Studies On Dynamic Course And Mechanism Of Lead And Cadmium In The Rhizosphere Of Tea Plants

The problem of pollution of lead and cadmium on tea has been cared bypeople. The research of the behavior and mechanism of lead and cadmium inthe rhizosphere of tea plants is important credential to control the pollution oflead and cadmium on tea plant in soil and forecast and estimate the ecologicaland environment risk of lead and cadmium in soil .Tea plants taken as experi-ment material , three experiments were managed by rhizosphere box culturesystem, solution culture and simulation indoors to study transport and confor-mation transformation of lead and cadmium in the rhizosphere of tea plantsand the mechanism. The main results are as follows:1. Lead was accumulated and cadmium was depleted in rhizosphere oftea plants after that were planted in the nature soil. Addition of lead canaccelerate accumulation of lead and addition of cadmium can decreasedeficiency of cadmium in rhizosphere of tea plants on nature soil. There wasdistinct interaction between lead and cadmium in rhizosphere and theinteraction was different in different soil .Treatment of cadmium canaccelerate accumulation of lead ,how ,treatment of lead can decreasedeficiency of cadmium in rhizosphere of tea plants in nature soil .In the soil oflead and cadmium treatment ,existence of cadmium advanced accumulation oflead in rhizosphere of tea plants ; the effect of lead on cadmium was differentwith the concentration of lead, when lead concentration was250mg/kg,exsistence of lead accelerated deficiency of cadmium ,while,deficiency of cadmium was decreased by lead which concentration was350mg/kg.Under the check, single and compound treatments of lead andcadmium, content of lead in rhizosphere increased with time, while content ofcadmium in rhizosphere decreased firstly and then, increased with time.2. There were phenomena of activation process of lead and cadmium inrhzosphere of tea plants in nature soil, percent of EXC-Pb and EXC-Cd (leadand cadmium exchangeable) was distinctly higher, while percent of Carb-Pband Carb-Cd (bound to carbonates), Pb and Cd residual and of FMO-Pb andFMO-Cd (bound to iron and manganese) were distinctly lower in rhizospheresoil than in bulk soil, the same as content. Treatments of lead and cadmiumhad distinctly effects on contents of EXC-Pb and EXC-Cd and their contentsincreased with the concentration of lead and cadmium added. There wasdistinct interaction between lead and cadmium in rhizosphere and theinteraction was different in different soil.The interaction between lead andcadmium in rhizosphere in nature soil resulted in the content of EXC-Pb beinghigher in the presence of Cd and that of EXC-Cd being also higher in thepresence of Pb, whereas there existed the transformation of EXC-Pb to Card-Pb, FMO-Pb and lead residual in the presence of Cd, of EXC-Cd, Card-Cd andFMO-Cd to cadmium residual in the presence of Pb in rhizosphere in soiltreated by lead and cadmium. 3. The phenomenon was observed that pH in rhizosphere in nature soilfell with time, however, pH in rhizosphere in soil treated by lead and cadmiumfirstly fell and then risen with time. The effects were different of lead andcadmium on pH in rhizosphere, at the beginning, pH in rhizosphere was lowerin soil of lead treatment than that in nature soil, while, the one was higher insoil of cadmium treatment than that in nature soil, and then the pH inrhizosphere both in soil of lead treatment and in soil of cadmium treatmentwas higher than that in nature soil. The hydroponic experiments showed thatthere was a acidifying process in rhzosphere of tea plants,while treatments oflead and cadmium could restrain acidification ,but, lead treatment of higherconcentration could advance the acidification at the beginning. 4.Stresses of lead and cadmium have distinct effects on exudation oforganic acids in roots of tea plants. Both lead of 20mg/kg and 60mg/kg andcadmium of 0.5mg/kg and 1mg/kg could make exudation amounts of oxalateacid, malate acid, succinate acid and total organic acids increase, while theamount of citrate acid decreased under the stresses of lead of 60mg/kg andcadmium of 1mg/kg. The exudation amount of malate acid had increased withtime moving on, while those of other organic acids and total organic acids haddecreased with time moving on.The hydroponic experiments showed that rootsof tea plants could initiatively regulate the exudation of organic acids underthe stresses of lead and cadmium, and both metabolism pathway an non-...