Absorption and volatilization of selenium by plants

Plants absorb selenium from soil and air through roots and leaves. At the same time, selenium is also discharged into the air through the leaves. This process of absorption and emission constitutes the metabolic mechanism of plants. The amount of selenium absorbed by plants and the structural state of selenium discharged from plants are different.

plants absorb selenium from soil and air and convert it into amino acids and protein, together with water-soluble inorganic selenium, which are transported to various organs of plants for storage (Shrift,1969). This kind of absorption and storage changes with the development of plants, and the absorption, transportation and storage of selenium by different plant species are different from root to leaf to stem to seed. Zheng Daxian (1988) conducted a pot experiment with barnyard grass. In dry land cultivation, roots > leaves > stems > grains, while in waterlogging cultivation, roots > grains > leaves > stems were found. The seed storage capacity of the latter cultivation method is obviously higher than that of dry land cultivation. According to the data from the United States, the selenium content of rhizomes and bulbs, fields, leafy vegetables, lavers, fruits and vegetables, and fruit trees is .48, .279, .11, .66, and .31 μ g/g in turn (Mikkelsen, 1989). Among them, the ability of field crops to absorb and store selenium is Cruciferae > Lolium perenne > beans > cereals (Hamilton,1964). Obviously, the selenium concentration of root crops is the highest in the soil, the selenium content of field crops and leaf vegetables crops closest to the ground is relatively low, and the selenium concentration of trees and fruits with the highest ground is the lowest. However, the selenium content in grains or seeds of grain crops is greater than that in stems and leaves. The absorption of selenium by wheat is concentrated in leaves in the early growth stage (before jointing stage) and enriched in wheat ears in the later stage, accounting for 62.3% ~ 62.9% of selenium in wheat plants (Zheng Jianguo, 1989). Vegetable crops also have a selenium accumulation process similar to that of wheat, and finally most of the soluble selenium is transferred to pods or seeds (Bisbjerg,1969). From this point of view, the selenium concentration in the roots and tuber crops of plants that can directly absorb and store selenium from the soil is naturally high, and the above-ground parts of plants need to transport selenium to various organs. Obviously, the parts closest to the ground (such as early leaves) have higher selenium content, while those parts higher from the ground (such as trees and fruits) have lower selenium content. In addition, for grain crops and vegetable crops, the selenium enrichment effect of grain is much stronger than that of leaves and stems (Bisbjerg,1969), which indicates that selenium tends to be enriched in grain with plant growth.

Plant roots have the ability to selectively absorb soil selenium. Because the root system needs energy to absorb Se (ⅵ) in soil, it is an active process, while it needs no energy to absorb Se (ⅵ), it is a passive process. However, some studies have confirmed that the ability of plants to absorb Se(Ⅵ) is 8 times that of plants to absorb Se(Ⅳ) when the same amount of Se is supplied (Bisbjerg,1969). This seems to indicate that when plants absorb soil selenium, it is carried out in an open system and a nonlinear evolution process.

table 1-9 comparison of selenium content in barnyard grass seeds under different cultivation methods μg/g

(according to Zheng Daxian, 1988)

In addition to the ability of the above plants to absorb and store selenium, the external environment is also an important factor affecting the absorption of selenium by plants. To sum up, there are mainly the following influencing factors: ① the total selenium content and available selenium content in soil; (2) crop cultivation methods; ③ Weather and climate change; ④ The level of soil pH and Eh; ⑤ Soil viscosity and organic matter content; ⑥ Synergy and antagonism of * * * elements. Generally speaking, in high selenium environment, high total selenium and high available selenium in soil can make plants absorb and store more selenium from soil. However, in the low selenium environment, sometimes the total selenium in soil is not very low (such as black soil and dark brown soil in Heilongjiang and Zhangjiakou), but due to the low available selenium, the selenium absorbed and stored by plants is very low. Regarding the influence of cultivation methods on plant absorption, Zheng Daxian's experiment has confirmed that the selenium absorbed by the same barnyard grass crop is much higher in waterlogging cultivation than in dry land cultivation, although the growth rate of waterlogging cultivation of barnyard grass grown in different soil types is different from that in dry land cultivation (Table 1-9). Zheng Daxian attributed this difference to the decrease of soil aeration conditions and redox potential. The effect of weather and climate on selenium absorption by plants has not been studied much, but this effect does exist. Yang Guangqi and Mao Daping once speculated that the severe drought in Enshi, Hubei Province from 1959 to 1963 caused a substantial increase in grain selenium (mainly corn selenium). This study has confirmed this conclusion through statistical analysis (see chapter 5, section 3). Soil pH and Eh values have the most obvious effects on selenium absorption by plants. The best condition for plants to absorb and utilize selenium is alkaline oxidation environment (high pH and Eh values). The selenium content of plants in alkaline soil is .1 ~ 1. μ g/g (dry weight), and in acidic soil, the selenium content of plants is .2 ~ 2. μ g/g (dry weight). The increase of soil organic matter and viscosity can reduce the ability of plants to absorb selenium, and the decrease of plant absorption caused by the increase of organic matter is more obvious than viscosity. Johnson's (1991) experiment on wheat showed that at high pH value, the clay content increased from 7% to 16%, and the selenium content of wheat grains decreased by 4% for every percentage point increase. The organic matter increased from 1.4% to 6.3%, and the selenium in wheat grains decreased by 9% for each percentage point increase. At low pH, each percentage point of the above process reduced the selenium content of wheat grains by 3% and 6% respectively. The elements and their compounds that can affect the absorption of selenium by plants in soil are, and iron-manganese oxides. The results show that under the condition of high concentration of S and Se in soil, the ratio enters plants first, and the absorption of S and Se by plants is inhibited. When the concentration of S and Se in soil is low, and can be absorbed by plants at the same time (Trelease,1938; Spencer,1982）。 The function of is similar to that of. Therefore, phosphorus application in low selenium soil can promote the absorption of selenium by plants (Fleming,198; Broyer,1972; Singh,1978）。 Fe-Mn oxides in soil can adsorb and fix a large amount of selenium to prevent plants from absorbing selenium, but the use of Fe-Mn oxides can desorb the adsorbed selenium and increase the absorption of selenium by plants (Peng An, 1988).

Plants not only absorb Se6+ and a small amount of Se4+, but also absorb soluble organic selenium. There is evidence that both non-selenium storage plants and selenium storage plants can absorb selenomethionine, selenium-cystine and other low molecular weight organic substances. A small amount absorbed by roots is quickly transported to the upper end of plants, and the form remains unchanged, while about 9% is metabolized into amino acids, dipeptides, tripeptides and other organic forms (Zheng Daxian, 1985; Liao Ziji, 1992).

It is another way for plants to gather selenium by absorbing selenium from the atmosphere. Especially in areas with high selenium concentration in the air. However, it is still difficult to confirm the proportion of selenium absorbed by plant roots and leaves. Most researchers focus on the absorption form of soil selenium by plant roots. In general, the selenium that may be absorbed from the atmosphere is not enough to affect the whole selenium state level of plants. However, foliar spraying selenium can quickly and effectively increase the selenium concentration of plants (Elrashidi,1989; Mikkelsen,1989) enlightens us that it can not be ignored that plant leaves inhale a lot of selenium from the air near garbage incineration sites and coal-fired factories. In Allen Steam Factory in Memphis TN, USA, it is calculated that 2.2μg/g of Se is contained in the boiler, .3% of Se is discharged with the cinder, 68% enters the fly ash particles, and the remaining 32% is combined with the aerosol (Elrashidi,1989). It can be seen that selenium in coal basically enters fly ash and aerosol after combustion, and about 6% of it enters the atmosphere. At least 1.1kg of selenium should be discharged into the air around the factory if the daily combustion is 1t.

the volatilization of selenium in plants is a kind of excretion from plants, mainly methyl selenide from the body. This effect was first discovered by Beath and others in 1935. Later, Lewis(1966), Evans, Haygarth(1968) and others successively studied it. It is found that not only selenium-storing plants growing in high selenium soil can metabolize volatile selenides, but also non-selenium-storing plants growing in low selenium soil can produce volatile selenides. Selenium compounds produced in higher plants are mainly dimethyl diselenide and dimethyl selenium. When these selenides are in high concentration, they will give off an odor similar to garlic. As far as we know, the plants that produce the most volatile selenium are rice, cabbage and cabbage, and the daily volatile selenium reaches 2 ~ 3μ g/m3. Followed by radish, oat, alfalfa, tomato, cucumber, cotton, eggplant, corn, etc., the daily volatile selenium is 3 ~ 1μ g/m3; The lowest is beet, pea, lettuce and onion, and the average daily volatile selenium is 15μg/m3.

On the one hand, plants absorb selenium from soil and atmosphere and store it in their organs; on the other hand, they convert part of selenium into volatile selenides through metabolism and excrete it. Whether this absorption and metabolism of selenium by plants can play a certain role in the growth and development of plants has not yet reached a final conclusion. But it can be imagined that plants can protect selenium-storing plants, especially non-selenium-storing plants, from selenium poisoning through volatilization in high selenium soil. In some low selenium areas in China, applying selenium fertilizer to soil or spraying selenium on crop leaves has achieved the effect of increasing yield and improving product quality (Liao Ziji, 1992; Yi Huying et al., 1991; Li Jiyun et al., 1991). But there are also many unsuccessful reports (Broyer,1968, 1972). Therefore, whether selenium can be used as an essential nutrient element in plants should be further studied. At present, more and more people tend to use selenium compound fertilizer (that is, adding selenium to NPK fertilizer), which not only increases crop yield, but also increases selenium content in crops.