Comparative Study on Grain Cadmium Content and Yield in Different Rice Varieties

Weiqing FU Peng DU Jing ZHOU Guiyuan MENG

Abstract ［Objectives］ The rice varieties with a low cadmium (Cd) content and a high yield were screened to provide reference for further experiments on moderate and high Cd-contaminated soils and local promotion.

［Methods］ Under the soil condition of a Cd content of 0.58mg/kg, differences in agronomic traits such as grain Cd content, yield, number of effective panicles, number of grains per panicle and 1 000-grain weight of 26 different rice varieties such as Huanghuazhan, Y Liangyou 9918 and T You 109, were studied by pot experiment.

［Results］ Such six varieties as Xiangyou 109, Tianfengyou 316, C Liangyou 651, Y Liangyou 3218, Y Liangyou 9918 and T You 109 had a relatively higher yield and relatively lower Cd content, which was because that their traits including panicle length, number of effective panicles and 1 000-grain weight were superior to other varieties. The correlation analysis between plant Cd content and yield showed that there was a negative correlation between Cd content and yield in rice.

［Conclusions］ This study provides a theoretical basis for cultivating suitable varieties in this area.

Key words Rice variety; Cadmium content; Yield; Screening

Cadmium (Cd) is the most biologically toxic heavy metal element, and is easy to enter human body through the enrichment of food chain, thus endangering human health［1］. Studies have shown that Chinas cultivated land that has been polluted by heavy metals has reached about 10%［2］, and for the level of heavy metal pollution in cultivated soil, the proportions of almost clean, clean, lightly polluted, moderately polluted and heavily polluted are  68.12%, 15.22%, 14.49%, 1.45% and 0.72%, respectively［3］. Among the 8 common heavy metal pollution elements, the probability of Cd pollution is the highest, being 25.20%［4］. In recent years, many countries have carried out more exploration and practice on the treatment of Cd pollution, including the biochar remediation method［5］, the application of soil acidification conditioner to improve the soil pH value［6］, the method of replacing topsoil with soil from other place［7］, phytoremediation method［8］and electrochemical soil remediation method［9］. There are few reports on the realization of current production activities in Cd-contaminated soils through breeding for Cd resistance.

Hunan is the province with the largest contents of non-ferrous metals in China. Due to the activity of long-term unreasonable mining and lack of awareness of cultivated land protection, the cultivated land around mining areas has been polluted by heavy metals to varying degrees［10］. In order to recover normal production activities in heavy metal-contaminated areas which aim to ensure the safety of Chinas grain production, China has started a comprehensive management of heavy metal-contaminated cultivated land restoration, and has carried out pilot work in Changzhutan area of Hunan Province［11］. In this study, with 26 rice varieties of three different types (conventional rice, two-line hybrid rice and three-line hybrid rice) as experiment materials, under the soil condition with a Cd content of 0.58 mg/kg, the differences of Cd content, yield and agronomic traits among different rice cultivars were studied by pot experiment. This study aimed at screening out rice varieties with a low Cd content and a high yield, providing a reference for further experiments of moderate and high Cd-contaminated soils and local promotion and cultivation, and providing a theoretical reference for breeding good varieties which suit the cultivation in  Hunan Province.

Materials and Methods

Experiment materials

Experiment varieties

The experiment selected 26 main rice varieties in central Hunan, including 4 conventional rice varieties, 9 two-line hybrid rice varieties, and 13 three-line hybrid rice varieties. The specific variety names are shown in Table 1.

Experiment soil

The soil was taken from the experiment field of Lengshuijiang Agricultural Bureau. It was collected at a depth of 20 cm, and the amount and area of the soil were measured. The collection, filling and transportation were carried out with clean tools.

Experiment methods

The tested varieties were planted in pots containing Cd-contaminated soil (each variety was divided into 3 groups, each of which was set up with 3 replicates), and unified normal management was adopted. After maturation, the plant height, panicle length, number of effective panicles, number of grains per panicle, 1 000-grain weight and seed setting rate of each variety were measured. After harvesting, the Cd uptake of each test variety was determined.

Experiment design

After germination of rice varieties, the rice seedlings were first raised in Cd-free rice fields. After 30 d, the seedlings with 3-4 tillers were transplanted into pots. These pots were required to be about 0.16 m2 at the surface (the diameter of the round pots was 45 cm), and the soil was added into the pots according to 15 kg per pot. The seedlings were transplanted according to 3 holes per pot, 2 plants per hole, and 5 pots per treatment. After transplanting, a 2-3 cm water layer (pure water produced by pure water equipment) was maintained on the potted soil surface during the whole growth period. The pots were neatly and compactly placed in a flat net room, which was provided with a protective net to do a good job in pest and rat control.

Determination items and methods

Determination of agronomic traits and yield: The plant height, panicle length, number of effective panicles, number of grains per panicle, 1 000-grain weight and seed setting rate were investigated at the maturation stage. Each plot was harvested and dried separately, and the actual grain yield was measured after natural air drying.

Determination of heavy metal Cd: The aboveground samples of rice were taken at the maturation stage of rice, and divided into two parts: grains, and stems and leaves, which were washed repeatedly with tap water and deionized water and dried to constant weight at 70 ℃. The dried samples were crushed with a stainless steel pulverizer and sieved through a 45 mesh sieve. The soil and rice samples were digested with an MARS microwave digestion system (manufactured by CEM, USA), and the contents of Cd, As and Hg were determined by inductively coupled plasma atomic emission spectrophotometer (ICP-AES). Specifically, 0.5 g of soil sample was accurately weighed into a digestion tank, which was then added with 5 ml of aqua regia and 3 ml of ultrapure water, and placed in a microwave digestion apparatus for digestion. After digestion, the liquid was filtered with a quantitative filter paper, obtaining the filtrate which was diluted to 50 ml. For rice samples, 0.5 g of a rice sample was added to a digestion tank, which was then added with 5 ml of concentrated nitric acid, 3 ml of ultrapure water and 2 drops of hydrogen peroxide and placed in a microwave digestion apparatus for digestion. After the digestion, the liquid was filtered with a quantitative filter paper and diluted to a volume of 50 ml. The reagents used in the analysis were all guarantee reagents.

Data statistics and analysis

The data were processed and analyzed using DPS and Excel software.

Results and Analysis

Analysis of differences in Cd accumulation and yield among different rice varieties

It can be seen from Table 1 that the average Cd contents and yields of different types of rice varieties ranked as three-line rice > two-line rice > conventional rice. The differences between the first two types of rice were not significant, but their values were significantly larger than those of conventional rice varieties. The amplitude of variation of Cd content in different types of rice varieties was 0.012-0.035 mg/kg, which was significantly less than the national standard value (0.2 mg/kg). There were 10 varieties with a Cd content less than or equal to 0.015 mg/kg, such as Tianfengyou 316, and 10 varieties with a Cd content in the range of 0.016-0.025 mg/kg, such as Huanghuazhan, and the Cd contents of other varieties were greater than 0.026 mg/kg. The yields ranged from 79.21 to 210.46 g/pot. The highest yield among these types was Xiangyou 109, which was extremely significantly higher than other varieties, followed by Tianfengyou 316 (196.04 g/kg), and Huanghua exhibited the lowest yield. According to the comprehensive analysis of Cd contents and yields of different types of rice varieties, the ideal rice varieties with a higher grain yield and a lower Cd content were Xiangyou 109, Tianfengyou 316, C Liangyou 651, Y Liangyou 3218, Y Liangyou 9918 and T You 109.

Different lowercase letters following data in the same column indicate a difference at the 0.05 significant level.

Comparison of main agronomic traits among different rice varieties

Table 2 shows the differences in main agronomic traits among different types of rice varieties. It can be seen from the table that the average values of plant height and number of effective panicles ranked as two-line hybrid rice > three-line hybrid rice > conventional rice varieties, and the average values of panicle length, number of grains per panicle, the seed setting rate and 1 000-grain weight were in order of three-line hybrid rice > two-line hybrid rice> conventional rice. There were some differences in agronomic traits among different rice varieties. The amplitude of variation of plant height was 85.5-118.5 cm, and the plant heights of such two varieties as T You 1655 and Y Liangyou 9918 were over 110 cm. The number of effective panicles ranged from  7.83 to  12.33 panicles/plant, and among the various varieties, four varieties, namely, Neixiang 2128, Y Liangyou 3218, Nei 5 You 263 and Shenliangyou 9588 all exhibited a number of effective panicles over 11 panicles/plant, which were superior to other rice varieties. The panicle lengths were in the range of 20.2-31.4 cm, and such 8 varieties as Chuanxiang 8, C Liangyou 651, Xiangwanxian 13, Tianyou 3301, Y Liangyou 3399, T You 109,  Xiangyou 109 and Y Liangyou 3218 all showed a panicle length greater than 29 cm. The number of grains per panicle ranged from 106.0 to 265.3 grains/panicle, and such 3 varieties as Xiangyou 109, C Liangyou 651 and Tianfengyou 316 had more than 260 seeds per panicle, which were better than other varieties. The seed setting rates were in the range of 86.42%-94.62%, and among the various varieties, Tianfengyou 316 had the highest seed setting rate, while Xiangwanxian16 and Nongxiang 18 had a relatively lower seed setting rates (about 86%). The 1 000-grain weight/ranged from 17.8 to 26.7 g, and such 6 varieties as Y Liangyou 9918, Zhunliangyou 527, Xinliangyou 6, T You 109, Tianfengyou 316 and Nongxiang 18 all exhibited a 1 000-grain weight above 26 g. Further analysis showed that the higher yield of Xiangyou 109 was due to its longer panicles and more grains per panicle, and the high yield of Tianfengyou 316 was due to its better seed setting rate, number of grains per panicle and 1 000-grain weight.

Agricultural Biotechnology2019

Correlation analysis between Cd content and yield of different rice varieties

To further understand whether heavy metal Cd stress has an effect on the yields of different rice cultivars, the Cd contents and yields of various rice varieties were subjected to correlation  analysis (Fig. 1). The results showed under the soil Cd content of 0.58 mg/kg, there was a certain negative correlation between Cd content and yield of rice, indicating that under the concentration of Cd pollution, the rice varieties with higher yield potential had relatively low Cd uptake and accumulation. Such result has important theoretical and reference significance for screening ideal rice varieties for field cultivation.

Conclusions and Discussion

Comparing the Cd contents and yields of various rice varieties, the result shows that there were six ideal rice varieties with a higher grain yield and a lower Cd content, namely Xiangyou 109, Tianfengyou 316, C Liangyou 651, Y Liangyou 3218 and T You 109; and 10 varieties showed a poor comprehensive effect, namely T You 1655, Xinyou 215, Zhunliangyou 527, Q You 1127, Fengliangyou 4, Nongxiang 18, Xiangwanxian 16, Jinyou 163, Shenyou 9588 and Fenghuazhan. Furthermore, the higher yields of these varieties are mainly because that their traits such as panicle length, number of effective panicles and 1 000-grain weight are superior to other varieties. The correlation analysis between Cd content and yield of mature rice plants showed that there is a negative correlation between Cd content and yield in rice at the maturation stage.

The test soil in this study was taken from the test field of the Agricultural Bureau of Lengshuijiang City (one of the areas with serious heavy metal pollution in Hunan Province), with a Cd content of 0.58 mg/kg. Twenty six rice varieties that had been promoted locally in a large area or being promoted were selected as materials, and during the test, measures were taken to prevent Cd pollution caused by other factors, such as irrigation with deionized water, so the test results were highly targeted and practical. However, since the results of this study were obtained under a low Cd-contaminated soil condition, it is not yet certain whether the Cd contents and yields of different rice grains still have such a relationship under moderate Cd-contaminated soil and high Cd-contaminated soil conditions, which still needs to be further researched and verified.

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