Effect Of Selenite And Selenate Application (Soil Or Foliar) On Transport,Transformation,and Distribution Of Selenium In Soil And Their Bioavailability In Wheat (Triticum Aestivum L.)

Selenium（Se）is an important trace element from the environmental and biological viewpoints.Through past few decades,the importance of Se to human and animal health is extensively studied and their possible role in plant metabolism is also discussed but their essentiality to plant is still controversial.Food consumption provides the principal route of Se intake for most of the population,the Se biofortification of crops has demonstrated to increase the Se in common dietary foodstuff and thus to enhance human nutrition.Se biofortification is an agricultural process that increase the accumulation of Se in crops,through plant breeding,genetic engineering,or use of Se fertilizers such as selenite and selenate.Wheat plays an important role in production of Se-rich foods to increase the Se intake of humans.A comprehensive understanding of the behavior and cycling of Se application in soil-plant system is necessary.For this purpose,a series of pot and field experiments were conducted to evaluate the Se distribution,transportation,accumulation and bioavailability in wheat.Different application levels and Se forms（selenite&selenite）with soil and foliar application were tested on various wheat varieties.The main points and the results are the following.1)This study investigated the effect of selenate and selenite on the distribution,transformation of Se fractions in soil,as well as the accumulation and availability of Se in each part of wheat plants.A pot experiment was conducted using different concentrations of exogenous selenite or selenate(0.5,1,2.5,5,and 10 mg Se kg^–1 soil).Sequential extraction was used to determine the Se fractions in soil,and different models were used to study the behavior of Se in soil and its availability to wheat.Results showed that the distribution and availability of Se in soil and its accumulation in wheat affected both by Se concentrations and forms of exogenous Se.In selenite-treated soil,the proportion of exchangeable and carbonate-bound Se（EXC–Se）（21%–42%）fraction increased compared to that in control（12%）,while organic matter-bound Se（OM–Se）（23–33%）and Fe-Mn oxide-bound Se（FMO–Se）（11%–15%）fractions decreased compare with those in control（37%and 32%,respectively）.In selenate-treated soil,soluble-Se（SOL–Se）fraction（30%–54%）increased and the OM–Se（9.8%–20%）and FMO–Se（4.7%–14.2%）fractions decreased compared with those in control.Residual Se（RES–Se）fraction was increased for selenite（7.4%–13.4%）and selenate（12%–20%）treatments compared with that in control（6.5%）.In comparison with control,the available Se（SOL-Se+EXC-Se）fraction increased by both selenite（32%–47%）or selenate（54%–72%）treatments.Moreover,at the same rate of Se application,Se availability was higher in wheat grown in selenate-treated soils than that in selenite-treated soils.The redistribution index(U_ts)of Se increased from 1（in control）to1.2–1.9 and 1.5-2 for selenite and selenate treatments,respectively;additionally,the mobility factor（MF）in selenate-treated soil was 40%–90%higher than that in selenite-treated soil.Furthermore,relative bonding intensity（I_R）for both selenite（0.38-0.45）and selenate（0.33-0.41）decreased compared with that in control（0.55）.These differences indicated that selenite and selenate varied in terms of fixation capacities in soil,in transformation and distribution of Se in soil fractions and,in their availability to plants.The results of Michaelis–Menten equation demonstrated the high affinity of leaf to selenate,and the high affinity of roots and grains to selenite.Selenate was dominant in nearly all parts of wheat plants and in each application level.However,the affinity of selenite to wheat grains suggests that selenite is a useful Se fertilizer that must be considered in biofortification programs.2)The study was aimed to investigate the effect of selenite and selenate on the uptake and distribution of selenium（Se）,as well as yield and photosynthetic properties in 8 varieties to find out the possible resistance and nutritional aspects of varieties to the application of Se.For this purpose,eight wheat varieties were selected and tested against low(1 mg kg^-1)and high(10 mg kg^-1)doses of both selenite and selenate,in a pot experiment.Application of both Se exogenous forms increased the uptake of Se in each part of the plant.In selenite treated soils,Se was mostly accumulated in grain and roots,while in selenate treated soils,Se was mostly accumulated in leaves and straw.Compared to other varieties,grain Se uptake was the highest in Zhengmai-9023for both selenite(553μg pot^–1)and selenate(3098μg pot^–1)treatments.This study also concluded that yield and photosynthetic properties were closely related to the application of Se.Photosynthesis can also be used as defense mechanism of wheat plant to reduce the adverse effects of Se toxicity in tolerant plants,which can clearly be observed in this study.Biomass grain and yield of almost all the varieties was decreased by the application of both selenite and selenate,except for Zhengmai-9023 at low Se rate,while high Se rate was toxic for all the varieties.On the basis of these results,it was concluded that Zhengmai-9023 was tolerant to Se toxicity and low Se enhanced yield and accumulated Se in grains.Waimai-52 was also slightly tolerant and can be beneficial in Se biofortification programs.Shixin-618 was slightly susceptible to Se fertilization among all varieties under study.The response of other varieties like Shaanmai-979,Jimai-22,Xiaoyan-22,Xumai-30,and Hengmai-5229 can also be considered for Se nutrition.Low Se fertilization rate(≤1 mg Se kg^–1)less affected the yield and accumulated a reasonable amount of Se in wheat varieties.3)The aim of the study was to compare the soil and foliar application of selenium at different stages to investigate the changes in Se concentration and yield of wheat in actual field condition.For this purpose,two Se fertilizer forms（selenite and selenate）with two application levels(20&100 g ha^–1)were applied as soil application at the time of sowing,and foliar application at two growth stages（before flowering;&before grain filling stage）.Results showed that there was no significant increase in grain yield and biomass yield for 20 and 100 g ha^-1 of Se application.There was only 2%-4%increase in grain yield while 3%-5%increase in biomass yield of wheat at various application levels of Se,as compared to Control.Yield of wheat was a little higher for foliar application method at flowering time as compared to soil application,while later foliar application before grain filling more reduced the yield a little.Se concentration was higher at tillering stage but it was reduced before flowering because of dilution effect.After harvest,Se concentration in all parts of wheat plant was increased with Se application irrespective of form,level or application method.4)The aim of the study was to investigate the Se-enriched wheat grains fractions（flour and bran）for Se concentration and grain quality by soil and foliar application of Se at different growth stages.Wheat grains were separated,and different fractions of flours（60%.70%and 80%）and bran（40%,30%and 20%）along with whole grain flour were investigated for Se concentration and grain quality as total protein content in these fractions.The Se concentration of each flour and bran fractions was significantly increased with the application of Se as compared to control,but the difference between the fractions was non-significant.The bran fraction of wheat grains had a little high Se content than other flour fractions and whole grain flour.The application of Se to foliage before flowering stage or even before the grain filling stage recorded the highest accumulation of Se in the grain and in the flour,but the foliar application should not be too delay to avoid detrimental impacts on grain quality.The protein was not affected by time or form of Se application except a little reduction was recorded at higher Se applied as selenite at a very late stage.Therefore,the application of Se fertilizer in the form of selenite and selenate should be encouraged to improve crop productivity,quality as well as produce Se-enriched food products.Wheat can be a best option in this regards because of its great importance and Se enrichment properties.Proper time,balance level and suitable Se form should be carefully considered before application to any crop species as well as cultivar according to the soil and environmental conditions.Se biofortification could help to minimize the risk of Se deficiency in human and animals and to avoid the health consequences attached with its deficiency or toxicity.