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 is different from the structural state of selenium discharged by plants.

Plants absorb selenium from soil and air and convert it into amino acids and protein, which are transported to various organs of plants for storage together with water-soluble inorganic selenium (Shrift, 1969). This kind of absorption and storage changes with the development of plants. Different plant species absorb, transport and store selenium differently, from roots to leaves to stems to seeds. Zheng (1988) conducted a pot experiment of barnyard grass. The results showed that the selenium content of dryland cultivation was root > leaf > stem > grain, while that of submerged cultivation was root > grain > leaf > stem. The seed storage capacity of the latter cultivation method is obviously higher than that of dry land cultivation. According to American data, the selenium contents of roots and bulbs, fields, leafy vegetables, laver, fruits and vegetables and fruit trees are 0.408, 0.279, 0.1/kloc-0, 0.066, 0.031μ g/g, respectively (Mikkelsen, 65433. Among them, the ability of field crops to absorb and store selenium is Cruciferae > Ryegrass > Beans > Cereals (Hamilton, 1964). Obviously, the selenium concentration of root crops is the highest in the soil, the selenium content of field crops and leafy 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 was concentrated in leaves at the early growth stage (before jointing stage) and enriched in ears at the later stage, accounting for 62.3% ~ 62.9% of the selenium content 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 roots and tuber crops of plants that can directly absorb and store selenium from soil is naturally high, and the aboveground 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 the parts higher from the ground (such as trees and fruits) have lower selenium content. In addition, for grain crops and vegetable crops, the selenium-enriched effect of grains is much stronger than that of leaves and stems (Bisbjerg, 1969), which shows that selenium tends to be enriched in grains with the growth of plants.

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 the absorption of Se (ⅵ) does not need energy, 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 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, which is a nonlinear evolutionary process.

Table 1-9 Comparison of Selenium Content in Seed of Zushima under Different Cultivation Methods μg/g

(According to Zheng, 1988)

In addition to the ability of plants to absorb and store selenium, the external environment is also an important factor affecting plants to absorb selenium. To sum up, there are mainly the following influencing factors: ① Total selenium content and available selenium content in soil; (2) crop cultivation methods; ③ Weather and climate change; ④ soil pH and Eh levels; ⑤ Soil viscosity and organic matter content; ⑥ Coordination and antagonism of * * * elements. Generally speaking, in high selenium environment, the content of total selenium and available selenium in soil is high, which can make plants absorb and store more selenium from soil. However, in low selenium environment, sometimes the total selenium content 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 content, the selenium absorbed and stored by plants is very low. Regarding the influence of cultivation methods on plant absorption, Zheng's experiment has confirmed that the selenium absorbed by the same barnyard grass crop in flooded cultivation is much higher than that in dry land cultivation, although the growth rate of barnyard grass grown in different soil types is different from that in dry land cultivation (Table 1-9). Zheng attributed this difference to the decrease of soil aeration conditions and redox potential. The effects of weather and climate on selenium absorption by plants have not been studied much, but this effect does exist. Yang Guangqi and Mao Daping once speculated that the severe drought in Enshi, Hubei Province for several years from 959 to 1963 caused a significant increase in grain selenium (mainly corn selenium). This study confirmed this conclusion through statistical analysis (see Chapter 5, Section 3). Soil pH and Eh have the most obvious effects on selenium absorption by plants. The optimum conditions for plants to absorb and utilize selenium are alkaline oxidation environment (high pH and Eh value). In alkaline soil, the selenium content of plants is 0.0 1 ~ 10.0 μ g/g (dry weight), and in acidic soil, the selenium content of plants is 0.02 ~ 2.0 μ g/g (dry weight). The increase of soil organic matter and viscosity will 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 experiment on wheat (1991) showed that at high pH value, the clay content increased from 7% to 16%, and the selenium content in wheat grain decreased by 4% with each percentage point increase. The organic matter increased from 1.4% to 6.3%, and the selenium content in wheat grains decreased by 9% with each percentage point increase. At low pH value, the selenium content in wheat grain decreased by 3% and 6% for every percentage point reduction in the above process. The elements and their compounds that can affect the absorption of selenium by plants in soil are, and iron-manganese oxides. The results showed that under the condition of high concentration of sulfur and selenium in soil, the ratio entered the plants first, and the absorption of sulfur and selenium by plants was 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 is similar to that of. Therefore, phosphorus application in low selenium soil can promote the absorption of selenium by plants (Fleming,1980; Brohier,1972; Singer, 1978). Iron-manganese oxides in soil can adsorb and fix a large amount of selenium, which prevents plants from absorbing selenium, but their use can desorb the adsorbed selenium and increase the absorption of selenium by plants (Peng 'an, 1988).

Besides Se6+ and a small amount of Se4+, plants also absorb soluble organic selenium. There is evidence that both selenium-free storage plants and selenium-containing storage plants can absorb low molecular weight organic compounds such as selenomethionine and selenocysteine. A small amount absorbed by roots is quickly transported to the upper part of plants, and the morphology remains unchanged, while about 90% is metabolized into organic forms such as amino acids, dipeptides and tripeptides (Zheng,1985; Liao, 1992).

It is another way for plants to collect 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 are concerned about the absorption form of soil selenium by plant roots. Generally speaking, the selenium that may be absorbed from the atmosphere is not enough to affect the overall selenium 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 Plant in Memphis, TN, USA, it is calculated that the boiler contains 2.2μg/g of Se, 0.3% of Se is discharged with coal cinder, 68% enters fly ash particles, and the remaining 32% is combined with aerosol (Elrashidi, 1989). It can be seen that selenium in coal basically enters fly ash and aerosol after combustion, and about 60% of it enters the atmosphere. If 10t is burned every day, at least 1. 1kg selenium should be discharged into the air around the plant.

The volatilization of selenium in plants is a kind of excretion of plants, mainly methyl selenide discharged from the body. This effect was first discovered by Beath and others in 1935. Later, Lewis( 1966), Evans, Haygarth( 1968) and others successively conducted research. It is found that not only selenium-enriched plants growing in high selenium soil can metabolize volatile selenides, but also non-selenium-enriched 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 emit a smell similar to garlic. As far as we know, the plants that produce the most volatile selenium are rice, Chinese cabbage and cabbage, and the daily volatile selenium reaches 200 ~ 300μ g/m3. Followed by radish, oats, alfalfa, tomatoes, cucumbers, cotton, eggplant, corn and so on. The daily volatile selenium is 30 ~100μ g/m3; The lowest is beet, pea, lettuce and onion, and the daily average volatile selenium is 15μg/m3.

On the one hand, plants absorb selenium from soil and atmosphere and store it in organs, on the other hand, they convert some selenium into volatile selenides through metabolism and excrete them. Whether this absorption and metabolism of selenium by plants can play a certain role in plant growth and development is still inconclusive. However, it is conceivable that plants can protect selenium-enriched plants, especially non-selenium-enriched plants, from selenium poisoning by 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 production and improving product quality (Liao,1992; Yi Huying et al.,1991; Li Jiyun et al., 199 1). But there are also many unsuccessful reports (Broyer, 1968, 1972). Therefore, whether selenium can be used as an essential nutrient element for plants needs further study. At present, more and more people tend to use selenium compound fertilizer (that is, adding selenium to NPK fertilizer), which not only improves crop yield, but also increases the selenium content in crops.