Mechanisms Of Selenium Influencing Cadmium Uptake,Translocation And Accumlation In Rice

Cadmium(Cd)is one of the most harmful and widespread heavy metals in agricultural soils,which enters from anthropogenic processes,then transferred to plant and accumulates in edible parts,leading to chronic toxic effects on human beings.Paddy rice(Oryza sativa.L)is the staple food for Chinese people,is a major source of Cd intake.Therefore,effective measures are needed to reduce rice Cd level.Selenium(Se)is an essential trace element for humans and animals.In recent years,the antagonism of Se on heavy metal stress has been observed in many different plants.In the present study,solution culture experiments,pot experiments and field experiments were applied to investigate the effects of Se on Cd uptake,translocation and accumulation in rice.The main findings were as follows:(1)The effects of selenite and selenate on Cd uptake and translocation were investigated in this experiment.The concentration-dependent Cd kinetics results suggested that selenite promoted Cd influx in 60 min,compared with no selenite addition,Vmax of Cd uptake increased by 13.8%in 10 ?mol L-1 selenite treatment;however,the presence of selenate approximately had no effect on the influx of Cd.Se addition slightly increased the uptake of Cd by rice seedlings at the initial Cd exposure(? 20 h),and decreased the Cd uptake and translocation from rice roots to shoots with exposure extending.It was also found that selenite was more effective in decreasing Cd uptake by plants than selenate.During the period of Se exposure(72 h),selenite decreased the amount of Cd taken up by per gram of rice root by 47.4%,compared with only 11.8%for selenate treatment.In addition,pre-Se treatment also decreased the Cd uptake and translocation from root to shoot.(2)The effects of Se on subcellular distribution of Cd in rice root and the translocation to xylem,as well as the responses of antioxidants to Cd and Se in rice root,were studied to improve our understanding of the alleviating phenomenon of Se mediated resistance to Cd stress in rice.Results showed that selenite addition increased the proportion of Cd distributed to cytoplasm by 23.4%,decreased the Cd distribution in organelle by 28.7%,and decreased Cd in xylem sap of rice by 55.9%.The activity of CAT in rice root was decreased,while H2O2 and MDA contents were elevated with Cd application relative to the control.Se addition mitigated Cd stress and increased GSH content.(3)This experiment was conducted to investigate the effects of iron plaque or/and Se on Cd uptake and translocation by rice.The presence of iron plaque decreased Cd in DCB extracts in Cd-alone or+selenate treatment,but not in +selenite treatment.It also decreased Cd uptake by rice.The addition of selenite decreased Cd in DCB extracts by 40.5%with iron plaque(24 h)but had no significant effect on that without iron plaque.(4)In order to determine the effectiveness of Se in regulating rice Cd uptake under various soil conditions that could significantly affect Cd bioavailability,this study investigated the effects of selenite fertilization(0,0.5 and 1.0 mg kg-1)on soil solution Cd dynamics and rice uptake in two Cd-contaminated soils(acidic soil and alkaline soil)under two different sets of conditions:aerobic and flooded.The plants were harvested at the seedling stage and at maturity to determine their Cd levels.Soil solutions were also extracted during the growing season to monitor Cd dynamics.The results showed that in the acidic soil(pH 5.25),the Cd concentrations in the soil solutions,seedlings,and mature rice were higher under aerobic than under flooded water management.The Cd content in the grain of rice grown in aerobic acidic soil soil was 12.7-13.5× higher than that raised in flooded acidic soil soil.In the alkaline soil(pH 7.26),however,flooding decreased Cd levels in the rice seedlings but not in the mature plants.Selenite additions to the alkaline soil decreased Cd concentrations in the soil solutions and in the rice plants,while decreased the rice grain Cd content by 45.2%and 67.7%-77.4%under aerobic and flooding conditions,respectively,relative to the control treatment;in the acidic soil,selenite additions decreased Cd in the rice seedling but not in mature plant.(5)Results showed that foliar application of Se could effectively enhance Se content,and decrease Cd content in rice grain.Compared with control,Se addition(15 g hm-2)decreased grain Cd by 7.9-18.9%.Additionally,significant negative correlation was observed between Cd and Se content in rice grain(r=0.7929,P<0.001).