Effects Of Malic Acid On Cd Accumulation In Rice

Rice is the largest planting crop in southen China.In recent years,due to the discharge of industrial waste water and gas,cadmium pollution in rice has broken out from time to time in some areas of southern China,which has caused serious adverse effects on rice production and rice processing industry.Therefore,how to reduce the risk of cadmium pollution in rice and ensure the safety of rice quality has become one of research hotspots.Malic acid,a small molecular acid,is an important organic acid produced in the metabolic process of organisms.In this study,effects of malic acid on reducing cadmium in rice plants by increasing the concentration of malic acid in rice rhizosphere or by foliar application with malic acid and their mechanism have been analyzed in order to provide a reference for the research and development of Cd-reducing foliar fertilizer for rice.The main results are as follows:1.Malic acid effectively inhibited the uptake of Cd by rice roots and the transport of Cd from roots to shoots.We found that Cd in rice seedlings were mainly distributed in the cell wall（F1）and cytosol（F3）.The cadmium content in organelles（F2）was relatively small.Addition of 0.5 to 1.5 mmol·L^-1malic acid in the rhizosphere significantly promoted the biomass of roots and shoots in rice seedlings,and significantly reduced the accumulation of Cd in the cell wall（F1）,organelle（F2）and cytosol（F3）of roots and shoots.Compared with control treatment,the transfer factor of Cd from roots to shoots was decreased by 1.8%-23.3%by adding malic acid,and the effect of adding 1.5 mmol·L^-1 malic acid was better than low concent malic acid.2.Under Cd stress,malic acid can promote the absorption and transport of Ca and the synthesis of amino acids in rice.The addition of different concentrations of malic acid in the rhizosphere significantly promoted the accumulation of Ca and the transfer of K and Mn from roots to shoots in rice seedlings,but significantly inhibited the accumulation of Zn.In addition,the content of glutathione（GSH）and amino acids such as Asp,Glu,Cys,Gly and Ala in shoots and roots of rice were significantly increas by adding different concentrations of malic acid,while the content of phytochelatins PC₂,PC₃and PC₄ was decreas with the increase of malic acid concentration.The effect of 1.5 mmol·L^-1 malic acid was significantly greater than that of 0.5 mmol·L^-1 malic acid.3.Spraying malic acid on leaves decreased cadmium content and increased amino acid content in rice.The application of 5.0 mmol·L^-1 malic acid at the flowering stage significantly reduced the Cd content in grains,panicle necks,nodes,and flag leaves by 2-3 times.The average Cd content in the four varieties of grains decreased from 0.26 to 0.14-0.09 mg·kg^-1,and the corresponding percentages of Cd decrease were 46.2%and 65.4%,respectively.The concentration of Cd in nodes decreased by 23.4%and 56.6%,respectively.After application with malic acid for three times,the glycine and valine content in the grains increased by 46.5%and 34.6%,respectively.4.Application with malic acid significantly affected the content of Ca,Fe,Mn and Zn and the transfer factors between adjacent organs of rice.The content of Mn,Fe and Zn in all organs was significantly decreased and the content of Ca was significantly increased by application with malic acid.The addition of malic acid also affected the essential element transfer factor of each variety.The Ca transfer factor of X24 and F36 from node to panicle neck increased by 28.4%and 13.0%,respectively;Zn transfer factors of 705,X24,F36 and TGW from neck to grain increased by 55.3%,33.3%,42.2%and 21.3%,respectively.5.Application with 5.0 mmol·L^-1 malic acid,zinc sulfate and malic acid plus zinc significantly decreased the cadmium content and increased the amino acid content in rice grains.Malic acid,zinc sulfate and malic acid with zinc reduced cadmium content by 14.6%,29.3%and 22.0%,respectively.After application with malic acid and malic acid plus zinc,the content of aspartic acid,glutamic acid,glycine,phenylalanine,isoleucine and valine was increased by 10.5%and 5.7%,13.7%and 7.5%,14.8%and 2.3%,14.6%and 7.1%,11.3%and 3.6%,and 23.9%and 4.1%,respectively.