Transformation Of Selenium In Soil And Plants, And The Mechanisms For The Accumulation And Resistance To Selenium In Tobacco Plant

Selenium(Se) is the beneficial element for plant growth and has been shown an essential nutrient for human beings. It has been reported that Se is an important ingredient of many enzymes and proteins in organisms. Se has biological functions such as improving immunity of human body, anti-aging and preventing cancers. Many diseases may be caused by Se deficiency. But it is widespread throughout the world for Se deficiency in soil. About 72% soil in China is severely lack of Se or in a state of low Se content. It is a way of economic, effective, safe and convenient to supplement Se by consumption of selenium-rich agricultural products in Se deficiency districts. The level of Se in food chain system and the content of Se in plant are related to the content, distribution, valences, forms and transformation of Se in ecological environment and soils. Application of different forms and valences state of Se affected the effectiveness of plant and soil ecological environment. Resently, increasing concerns from the scientists and public have been paid for the availability and the influence of Se to plants and soils. Investigations on the availability and mechanisms for absorption, accumulation of Se in plants will contribute to the understandings of tolerance mechanisms, and also benefit to provide more theoretical knowledge in the transformation mechanism of Se in soils.In this study, pot experiments and hydroponic culture have been conducted to investigate: 1) the absorption and transformation of different valences state of Se in hyperaccumulation plant and tobacco; 2) the effects of Se to soil ecological and the transformation mechanism; 3) the effects of Se to physiological characteristics and resistant mechanism of tobacco plant; 4) the availability of different kinds of Se to Cardamine violifolia. In this study, the employed plants were tobacco plant and Cardamine violifolia, at the same time, physiological and chemical determination methods, hydride generation atomic fluorescence spectrometer test tcehnology and the subcellular fractionation technology were also used. The main results were as follows:(1) The morphological characters and the biomass of tobacco plant could be improved in Se concentration of 1-10 mg/kg, but it was negative in high Se concentration(20-30 mg/kg) and the effects of Se(VI) was significant serious to Se(IV). The Se content in different parts of tobacco plant was remarkably increased by applying selenite and selenate, and the applying of Se(VI) was 3 to 30 times more than Se(IV) in the same total Se treatments. The order of distribution density and enrichment ability of Se in different parts of tabacco plant was respectively leaves> roots > stalks with the treatment of Se(IV) and Se(VI). Michaelis-Menten model fitting results indicated that estimated maximal Se content of leaves, stalks and roots in selenate-added soils were 1.8, 15.8 and 21.0 times larger than in selenite-added soils. The bioavailability and affinity of selenate was higher than selenite for tobacco plant. The Se species were increased significantly by applying the different valences state of water-soluble Se. The species of carbonate bound and Fe-Mn oxides bound were improved in selenite-added soil, while the soluble form and exchangeable were increased in selenate-added soil. The transforming amount of different valences was low in Se-added soils.(2) The treatment of Se in low concentration(< 10 mg/kg) improved soil catalase activities and neutral phosphatase activities and reduced soil urease activities and sucrase activities. High Se concentration(>10 mg/kg) reduced soil urease activities, sucrase activities and catalase activities, and the inhibitory effect was Se(VI) > Se(IV). The number of soil microorganisms and microorganism diversity were better in the condition of low Se treatments than high Se treatments. It was showed that the pollution level of Se(VI) and Se(IV) were represented by the decreased of soil urease activities(-0.916**) and sucrase activities(-0.871*), respectively. The Se content of water-soluble Se in soil was the most effect with the soil enzyme activities(significant negative correlation). The decrease of urease activities and sucrase activities, and the reduce of microbial quantity(especially the number of bacteria and fungi) were one of the reason for soil ecological deterioration of Se pollution.(3) The results showed that low concentration(<2mg/L) of Se improved the growth of tobacco plant, but it has obvious inhibitory effect to tobacco plant at high Se concentration(>5 mg/L) and the negative effects were Se(IV) serious than Se(VI) at the same concentration of Se. The content and accumulation of Se in different parts of tobacco plant increased significantly with the application of different valences of Se. It was found that the Se distribution in the stems and leaves with the treatment of Se(VI) was higher than Se(IV), but the Se distribution in roots was opposite. The Se forms in leaves were mainly organic Se, and the Se(IV) was the dominating form of inorganic Se. The affinity of selenate was higher than selenite for the leaves and stalks and opposite for the roots of tabacco in the hydroponic conditions. In leaves, Se was mainly stored in the cytoplasm, and then the cell wall, again for the mitochondria, chloroplasts at the last. The Se content of leaves, cell wall and cytoplasm of Se(VI) treatment was higher than Se(IV) treatment. It was showed that the absorption ability and tolerance of Se(VI) were better than Se(IV) for tobacco plant. This illustrated that Se was more easy to synthetic protein or amino acids and be fixed by cell wall to alleviate the toxicity of Se for the treatment of Se(VI) than Se(IV).With the increased of Se(VI) and Se(IV), enzyme activities of SOD was increased, POD was decreased, CAT was rapidly increased first and then decreased compared with the control. It was showed that the detoxification process of high concentration of Se relative to the SOD enzyme activities, and CAT enzyme activities only had the effects of removing H2O2 processing in low concentration of Se. The stomatal closure and interruption of CO2 were the main limiting factors of weaken photosynthesis by the treatment of high Se(VI) and Se(IV).In the treatment of low Se concentration(<2mg/L), the content of MDA decreased and the root activity increased, which showed that the resistance of tobacco plant was enhanced. The content of MDA was increased obviously and root activity decreased with the treatment of high Se concentration(>5mg/L), which meant that the high Se concentration was toxic to tobacco plant, and the toxicity was Se(IV) serious than Se(VI) at the same concentration. The reasons of above were the content of proline and the activities of SOD in the treatment of Se(VI) was increased more than Se(IV), and there was a difference between the absorption and fixtion of Se(IV) and Se(VI) for tobacco plant.(4) A pot experiment was carried out to study the effects of applying selenite and selenium ore powders on the absorption and transformation of Se in Cardamine violifolia. The results showed that the Se content and accumulation were significantly increased with the using of selenite and selenium ore powders, and the Se accumulation of different parts was leaf > stem > root. Compared with two kinds of Se, the effects of increasing Se content was better with applying selenium ore powders than selenite when the dosage of Se was low(<10mg/kg), and it was opposited when the dosage of Se was high(>10mg/kg). The trend of inorganic Se into organic Se ratio of leaf was improved with the applying of selenium ore powders than selenite. The Se availability was rised by the proper application of Se. The ligand-exchangeable and residual selenium of soils were main respectively increased by using of selenite and selenium ore powders. The conversions of Se in leaves was significant negative correlation(-0.779*) to the ratio of availability Se to total Se in soil, which showed that it had a relationship of the difficulty of plant to absorb Se and the conversions of Se in leaves.