| China is a tea production and trading power and tea industry has played an important role in the national economy and international trade. In recent years, the issue of tea quality safety has become one of the bottlenecks of tea industry development in China, in which lead contamination of tea has been attracted attentions widely. Lead (Pb) is a physiological and neurological toxin affecting almost every important organ and system in the human body. Therefore, it is very important to study on lead pollution in tea for sustainable development of Chinese tea industry and health of consumers.Tea polyphenols (TP) are the main compounds and natural metal organic ligands in tea plant. It is worthwhile to make a further research on tea polyphenols'environment function to regulate the availability of heavy metals. Therefore, in this work the relationship between TP and accumulation characteristics of lead in different varieties of tea plant was investigated and assessed firstly, and then the pot soil experiment and solution culture experiment were carried out to study regulation mechanisms of TP on the bioavailability of lead in tea plant. The main results obtained are as follows:(1) It was made clear for characteristics of lead accumulation in different varieties of tea plant and its influencing factors and found that TP content in the soil and tea plant was closely related to lead accumulation amount in tea plant. There were significantly differences among the lead content in different varieties, the same as lead content in different organs. Soil pH, organic matter content, chemical species of lead and lead adsorption capacity of leaf were important factors affecting lead accumulation difference in different varieties of tea plant. There was significant difference between TP content in old leaves/old roots and that in the soil of root zone for different varieties of tea plant. Polyphenol content in the soil of root zone had significant correlation with soil pH, organic matter content, chemical species of lead, and lead adsorption capacity of leaf. The content of TP in tea plant was also significantly related to lead adsorption capacity of tea leaf and lead content in tea plants.(2) TP had obviously regulative effect on the uptake, accumulation, and detoxification of lead in tea plant. Under low concentration of lead, growth and development, photosynthesis, and biomass production of tea plant were promoted in a certain level, but inhibited under high concentrations. With the increase of the concentration of lead, the lead content in tea plant increased, but there was little change of lead distribution in the different organs of tea plant. With TP treatment, the growth and development condition of tea plant was significantly improved and biomass production increased although under lead pollution. Under low concentration of TP, the accumulation amount of lead in tea plant was reduced, but uptake amount of lead by roots was increased and the lead content of leaves was reduced with high concentrations of TP.(3) Under lead stress, some disturbances in ultrastructure of drganelles were found. The overall structures of cells were significantly changed and not complete, a large number of organelles even disappeared. The structures of chloroplast, nucleus, mitochondria and other organelles were markedly damaged. Plasmodesmata structure was changed significantly, a mass of endoplasmic reticulum disappeared. Abundant cloudy black depositions were found in cell wall. It was worth mentioning that the deposition particles were fine and well-distributed, mainly in the cell wall corners. After TP treatment, the overall structure of tea leaf, root tip cells damaged by lead stress had been taken a significant turn for the better. The overall-structure-condition of the chloroplast, mitochondrion was nearly same as normal state. And it was a remarkable phenomenon that block or granular black depositions were found in the cell wall, plasmodesmata, nucleus, endoplasmic reticulum, which were distinctly different from depositions under only lead stress treatment. (4) TP played an obviously regulative role on soil physical and chemical properties, and thus it had an effect on the lead bioavailability of soil. After TP was added to the soil, the soil pH values decreased, the contents of polyphenols and organic matter increased. The soil mineral phase was changed which showed a more complete crystal, and particularly redox reaction occurred between Fe-Mn oxides and polyphenols. The chemical species of lead in soil had been significantly transformed, the proportion of availability species of lead increased under low concentration of lead, but decreased under high concentrations. Chemical bound species of lead in soil had been transformed from the large proportion of lead phosphate to the combined species of TP-lead. Dry matter production and lead accumulation of tea plants was closely correlative to the changes of soil physical and chemical properties, and bioavailability of lead caused by TP treatment. (5) TP had a significant effect on the micro-area distribution and transformation of lead in tea plant. Under only lead treatment, lead was deposited mainly in the cell wall of root tip cells, while on the tissue level lead was mainly deposited in the spongy tissue and lower epidermis of leaves, and epidermis, endodermis and pericycle of root tips etc. After TP treatment, lead was deposited mainly in the vacuole of root tips, and on the tissue level the content of lead in the upper and lower epidermis of leaf was significantly higher than that in mesophyll tissue. The distribution of lead in the vertical vein of leaf and different tissues of the root tips was changed obviously. Direct damage of the important cells and tissues were avoided by the re-distribution of micro-area distribution of lead in tea plant, which may be one of the important mechanisms of TP to reduce lead toxicity for tea plant.Fourier transform infrared spectroscopy studies showed that tea roots and leaves had diverse functional groups, including hydroxyl, carboxyl, amino, amide, methyl, carbonyl, and ether group, etc. Functional groups in tea plant tissues like hydroxyl, amide, carbonyl, etc. played an important role in the absorption, accumulation, and detoxification of lead in tea plants. Especially after adding TP the content of these functional groups significantly increased, which accordingly inhibited the toxicity of lead.Under only lead treatment, combined species of the lead in root of tea plant were mostly Pb-TP, chloride, lead, Pb-theanine, Pb-fulvic acid and phosphoric acid lead, etc. After TP treatment, the ratio of binding species of Pb-TP greatly increased, which showed that TP had an important role in the regulation of the lead molecular species in tea plant.
|
PDF Full Text Request