Mechanisms Of Organic Selenium Uptake,Translocation And Speciation Transformation In Rice And Wheat

Selenium（Se）is an essential trace element for human and animal health.Globally,it has been estimated that 0.5 to 1 billion people may have inadequate intakes of Se.Cereal,especially rice and wheat,are the primary source of Se in most diets,and they contribute 70%of Se intake in low-Se intake areas of China.Therefore,biofortification of cereals provide a safe and effective method of increasing human Se intake.In the present study,rice（Oryza sativa L.）and wheat（Triticum aestivum L.）were targeted species.Solution culture experiments and HPLC-ICP-MS techniques were applied to investigate the mechanisms of organic Se uptake,translocation and speciation transformation in crops.Field experiments were used to investigate the effect of different forms of Se fertilizers on Se accumulation,distribution,and residual effect in subsequent crops.Both the results of solution culture experiments and field experiments could provide theoretical basis for developing Se-enriched products.The main results were as follows:（1）The results of concentration-dependent kinetics of Se uptake experiment showed that different forms of Se significantly affected the Se uptake rate,V_max and K_m of rice roots.The time-dependent kinetics of organic Se uptake in roots revealed that the uptake of SeMet by rice roots was described satisfactorily by the Michaelis-Menten equation,but a linear equation was fitted to the SeOMet uptake with regression coefficients of 0.9920.In addition,the different pH values of nutrition solutions significantly affected the Se uptake rate by wheat root,but did not significantly affect the proportion of Se distributed to shoots.（2）Compared with control,the addition of AgN03 significantly inhibited the root uptake of SeMet by 93.1%（P<0.05）,while the addition of AgN03,TEACl and CCCP significantly inhibited the root uptake of SeOMet by 41.7%,45.6%and 30.4%（P<0.05）.The result of interaction effect experiment showed that the root Se concentration and uptake rate of SeMet treatment were significantly 10-11 times and 12-13 times higher than SeMet treatment and SeMet/SeOMet treatment（P<0.0001）.Root Se concentrations in both organic Se treatments were not significantly affected by Se species and culture mediums,as well as no interactions between these two factors（P>0.05）.On contrary,the Se concentrations of shoots were significantly affected by Se species and culture mediums（P<0.05）,but without interaction between these two factors（P>0.05）.（3）The Se concentrations of wheat roots and shoots were significantly affected by treatment time and organic Se species（P<0.05）,as well as strong interaction between these two factors（P<0.05）.The results of selenium speciation showed that forms of MeSeCys and SeMet,were the dominant species in the organic Se-treated plants and accounted for approximately 14.3-97.2%of the total Se.Additionally,Se（Ⅳ）、SeMet、MeSeCys and another unidentified Se species were detected in the xylem sap of wheat.（4）In the field experiments of wheat and maize,sodium selenite,sodium selenate,selenomethionine,chemical nano-Se,humic acid+sodium selenite,and compound fertilizer+ sodium selenite were applied once at the flowering stage of wheat and at the bell stage of maize.Compared with the control,foliar Se applications significant increased the grain Se concentrations of wheat and maize by 0.02-0.31 and 0.07-1.09 mg kg-1 DW,respectively.Wheat and maize grain Se recoveries ranged between 3.0-10.4 and 4.1-18.5%,respectively.However,Se concentrations in the grain of subsequent wheat and maize significantly decreased by 77.9%and 91.2%,respectively.The change of Se concentration in soil was a dynamic process with Se depletion after harvest of maize.（5）In the field experiments of rice,Se concentrations in rice grain were increased by 0.004-0.043 mg Se kg^-1 dry weight for each gram of Se added per hectare.The grain Se concentration,grain Se recovery and the straw-to-grain transfer factor（TF）in SeMet treatment were higher than those of other Se treatments.And the distribution of Se in different parts of rice ranked:straw>grain>husk.In addition,there was positive correlation between grain Se concentration and the application dosage of Se fertilizers.