of nine soybean cultivars， indicating that soybean seeds have certain adaptability to low concentrations of salt， which may be related to the promotion effect of low concentrations of salt on cell membrane regulation. However， with the increase of salt concentration， seed germination rate， plant height and radicle length demonstrate a descending trend， suggesting that high concentrations of salt stress significantly inhibit the germination of soybean seeds， which maybe because high concentrations of salt can cause ion poisoning to the seeds， thus inhibiting seed germination[20].
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[9] SHAO GH， SONG JZ， LIU HL. Preliminary studies on the evaluation of salt tolerance in soybean varieties[J]. Scientia Agricultura Sinica， 1986， （6）：30-35.（in Chinese）
[10] ZHAO X， YANG XJ， SHI Y， et al. Ion absorption and distribution of symbiotic Reaumuria soongorica and Salsola passerina seedlings under NaCl stress[J]. Acta Ecologica Sinica， 2014， 34（4）： 963-972.（in Chinese）
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[13] YU BJ， LUO QY， CAO AZ， et al. Comparison of salt tolerance and ion effect in cultivated and wild soybean[J]. Journal of Plant Resources and Environment， 2001， 10（1）：25-29.（in Chinese）
[14] GAI RY. Identification of salt tolerance and diversity analysis of soybean germplasm resources[D]. Beijing： Chinese Academy of Agricultural Sciences， 2007.（in Chinese） [15] KOU H， CAP MJ， NA GQ. Preliminary study on comprehensive evaluation of salt tolerance for soybean during seedlings stage[J]. Rain Fed Crops， 2007， 27（5）： 352-354.（in Chinese）
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Key words Soybean； Bred varieties； Salt tolerance evaluation
Previous studies have shown that the cultivation of new salt-tolerant crop varieties is one of the most economical and effective ways to improve the utilization efficiency of saline-alkali soils[1-3]. Carrying out research on salt-tolerance in existing germplasm resources and cultivating new salt-tolerant varieties to improve the utilization efficiency of salinized land is an effective way to increase grain output and to develop sustainable agricultural development.
Soybean is the main oil crop in China， which is rich in fat， proteins and a variety of biologically active substances that are beneficial to health such as soy isoflavones， soyasaponin and vitamin E. Soybeans are moderately salt-tolerant crops[4-5]. Salt stress is an important abiotic stress that affects soybean yield. Soil salinization causes inestimable losses to soybean yield and quality[6-8]. Therefore， screening salt-tolerant germplasm resources is important to the cultivation of salt-tolerant soybean varieties.
To accurately evaluate the salt tolerance of soybean varieties， it is necessary to identify the salt tolerance of soybean at various stages. The primary problem that actually exists in production is whether the seeds can germinate from saline-alkali topsoil and whether the seedlings can survive normally. Moreover， plants at the germination stage are most sensitive to salt throughout the whole growth period， and the salt damage is reduced subsequently[9-10]. Zhang et al.[11] analyzed the salt tolerance of soybean at the germination stage and compared the salt tolerance of different soybean cultivars， which provided important theoretical basis for revealing the physiological mechanism of salt tolerance of soybean at the germination stage and cultivating salt-tolerant soybean varieties. Indoor identification of salt tolerance has the advantages of short operation time， strong operability， short cycle， high efficiency， strong repeatability， and low environmental impact， which can be used to identify the salt tolerance of large batches of soybean varieties. Significant progresses have been made in the identification and evaluation of salt tolerance of soybean germplasm resources In China[12-18]. In this study， the salt tolerance of different soybean cultivars at the germination stage under salt stress was evaluated to analyze the differences in salt tolerance among different soybean cultivars and to screen salt-tolerant materials， aiming at providing data reference and parent materials for physiological research and cultivation of salt-tolerant soybean. The salt tolerance of nine soybean cultivars bred in Shandong Province was evaluated at the germination stage， and five highly salt-tolerant materials were initially screened， which provided high-quality germplasm materials for improving the salt tolerance of soybean varieties.
Materials and Methods
Experimental materials
Nine soybean cultivars with large cultivation area in Shandong Province were collected as experimental materials： Qihuang 30， Qihuang 31， Shengdou 10， Hedou 12， Gaofeng 1， Lindou 10， Hedou 28， Weidou 9， Qihuang 34.
Experimental design and implementation
A two-factor experimental design was performed at different levels. Factor 1： treatment solution， including CK （distilled water）， T1 （NaCl solution， 5 g/L）， T2 （NaCl solution， 10 g/L） and T3 （NaCl solution， 15 g/L）； factor 2： nine soybean cultivars. Plump soybean seeds with consistent growth were selected， disinfected with 70% ethanol for 2 minutes， and rinsed with distilled water three times； 20 ml of corresponding treatment solution was added into the germination box with three layers of filter paper， so that the treatment solution was fully absorbed by filter paper and overflowed slightly； 100 soybean seeds were uniformly placed in the germination box， covered with two layers of filter paper that was moistened with the corresponding treatment solution， covered with the lid， and cultured in an incubator at （25±1）℃. The seeds were rinsed with the corresponding solution and filter paper was replaced every 24 h to maintain constant concentration of NaCl solution and good ventilation. Each treatment was repeated three times.
Determination indicators and evaluation methods
Determination of germination indicators
The number of germinated soybean seeds in each treatment was recorded to calculate the germination rate and the relative salt damage index according to the following equations.
Germination rate = Number of germinated seeds within 7 days/Total number of seeds × 100% Relative salt damage index = （Germination rate in control group-Germination rate in treatment group）/Germination rate in control group × 100%
In accordance with Descriptors and Data Standard for Soybean （Glycine spp.）[19]， salt tolerance grades were divided according to the relative salt damage index of different soybean cultivars at the germination stage （Table 1）. Smaller salt damage indexes suggest stronger salt tolerance and higher salt tolerance grades.
Determination of growth indicators
After 7 days of culture， growth indicators of soybean were determined. The plant height and radicle length of soybean seedlings were measured with a ruler. The number of fibrous roots was recorded. Ten seedlings of each cultivar in each treatment were selected as samples. The data were averaged with three replications.
Data processing and statistical analysis
Statistical analysis and differential significance test were performed using DPS 7.05 software package； Microsoft Excel 2007 software was used for mapping.
Results and Analysis
Effect of NaCl stress on seed germination rate of soybean cultivars bred in Shandong Province
As could be seen in Table 2， the germination rate of soybean in CK group varied from 90.0% to 100.0%， with an average of 95.19% and a range of 10.0%， which was relatively improved by 10.0%. The germination rate of Qihuang 34 was 100%. The germination rate of Qihuang 31， Shengdou 10， Hedou 12， and Gaofeng 1 reached 96.67%， which was significantly higher than that of Qihuang 30， Lindou 10， Hedou 28， and Weidou 9.
The germination rate of soybean in T1 treatment varied from 85.00% to 96.67%， with a range of 11.67%， which was relatively improved by 12.07%. The germination rate of Shengdou 10 and Gaofeng 1 was the highest， which was significantly higher than that of other seven cultivars； the germination rate of Qihuang 30， Qihuang 31 ， Hedou 28 and Weidou 9 was 85.00%， indicating that these soybean cultivars were relatively sensitive to NaCl treatment； the differences between soybean cultivars were statistically significant. Compared to CK group， the germination rate of soybean in T1 treatment was reduced by 5.56% on average， which varied from 0.00% to 11.93%， with a range of 11.93%.
The germination rate of soybean in T2 treatment varied from 60.00% to 93.33%， with a range of 33.33%， which was relatively improved by 35.71%. The germination rate of Gaofeng 1 and Lindou 10 was the highest， which was significantly higher than that of other seven cultivars； the germination rate of Qihuang 30， Hedou 28， Weidou 9 and Qihuang 34 was lower than 75.00%， indicating that these soybean cultivars were relatively sensitive to NaCl treatment； the differences between soybean cultivars were statistically significant. Compared to CK group， the germination rate of soybean in T2 treatment was reduced by 16.28% on average， which varied from 1.75% to 33.33%， with a range of 31.58%. The germination rate of soybean in T3 treatment varied from 8.33% to 78.33%， with a range of 70.00%， which was relatively improved by 89.37%. The germination rate of Shengdou 10 and Hedou 28 was the highest， which was significantly higher than that of other seven cultivars； the germination rate of Qihuang 30， Qihuang 31， Weidou 9 and Qihuang 34 was lower than 54.00%， indicating that these soybean cultivars were relatively sensitive to NaCl treatment； the differences between soybean cultivars were statistically significant. Compared to CK group， the germination rate of soybean in T3 treatment was reduced by 46.64% on average， which varied from 17.54% to 90.61%， with a range of 73.07%.
Effect of NaCl stress on seedling growth of soybean cultivars bred in Shandong Province
As could be seen in Table 3， the plant height of soybean in CK group varied from 8.64 to 4.61 cm， with a range of 4.03 cm， which was relatively improved by 46.64%. In T1 treatment， seedling growth of various cultivars was inhibited； the plant height of soybean varied from 5.68 to 2.67 cm， with a range of 3.01 cm， which was relatively improved by 53.00%. In T2 treatment， seedling growth of various cultivars was inhibited； the plant height of soybean varied from 3.92 to 1.73 cm， with a range of 2.19 cm， which was relatively improved by 55.87%. In T3 treatment， seedling growth of various cultivars was inhibited； the plant height of soybean varied from 2.25 to 0.74 cm， with a range of 1.51 cm， which was relatively improved by 67.11%.
Compared to CK group， the plant height of soybean in T1， T2 and T3 treatments was averagely reduced by 35.94%， 56.38% and 73.69%， with a range of 34.44% （25.18%-59.62%）， 40.85% （35.42%-76.27%） and 38.51% （48.00%-86.51%）， respectively.
Effect of NaCl stress on root growth of soybean cultivars bred in Shandong Province
As could be seen in Table 4， the radicle length of soybean in CK group varied from 6.44 to 3.29 cm， with a range of 3.15 cm， which was relatively improved by 48.91%； the radicle length of soybean in T1 treatment varied from 6.10 to 2.71 cm， with a range of 3.39 cm， which was relatively improved by 55.57%； the radicle length of soybean in T2 treatment varied from 1.57 to 0.50 cm， with a range of 1.07 cm， which was relatively improved by 68.15%； the radicle length of soybean in T3 treatment varied from 1.37 to 0.13 cm， with a range of 1.24 cm， which was relatively improved by 90.51%.
Compared to CK group， the radicle length of soybean in T1， T2 and T3 treatments was averagely reduced by 12.29%， 78.02% and 90.57%， with a range of 53.54% （-10.37%-43.17%）， 20.69% （70.13%-90.82%） and 18.7% （77.48%-96.18%）， respectively. As could be seen in Table 5， the number of fibrous roots in CK group varied from 26.78 to 13.22， with a range of 13.56， which was relatively improved by 50.63%； the number of fibrous roots in T1 treatment varied from 19.00 to 4.22， with a range of 14.78， which was relatively improved by 77.79%； the number of fibrous roots in T2 treatment varied from 1.11 to 0， with a range of 1.11， which was relatively improved by 100%.
Compared to CK group， the number of fibrous roots in T1， T2 and T3 treatment was averagely reduced by 43.11%， 98.87% and 100.00%， with a range of 50.78% （13.85%-64.63%）， 4.35% （95.65%-100.00%） and 0.00% （100.00%-100.00%）， respectively.
Classification of salt tolerance of nine soybean cultivars at the germination stage
As shown in Fig. 1， nine soybean cultivars in T1 treatment were highly tolerant to salt stress. In T2 treatment， Qihuang 31， Shengdou 10， Hedou 12， Gaofeng 1 and Lindou 10 were highly tolerant to salt stress； Qihuang 30， Lindou 10， Hedou 28， Weidou 9 and Qihuang 34 were relatively tolerant to salt stress. In T3 treatment， Hedou 28 was highly tolerant to salt stress； Shengdou 10， Gaofeng 1 and Lindou 10 were relatively tolerant to salt stress； Hedou 12， Weidou 9 and Qihuang 34 were moderately tolerant to salt stress； Qihuang 30 and Qihuang 31 were sensitive to salt stress.
According to the average relative salt damage index in three treatments， Shengdou 10， Hedou 12， Gaofeng 1， Lindou 10 and Hedou 28 are highly salt-tolerant cultivars； Qihuang 30， Qihuang. 31，
Weidou 9 and Qihuang 34 are relatively salt-tolerant cultivars. The results indicate that these nine soybean cultivars bred in Shandong Province exhibit high levels of salt tolerance.
Correlation analysis of relative salt damage index and different growth indicators of soybean under different salt stresses
In order to investigate the correlations between relative salt damage index and different growth indicators of soybean under different salt stresses， correlations between relative salt damage index and germination rate， radicle length， plant height， number of fibrous roots were analyzed （Table 6）. In T1， T2 and T3 treatments， relative salt damage index and germination rate exhibited an extremely significantly negative correlation， with a correlation coefficient of -1.00**； relative salt damage index was negatively correlated with the radicle length and plant height； moreover， there was no significant correlation between relative salt damage index and number of fibrous roots. Conclusions and Discussion
The improvement of salt tolerance of crops is an important way to ensure high yield of crops under salt stress. Screening salt-tolerant varieties using existing germplasm resources is a simple and effective breeding method. At present， the salt tolerance of crops is mainly improved by improving cultivation techniques and breeding salt-tolerant varieties. The tolerance of soybean to saline-alkali stress at the germination stage reflects the comprehensive ability of soybean seeds to absorb water and sprout under stress conditions.
Both seed germination and seedling growth are based on embryo growth. The growth of embryos is resulted from the coordination of all physiological and biochemical systems within seeds. Therefore， the germination rate is a comprehensive reflection of seed vitality. Higher seed germination rate under saline-alkali stress indicates stronger comprehensive ability of seeds to absorb water and sprout. Under 5， 10 and 15 g/L NaCl stresses， salt damage index of soybean seeds varied significantly. Although soybean varieties exhibiting salt tolerance at the germination stage may not be salt-tolerant in the whole growth period[12]， the tolerance at seedling stage is more important to production practice.
Based on comprehensive analysis of various growth indicators， under three concentrations of salt stress， Shengdou 10， Lindou 10 and Hedou 28 exhibit relatively strong salt tolerance， indicating that these cultivars are relatively salt-tolerant and highly salt-tolerant cultivars； Hedou 12， Gaofeng 1， Weidou 9 and Qihuang 34 are moderately salt-tolerant cultivars； Qihuang 30 and Qihuang 31 are relatively sensitive to salt solution under 15 g/L NaCl stress.
During the process of seed germination， low concentrations of salt stress have no significant inhibitory effect on the germination rate of nine soybean cultivars， indicating that soybean seeds have certain adaptability to low concentrations of salt， which may be related to the promotion effect of low concentrations of salt on cell membrane regulation. However， with the increase of salt concentration， seed germination rate， plant height and radicle length demonstrate a descending trend， suggesting that high concentrations of salt stress significantly inhibit the germination of soybean seeds， which maybe because high concentrations of salt can cause ion poisoning to the seeds， thus inhibiting seed germination[20].
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