Mechanisms Of Lead Uptake/Accumulation And Tolerance In Tea Plant (Camellia Sinensis L.)

Lead has been regarded as one of the widespread heavy metal pollutants in the environment because of its large-scale exploitation and use, which poses a serious threat to both the natural environment and human health. High in nutritional value and beneficial to human health, tea has won the great popularity all over the world. As a non-alcoholic beverage, tea consumption is only second to water. Pb, with its unique characteristics, has become a research hotspot as the main heavy metal pollutant in the tea. Therefore, the research about the mechanisms of lead uptake/accumulation and tolerance in tea plants is of great significance for controlling the tea Pb pollution, promoting the healthy development of the tea industry as well as guaranteeing the consumers’health.In this study, we made a further probe into the mechanisms of Pb uptake and tolerance in tea plants, with the aim to reveal the variety differences in the Pb toxic effect and Pb chemical forms and subcellular distribution in tea plant, the mechanism of Pb absorption, distribution and accumulation in different tea plant organs, and the role of root cell walls in Pb accumulation and the micro-distribution of both Pb and other elements in different tea plant organs. The main results are as follows:(1) It was made clear for characteristics of lead accumulation in differernt tea plant varieties and the factors influencing lead absorption. Different varieties of tea plants growing in the same environment showed some differences in Pb accumulation and distribution. There were three patterns of Pb distribution in different organs among the eight varieties. The rhizosphere speciation of Pb was similar for all tea plant varieties, with Pb being predominantly in the residual and oxide-bound forms, followed by carbonate-bound forms. Analyses indicate that exchangeable and carbonate-bound Pb fractions appeared to be readily bioavilable for tea plants. There were significant differences in lead chemical speciation, soil pH, organica matter content between different tea plant varieties, which were important factors influencing lead uptake and accumulaton by tea plant. Some certain differences were also found in Pb absorption capacitiy of mature leaves in different tea plant varieties.(2) Under the same treatment, there appeared to be differences between LJ and YS in root morphology, plant growth, net photosynthetic rate, root acivity and membrane permeability. We found the following Pb toxic effects on the ultrastructure of root cells including disappearance of nuclear, mitochondrial swelling or disappearance of cristae, membrane invagination and cell wall thickening; the Pb toxic effects on the ultrastructure of shoot cells including chloroplast deformation, disappearance of grana lamellae, thylakoid disorder and appearance of large lipid particles; the Pb toxic effects on the ultrastructure of leaf cells including chloroplast damage, mitochondrial swelling and so on. Our comparative study indicated that there were some differences in the leaf cell structure of the two varieties under Pb stress. Besides, the forms of Pb sediments in root cells were also different. The Pb tolerance ability of YS was relatively stronger than LJ.(3) Fourier transform infrared spectroscopy studies show that Pb was found to be mainly bound with the carboxyl group in the root cell walls. At the same time, the protein and pectin might also be involved in binding Pb. Carboxyl group played a greater role in binding Pb for LJ, while amino group played a greater role for YS. And pectin in root cell walls of the two varieties showed the same effect on binding Pb. Some differences existsed in the carboxyl group, amino group and protein percentage between the two tea plant varieties.(4) Roots played a significant role in fixing the Pb by tea plants. Pb content in different tea plant organs showed the same distribution order:roots> stems> young leaves> mature leaves. But the analysis found that there was a difference in Pb content variation in the leaves of two tea plant vareities and also Pb absorption capacity between fine roots and coarse roots. Compared with the control, Pb stress promoted the absorption of Cu and Fe, but inhibited the absorption of Mn and Mg by roots of two tea plant varieties. Besides, the two varieties showed different characteristics in the absorption of Ca and Zn. The results proved that the absorption of Pb in tea plants might not be an active process. LaCl3, an ion(Ca) channel inhibitor, significantly inhibited the absorption of Pb by roots, which indicated that the Pb absorption had a close relationship with ion(Ca) channels. Synchrotron radiation X-ray fluorescence scan results also showed that, to some extent, the Ca ions inhibited the absorption of Pb by roots.(5) Pb stress affected the uptake of other elements by tea plants. Compared with the control, Pb stress promoted the absorption of Cu and Fe, but inhibited the absorption of Mn and Mg by roots of two tea plant varieties. Besides, the two tea plant varieties showed different characteristics in the absorption of Ca and Zn. The Fe content in the YS root epidermis was relatively higher than other parts of the root, but Ca content was higher in the interface of YS root endodermis and vascular cylinder. No remarkable distribution rules were found about Ca and Fe in LJ root cross section under Pb stress. Under normal conditions of hydroponic culture, the highest content of elements such as Fe, K and Mn was found in the region close to the adaxial epidermis of LJ and YS leaf (palisade tissue). While under the Pb stress, the distribution rule, i.e., the element content appears higer in leaf edges (upper and lower epidermis) was weakened.