Effect Of Selenium Application Methods On Selenium Uptake,transport And Speciation In Wheat (Triticum Aestivum L.) And Its Mechanism

Selenium（Se）is known to be an essential trace element for mammals,while the daily Se intake of 39%-61%of Chinese residents is still lower than the WHO/FAO recommended amount(55μg d^-1).Wheat（Triticum aestivum L.）is the main grain crop with strong Se-enrich ability among cereal crops,while the grain Se content is generally low due to low Se content in soil or low Se bioavailability.Therefore,wheat Se biofortification has attracted attention from many disciplines.However,there is a lack of systematic research on Se biofortification of wheat by different Se application methods.Therefore,in this study,pot experiment and field experiment were combined to systematically compare the effect of different Se application methods（different methods,types,concentrations,and stages）on wheat yield,uptake,and translocation for the first time,and the differences of flour extraction rate of flour（70%and100%wheat flour）by the different Se application methods were also compared.The main points and results are the followings.1.The effects of the difference of Se application on the uptake and distribution of Se in different parts of wheat were systematically compared.The plant height,effective panicle number,and flower axis length of wheat were significantly decreased by soil selenate of high concentration(2 mg kg^-1)compared with the corresponding low concentration(0.5 mg kg^-1)treatment.Soil selenite application of high concentration increased wheat grain yield by about6%compared with the control treatment,which obtained the highest grain yield.Application of Se significantly increased the Se content in each part of wheat.Compared with selenite treatment,selenate treatment significantly increased the Se content of nine parts of wheat,indicating that the higher bioavailability of selenate,which is related to the increased transport of wheat from the flower axis to the grain.Se content in wheat grains at the pre-filling stage was significantly increased by 6%-30%compared with that at the pre-flowering stage.The Se concentration in the first node is higher than that in the first internode,irrespective of the Se application methods,which confirmed that the first node plays an important role in the translocation of Se from xylem to phloem.This study showed that wheat could be as an ideal crop for Se biofortification.2.The effects of different Se application methods on grain Se speciation and bioavailability of wheat were clarified.Se Met（87%～96%）and Se Cys₂（4%～13%）were the main speciation of Se in wheat grains,which indicated that wheat could efficiently convert inorganic Se into organic Se.Soil application of high concentration of selenite increased the proportion of Se Met in wheat grains by 6%compared with corresponding low concentration treatment.The proportion of Se Met in wheat grains was decreased by 12%by high concentration of selenate compared with the corresponding low concentration.The results showed that selenate was more effective,and high concentration of exogenous Se reduced the proportion of organic Se.Different processing methods can cause the loss of Se in wheat,the loss rate of Se in 70%flour compared with that in whole wheat was between 12%-68%.Further,the Se bioaccessibility of whole wheat and 70%flour with different Se treatments was between6%-38%,while the Se bioaccessibility of 70%flour is higher than the whole wheat.The Se bioaccessibility of wheat treated with selenate was higher than that of selenite treatment,and the Se bioaccessibility of whole wheat and 70%flour in intestinal fraction was higher than that in gastric fraction.3.The effect of foliar Se application on wheat uptake,translocation,and speciation distribution was confirmed.Different forms of foliar Se application had no significant effect on wheat plant biomass and grain yield.Selenium distribution in wheat was in the descending order of leaf>root>grain>glume>stem when selenite was foliar applied,and it ranked as leaf>grain>glume>stem>root when selenate was foliar applied.Foliar application of either selenite or selenate at pre-filling stage was more efficient at increasing Se content in wheat grain than at pre-flowering stage.Se Met was the main organic Se species in wheat grain,accounting for about 67%-86%of total Se.Thus,we suggest that foliar application of selenate at 20 g ha^-1 at pre-filling stage is the most efficient way for Se biofortification in wheat.In conclusion,exogenous Se application significantly increased the Se content in various parts of wheat,and the bioavailability of selenate was higher than that of selenite,and the grain Se content of wheat was higher in the pre-filling than in the pre-flowering.The grain Se speciation of wheat was mainly as Se Met in soil or foliar Se application,and the Se content of whole wheat was higher than that of 70%flour,while the Se bioaccessibility of 70%flour is higher than the whole wheat.Therefore,different Se application treatments had different effects on Se uptake,transport,and speciation distribution of wheat grains,which should be considered comprehensively in Se biofortification or phytoremediation.