Screening and Analysis of Cadmium-tolerant Rice Variety Resources

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1.College of Agriculture and Biotechnology, Hunan University of Humanities, Science and Technology, Loudi 417000, China; 2.Hunan Hybrid Rice Research Center, Changsha 410125, China; 3.Southern Regional Collaborative Innovation Center for Grain and Oil Crops, Changsha 410128, China

AbstractIn order to find out the difference in cadmium tolerance of different rice varieties in central Hunan, 26 varieties of conventional rice, two-line hybrid rice and three-line hybrid rice were used as materials. The effects of cadmium stress(0.5 mmol/L)on germination percentage, germination index, vigor index, root length, bud length and fresh weight of root buds of rice seeds were studied by indoor bioassay. The results showed that cadmium stress had little effect on seed germination rate and germination index of different rice varieties, but had obvious effect on vigor index, root and bud growth, and the inhibitory effect of cadmium stress on root was significantly greater than that on bud. There were some differences in the mean values of six indexes among different types of rice varieties, but the differences were not significant. The tolerance responses of 26 varieties to cadmium stress were quite different. According to the cluster analysis of comprehensive effects of stress, they could be divided into three types: sensitive type, intermediate type and tolerant type. Among them, the sensitive rice varieties were Xiangwanxian 16, Y Liangyou 9918, C Liangyou 651 and Xin You 215, accounting for 15.38% of the total tested varieties; the tolerant varieties were Y Liangyou 1 and Neixiang 2128, accounting for 7.69% of the tested varieties; the other varieties fell within tolerant intermediate type, accounting for 76.92%.

Key wordsRice varieties, Cadmium tolerance, Variety resources

1 Introduction

Rice is the main food crop in China, and more than two thirds of people in China take rice as the staple food, so the stable yield and high yield of rice is of great significance to ensuring food security and social stability. With the rapid development of industry and agriculture and the need for the development and utilization of mineral resources in Hunan, the pollution of heavy metals in Hunan is becoming more and more serious. Cd, Hg, Pb and other heavy metals have become the most serious pollutants endangering farmland, and have seriously threatened agricultural production and food safety in China[1]. Heavy metal cadmium is a non-essential element for plant growth, but it has been widely concerned because it is easily absorbed and enriched by plants and endangers human health through the food chain[2]. Some studies have shown that cadmium has become the most common heavy metal pollution element in farmland soils in Hunan Province. In seriously polluted areas, the cadmium content of early and late rice reached 5.33 and 9.36 mg/kg, respectively, and the cadmium content of polluted soil in Hunan Lengshuijiang was as high as 2.5-181.6 mg/kg[3-4]. At present, a large number of studies have found that different rice varieties have great differences in cadmium absorption and accumulation[5-11]. The evaluation of seed cadmium tolerance of rice varieties is the basis of breeding of rice varieties with cadmium tolerance, and it is also widely used in the world[12-14]. Therefore, the study on the difference in cadmium tolerance potential of rice seeds with different variety types and different genotypes has important reference value for breeding cadmium-tolerant rice varieties. In this experiment, 26 rice varieties mainly cultivated in central Hunan were used as materials to study the difference in seed stress among rice varieties under cadmium stress. This study provides a scientific theoretical basis for screening and breeding cadmium-tolerant rice varieties and selecting rice varieties with low absorption and accumulation in polluted farmland.

2 Test materials and methods

2.1 Test materialsIn the experiment, 26 main rice varieties were selected in central Hunan, including 4 conventional rice varieties (Huanghuazhan, Xiangwanxian 13, Xiangwanxian 16, Nongxiang 18), 9 two-line hybrid rice varieties (Y Liangyou 9918, Y Liangyou 3218, Y Liangyou 3399, Xin Liangyou 6, C Liangyou 651, Y Liangyou 7, Feng Liangyou 4, Zhun Liangyou 527, Lu Liangyou 267), and 13 three-line rice varieties (T You 109, T You 1655, Shen You 9588, Tian You 998, Tian You 3301, Q You 1127, Xiang You 109, Jin You 163, Tianfeng You 316, Chuanxiang 8, Neixiang 2128, Nei 5 You 263, Xin You 215).

2.2 Experimental designThe determination method is based on the research methods of He Junyuetal.[7]. In this experiment, 0.5 mmol/L cdcl2was selected as the treatment concentration, which was mainly based on the following two aspects. First, the preliminary experiments were carried out on different concentration treatments, and the results showed that when the concentration was greater than 0.75 mmol/L in cdcl2, the root and bud of most rice seeds rotted, the seed vigor index was low, and there was no significant difference among varieties. Second, it makes reference to the research of relevant scholars in this field[7].

2.3 Items and methods of determinationAfter 6 d of culture, the root length and seedling length of seeds were measured accurately and weighed accurately by electronic balance. The related indices are calculated as follows:

Germination rate=(Number of germinated seeds-Number of seeds tested) × 100%;

Germination index=Σ(Germination number in a few days-Number of germination days);

Vigor index=Seedling root length (cm) × Germination index;

Stress index=1-(Treatment value-Control value).

3 Results and analysis

3.1 Effects of cadmium on seed germination, germination index and vigor index of different rice varietiesTable 1 showed that under 0.5 mmol/L cadmium stress, the average seed germination rate of conventional rice and two-line hybrid rice was 91.88% and 95.04%, respectively, which was 3.45% and 3.50% lower than that of the control, respectively, and the stress index of seed germination rate was the same (0.04). The results showed that cadmium had little effect on seed germination of conventional rice and two-line hybrid rice in this concentration. However, the average seed germination rate of three-line hybrid rice increased slightly compared with that of the control, which showed that the seed germination of this type of rice varieties was less affected by stress and it could promote its germination to a certain extent, especially for T You 109, Tianfeng You 316 and Chuanxiang 8 with the promotion rate of more than 5%.

Under 0.5 mmol/L cadmium stress, the average germination index of the three types of rice varieties was 19.52, which was 1.62 lower than that of the control, and the seed germination stress index was 0.08. There was no significant difference in the average value among the three types of rice varieties. The results showed that cadmium stress had little effect on the germination process of rice varieties. Compared with the germination rate and seed germination index, the vigor index of all rice varieties treated with the same concentration of cadmium was significantly lower than that of the control, and the degree of inhibition was very significant (t=23.16，P<0.01). There was no significant difference in seed vigor index and stress index among the three types of varieties. The average seed vigor index of all varieties was 14.81, 23.58 less than that of the control, and the stress index was 0.59. Among them, the vigor index of Xiangwanxian 16, C Liangyou 651, Zhun Liangyou 527, T You 1655 and Jin You 1635 was most obviously inhibited, and the seed vigor stress index was above 0.7.

Table 1 Effects of cadmium on seed germination, germination index and vigor index of different rice varieties

TypeVarietyGermination rate∥%CKCdIGermination indexCKCdIVigor indexCKCdIConventional riceHuanghuazhan92.3084.300.0920.3018.100.1128.4214.300.50Xiangwanxian 1396.6096.300.0020.7018.800.0932.7112.970.60Xiangwanxian 1698.3090.500.0821.9020.400.0748.4013.870.71Nongxiang 1894.1096.40-0.0221.5020.600.0432.4717.300.47Average95.3391.880.0421.1019.480.0835.5014.610.57Two-line hybrid riceY Liangyou 991898.5096.200.0221.6020.100.0747.3015.880.66Y Liangyou 321898.8098.100.0121.2020.500.0359.7818.660.69Y Liangyou 339998.4090.700.0820.8019.100.0826.0013.370.49Xin Liangyou 696.2096.600.0021.0019.200.0928.3513.630.52C Liangyou 651100.0096.400.0420.5018.900.0862.5313.040.79Y Liangyou 798.3094.300.0420.1019.100.0535.9813.750.62Feng Liangyou 498.5094.400.0421.4018.700.1332.5314.030.57Zhun Liangyou 52798.2096.100.0221.9020.300.0755.8514.010.75Y Liangyou 1100.0092.600.0720.8019.600.0622.6713.520.40Average98.5495.040.0421.0319.500.0741.2214.430.61Three-line hybrid riceT You 10986.2096.20-0.1221.8019.600.1032.2815.090.53T You 165598.7096.500.0220.9018.400.1241.1712.510.70Shen You 9588100.00100.000.0021.9021.300.0345.9916.610.64Tian You 99898.5096.800.0221.6020.500.0537.8013.940.63Tian You 330190.6094.40-0.0421.3018.600.1328.1212.830.54Q You 1127100.00100.000.0021.4020.500.0432.5317.220.47Xiang You 109100.00100.000.0020.2018.800.0734.1413.350.61Jin You 16398.3092.500.0621.7018.900.1351.6514.930.71Tianfeng You 31688.2092.70-0.0521.6020.200.0644.5016.160.64Chuanxiang 880.5084.40-0.0520.7019.500.0637.6716.580.56Neixiang 212892.8094.60-0.0221.7020.400.0631.6819.990.37Nei 5 You 26396.6092.800.0420.9019.400.0751.0018.240.64Xin You 21592.5086.500.0621.2018.500.1331.8012.580.60Average94.0794.42-0.0121.3019.580.0838.4915.390.59Total average95.9893.780.0221.1419.520.0838.4014.810.59

3.2 Effects of cadmium on seedling growth quality of different rice varietiesIt can be seen from Table 2 that the average root length of conventional rice, two-line hybrid rice and three-line hybrid rice under cadmium treatment was 0.67,0.62 and 0.77, respectively, which was 61.93%, 69.46% and 57.29% lower than that of the control, respectively. The average stress index of root length was between 0.59 and 0.65, and the difference was not significant.The root fresh weight of three types of rice seeds decreased significantly under cadmium treatment, and the average values of conventional rice, two-line hybrid rice and three-line hybrid rice were 0.59,0.51,0.78, respectively. Compared with the control, it decreased by 1.73, 1.91 and 1.26 mg, respectively. The root fresh weight stress index was between 0.59 and 0.71, and the inhibition of two-line hybrid rice was the most obvious, while three-line hybrid rice was relatively less affected. Among all the tested rice varieties, the fresh weight of seed root of 8 rice varieties (Xiangwanxian 16, Y Liangyou 9918, C Liangyou 651, Y Liangyou 7, Zhun Liangyou 527, Tian You 998, Tian You 3301 and Xin You 215) was most obviously affected by stress effect, and the stress index was above 0.80.

Cadmium treatment had a great effect on the bud length and fresh weight of the three types of rice, and the average value was significantly lower than that of the control, and the stress indexes of bud length and fresh weight were 0.23-0.28, 0.27-0.32, respectively. The inhibitory effect on conventional rice seeds was the most obvious, while the average value of two-line and three-line hybrid rice varieties was not significantly different. Of the 26 varieties tested, the bud length of 10 varieties (Nongxiang 18, Y Liangyou 3399, Zhun Liangyou 527, Y Liangyou 1, T You 109, Shen You 9588, Tian You 998, Tian You 3301, Jin You 163, Xin You 215) and the bud fresh weight of 4 varieties (Y Liangyou 3218, Zhun Liangyou 527, Y Liangyou 1 and Nei 5 You 263) were slightly inhibited, and the stress index was below 0.20. Comparatively speaking, cadmium stress had obvious inhibitory effects on bud length of 3 varieties (Xiangwanxian 16, Y Liangyou 9918, Xin Liangyou 6) and bud fresh weight of 5 varieties (Xiangwanxian 16, Y Liangyou 9918, Xin Liangyou 66 and Y Liangyou 9918, Y Liangyou 3399, Xin Liangyou 6, T You 1655 and Q You 1127). It was found that cadmium had a strong inhibitory effect on rice root growth, and the inhibitory intensity was smaller than that on bud growth. Among the tested varieties, the intensity of inhibition varied with the genetic differences of rice varieties themselves.

Table 2 Effect of cadmium on seedling growth quality of different rice varieties

TypeVarietyRoot length∥cmCKCdIRoot fresh weight∥mgCKCdIBud length∥cmCKCdIBud fresh weight∥mgCKCdIConventionalHuanghuazhan1.400.750.461.310.540.592.681.860.318.435.590.34riceXiangwanxian 131.680.690.591.500.540.642.861.970.318.665.670.35Xiangwanxian 162.210.610.724.840.570.882.981.900.369.476.190.35Nongxiang 181.730.630.641.630.710.563.172.680.1511.128.240.26Average1.760.670.602.320.590.672.922.100.289.426.420.32Two-lineY Liangyou 99182.190.450.792.830.510.823.782.260.4013.456.530.51hybrid riceY Liangyou 32182.940.910.692.090.670.684.053.130.238.639.90-0.15Y Liangyou 33991.250.890.292.320.780.663.092.580.1712.326.840.44Xin Liangyou 61.440.710.511.970.670.664.072.660.3514.368.190.43C Liangyou 6513.180.720.774.700.360.923.462.540.2710.096.810.33Y Liangyou 71.850.350.811.930.260.873.342.330.309.856.710.32Feng Liangyou 41.620.360.781.150.520.553.812.790.2712.268.030.35Zhun Liangyou 5272.550.380.853.830.060.983.563.030.1510.7510.360.04Y Liangyou 11.270.800.370.980.730.262.602.510.039.178.010.13Average2.030.620.652.420.510.713.532.650.2411.217.930.27Three-lineT You 1091.800.660.632.010.490.763.312.770.169.677.540.22hybrid riceT You 16552.080.610.712.140.790.633.632.700.269.635.350.44Shen You 95882.100.750.642.230.880.614.133.370.1814.4711.370.21Tian You 9981.890.760.601.960.350.823.032.730.1010.347.630.26Tian You 33011.520.800.471.230.080.933.272.640.198.777.010.20Q You 11271.600.860.461.211.080.113.702.570.3116.779.010.46Xiang You 1091.740.590.661.310.420.683.933.080.2211.168.410.25Jin You 1632.380.700.712.840.830.714.263.470.1913.539.090.33Tianfeng You 3162.061.110.462.171.170.463.682.800.2410.467.740.26Chuanxiang 81.950.870.552.450.630.743.342.360.2911.438.420.26Neixiang 21281.521.050.311.752.36-0.354.012.760.3114.058.900.37Nei 5 You 2632.440.880.643.350.740.784.613.060.348.639.34-0.08Xin You 2151.690.310.821.930.350.822.832.370.1618.347.340.60Average1.910.770.592.040.780.593.672.820.2312.108.240.29Total average1.900.680.612.260.630.663.372.520.2510.917.530.29

3.3 Comprehensive evaluation and cluster analysis of the inhibitory effects of cadmium on the germination of different rice varietiesThere was no significant difference in the average comprehensive inhibitory effect among the three types of rice varieties under cadmium stress, but it was the lowest for three-line hybrid rice varieties, indicating that the seeds of this kind of rice were relatively more tolerant to cadmium stress. However, the inhibitory effects of cadmium on seed germination and growth of different rice varieties were different. Among them, the seed inhibition effects of Xiangwanxian 16, Y Liangyou 9918, C Liangyou 651 and Xin You 215 were stronger, all above 0.45. However, the stress effects on Y Liangyou 1 and Neixiang 2128 were less than 0.2, indicating that cadmium had a weak inhibitory effect on their germination.

26 varieties were divided into three different types by cluster analysis. (i) Sensitive type: this kind of varieties accounted for 15.38% of the tested varieties, and their representative varieties were Xiangwanxian 16, Y Liangyou 9918, C Liangyou 651 and Xin You 215; (ii) Intermediate type: this kind of varieties accounted for 76.92% of the total varieties tested; (iii) Tolerant type: this kind of rice varieties accounted for 7.69% of the tested varieties, and the representative varieties were Y Liangyou 1 and Neixiang 2128 (Table 3).

Table 3 Comprehensive evaluation and cluster analysis of the inhibitory effects of cadmium on the germination of different rice varieties

TypeVarietyComprehensive effect∥%Conventional riceHuanghuazhan0.34Xiangwanxian 130.37Xiangwanxian 160.45Nongxiang 180.30Average0.37Two-line hybrid riceY Liangyou 99180.47Y Liangyou 32180.31Y Liangyou 33990.32Xin Liangyou 60.36C Liangyou 6510.46Y Liangyou 70.43Feng Liangyou 40.38Zhun Liangyou 5270.41Y Liangyou 10.19Average0.37Three-line hybrid riceT You 1090.33T You 16550.41Shen You 95880.33Tian You 9980.35Tian You 33010.35Q You 11270.26Xiang You 1090.35Jin You 1630.40Tianfeng You 3160.30Chuanxiang 80.35Neixiang 21280.15Nei 5 You 2630.35Xin You 2150.46Average0.34Total average0.36

4 Conclusions

The results of this experiment showed that cadmium stress (0.5 mmol/L) could only promote the seed germination of a small number of different types of rice varieties, and the germination rate, germination index and vigor index of most rice seeds were inhibited to varying degrees. On the whole, the germination rate