Uptake,Transformation And Bioaccessibility Of Selenium And Cadmium In Rice And Vegetables

Rice（Oryza sativa L.）is one of the most important food crops for human beings,whose products are consumed by more than half of the world’s population,and its total output is third only to wheat and corn.Cucumber（Cucumis sativus L.）,long bean（Vigna unguiculata（Linn.）Walp.）and radish（Raphanus sativus L.）are three main vegetables with high nutritional value and are widely planted.Selenium is a micronutrient that plays an important role in the growth and development of humans and animals,which also has a positive impact on plant growth.However,the distribution of selenium resources on the earth is not balanced.For residents in low selenium areas,there is an effective way to supplement selenium by ingesting selenium-rich foods.Due to the associated relationship between selenium and cadmium,selenium-enriched agricultural products are often accompanied by serious cadmium pollution.Therefore,the uptake and transformation mechanism of selenium and cadmium by crops,as well as the transformation and bioavailability of selenium and cadmium in the processing of selenium-enriched agricultural products,deserve further study.This study took the most common selenium-enriched agricultural products such as rice and vegetables as the research objects to study absorption and accumulation of selenium and cadmium,as well as their transformation and bioavailability during processing and to clarify the changes of selenium,cadmium and nutrition contents during the processing.And the study is hoped to provide reference opinions for scientific selenium supplementation and the formulation of production technical specifications for related selenium-enriched products.The main findings are as follows:（1）This study investigated the effects of exogenous selenium on the growth,antioxidant response and gene expression of cadmium-stressed rice.The results showed that foliar spraying of exogenous selenium increased the absorption and accumulation of selenium in rice,while reducing the absorption of soil cadmium.Selenium application also increased the net photosynthetic rate（Pn）,transpiration rate（Tr）,stomatal conductance（Gs）,chlorophyll a and total chlorophyll content of rice leaves at the tillering stage,and relieved the inhibitory effect of cadmium on photosynthesis.The application of exogenous selenium increased the activities of superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）in rice,increased the content of reduced glutathione（GSH）and ascorbic acid（AsA）,decreased Malondialdehyde（MDA）formation.Transcriptome analysis showed that the addition of exogenous selenium up-regulated the expression of genes related to phosphate and sulfate transport and antioxidant systems,and application of exogenous selenium in rice under cadmium stress caused responses to pathways such as cellular metabolism,photosynthesis,and signal transduction.（2）This study investigated the effects of exogenous selenium addition on selenium and cadmium content,morphological transformation and nutritional quality in rice during processing.The results showed that foliar spraying of exogenous selenium significantly increased the selenium content of rice,which was 1.33-17.86 times higher than the control in brown rice,2.74-24.33 times in polished rice,and 1.21-21.42 times in steamed rice.The selenium speciation analysis showed that the selenium in milled rice was mainly organic selenium,and the content of selenomethionine（SeMet）was the highest,followed by selenocystine（SeCys₂）.After applying selenium,the cadmium content in brown rice decreased by 12.98%-27.67%compared with the control group,and the cadmium content in polished rice and steamed rice decreased by 12.77%-24.20%and 2.80%-21.40%,respectively.The application of exogenous selenium significantly increased the protein content of rice,but had little effect on the amylose content（3）This study analyzed the changes of Seand Cd content,morphology and bioavailability of rice during processing.The results showed that the rice milling and cooking process resulted in the loss of selenium,which was 14.11%-23.86%and 5.67%-19.98%,respectively.The bioaccessibility of selenium in rice with higher selenium content was also relatively higher,and the bioaccessibility of selenium decreased to a certain extent after cooking.Compared with polished rice,the relative contents of SeMet and inorganic selenium decreased in steamed rice,and SeCys₂ increased.During rice milling and cooking,cadmium content was lost by 8.36%-14.35%and 2.90%-9.13%,respectively,and the bioaccessibility of cadmium also decreased after processing.（4）The application of exogenous selenium in the field can effectively promote the growth and development of cucumbers,long beans and radishes.The selenium content in the edible parts of vegetables was increased by 3.19-5.34 times,the cadmium content was reduced by 9.48%-47.86%,and the nutritional quality of soluble sugar,soluble protein and vitamin C was improved.During the series of factory processes such as pickling,cutting,washing and pressing,mixing,filling and sterilization,the selenium content of vegetables gradually decreased,and the whole process decreased by 66.06%～78.30%.Bioavailability of selenium reduced after processing.During processing,the cadmium content was reduced by 11.16%～16.97%,the soluble sugar,soluble protein and vitamin C also gradually decreased.