Effect Of Selenium And Cadmium Application On Nutritional And Physiological Aspects Of Rice

Cadmium is a serious threat to the environment and rice can accumulate a significant amount of cadmium in polished rice over the permitted range of 0.2 mg kg-1(GB 2762-2017).In China,13 mha area is not utilizable and affected by cadmium.Selenium is a well-known important candidate to mitigate cadmium toxicity.Se-rich diets have gained attention recently,but the potential of Se-rich rice in mitigating Cd stress needs further investigation.The purpose of the study was to conduct field trials evaluating varietal response for cadmium and selenium on nutritional contents of the rice.Additionally,Selenium biofortified rice and low responsive Se varieties comparisons were the focus under study.The varieties were evaluated for elemental accumulation and environmental effect,under series of high and low(cadmium and selenium)treatment levels and with different sowing dates regarded as(early and late sown).Accretion in metals and the Cd mitigation effects were observed in soil and rice components viz.,roots,stem,leaves and panicle parts(panicle straw,husk,rice bran,endosperm and embryo).Four independent experiments in field were conducted.After measuring the contents in various components and implementing a range of statistical analysis and association studies between different components.Following conclusions are hence,presented1.The increased fertilizer treatment in soil bulk linearly increased the metal content in rice grain.Approximately,50-70%of metal was recovered in rice tissues while 5-20% of metal applied was leached down into the soil.A Se concentration of 0.4 mg kg-1could significantly improve total Se content in grain and mitigates Cd toxicity(1 mg kg-1)below the permitted-range.Panicles and roots were more active for total Se accumulation in Se-rich and non-Se rich rice,respectively.Polishing and milling operations can significantly reduce the Cd content,as rice bran in rice tissues accumulated most of the metal's residues.The late matured rice cultivars consumed more heat-units and more metal content were assessed in them.Collectively,it was found that Se can mitigate Cd toxicity but the rice cultivation at T₂(high Cd;2 mg kg-1 and high Se;1 mg kg-1)increased the metal uptake capability and health-risk index in polished rice with Se content heightened over permitted range of 0.04–0.30 mg kg-1(GB/T 22499-2008).2.The implications of Se and Cd on rice nutritional content,deviations in chlorophyll content mg/g(SPAD),photosynthetic characteristics(net photosynthesis rate(Photo;P_N),stomatal conductance(Cond;g_s),intercellular CO₂(C_i),transpiration rate(Trmmol;E),water use efficiency(WUE)and stomatal limitation to CO₂(SlCi))at different stages(tillering,heading,flowering,50%maturity stage)and grain yield were assessed in high Cd(4,8 mgkg^-1)and high Se(1 mgkg^-1)treatment groups.It was found that Cd treatment improved the water use efficiency and reduces the transpiration rate in non-Se rice.Application of 1 mgkg^-1 sodium selenite decreased uptake of cadmium content and increased photosynthetic activity in rice while grain yield was insensitive to metal applied.A high Se treatment(1 mgkg^-1)along with high Cd(4 and 8 mgkg^-1)can result in the elevated Se accumulation of(4 mgkg-1)and Cd (2 mgkg^-1)in polished rice.However,further molecular studies are required,to completely access the Se hyperaccumulation behaviour in rice tissues at high Cd soil stress.3.The impact of heat units on the accumulation behaviour in two environments was assessed.Se and Cd were applied in three groups(T0;0:0,T1;0.4:1,T2;1:2)mgkg-1.As the time of planting and heat units consumed by the crop to attain its physiological maturity appears to be different.The quantity of Cd gets accumulated at different levels of treatment was more than Se even in the same rice cultivar.The plant sown earlier accumulate more Se than Cd while the plants uptake more Cd when sown 1 month later.The plant completes the critical developmental phases(Tillering,heading and flowering)within 20-30 days interval depending on the variety.But the late-sown plants interchange the transitional phase(tillering and heading)within 5-8 days interval.These quick transitional changes may lead to the uptake of an ample amount of Cd content in rice grain even at a low level of Cd stress(1 and 2 mgkg-1)due to multiple environmental factors,making rice unsafe for edible purposes.The proper time of planting could be effective in timely acclimatization of Se and Cd sequestration and translocation in rice various components.4.The results revealed by the foliar sodium selenite treatment on critical growth stages indicated that Se application could significantly improve the total and organic selenium in grain.Application of 10 mg L^-11 sodium selenite disclosed most organic selenium(0.03mg kg^-1)in endosperm.Association studies between leaves sodium selenite application and other plant parts(brown rice and glume)uncovered positive relation for total selenium accretion in order;glume>rice bran>brown rice>endosperm>embryo,but behaviour of organic selenium differs in a pattern such as;endosperm>embryo>rice bran>glume.Meanwhile,75-85%of Se found in endosperm and embryo was organic in nature.Integrating all these results,10 mg L^-1 sodium selenite was found effective foliar fertilizer for organic selenium biofortification(0.06⁰.08 mgkg-1)in brown rice of non-Se rich rice,while the natural uptake capability of Se-rich rice was stronger.In summary,the selenium fertilizer can mitigate cadmium accumulation in rice different parts.Under the natural field environment,Selenium rich rice cultivars(5097A/R2035,5097B)were more responsive to selenium uptake while also mitigates cadmium accumulation in rice.While the non-selenium rice(GangYou 725)accumulated selenium contents in endosperm below the standards.Exogenous selenium fertilizer can biofortified non-Se rice for selenium element,but the extent of cadmium tolerance varies in different materials.In the determination of Se speciation(organic and inorganic),the performance of selenium-rich rice was far better than non-selenium rice.74⁷6%organic selenium was observed in brown rice of Se-rich rice than non-Se rich rice which was 39⁵2%.While 80%of selenium found in rice bran of non-selenium rice was inorganic in nature.In the process of utilization of cadmium affected land and biofortification with Selenium.Bio-fortified rice(5097A/R2035 and 5097B)were found promising.As have more selenium in their endosperm than cadmium and environmentally friendly,while can accumulate more organic selenium in natural environmental conditions as disclosed by foliar experimentation.Results of present investigation revealed that Se-biofortified rice can be utilized in low Cd effected areas,with the use of 0.4 mgkg^-1 selenium fertilizer to avoid Se hyper-accumulation and deficiency in grain.