These interactions can promote each other or inhibit each other. Therefore, many scholars believe that studying the interaction between heavy metals and nutrient elements is not only conducive to solving the problem of heavy metal pollution in crops, but also of great significance for correctly understanding the toxic effects of heavy metals and reasonably and scientifically solving the contradiction between nutrient element utilization and heavy metal accumulation.
1 Interaction between nutrients and heavy metals
The interaction between heavy metals and nutrients is a frontier scientific problem in the field of heavy metal pollution ecology in recent years. Most heavy metals in soil have antagonistic effects on cationic nutrients, but may have synergistic or antagonistic effects on anionic nutrients.
The interaction between heavy metals and nutrients in crops is complex. Nutrient elements such as nitrogen, phosphorus and potassium play an important role in the synthesis and metabolism of important substances such as protein and nucleic acid in crops. The lack of nutrients in crops will lead to the disorder of substance metabolism, which will affect the growth of crops and the output of agricultural products.
With the increasing heavy metal pollution in farmland soil, heavy metal stress interferes with the utilization of crop nutrients and becomes the main reason for crop nutrient deficiency or decreased bioavailability. In recent 30 years, many scholars who study plant nutrition and plant physiology have conducted extensive research on the interaction between nutrients and heavy metals in plants, and found that increasing the supply of nutrients such as nitrogen, phosphorus and potassium can improve the enzyme system and metabolic process in crops and alleviate the impact of heavy metal stress to some extent.
Selenium can promote the formation of antioxidant substances in crops, increase the resistance of crops to harmful substances such as heavy metals and reduce absorption. SCH？ TZENDUBEL et al. found that selenium and cadmium can combine with sulfhydryl moiety of cysteine in some protein. The supply of exogenous selenium can increase the glutathione content of glutathione peroxidase (GSH-Px) in rice and promote the combination of cadmium and sulfhydryl groups.
Selenium and cadmium may also form CdSeO3, which reduces the solubility and absorption of cadmium. The combination of selenium and other heavy metals can also produce insoluble compounds, inhibit the absorption of heavy metals by crops and reduce the accumulation of heavy metals in plants. Selenium may also promote the removal of heavy metals by cells with active crop metabolism or affect the transport of heavy metals in their bodies by changing the permeability of cell membranes to heavy metals.
Therefore, the application of selenium fertilizer can reduce the risk of cadmium pollution in rice and other crops. The pot experiment of SHANKER et al. found that selenium can reduce the absorption of mercury by radish, probably because selenium and mercury form insoluble compounds in soil.
Silicon can promote the growth of crops, improve their stress resistance and significantly reduce the migration of cadmium and lead in rice and other crops. Silicate ions contained in silicon fertilizer applied to soil can react chemically with cadmium to form silicate which is not easily absorbed by plants and precipitate, thus reducing the absorption of heavy metals such as cadmium by crops.
Silicon, as a nutrient element of rice, can increase chlorophyll content in rice leaves, improve root activity and reduce cell membrane permeability, thus improving rice resistance to heavy metals. Spraying nano-silicon preparation on leaves can alleviate the toxic effect of heavy metals on rice, and the absorption of cadmium, lead, copper and zinc by seeds after spraying silicon preparation is obviously reduced. Another study shows that preventing cadmium from migrating upward due to the precipitation of silicon and cadmium can significantly reduce the migration ability of cadmium in soil, thus reducing the accumulation of cadmium in the aboveground parts of crops and reducing the cadmium content in rice.
Spraying nutrients such as nitrogen, phosphorus and potassium on leaves can reduce or eliminate the toxic effects of heavy metals on crops and reduce the absorption and accumulation of heavy metals by crops. The results showed that the application of nitrogen fertilizer can alleviate the growth inhibition of heavy metals such as lead and zinc on leaves and roots of winter wheat seedlings, and it will be enhanced with the increase of nitrogen application rate.
Foliar spraying of phosphorus fertilizer can improve the symptoms of crop phosphorus deficiency caused by lead poisoning, and soil application of phosphorus can reduce the bioavailability of lead, which has been verified in spinach, carrots, oats and ryegrass. Ding Lingyun and other studies show that foliar spraying of KH2PO4 can increase rice yield and reduce the accumulation of lead, zinc and cadmium in rice. SINGH and others found that the application of potassium fertilizer can obviously reduce the zinc concentration in wheat plants. When studying the migration pathway of cadmium in wheat cells, it was found that the chief operating officer on the surface of cell wall-easily combined with cadmium, so that cadmium was trapped on the cell wall and did not enter the cell membrane.
Calcium, magnesium and zinc can compete with heavy metals for absorption and transportation sites in crops. Studies have shown that high concentrations of heavy metals will inhibit the absorption and transportation of nutrients such as calcium and magnesium by crops. For example, the cadmium content in the stems and leaves of wheat seedlings treated with cadmium increased significantly, while the contents of nutrients such as calcium and magnesium decreased significantly. The increase of zinc concentration can reduce the concentration of magnesium and other elements in crops.
Therefore, the adequate supply of nutrients such as calcium and magnesium is conducive to reducing the toxic effects of heavy metals. This is because calcium and magnesium in crops are beneficial to maintain the normal osmotic system of root cells.
Under cadmium stress, comparing the situation of adding calcium and not adding calcium, it was found that adding calcium could significantly increase the calcium content in the organelles and cytoplasm of maize roots and leaves, but significantly reduce the cadmium content; Without calcium, cadmium will lead to chloroplasts without grana, or the arrangement of grana lamellae will be disordered, and hungry granules will increase. This is because calcium plays a very important role in maintaining the normal structure and function of maize leaves under cadmium stress.
Iron can affect the chloroplast function of crops, coordinate physiological functions and affect the absorption and transportation of heavy metals. Therefore, by foliar spraying to improve the iron supply of crops, the accumulation of heavy metals such as cadmium can be reduced to some extent. Under normal circumstances, if the iron content in crops is sufficient, the content of heavy metals such as manganese, copper, zinc and cadmium will be low; However, due to iron deficiency, the contents of heavy metals such as manganese, copper, zinc and cadmium are high, which may be related to the expression of iron transporter gene.
COHEN et al. studied the absorption kinetics of cadmium by peas under iron deficiency and iron deficiency conditions. The results showed that the maximum initial absorption rate of cadmium by peas under iron deficiency was nearly 7 times that under iron deficiency. The difference of cadmium absorption under different iron supply conditions may be related to the expression of IRT 1 gene. IRT 1 is an iron transporter gene cloned from Arabidopsis thaliana. Iron deficiency can induce its expression, promote the absorption and transport of iron, and also promote the absorption and transport of heavy metals such as cadmium. The increase of cadmium absorption rate under iron deficiency may also be related to the activation of proton pump in plasma membrane. As a trace element fertilizer, FeSO4 _ 4 can not only increase crop yield, but also reduce cadmium accumulation in crops.
The research shows that rare earth elements can alleviate the stress of heavy metals to some extent. Spraying neodymium or its complex on leaves can reduce the damage of lead to mung beans, Chinese cabbage and spinach. Spraying lanthanum on the leaves of kidney bean and corn seedlings under cadmium stress can reduce the damage of cadmium to seedlings. Foliar spraying 100 mg/L lanthanum-glycine complex can reduce the damage of cadmium to Chinese cabbage, increase the photosynthetic rate, Hill reaction activity and nitrate reductase activity of Chinese cabbage, increase the contents of chlorophyll and nucleic acid, reduce the contents of malondialdehyde and cadmium, and reduce the permeability of plasma membrane.
In addition, the application of gibberellin, diethyl aminoethanol hexanoate, aminolevulinic acid, salicylic acid, proline, glycine, betaine and other physiological regulatory substances can also reduce the toxic effects of heavy metals on crops. Applying cytokinin 6- benzylamino adenine can alleviate the toxic effect of mercury on Alternanthera philoxeroides. The application of salicylic acid and abscisic acid can alleviate the toxic effect of cadmium on barley seedlings. Spraying NAA can reduce the contents of malondialdehyde and proline, membrane lipid peroxidation and protein hydrolysis, decrease the activity of peroxidase (POD) and increase the activity of nitrate reductase under cadmium stress.
Plants have a complex mechanism to accurately adjust the absorption, transportation and excretion of elements to adapt to the changes of the external environment. At present, the ways of plant element balance and regulation network are not very clear. In the past, people's research often stayed on two or more elements, but in fact, the balance of elements in plants is a very complicated process. Some people think that the ion omics method developed in recent years can provide a means to further reveal the interaction mechanism between nutrient elements and heavy metals in plants.
At present, although the relationship between heavy metals and crop nutrients has been widely discussed, the research on the related mechanism is not in-depth as a whole. Different research conditions, different research methods and different tested crops lead to different and even contradictory conclusions. The interaction between heavy metals and nutrients needs further study.
2. Development status of leaf physiological resistance control technology.
In recent years, it is a new direction to prevent the accumulation of heavy metals in crops by using physiological barriers on crop leaves. By depositing or chelating heavy metals on the cell wall of crops, the resistance of crops to heavy metals can be improved, and the transfer of heavy metals to the food chain can be reduced or even completely blocked. This technology is favored by domestic researchers because of its advantages of low cost, environmental friendliness and convenient operation. Judging from the existing domestic related patents, the existing foliar physiological barrier agents are roughly as follows:
(1) Leaf physiological barrier containing silicon (including silicone and inorganic silicon). The related patents are103789114a (patent number, the same below), 10 1907029A, 103868.
(2) Physiological barrier of selenium or rare earth elements on leaves. The related patents are 10380409 1A, 102356739A, 104788 156A,1043222.
(3) A foliar physiological barrier agent with nitrogen, phosphorus, potassium and conventional trace elements (calcium, magnesium, iron, boron, manganese, zinc, molybdenum, titanium and sulfur) as the main components, and the related patents are 103749223A,1033149993.
(4) Physiological barrier of crop leaves, the related patents are 10339240 1A,10402592102356739a, 102550365438+3A,/kloc. 103980030A， 103936495A， 102653486A， 102653485A， 10422 1796A， 104322。
At present, China's institutions engaged in the development of crop leaf physiological barrier products and the study of experimental results are mainly concentrated in the eastern and central and southern regions, mainly universities and scientific research institutions, as well as some grass-roots agricultural departments. The commonly used experimental crops are mainly rice, vegetables, melons and fruits. The heavy metals involved are mainly cadmium, and lead, arsenic and mercury are also involved.
Because the problem of cultivated land resources in Europe and America is not very prominent, there are not many related patents. Only three related patents were retrieved from websites such as the US Patent Office, the European Patent Office and WIPO of the World Trade Organization, among which China scholars declared 1.
From some preliminary test results, the effect of using leaf physiological resistance control technology varies with different producing areas and crop categories.
The experimental results of Li Fangbai and others show that foliar spraying silicon fertilizer can increase rice yield by 29.6%, reduce rice arsenic by 28.2% (mass fraction, the same below) and cadmium by 40.2%.
Pot experiments by Wang Shihua et al. showed that after spraying silicon fertilizer, cadmium in rice grains decreased by 17% ~ 53%, and lead, zinc and copper decreased by 26% ~ 4 1%, 29% ~ 34% and 45% ~ 53% respectively.
The experiment in Liu Jie showed that spraying foliar silicon fertilizer with cadmium detergent could increase rice yield by 5% and reduce cadmium by 40%, which inhibited the migration of heavy metals from leaves to grains.
Cui Xiaofeng and others found that foliar spraying silicon fertilizer can promote the growth of lettuce and increase the activities of POD and SOD, and the cadmium and lead in the aboveground parts of lettuce decreased by 33.5% ~ 40. 1% and 55.2% ~ 63.3% respectively.
Liu's research showed that the contents of cadmium, arsenic and lead in Shuidong mustard decreased by 24.5%, 26.4% and 22.5% respectively after spraying silicon fertilizer on the leaves. At the same time, spraying silicon fertilizer and cerium fertilizer reduced cadmium, arsenic and lead by 42.6%, 40.0% and 36.8% respectively.
The research in Liu Jizhen showed that the cadmium content in potted peppers decreased 13.4% ~ 26. 1% after spraying silicon.
Foliar spraying selenium fertilizer also has obvious effect on the decline of heavy metals in crops. Jiang Bin research showed that spraying nano-selenium fertilizer and selenium+chitosan compound fertilizer on leaves reduced the cadmium content of hydroponic lettuce by 19.24% and 2 1. 13% respectively. After spraying selenium fertilizer on rice leaves, rice yield increased by 16.2%, and cadmium decreased by 8.6% ~ 17.8%. After foliar spraying selenium fertilizer on vegetables and melons, cadmium in tomatoes and garlic seedlings decreased by 24.46%. The cadmium content in tomatoes and cucumbers decreased by 60.6% ~ 75.8%. In watermelon, cadmium decreased by 6.6 1% ~ 66. 13%, lead decreased by 4.55% ~ 83.33%, and the content of malondialdehyde, a product of membrane lipid peroxidation, also decreased. Cadmium in strawberry leaves and fruits decreased by 10.20% ~ 94.65% and 18.33% respectively, and lead decreased by 38.86% ~ 76.80% and 77.7 1% respectively. The contents of cadmium, lead and mercury in persimmon decreased significantly. The cadmium content in Chinese cabbage and pepper decreased 1.9% ~ 20.6% by spraying Se-Si-Mo compound fertilizer. Combined application of selenium and silicon can increase rice yield by 43.8% and reduce arsenic content in grain by 46%.
Spraying rare earth elements on leaves can alleviate the symptoms of cadmium poisoning and lead poisoning in corn, mung bean and Chinese cabbage. Cadmium and lead in the shoot of lettuce decreased by 365,438 0.7% ~ 45.3% and 26.7% ~ 765,438 0.4% respectively. Cadmium in tomato decreased by 19.4% ~ 37.0%, and cadmium in cucumber decreased by 32.0% ~ 49.8%. Copper, zinc, cadmium, lead and nickel in rape stems and leaves decreased by 2.9 1% ~ 7.82%, 2.9 1% ~ 6.99%, 7.26% ~ 20.92%, 6.32% ~ 15.79% respectively. Cadmium, arsenic and lead in Shuidong mustard tuber decreased by 22.9%, 26.0% and 32.5% respectively. Spraying zinc on leaves reduced the cadmium content of lettuce by 37.02%. Cadmium in tomato decreased by 37.0 1%, and had no obvious effect on lead. Cadmium in brown rice decreased by 465,438+0.9%. Spraying iron on the leaves reduced the contents of cadmium, lead and copper by 4.3% ~ 35.5%, 6. 17% ~ 50.30% and 8.34% ~ 33.40% respectively. The cadmium content in tomato fruit decreased by 2.8% ~ 8.2%.
3 Application prospect
Heavy metals and many essential nutrients affect the functions of both sides, and nutrient supplementation can reduce the toxic effects of heavy metals on crops. Silicon, selenium, nitrogen, phosphorus, potassium, calcium, magnesium and some trace elements are the beneficial elements of crops. Using foliar spraying as a physiological barrier can not only prevent the accumulation of heavy metals in crops to a certain extent, but also promote the growth, yield increase and quality improvement of crops. Because of its low cost and convenient operation, many researchers believe that foliar spraying is a convenient and effective physiological barrier method for moderately and lightly polluted soil and has broad application prospects.
Compared with foreign countries, the discussion on leaf physiological barrier agents in China is wide, and the research and development of physiological barrier agents is in the forefront of the world, especially in the application research of heavy metal control in rice. However, the basic theoretical research on controlling heavy metal content in crops by leaf physiological barrier is not as good as that abroad. At present, the technology of regulating the accumulation of heavy metals in agricultural products by using the physiological barrier of crop leaves in China is not mature enough, most products lack extensive field test evaluation, the application effect is not stable enough, and the application methods need to be improved. Due to the lack of unified and standardized standards, there are many varieties of foliar physiological barrier agents in China at present, which makes users at a loss. It is urgent to establish a standardized standard for foliar physiological barrier agents. At the same time, due to the lack of standardized application technology, users can not achieve the due application effect because of irregular operation. Therefore, we should focus on strengthening the following research:
(1) Research and development of efficient physiological barrier for heavy metal contaminated crop leaves in farmland. On the basis of previous studies, the control effects of different physiological barrier agents on heavy metals in agricultural products were deeply discussed in order to screen out efficient physiological barrier agents for crop leaves.
(2) Study on the application scope of physiological barrier of crop leaves. In different regions, different rotation systems and different water and fertilizer management conditions, the effects of foliar physiological barrier on heavy metal absorption by crops were studied to determine its application scope, and the effects of environmental factors (temperature, humidity, light, soil fertility), spraying time, spraying dose and concentration, spraying times and so on on on heavy metal absorption by crops were discussed.
(3) Screening additives to improve the efficacy of leaf physiological barrier and developing compound leaf physiological barrier. Through the application of leaf physiological barrier, surfactant, complexing agent and plant growth regulator, the formula of blocking heavy metal absorption with low toxicity and high efficiency was screened out. On this basis, the physiological barrier of compound leaves was further studied through the matching test of various formulas.
(4) Strengthen the field demonstration of leaf physiological barrier and study on application technical specifications. Selecting mature leaf physiological barriers can be used for field demonstration research in many places, and the application technology can be constantly revised in practical application to improve its efficiency, and finally relevant technical regulations can be formed.