Advances in Research on Effect of Heavy Metals Cd and Pb on Wheat and Mitigation Effect of Conditioners

Shuyan SONG, Hao LIU, Belekz·Matikurban

1. College of Agronomy & Resources and Environment, Tianjin Agricultural University, Tianjin 300384, China; 2. Department of Agricultural Sciences, Hetian Polytechnic, Hotan 848000, China

Abstract In this article, the effect of heavy metals Cd and Pb on seed germination, seedling growth and yield of wheat is investigated, and the effect of related conditioners on improving soil and alleviating Cd and Pb stress is explored.

Key words Wheat, Heavy metal stress, Cd, Pb, Conditioner

1 Introduction

By the end of 2019, the planting area of wheat in China had reached 23.73 million ha. With the advancement of China’s modernization, the ensuing environmental pollution and ecological damage cannot be ignored. The overall farmland environment in China is deteriorating. From the perspective of pollution characteristics, heavy metals are the main pollutants of farmland soil and water body. Wheat roots absorb heavy metals under certain conditions, and heavy metals not only affect plant growth, but also accumulate in plant organs. After being absorbed by the human body, they will inevitably harm human health.

2 Sources of heavy metals and their harm to human body

2.1 Sources of heavy metals in farmland

2.1.1

Sewage irrigation. Under normal circumstances, when sewage is used to irrigate farmland, heavy metal ions in sewage will remain in the soil. According to data from the Ministry of Agriculture, China, as of 2000, China’s sewage irrigated area had reached more than 4.3 million ha. The accumulation of heavy metals in a large amount of farmland due to sewage irrigation can no longer be ignored.

2.1.2

Air pollution. The waste gas emitted from mineral mining and heavy industrial production (such as non-ferrous metal processing, metal smelting) contains a large amount of heavy metal particles. These pollutants enter the soil from the atmosphere through dry deposition or wet deposition. In addition, air pollutants tend to move with the wind to cause large-scale pollution.

2.2 Harm of heavy metals to human body

Cd enters the human body and is mainly distributed in the bones, with a long half-life. Itai-Itai disease is caused by the long-term consumption of Cd-contained rice, which leads to the loss of Ca in the body and eventually makes people become ill. Cd is also distributed in the liver and pancreas of the human body, leading to the occurrence of many chronic diseases.

Chronic Pb poisoning is very common, mainly disrupting the body’s enzyme system and destroying human body functions. The brain is the target organ of Pb. If the blood Pb content exceeds the standard, it will cause serious damage and impact on the nervous system, such as causing retardation of the intelligence and development of school-age children. In addition, Pb can also affect the human immune system and cause the body’s own immunity to decline.

3 Effect of Cd and Pb on growth of wheat

3.1 Effect on germination of wheat seeds

The germination of wheat seeds marks the beginning of the wheat growth phase. It is also the earliest stage when plants are exposed to heavy metals. According to research by Zhou Qiufeng

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, the germination rate and germination potential of wheat were obviously reduced with the increase of Cd ion concentration, and they were negatively correlated with the increase of Cd ion concentration. However, there are also some opposite conclusions. Zhu Dumin’s researchshows that in the case of applying low-concentration Cd (2 and 5 mg/L), the germination rate of wheat was increased compared with the control, showing a promoting effect. Only when the Cd concentration reaches a certain level will it have a significant inhibitory effect on wheat germination, and will the germination rate show a gradual downward trend. Research by He Junyu

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shows that the effect of Cd on the germination of wheat seeds is not only related to concentration, but also related to wheat varieties. Different wheat varieties have different responses to Cd. In general, when the concentration of Cd compounds is greater than 50 mmol/L, wheat seed germination and growth will be severely affected and inhibited from the beginning.

The research of Zhang Yali and Wang Linshengon the seed germination and seedling growth of spring wheat and durum wheat shows that wheat seeds had a certain tolerance to heavy metals, but after exceeding a certain limit, with the increase of the concentration of heavy metal ions, the germination index and vigor index of wheat decreased.

3.2 Effect on growth of wheat seedlings

Studies have shown that both Cd and Pb have a stress effect on the growth of wheat seedlings, and the six indexes of seedling root length, shoot length, root length/bud length, root weight, shoot weight and root weight/bud weight all show a decreasing trend. In terms of the effect on the length and weight of wheat seedlings, the inhibitory effect of Cd and Pb on the length of wheat seedlings is greater than that on the weight. As the root/shoot ratios decrease continuously, it could be concluded that Cd and Pb are more harmful to the growth of wheat roots than to young shoots. This is mainly due to the adsorption and fixation of large amounts of heavy metal ions by wheat roots, hindering the migration of heavy metal ions to the above-ground part and reducing the damage to the buds. As a result, the inhibition of the growth of wheat roots is significantly stronger than that of buds. From the perspective of wheat seedling growth, even if low concentrations of heavy metals have limited impact on seeds, the germination rate and germination vigor are not significantly reduced, and the germination rate of some varieties even increases slightly, the growth of wheat seedlings in such environment will be inhibited after germination. Under the environment of low concentration of Cd, the seedling growth of some wheat varieties such as Luohan 6, Zhongyu 10 and Yunong 949 is better than the control. However, this positive promotion effect is limited and will gradually disappear over time.

3.3 Effect on dry matter accumulation and yield of wheat

Cd and Pb stress affect the subsequent growth of wheat, manifested as a decrease in plant height and a decrease in biomass of the above-ground part, affecting normal heading of wheat. After Cd treatment, wheat heading and flowering time are about 5 d later than the control. Heavy metal stress can also affect wheat yield, as well as leaf area index, effective spike number, grain number per spike and 1 000-grain weight. The effect of Pb and Cd on wheat yield is mainly reflected in their effect on tillering ability. According to the research of Huang Pingxia, as the concentration of Cd and Pb ions increased, the number of effective ears of wheat showed a downward trend.

The direct effect on the grain number per spike and grain weight per spike is mainly reflected in the process of grain filling. The source of wheat grain filling material can be divided into two parts according to the sequence of formation time. One part is the assimilation products stored in the vegetative organs before flowering and transferred to the grains after flowering. The other part is the products of assimilation after flowering, including the photosynthetic products directly transferred into the grains and storage materials formed after flowering and then transferred to the grains. The transfer of dry matter affects the quality of wheat grains. During grouting, Cd ions may damage the assimilation product transport pathway of "leaf-stem sheath-ear". As a result, the assimilation products cannot be smoothly transported from the leaves and stem sheaths to the ears, and part of them remain in the leaves and stem sheaths. Due to the obstruction of dry matter transportation, the development of wheat ears will be inevitably affected, leading a decline in yield.

4 Slow release effect of conditioners on soil Cd and Pb ions

4.1 Calcium carbonate and fly ash

Calcium carbonate and fly ash can effectively alleviate the toxicity of Cd and Pb to wheat. Fly ash is rich in alkaline metals such as K, Ca and Mg. Increasing the soil pH value also changes the physical and chemical properties of the soil, promoting the enhancement of the activity of some microorganisms in the soil and promoting the conversion of nutrients. In addition, Ca and Mg are essential elements for plant growth. Ca ions can produce resistance when plant roots absorb Cd and Pb. That is, when Ca ions enter, the absorption of heavy metal elements by plant roots will decrease. At the same time, the absorption of Ca will increase. In addition, Ca ions can participate in the construction of wheat cell wall and form insoluble precipitate with pectin to prevent the intrusion of Cd and Pb ions.

4.2 Clay minerals

Clay minerals are rich in reserves. They are collectively referred to as hydrated aluminosilicates, and are a type of natural cleaning agent, widely used in the improvement of soil. The principle is that clay mineral particles are small in size and large in specific surface area, and they form colloids with the soil solution in the soil, achieving soil improvement with their excellent adsorption and ion exchange. The main function of clay minerals as conditioner is to carry out adsorption, ion exchange, coordination reaction, co-precipitation with heavy metal ions, so as to achieve the effect of fixing heavy metals.Sepiolite is a magnesium silicate clay mineral with strong adsorption capacity. According to the study by Hao Jincai

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, when sepiolite was applied, the contents of available Pb and available Cd in the soil were reduced by 19.2% and 30.8%, respectively compared with the control. This shows that sepiolite has a good remediation effect on the soil with moderate Cd pollution and high Pb pollution.

4.3 Biochar

Biochar is a kind of carbon-rich, stable aromatized solid substance formed by crop straw or other biomass materials through cracking under hypoxia or anaerobic conditions at less than 700 ℃. Because biochar has a stable soil aromatization structure in the soil, bacteria and fungi in the soil are difficult to degrade biochar, effectively preventing the combined heavy metal ions from re-entering the soil due to the decomposition of biochar by microorganisms, prolonging the consolidation time, and alleviating soil Cd and Pb pollution. Soil pH affects the adsorption effect of biochar on Cd and Pb. According to research by Huang Min

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, after biochar was applied to soils with different pH values, the available Cd and Pb contents in the soil were significantly reduced, the reduction of available Pb content (58.6%) was significantly greater than that of available Cd content (50.05%), and the reductions in acidic soil were significantly greater than those in neutral soil and alkaline soil.

5 Conclusions

In general, low concentrations of Cd and Pb can promote the germination rate and germination potential of wheat. But when they reach a certain concentration, they show an inhibitory effect on the germination of wheat seeds. As the concentration increases, the inhibitory effect becomes more obvious. At the same concentration, Cd shows a stronger impact on the germination of wheat seeds than Pb. This may be due to the strong migration ability of Cd in the soil, making it easier to enter wheat roots. Compared with the blank control, the plant height of wheat treated with Cd and Pb decreased to varying degrees, and the yield of wheat also decreased with the increase of the concentration of heavy metals. However, some wheat varieties have better tolerance to Cd and Pb, and can withstand relatively high concentrations of heavy metals. For example, the seedling growth of Zhongyu 10 is promoted at low concentrations of heavy metals, and the promoting effect is weakened with time. Therefore, varieties that are more tolerant to Cd and Pb can be promoted as tolerant varieties.

There are a wide range of wheat producing areas in China, and the soil quality and soil environment in different areas are obviously different. The actual conditions of the effective state of Cd and Pb are also different. Research shows that the lower the soil pH value and Eh value, the higher the effective state of Cd and Pb. The higher concentration of cations in the soil, to a certain extent, can absorb more Cd ions and Pb ions. Calcium carbonate, fly ash, clay minerals and biochar have good effects on soil improvement. However, it depends on the specific situation when they are used. Excessive use will often lead to the opposite result. Most of the conditioners have a good improvement effect on the acidic soil in southern China, but have poor effect on neutral and alkaline soil. A few improvement materials such as sulfhydryl modified palygorskite can be applied in neutral soil and have good improvement effect.

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