Accumulation of Heavy Metals in Agricultural Crops and Ecological Series of Crops Placement

: The accumulation of toxic elements in vegetables and melons grown in agriculture, Brassica rapa - turnip, Solanum lycopersicum - tomato, Citrullus lanatus - watermelon, Capsicum annuum - bell pepper, Daucus carota - carrots, Cucurbita pepo - pumpkin, Cucumis melo - melon, and also Prunus armeniaca - apricot from fruit trees were analyzed. The excess of maximum allowable concentrations in agricultural crops of the element As by 1.65-1.75, Cd - 1.6-2.3, Cr -1.2-2.35, Cu -1.6-3.3, Ni - 1.16-3.53, Pb - 1.54-3.08, Al - 1.36-3.5, Sb - 2.0-33, Se - 1.1-3.3 times was established. The maximum allowable concentration of mercury in vegetables and melons was equal to 0.02 mg/kg, and in the chosen plants this indicator was close to the maximum allowable concentration (MAC). An ecological series of vegetable and melon crops (tomatoes → pumpkin → turnips → bell peppers → melons → watermelons → carrots) has been developed for their placement on fields contaminated with heavy metals Se, As, Pb, Cd, Zn, included in the first class in terms of the degree of danger to human health, while Ni, Cu, Cr metals were from the second class, and metal Mn from the third class. Agricultural crops in the ecological series are placed in inverse proportion to the regularities of the hyper accumulation of heavy metals in them.


Introduction:
An increase in the production of agricultural crops leads to soil pollution with their residues and emissions from the environment, accumulation in large quantities of various chemical compounds and has a negative effect on the ecological state of the soil cover. The priority tasks for providing the population with ecologically clean products are soil cleansing, as well as increasing crop yields and improving the quality of agricultural products 1 .
Ecological monitoring is the observation, assessment and forecasting of future changes in environmental pollution as a result of anthropogenic impact. Ecological monitoring is a priority when assessing the contamination of selected soils and agricultural products with heavy metals. This is due to the fact that for several decades the use of mineral fertilizers in agriculture, as well as the release of waste from various areas of anthropogenic impact, led to their accumulation in the soil cover 2 .
Essential elements such as Cu, Zn, Ni, Co, Mn and others that enter the body are largely absorbed due to their participation in the structure of many enzyme systems and proteins with protective, transport and regulatory function. High levels of micronutrients also lead to intoxication 3 . Manganese in plants predominantly activates the action of various enzymes (or is a part of them), which are of great importance in oxidationreduction processes, photosynthesis, respiration, etc. Various agricultural crops take out from 100 Ecological assessment is a determination of the degree of suitability (favorableness) of the natural and landscape conditions of the territory for human habitation and any type of economic activity. Comprehensive ecological assessment includes landscape differentiation of the territory and analysis of the stability of landscapes to anthropogenic impact, determination of anthropogenic load, assessment of environmental pollution, and determination of the severity of the ecological situation 6 .
Among them, the soil cover plays a special role. Such pollutants accumulate in the atmosphere in the form of heavy metals and enter the soil. As a result of the impact of anthropogenic factors, we create two types of cases, consciously and unconsciously, the composition of the soil is ecologically unsuitable and polluted 7 . The distribution of heavy metals over the organs of plants is determined mainly by the properties of metals and the specific characteristics of plants but in most cases does not depend on seasonal and edaphic conditions. According to some authors, the distribution of metals in a plant is determined by the peculiarities of the genotype to a greater extent than the process of accumulation of elements. In this regard, plant species and even their varieties can differ significantly in the distribution of heavy metals between organs. In general, the distribution of metals in plants takes place as follows: root> stem> leaf> fruits (seeds). On average, aboveground organs contain 10-15, and according to some sources, 20 times less heavy metals than plant roots. The ability of the roots to retain heavy metals limits their further transport to the aboveground organs. At the same time, many studies show that with an increase in the content of heavy metals in the environment, their accumulation increases both in the roots and in the aerial part -stems, leaves and generative organs 7 .
According to many scientists, plants are the main key in the ecological chain relating all objects of the biosphere. The absorption of various kinds of toxic elements by plants, including heavy metals, is the most dangerous.
The eating of plants collected in contaminated areas can threaten the health of the population, negatively affecting the work of internal organs and the physiological processes taking place in them. From plant raw materials, heavy metals pass into food, and then enter the human body. Therefore, the study of ecological cleanliness of plant materials and extracts based on them is relevant 8 . Composting is very cheap and reliable techniques for the solid waste containing organic matter. If the compost contains contaminants such as heavy metal (HM) then it will be harm to the environment. Heavy metals are toxic to soil, plants, aquatic life and human health if their concentration is high in the compost. HMs exhibit toxic effects towards soil biota by affecting key microbial processes and decrease the number and activity of soil microorganisms. Even low concentration of HMs may inhibit the physiological metabolism of plant. Uptake of HMs by plants and subsequent accumulation along the food chain is a potential threat to animal and human health. Contaminants in aquatic systems, including HMs, stimulate the production of reactive oxygen species (ROS) that can damage fishes and other aquatic organisms. Hence the compost has to be used for agriculture it should be free from HMs. Therefore, the present study evaluated the effects of HMs containing compost on soil, plants, human health and aquatic life 9 . In urban lands, the presence of heavy metals (HMs) mainly originated from industrial emissions and traffic, whereas in rural lands the HMs pollution came from warfare activities, sewage sludge, mining, drilling, electroplating, tannery, fertilizers and pesticides. The use of HMs polluted lands for crops cultivation primarily causes reduction in the total yield and results in edible parts contamination, which harmfully disturb human health. There are important health risks associated to wastewater use for agriculture, which is a probable source of HMs such as Fe, Cu, Mn, Zn, Cd, Ni, Cr, and Pb 10 . Human can get "his share" of HMs not only directly from the inhaled air and soil dust, but also through food produced on contaminated agricultural lands 11 . Monitoring studies for 1996-2006 in the city of Yerevan, the accumulation of certain elements (Pb, Ni, Cu) in vegetables in concentrations exceeding many times the maximum allowable concentrations was revealed. Thus, the lead content was the most high in basil and exceeded the maximum allowable concentration (0.5 mg/kg) by more than 10 times, in pepper -3 times, and in tomato -4.4 times 12 . The highest measured chromium content exceeded the median value by almost 10 times. The content of arsenic in the soil exceeded the limit value of 25 times. The main purpose was the assessment of the safety of vegetable food for the prevention of potential risks for the population health. The accumulation of nickel and lead in vegetables in concentrations exceeding maximum permissible limits was found out. The high concentrations of lead and mercury were identified in mulberry fruits 14 .
According to the estimates of the World Health Organization, the average intake of aluminum in the human body from all possible sources (water, food, packaging, air) ranges from 11 to 136 mg per person per week. For European countries, this index is 11 -91 mg per person per week. According to the EFSA estimate, depending on the scenario taken into account, the consumption of five aluminum-containing food additives (aluminum-ammonium sulfate (E523)): acidic sodium aluminophosphate (E 541); sodium aluminum silicate (E 554); calcium aluminosilicate (E 556); aluminosilicate (kaolin) (E 559)) by the population of different age groups (children of primary and school age, adolescents, adults, the elderly) ranges from 2.3 to 76.9 mg/kg of body weight per week on average and from 7, 4 to 145.9 mg/kg body weight per week for 95% of the populat ion 15, 16 . Aluminum is one of the most abundant metals on our planet. It is widely used in industry, it is also found in the air as an oxide. And aluminum is also found in food, dishes, drugs and does not pose a particular threat. This metal penetrates into the body along with food, in the form of food additives or during cooking in aluminum dishes, using foil for baking, using packaging boxes, vegetables and fruits canned in cans. Its effect on the body is expressed in the formation of phospholipids, which slow down the activity of enzymes, which can cause poor memory and Alzheimer's disease 17 .
For vegetable and melons gourds, there is a unit for the maximum allowable concentration of heavy metals (MAC), which is defined as 30 mg of aluminum per kilogram of product 18 .
Arsenic is a highly toxic substance, a wellrecognized poison that is used as a drug to fight anemia, to improve appetite. Arsenic enters the body with vegetables and fruits, which have absorbed it from the ground in which it has accumulated. Usually this poisonous substance strikes the liver, skin, hair, nails. May cause cancer of the larynx, eyes and leukemia 17 . Cadmium is one of the toxic elements (similar to mercury), capable of actively accumulating in the body in large quantities. Cadmium can cause muscle pain and fragility of bones, kidneys and lungs. It accumulates in the kidneys, lungs and adrenal glands and can cause cancer, kidney failure, infertility and bone deformities 17 .
According to the data obtained by Ramankova A.A. and Batlutskaya I.V, the maximum concentrations of cadmium (Cd) accumulate in the leaves and roots, the maximum concentration of lead (plumbum) -in the inflorescences and leaves of the analyzed plants 19 .
The average content of the copper in the soil is 15-20 mg/kg. The total world reserves of copper in ores are estimated at 465 million tons. The total content of copper in the phytomass of the continents is 25 million tons, the mass fraction of copper in plants is 2•10 -4 %. Soil solutes contain copper at a concentration of 0.001-0.06 mg/l. The maximum allowable concentration of copper in vegetable and melon crops is 10 mg/kg 17 .
Mercury is highly toxic to all types of life. Mercury vapor is phytotoxic and manifests itself in slowing down the growth of branches and roots of plants and accelerating the aging of plants. This is one of the trace elements that is always present in the human body. In the absence of contact with mercury, about 1 mg% mercury was found in the kidneys in 67% of the cases studied and about 1 mg% mercury in the liver in 33%. In industrial settings, the ingestion of mercury through the respiratory tract into the body in the form of vapor or dust is of paramount importance 17 .
Nickel is the main route for food to enter the body. These include cocoa and tea, chocolate, milk and milk products, legumes, nuts, seeds, whole grains, buckwheat, oatmeal, fish, seafood, meat, intestines, eggs, mushrooms, apricots, currants, cherries, onions and dill, sorrel, salads, carrots and some other vegetables. One person receives up to 0.6 mg of nickel per day; but, according to scientists, a person needs from 100 to 300 mcg. If a little nickel gets into the body, then the blood sugar level may rise slightly, but the hemoglobin level will decrease; the growth of children has slowed down. When nickel enters the body with food, it can cause the development of tumors and mutations that pass through the cell 20 . Lead is one of the most toxic mineral elements. It is believed that up to 85% of lead in the human body comes from food. Although under natural conditions it is present in all plant species, the role of this element in metabolism has not yet been identified. The normal concentration of lead in plants is in the range of 0.5-10 mg/kg, toxic -30-300 mg/kg 21 . This work was performed via (i) analysis of the amount of toxic heavy metals in vegetable and melon crops, (ii) determination of the state of contamination with heavy metals, (iii) based on the state of accumulation of heavy metals in vegetable and melon crops, the development of an ecological series of vegetable and melon crops, which determines their placement on agricultural land according to the degree of their ability to accumulate heavy metals, that is, on fields heavily contaminated with heavy metals, place crops that weakly accumulate heavy metals. Such placement of crops reduces the accumulation of toxic heavy metals in them.

Materials and Methods:
The present work has used materials of local agro-ecological monitoring, held in the Guzar district of the Kashkadarya region. The climate of the studied area, Guzar district, located in the southeast of the Kashkadarya region of the Republic of Uzbekistan, is continental. The average annual temperature is 16°C. The average temperature in January is 1.9°C, the lowest temperature is -23°C. The average temperature in July is 26.6°C, the highest temperature is 46°C. The average annual rainfall is 285 mm. Most of the precipitation falls in spring and winter. The growing season is 272 days. The soils are light and typical gray soils. The irrigated area of the Burkhan farm, where crops were studied, is 47.4 ha, and the area of the Guzar White Horse farm is 180 ha, the fields are located at an altitude of 415-430 meters above sea level (Fig.  1). The concentration of toxic elements in crop products was measured in accordance with SanPiN No. 0366-19 22 . The content of heavy metals in crop products was determined according to the generally accepted method 16 . Analysis of results was performed on the basis of a computer program of statistical analysis, which was carried out on 24 elements of the periodic table. Physicochemical analyzes were carried out on the consumable parts of all plants selected for the content of heavy metals according to the method of 23 . Analyzed As, Cd, Cr, Cu, Hg, Ni, Pb, Al, Fe, Sb, Se and Zn, are the maximum allowable concentration of accumulated elements in plants 22,24 .
The exchange of lead in the human and animal body has been studied very little. Its biological role is also not completely clear. It is known that lead enters the body with food (0.22 mg), water (0.1 mg) and dust (0.08 mg). Usually, the lead content in the body of men is about 30 mcg %, and in women it is about 25.5 mcg %. From a physiological point of view, lead and almost all of its compounds are toxic to humans and animals. Lead, even in very small doses, accumulates in the human body, and its toxic effect gradually intensifies 25 .

Results:
To analyze the elements, the accumulation of which is observed in vegetable and melon crops grown in agriculture, Brassica rapa -turnip, Solanum lycopersicum -tomato, Citrullus lanatuswatermelon, Capsicum annuum -bell pepper, Daucus carota -carrot, Cucurbita pepo -pumpkin, Cucumis melomelon were selected, Prunus armeniaca -apricot was selected from fruit trees.
The turnip is a root vegetable, its root vegetable contains the element Al (aluminum) in the amount of 104.6 mg/kg, that is, 3.5 times higher than the MAC. In addition, among the selected crops, the content of 59.4 mg/kg was found in carrots, which is 1.98 times higher than the MAC. In a separate study of pieces of watermelon, the following results were obtained. The highest indicator was found in the core of watermelon -41 mg/kg, in the seed part of the watermelon -12.3 mg/kg, in the green peel of the watermelon -15.0 mg/kg, in the white part -18.7 mg/kg. The average content of the aluminum element in watermelon, 21.81 mg/kg, may not exceed the maximum allowable concentration, but in the part of watermelon consumed by humans, this element turned out to be 1.37 times higher than the MAC. It was found to be 18.2 mg/kg in the central part of the melon kernel, 17.2 mg/kg in the seed part and 31.6 mg/kg in the peel part of the melon. In vegetables and melons gourds, as well as in apricots, high concentrations of the Al element were found, exceeding the MAC. The highest concentration of the Al element was found in the turnip plant, which poses a danger to human health (Fig. 2. and Table  1).

Figure 2. Accumulation of the Al element in agricultural crops.
Considering that the maximum allowable concentration of As (arsenic) for vegetable and melon crops is 0.2 mg/kg, based on the abovementioned analyzes of agricultural products, we can say that the arsenic content in tomatoes, in the hearts of melons and watermelons, in red carrots and bell peppers is slightly higher than the MAC.
Melon kernels turned out to be within the MAC, but in the melon peels 1.65 times more, watermelon kernels, red carrots and tomatos -1,1 times, and bell pepper -1.75 times the MAC. No arsenic accumulation was observed in the fruit tree plant (Fig. 3. and Table 1). The maximum allowable concentration amount of Cd (cadmium) for vegetable and melon crops is 0.03 mg/kg, at the same time, it was found that in all monitored plants, the content of the element exceeds the maximum allowable concentration (MAC). The highest indicator was found in melon peel, namely 0.07 mg/kg, which is 2.3 times higher than the MAC (Fig.4. and Table 1). than the MAC, and in tomatoes this indicator is 2.15 times higher than the MAC. It was found that peels and kernels of bell pepper and pumpkin do not exceed the MAC, but in the watermelon kernel 0.18 mg/kg, and in the white part -2.25 times more than the MAC (Fig.5. and Table 1).

Figure 5. Accumulation of the Cr element in agricultural crops.
The accumulation of the element Cu (copper) in the tomato plant is 3.3 times higher than the MAC, in the melon kernel -by 7.0 mg/kg more than in the watermelon kernel. The element copper accumulates in greater quantities than in fruit trees, and differs twice as much. The content of the element copper in vegetables and melon crops forms the following descending series: tomato → turnip → pumpkin → bell pepper → watermelon → carrot → melon (Fig.6. and Table 1). The maximum allowable concentration for vegetables and melon crops for the element of Hg (mercury) is 0.02 mg/kg, and it can be seen that in individual plants these values are close to the allowable concentration. Element mercury observed in the peels of melon and pumpkin in the range of 0.0204-0.0201 mg/kg. In the green peel of the watermelon, 0.181 mg/kg was noted. In ecologically suitable parts for consumption, the mercury content is about 0.0037-0.0142 mg/kg. From an ecological point of view, it can be said that the mercury element in the soil is less than the MAC, and it was found that some plants have an accumulation process in the bark layer (Fig.7. and Table 1).

Figure 7. Accumulation of the Hg element in agricultural crops.
The MAC for the mobile form of Ni (nickel) in the soil is 4 mg/kg, and the MAC for vegetables and melon crops is 0.5 mg/kg. When assessing the ecological value of crops grown in the study area, one should pay attention to the toxic elements in the plant. The nickel content in tomatoes and pumpkins is 1.608-1.761 mg/kg, which, respectively, is 3.22-3.52 times higher than the MAC. The amount of nickel in the pumpkin peel is also 0.389 mg/kg. When studying a watermelon by dividing it into parts, the green peel, white peel, core and seeds, only in the white part of the watermelon the element is 1.37 times more than the MAC. However, it have been proven, that edible kernels to be harmless to the environment. The element nickel is 1.2 times higher than the allowable concentration in seeds and kernels of melons, 1.26-1.25 mg/kg in red carrots and turnips, 0.866 mg/kg in bell peppers, which is 1.2-3.5 times higher the MAC in vegetables and melon crops (Fig.8. and Table 1). The maximum allowable concentration for vegetables and melon crops for the element of Pb (lead) is 0.5 mg/kg. In different parts of the watermelon, the element of lead is contained in different amounts, the highest lead content in the kernel of the watermelon is 1.1 mg/kg, which is 2.2 times more than the MAC, in the white part the accumulation is 0.52 mg/kg. In the melon kernel, the lead content is 0.783 mg/kg, which is 1.56 times more than the MAC, and 1.542-1.49 mg/kg is accumulated in tomatoes and bell peppers, on average 3.08 times more, than MAC ( Fig.9. and Table 1). Only in turnip plants, the Zn (zinc) element is 1.7 times higher than the MAC, and the Fe (iron) element in the white part of watermelon and tomatoes is 1.3-1.4 times higher than the MAC. Cr -chromium, Co -cobalt, Cd -cadmium, Be -beryllium, As -arsenic, ND-not detected. Al -aluminum, Fe -iron, Hg -mercury, Ni -nickel, Pb -lead, ND-not detected. B -boron, Ba -barium, Cu -copper, Li -lithium, Mn -manganese, ND-not detected. Mo -molybdenum, Sb -antimony, Se -Selenium, Sr -Strontium, Tl -Thallium, ND-not detected. U -uranium, V -vanadium, W -wolfram, Zn -zinc, ND-not detected.

Open
The recommended concentration levels of the analyzed heavy metals by FAO/WHO in vegetables is shown in Table 2 26 .

Discussion:
This state of vegetable and melon crops is ecologically dangerous for consumption and can be the cause of various diseases of the human body. Based on the accumulation of heavy metals in vegetables and melons grown in agriculture, it is recommended to compose a series of plants with a decreasing accumulation of heavy metals in them, to place plants according to ecological state of territories contaminated with toxic elements. Likewise, the turnip had the lowest lead accumulation in the -0.44 mg/kg, which turned out to be less than the MAC 24 .
On the basis of the analyzes presented in Table 1, an ecological series of the agricultural crops with MAC of heavy metals was developed for the plant composition. As already mentioned at the end of the introduction, the ecological series of agricultural crops was developed based on the degree of their ability to accumulate toxic heavy metals in themselves. Therefore, in areas contaminated with Al metals (MAC 30 mg/kg), in addition to turnips and carrots, it is recommended to plant melons → watermelons → bell pepper → pumpkin → tomato. This is due to the fact that it was found that the accumulation of the element aluminum in these plant organs is small. It was found that all types of crops grown on soils contaminated with ions of the element arsenic contain a large amount of the element, and based on the data, it is not recommended to collect vegetables and melon cultures. It is not recommended to grow vegetables and melons, as all products grown on soils contaminated with cadmium and chromium ions contain a large amount of the elements Cd, Cr and Cu. This is due to the fact that an excess of allowable elements in the composition of the soil leads to the accumulation of large amounts in all types of vegetables and melon cultures 27 .
The accumulation of heavy metals in the above-mentioned plants is less concentrated only in the organs of watermelon plants and exceeds the allowable norm in all other plant organs. In soils contaminated with the element nickel, it is recommended to plant watermelon and melons from melons.
HM concentrations in Solanum lycopersicum and Solanum melongena have been found to be higher than permissible limits of FAO.

Conclusion:
It is recommended to plant technical crops when the content of lead ions in the soil is higher than the MAC, but it is not recommended to grow vegetables and melon cultures.
Taking into account the accumulation of heavy metals when studying the state of pollution by toxicants of agricultural crops, it is not recommended to plant plants 1, 2, 3 in a series. This is due to the fact that the absorption of toxic heavy metals by plants is dangerous to human health.
All the obtained results of analyzes showed that, from an ecological point of view, the accumulation of toxicants in plant organs occurs as a result of direct and indirect impacts of anthropogenic factors, which ultimately have a negative impact on people themselves through the ecosystem.
In conclusion, it should be noted that all elements with a toxic effect, such as heavy metals in irrigated soils, have a negative impact on the environment, on agricultural products, and also accelerate the process of migration through them.

Authors' declaration:
-There is no conflict of interests on the article.