Effects Of Anion Attendant,pH Value,and Concentrations Of Ca And K In Rhizosphere On Cd Uptake And Tanslolcation In Rice Root

Cadmium?Cd?contamination in soil has become one of the environmental issues as a result of different human activities,such as the application of municipal sewage sludge,atmospheric deposition of Cd,mining,application of chemical fertilizers and pesticides containing Cd.Cd in the soil is easily taken up and accumulated in plant tissues,resulting in reduced crop yield and also threatening human health via food chain.Therefore,the goal of developing of elite rice cultivars with consistently low cadmium content has been strongly highlighted in the research of crop science and rice breeding project in our country.In general,the whole process of Cd accumulation from the soil to above-ground plant parts can be divided into 4 steps,including?1?root Cd uptake from soil,?2?root-to-shoot translocation via xylem,?3?Cd sequestration and?4?Cd accumulation in above-ground tissues or edible parts through phloem.Over the last decade great progress has been achieved in this respect,and numerous transporter genes like OsNramp1,OsNramp5,OsHMA2 etc.have been identified and function-verified.However,the physiological regulation mechanism of Cd uptake and translocation in rice is far to be understood.In the present study,therefore,a low Cd accumulating rice varieties,Xiushui 09,was used to investiagte the effects of different environmental factors on cadmium uptake and translocation in rice through electrophysiological,physiological and biochemical techniques.The main results are summarized as follows:?1?With the attendance of nitric acid?NO₃^-?in the solution,rice plant showed much higher Cd content and net Cd²⁺flux in the root,in comparison to those plants grown with the attendance of chloride?Cl^-?and sulfate(SO₄^2-).Furthermore,the attendance of NO₃^-significantly increased the affinity rice root to Cd²⁺,resulting in more Cd taken up by rice root.?2?The change of pH value in the solution significantly affected the uptake of Cd,due to the regulation of pH on the activity of P-type ATPase of rice root.There was an optimal range of pH value for Cd uptake by rice root,with the highest Cd uptake capacity of rice root observed under pH 5.5-6.5.?3?The external Ca concentration showed a significant regulatory effect on the uptake and translocation of Cd in rice.The external Ca²⁺was able to reduce the uptake of Cd²⁺by rice root,not only by competing with Cd²⁺for transmembrane transport of calcium channels in rice roots,but also by reducing the expression levels of OsNramp1and OsNramp5.In addition,Ca²⁺could reduce the root-to-shoot translocation of Cd²⁺by decreasing the the expression levels of OsHMA2.?4?The external K concentration also considerably changed the uptake and translocation of Cd in rice.K might affect the uptake of Cd by rice root through decreasing the expression of OsNramp1.However,there was no directly relationship between the root-to-shoot translocation of Cd²⁺and the expression level of OsHMA2 in rice.In addition,the effect of K on Cd uptake and translocation varied with the growth stages of rice plants.Taken together,the attendance of anions,pH value and the external concentrations of Ca and K in the rhizosphere were all showing the significant regulatory effects on Cd uptake and translocation in rice plant,suggestive of the great potential of these rhizosphere factors in the practice of exploring new techniques recudcing Cd accumulation in rice grains during the production of low-Cd rice.