| Heavy metal pollution and its impact on the environment has led to serious concern for agriculture products safety and human health. Understanding the phytoavailability of heavy metals in the rhizosphere and how plants transport heavy metals are both essential to controlling metal uptake by crops and promoting remediation efficiency by metal hyperaccumulators. Soil organic matter (SOM) and pH are two key factors influencing metal mobility and bioavailability in rhizosphere soils. The effects of humic acid (HA), an important component of SOM, on metal phytoavailability are still poorly understood, due in large part to its heterogeneity and complexity. Tea is one of the most widely consumed beverages in the world, and its health benefits have received a great deal of attention. Lead (Pb) is a physiological and neurological toxin that can cause detrimental effects in almost every organ and system of the human body. In this study, Pb concentration in different varieties of tea leaves and their infusions was firstly investigated. The two most popular tea varieties in Zhejiang Province (C. sinensis cv. Longjing43and Yingshuang) were then chosen as the analytical object of this study. The impact of exogenous HA on Pb uptake, translocation, accumulation by tea plants and Pb toxicity was elucidated by using inductively coupled plasma mass spectrometry (ICP-MS), synchrotron radiation X-ray fluorescence (SRXRF) spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, and isobaric tags for relative and absolute quantitation (iTRAQ). Results from this series of studies would provide theoretical basis and technological support for minimizing the threat of Pb introduction to the food chain, and also promoting phytoremediation efficiency by metal hyperaccumulators. The main results of this research are as follows:(1) Pb concentration in tea leaves and their infusions was investigated. Total Pb content in young leaves was between4.64-20.26mg kg-1and the propotion of adsorbed Pb was between13.31%-48.05%among different varieties of tea plants. Total Pb concentrations in young leaves had positive correlation with soil Pb content and negative correlation with soil pH, whereas no correlation between Pb accumulation and SOM was observed. The average amount of Pb transferred through infusion increased with time from8.24%(1minute) to 17.58%(4hours).(2) The correlation of carbon content in different fractions of SOM with Pb concentration in various parts of the tea plant was analyzed. In all tissues tested, Pb concentrations were positively related to water soluble substances (WSS) and fulvic acid (FA) in the rhizospheric soil. In young leaves, young shoots and roots, Pb concentrations were negatively correlated to soil microbial biomass (SMB). No significant relationship was found between the various fractions of SOM in the bulk soil and Pb concentration in tea plants, and neither was observed of the SOM and HA in the rhizosphere soil with Pb concentration in plants tissues. In addition, SOM content in rhizosphere soil is not necessarily higher than that in bulk soil. Our results showed that in the pots that had not been spiked with Pb, the rhizospheric soil was lower than the bulk soil in SOC, whereas in the pots that had, a reverse trend was observed.(3) The adsorption characteristics of Pb onto HA and cell wall (CW) was compared. Adsorption of Pb to HA can be described by Langmuir equation, and adsorption to CW can be described by the Freundlich model. HA shows higher adsorption capacity for Pb than CW, but CW has higher binding affinity with Pb than HA. Fourier transform infrared spectroscopy studies show that Pb was found to be mainly bound with the carboxyl group in HA, and hydroxyl group in CW. HA could be involved in Pb adsorption to CW through ternary complexes (CW-Pb-HA) formation.(4) The impact of HA on the Pb accumulation and speciation transformation in tea plants was clarified. Pb uptake by plant roots and its translocation to shoots was promoted with the increase addition of HA in a certain range of concentration. As the concentrations of HA transcend their corresponding threshold, Pb uptake and its accumulation in plant tissues decreased. HA promoted Pb transport from roots to shoots and Pb accumulation in leaves through their vascular tissues. Pb bound to celllulose and Pb phosphate are the main form of Pb species in plant roots. As for Pb species in plant shoots, more than half of the Pb are bound to oxalate and theanine. The addition of HA increased the propotion of Pb bound to cellulose both in root and shoot tissues, but decreased the propotion of Pb bound to phosphate, oxalate and theanine. (5) Molecular mechanism of HA promoting plant growth and alleviating Pb stress was uncovered. Our results show that HA increased plant biomass and mitigated Pb stress which caused reduction of plant growth, inhibition of leaf photosynthesis, and the damage of cellular ultrastructure. However, the alleviation effect of HA was decreased with the increasing concentration of Pb in plant tissues. The CW of plant roots is the first protective barrier against Pb stress. Compared with Pb treatment, the addition of HA plus Pb increased total sugar and uronic acid contents of pectin, decreased arabinose and galactose contents of root cell wall. iTRAQ analysis indicated that a total of1530proteins with false discovery rate (FDR) less than1%were identified in plant root tissues. A total of52proteins (20up-regulated and32down-regulated) were identified as differentially expressed. Bioinformatics analysis showed that these differentially expressed proteins are mainly involved in antioxidant defence, signal transduction, carbohydrate and sulfur compound metabolism, and so on. These findings shed light on the molecular mechanisms involved in the alleviation effects of HA on Pb toxicity to tea plants. |
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