Fluoride Accumulation In Tea Plant And Its Physiological Response Mechanism

Tea infusion is one of the most popular beverage in the world. But with investigation offluoride in tea infusions, tea health safety causes more and more attention. Tea plant is afluoride accumulation species. In natural growth condition, tea plant could accumulate farmore fluoride than other plant species. Many researchers perfoemed studies on fluoride andtea, mainly focused on the fluoride leaching model from tea, the charcter of fluorideaccumulation in tea plant, the adsorption property of tea garden soil for fluoride, theinteraction of fluoride-aluminun on the uptake of fluoride by tea plant. This work investigatedfluoride accumulation in forty-four tea cultivars planted in same garden and studied thefactors that might influence the accumulation of fluoride by tea plants; Two cultivars whichhad inverse property in uptake of fluoride, named higher fluoride cultivar and lower fluoridecultivar, were studied by comparing protective enzymes and AsA-GSH cycle system underfluoride treatments. By contrasting leaf photosynthetic capacity, chlorophyll content, leafanatomical structure under fluoride treatment, this work analysed fluoride tolerancedifferences between tea cultivars and the position of fluoride damage in leaf；By detecting thefluoride and calcium contents in tissue and their distribution ratios, this work discussed therole of calcium in fluoride tolerance mechanism of tea tree; By testing the changes ofcatechins and volatile components in tea made from two cultivars under fluoride treatment,the influence of fluoride on the tea quality was studied; The dynamics models of fluoriderelease from tea rhizosphere soil were studied by simulation of small molecule organic acidsecretion from root. The response of organic acids secretion to fluoride treatments wasdiscussed by compared the changes of organic acids secretion contents and enzyme activities.The main results are as follows:1. There were significant differences in leaf fluoride contents of forty-four tea varieties.Tea leaf was the major organ of fluoride accumulation. Fluoride content was highest in matureleaf with a mean value of1121mg·kg-1and followed by tender leaf (the third leaf) with655mg·kg-1, which were301and175times higher than that in root. Variable coefficients of rootand twig fluoride contents were36.9%and36.0%, respectively, which were higher than thatof the tender leaf (22.2%) and the mature leaf (13.3%). Set leaf fluoride contents as variables,forty-four tea varieties were divided into three groups of high-fluoride, medium-fluoride and low-fluoride. In low-fluoride group, ten tea cultivars were found as: Japanese Xiao tea,Xixiang dahe11, Changlin daye, Xianggu liaobaihao, Zhenong113, Fudingdabai, Xixiangdahe6, Huangyezao, Soubei, Zhangke1. Tea varieties differed in fluoride accumulation whichmight be controlled by tea genotype. The main branch angle and lateral branch angle of teaplants were controlled by gene at moderate to strong degree in all of the phenotypic traits.This study found that tea branching angle could influence fluoride accumulation, which mightbe related to apical dominance and hormone distribution in tea plants, resulting in differencesof growth and absorption of elements by tea plant. In addition, fluoride absorption by tea treeswere also associated with root activity.2. Higher and lower fluoride cultivars were different in response to fluoride in theactivities of antioxidant enzymes and AsA-GSH cycle system. Under fluoride treatments,hydrogen peroxide content and superoxide anion generation rate were higher in higherfluoride cultivar than those in lower fluoride cultivar, which indicated that higher fluoridevariety suffered more active oxygen damage than lower fluoride variety. While the activitiesof antioxidant enzymes, such as superoxide dismutase, catalase, ascorbic acid peroxidase,single dehydrogenation ascorbic acid reductase and glutathione reductase, were lower inhigher fluoride cultivar than those in lower fluoride cultivar. It was also similar changes inantioxidants, including reduced ascorbic acid, oxidized ascorbic acid, reduced glutathione andoxidized glutathione levels between both cultivars. All of these showed that lower fluoridecultivar performed a more effective antioxidant protection system in response to fluoride.3. Fluoride caused fatty acid peroxide in cell membrane, reduced the proportions oflonger chain and unsaturated fatty acids, increased the proportions of shorter chain andsaturated fatty acids, increased malondialdehyde content as a product of fatty acid degradation,reduced chlorophyll content and resulted in a reduction of photosynthetic rate. By theobservation of leaf anatomical structure, this study found that fluoride damage was lighter inlower fluoride cultivar than that in higher fluoride cultivar. Cambium cells between the xylemand phloem in leaf was the main point of fluoride damage. And the mesophyll cells were alsodamaged by fluoride leading to a loose leaf tissue structure.4. Along with the increase of fluoride concentration, total phosphorus content increasedand total nitrogen content reduced, which might be related with the increased uptakes ofphosphate and reduced uptakes of nitrate and ammonium. Fluoride could also reduce theabsorptions of mineral elements, such as potassium, calcium, magnesium, iron, zinc, by teaseedlings. Tea seedlings could adjust the distributions of these elements to adapt to fluoridetreatment. Fluoride mainly accumulated in the cell walls in tea leaf, its accounts for48-57%of total fluoride, organelles accounts for23-38%, and cytoplasm accounts for11-21%. Calcium was mainly distributed in the cytosol, its accounts for52-73%of total calcium,followed by organelles with amount of16-25%, and the lowest amount was tested in cell wall,only occupied10-23%. With the increase of fluoride concentration, distribution ratio offluoride in cell walls increased and the distribution ratio of calcium from cytosol to cell wallalso increased, showing that calcium was involved in the bound of fluoride in cell wall, whichmight combine with fluoride to form an insouble CaF2and reduced the toxicity of fluoride.5. Influence of fluoride on the tea quality made from higher and lower fluoride teacultivars were different. Fluoride mainly decreased EGC and EC contents in higher fluoridecultivar, while only EGCG content in lower fluoride cultivar was decreased. In aspect ofvolatile compounds, fluoride increased leaf aldehydes, alcohols and esters contents in higherfluoride cultivar, but decreased those contents in lower fluoride cultivar. However, due tohigher phenols and volatiles in leaf in lower fluoride cultivar, it still maintained higher levelsof phenols and volatiles than higher fluoride cultivar after fluoride treatment. Effect offluoride on tea quality was related with tea varieties.6. By simulating a sustained secretion of organic acids by tea root, the effect of organicacids on the release from tea rhizosphere soil was studied. The result showed that the fluoriderelease kinetics model fit to an Elovich equation. Fluoride release was divided into a0.5-10hquick release period and a10-180h slow release period. These phenomenons were related tothe reaction parts between the organic acids and soil. Fluoride supply reduced the secretion ofmalic and citric acids, this might be related to the decrease of malic and citric acids contentsin root. Malate dehydrogenase and citrate synthase avtivities in leaf were higher than in root,which might promote the synthesis of malic and citric acid in the leaves and promoted thetransfer of malic and citric acids into root to participate organic acids secretion and adapt tothe outside fluoride.