Effects Of Organic Acids And Iron Plaque Outside Roots On Fluoride Adsorption And Accumulation In Tea Plants

Tea plant （C. sinensis L.） is a high fluoride-accumulator plant. More than90percent of fluoride accumulated in tea plant was concentrated in tea leaves, especially in old leaves. People who drink brick tea which is made of old leaves will frequently suffer from in take too high fluorine in tea liquid. Most of fluoride in tea plants is absorbed from soil by tea root, and the effect of rhizosphere micro-environment on the fraction, adsorption and accumulation of fluoride from soil will provide useful data for regulating the accumulation of fluoride in tea plant.In this paper, the impacts of aluminum （Al） concentrations on root cell membrane permeability, plasma membrane permeability and components of organic acids in root exudates of tea plant were investigated by hydroponics methods. And the effects of Fe²⁺ concentrations, incubation time, pH and organic acids including oxalic acid, malic acid and citric acid on iron plaque on root surfaces and fluoride adsorption and accumulation in tea plants were also studied. The results revealed that:1) The low Al concentration （20mg/L） could enhance the cell membrane stability of tea root, and the membrane permeability was significantly declined with deficiency （0mg/L, CK） and high （100mg/L） Al treatments. Proper amount of Al could improve the tea root activity. And pH of rhizosphere was significantly decreased as increasing Al concentration.2) The total organic acids firstly decreased and then increased with the increase of Al concentrations. About85%⁹3%of the total organic acids in root exudates of tea plant were oxalic acid, malic acid and citric acid. Compared with7.44mg/g DW of CK, the oxalic acid secretion remarkably reduced84.7%and34.3%, with the low and high concentrations of Al treated. Compared with1.62mg/g DW of CK, the malic acid of root exudates increased by121.1%in low Al concentration, and decreased by40.9%in high Al concentration. There wasn’t significant influence on citric acid, containing about3.5⁴.5mg/g DW.3) Most of the iron plaque induced on tea roots was concentrated between0.20cm and0.50cm from the root tip. As the Fe2+concentrations and incubation time increased, the amount of iron plaque was increased, but negative relationships were found when pH as well as organic acids concentrations added. The highest quantities of iron plaque were found in the conditions of30mg/L of Fe2+concentration and incubation for12h at pH4.5.4) The effect of three organic acids in root exudates desorption of iron plaque on root surface indicated that: when the concentrations of organic acid were in2⁵μ mol/L, it follows the rule of citric acid＞malic acid＞oxalic acid when the concentration was in5⁸μmol/L, it follows the rule of malic acid＞oxalic acid＞citric acid it follows the rule of oxalic acid＞malic acid＞oxalic acid as organic acids were in high level. About90percent of iron plaque in root surface desorbed when the concentration of organic acids was increased to12μmol/L.5) There was a positive relationship between fluoride adsorption and accumulation in tea plants and quantities of the iron plaque on root surface when the amount of iron plaque was2.4to13.6mg/gRDW. Compared with CK without iron plaque and organic acids treatment, the amount of fluoride accumulated in tea plants was increased between42.3%and103.7%with only Fe2+added in incubation solution. The quantities of fluoride detected in tea plant were increased from101.7%to243.0%when the organic acids and iron plaque added in incubation solution together.