as early as the 197s, people have noticed that heavy metals and nutrients in crops are often absorbed or stored by the same transport system because of the similarity of chemical properties or the correlation of metabolic pathways, and the symptoms of heavy metal poisoning in crops are often very similar to the symptoms of lack of some nutrients. The supply level of different nutrient elements can greatly affect the transportation and accumulation of heavy metals in crops.
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 issue in the field of heavy metal pollution ecology in recent years. Heavy metals in soil are mostly antagonistic to cationic nutrients, while they may be synergistic or antagonistic to anionic nutrients.
The interaction between heavy metals and nutrient elements in crops is complicated. Nutrient elements such as nitrogen, phosphorus and potassium play an important role in the synthesis and metabolism of protein, nucleic acid and other important substances in crops. The lack of nutrient elements in crops will lead to the disorder of substance metabolism, thus affecting 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, which has become the main reason for the lack of crop nutrients or the decrease of bioavailability. In recent 3 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 their resistance to harmful substances such as heavy metals and reduce their absorption. SCH? TZENDUBEL and others found that selenium and cadmium can partially combine with sulfhydryl groups 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 from cells with active metabolism of crops or affect the transport of heavy metals in their bodies by changing the permeability of cell membranes to heavy metals.
Therefore, applying 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 silicates which are 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's 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 in seeds is significantly reduced after spraying silicon preparation. 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.
foliar spraying of nutrients such as nitrogen, phosphorus and potassium can reduce or eliminate the toxic effects of heavy metals on crops and reduce the absorption and accumulation of heavy metals in crops. Studies have shown that the application of nitrogen fertilizer can alleviate the growth inhibition of heavy metals such as lead and zinc on the 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 phosphorus deficiency in crops caused by lead toxicity, 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 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 COO- on the surface of cell wall was easy to combine 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 transport sites in crops. Studies have shown that high concentrations of heavy metals can inhibit the absorption and transport 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, adequate supply of nutrients such as calcium and magnesium is beneficial to alleviate 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 with that of 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 decrease the cadmium content. Without calcium, cadmium can lead to chloroplast without grana, or the arrangement of grana lamellae is disordered, and the number of hungry granules increases. 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, improving the iron supply of crops by foliar spraying can reduce the accumulation of heavy metals such as cadmium in their bodies 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, the content of heavy metals such as manganese, copper, zinc and cadmium is high due to iron deficiency, which may be related to the expression of iron transporter gene.
COHEN et al. studied the cadmium absorption kinetics of peas under the conditions of iron deficiency and iron sufficiency. The results showed that the maximum initial absorption rate of cadmium by peas under the conditions of iron deficiency was nearly 7 times that under the conditions of iron sufficiency. The difference of cadmium absorption under different iron supply conditions may be related to the expression of IRT1 gene, which 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. FeSO4 _ 4, as a trace element fertilizer, can not only increase the yield of crops, but also reduce the accumulation of cadmium in crops.
The research shows that rare earth elements can alleviate heavy metal stress to some extent. Spraying neodymium or its complex on leaves can alleviate 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 application of 1 mg/L lanthanum-glycine complex can reduce the damage caused by cadmium to Chinese cabbage, improve 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 plasma membrane permeability.
In addition, the application of physiological regulating substances such as gibberellin, diethyl aminoethanol caproate, aminolevulinic acid, salicylic acid, proline, glycine and betaine 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 reduce the toxic effect of cadmium on barley seedlings. Spraying NAA can reduce the contents of malondialdehyde and proline in leaves of soybean seedlings under cadmium stress, reduce membrane lipid peroxidation and protein hydrolysis, reduce the activity of peroxidase (POD) and increase the activity of nitrate reductase.
plants have complex mechanisms to precisely regulate the absorption, transport and excretion of elements to adapt to the changes of external environment. At present, the ways of plant element balance and regulation network are still unclear. In the past, people's research often stayed in two or a few 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 nutrient elements in crops has been widely discussed, the research on the related mechanism is not in-depth as a whole. Different research conditions, research methods and tested crops lead to different or even contradictory conclusions. The interaction between heavy metals and nutrients needs further study.
2 development status of physiological resistance and control technology of leaf surface
It is a new direction of research on prevention and control of heavy metal pollution in farmland in China in recent years to prevent and control the accumulation of heavy metals in crops by using physiological resistance agent of leaf surface, which mainly reduces or even completely blocks the transfer of heavy metals to the food chain by depositing or chelating heavy metals on crop cell walls and improving the stress resistance of crops. This technology is favored by domestic researchers because of its advantages of low cost, environmental friendliness and convenient operation. According to the existing patents in China, the existing physiological barrier agents for foliage can be roughly classified into the following categories:
(1) physiological barrier agents for foliage based on silicon (including silicone and inorganic silicon), and the related patents are 13789114A (patent number, the same below), 119729A, 13864531A, 1575763A and 113.
(2) Foliar physiological barrier agent composed of selenium or rare earth elements, related patents are 138491A, 12356739A, 14788156A, 14322335A, 11597191A, 143815A, 13766182A and 12964177A.
(3) A foliar physiological barrier 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 13749223A, 13314999A, 13314693A and 15139A.
(4) The related patents are 1339241A, 1425921A, 12356739A, 1255313A, 14823738A, 1194112A, 13983A, 13936495A
At present, the institutions engaged in the product development and experimental effect research of crop leaf physiological barrier in China are mainly concentrated in the eastern and south-central regions, mainly universities and scientific research institutions, and some grass-roots agricultural departments. The commonly used experimental crops mainly include 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 the World Trade Organization's Intellectual Property Organization, and one of them was declared by China scholars.
According to some preliminary test results, the effect of using foliar physiological resistance control technology varies with different producing areas and crop categories.
The experimental results of Li Fangbai and others showed that foliar spraying silicon fertilizer increased rice yield by 29.6%, rice arsenic decreased by 28.2% (mass fraction, the same below) and cadmium decreased by 4.2%.
The pot experiment of 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% ~ 41%, 29% ~ 34% and 45% ~ 53% respectively.
The experiment in Liu Jie showed that spraying the foliar silicon fertilizer of Cadmium-lowering Ling on the leaves increased the yield of rice grains by 5%, and cadmium decreased by 4%, 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, increase the activities of POD and SOD, and reduce the cadmium and lead of lettuce aboveground by 33.5% ~ 4.1% and 55.2% ~ 63.3% respectively.
Liu Chuanping's research showed that cadmium, arsenic and lead in Shuidong mustard decreased by 24.5%, 26.4% and 22.5% respectively after foliar spraying silicon fertilizer. At the same time, spraying silicon fertilizer and cerium fertilizer decreased cadmium, arsenic and lead by 42.6%, 4.% and 36.8% respectively.
Liu Jizhen's research showed that cadmium in potted peppers decreased by 13.4% ~ 26.1% after spraying silicon.
foliar spraying selenium fertilizer also has obvious effect on the reduction of heavy metals in crops. Jiang Bin research showed that spraying nano-selenium fertilizer and selenium+chitosan compound fertilizer on leaves decreased cadmium content in hydroponic lettuce by 19.24% and 21.13% respectively. After spraying selenium fertilizer on rice leaves, rice yield increased by 16.2% and cadmium decreased by 8.6% ~ 17.8%. After spraying selenium fertilizer on the leaves of vegetables and melons, the cadmium in tomatoes and garlic seedlings decreased by 24.46%. Cadmium in tomato and cucumber decreased by 6.6% ~ 75.8%; In watermelon, cadmium decreased by 6.61% ~ 66.13%, lead decreased by 4.55% ~ 83.33%, and the content of malondialdehyde, the product of membrane lipid peroxidation, also decreased. Cadmium in strawberry leaves and fruits decreased by 1.2% ~ 94.65% and 18.33% respectively, and lead decreased by 38.86% ~ 76.8% and 77.71% respectively. The contents of cadmium, lead and mercury in persimmon decreased significantly. Spraying selenium, silicon and molybdenum compound fertilizer, cadmium in Chinese cabbage and pepper decreased by 1.9% ~ 2.6%. Combined spraying of selenium and silicon can increase rice yield by 43.8% and reduce arsenic in grains by 46%.
Foliar spraying of rare earth elements can reduce the symptoms of cadmium toxicity and lead toxicity of corn, mung bean and Chinese cabbage. Cadmium and lead in the aboveground part of lettuce decreased by 31.7% ~ 45.3% and 26.7% ~ 71.4% respectively. Cadmium in tomato decreased by 19.4% ~ 37.%, and cadmium in cucumber decreased by 32.% ~ 49.8%. Copper, zinc, cadmium, lead and nickel in rape stems and leaves decreased by 2.91% ~ 7.82%, 2.91% ~ 6.99%, 7.26% ~ 2.92%, 6.32% ~ 15.79% and 7.69% ~ 21.31% respectively. Cadmium, arsenic and lead in Shuidong mustard decreased by 22.9%, 26.% and 32.5% respectively. Spraying zinc on leaf surface reduced the cadmium content of lettuce by 37.2%. Cadmium in tomatoes decreased by 37.1%, which had no obvious effect on lead.