The Mechanism Of Citric Acid Inhibiting Cadmium Absorption And Transport In Rice

Rice is an important food crop in China,while vegetative organs of rice have a strong ability to accumulate cadmium?Cd?in acid soil,and they can transport some Cd to rice grains.So,it is of great significance to reduce the Cd content and improve the quality of rice grains to ensure the food security of China.This experiments were performed to study the mechanism of Cd resistance in rice vegetative organs and the mechanism of foliar application with citric acid?CA?on reducing Cd in rice,in order to provide theoretical basis for the research and development of the foliar fertilizer which can reduce Cd content in rice.The main results were as follows:1.Vegetative organs of rice can prevent Cd from flowing into grains by altering Cd chemical forms.In farmland with different levels of Cd pollution,Cd content in roots and nodes was much higher than that in other organs.More than 75%Cd could be converted into insoluble fraction?Cd extracted by NaCl and residual Cd?by vegetative organs.Accumulation of soluble Cd was always companied by decrease of Ca and increase of Mn in roots,uppermost nodes and rachises.A small increase of soluble Cd from 0.05 to 0.1 mg·kg-1 caused a sharp decline of Ca:Mn ratio in roots and rachises,and increasing Ca:Mn ratio may be special way to alleviate Cd toxicity by establishing new ionic homeostasis in rice plants.2.Foliar application with citric acid at the flowering stage can significantly reduce Cd content in mature rice.In farmland with higher Cd pollution?2.04 mg·kg-1?,foliar application with 1 mM citric acid and 5 mM citric acid could significantly reduce the Cd content in rice grains,the effect of 5 mM citric acid was better.5 mM CA made Cd content in grains and rachises decreased by 53%and 37%,respectively.The content of Cd in organs was negative correlated with the content of citric acid in them.Foliar application with 5 mM citric acid resulted in a significantly reduction of soluble Cd content in vegetative organs,as well as the translocation factors?TFs?of Cd from leaves to nodes.5 mM citric acid made the content of Mn in grains,uppermost nodes and roots increased,made the ratio of Mn to Cd in rachises,uppermost nodes,falg leaves and roots significantly increased.3.Foliar application with citric acid can significantly promote the expression level of NRAMP and the synthesis of amino acids,inhibit of Cd transportion to rice grains.Pot experiments were carried out in polluted soil with Cd content of 0.1-2.4 mg·kg^-1,foliar application with citric acid efficiently raised Mn content in grains and protoplast fraction of nodes,and decreased Cd content in grains,rachises and nodes,and resulted in a great increase of Mn:Cd ratio in grains and nodes of both HZ and P7.Expression level of OsNramp5 was much higher than that of OsNramp1,2,3 in panicles of HZ and inhibited by Cd stress.Foliar application with citric acid efficiently enhanced the expression level of OsNramp2,3,5 in panicles.Quantities of some AAs were negatively and significantly correlated with content of Cd in grains.Exogenous citric acid alleviated the negative effect of Cd on the synthesis of Glu,Met and Val in grains.These results indicate that foliar application with citric acid can regulate the transport of Cd and Mn in the opposite directions by enhancing expression of OsNramp2,3,5 and promoting synthesis of amino acids in rice grains.4.Increasing the concentration of citric acid in the rhizosphere significantly reduced the uptake of Cd by roots and inhibited of Cd transportion to the shoot.The results of hydroponic experiments indicated that under different Mn content in solution,application with 0.65 mM citric acid could significantly reduce Cd content in shoots and roots of rice seedlings by 35.7%-50.4%,9%-13.6%,respectively;significantly reduce soluble Cd content and percentage in shoots and roots of rice seedlings,thus significantly inhibit the transport of Cd;increase the ratio of Mn to Cd in shoots and roots of rice seedlings.0.65 mM citric acid could alleviate the inhibition of higher Mn on Zn uptake by roots of rice seedlings.When Mn content in solution was 80?M,0.65 mM citric acid resulted in a reduction of Mg and Fe content in shoots and roots of rice seedlings.Higher Mn and Mn+0.65 mM citric acid in solution could increase glutathione and cysteine in shoots and roots of rice seedlings.