Potential Of Increasing Wheat Grain Zinc By Selecting Cultivars And Mobilizing Soil Zinc In Dryland

The human zinc deficiency,afflicting billions of people,has aroused worldwide concern.The main solution to this problem is grain Zn biofortification of staple food crops like wheat.The most cost-effecive biofortification measure is breeding cultivars with both high yield and high grain Zn concentration.However,in rainfed dryland calcareous soil,it is difficult to simultaneously achieve high yield and high grain Zn of 40 mg kg^-1by breeding.One reason for this is the low soil available Zn or Zn availability,the other is that tested cultivars cannot efficiently absorb Zn from soil or translocate Zn from vegetative parts to grain.So the limieted shoot Zn uptake is not sufficient to simultaneously support high yield and high grain Zn.In the present study,dryland low-Zn calcareous soil and winter wheat were tested,to investigate the effect and mechanism of increased grain Zn concentration by mobilizing the intrinsic Zn in soil.Further,the cultivars with similar yields and large grain Zn variations were subjected for deep analyses,to find out the suitable Zn biofortification measures for high-yield wheat cultivars in rainfed dryland.The main results are given below.（1）Addition of ethylene diamine tetraacetic acid（EDTA）to low-Zn calcaresous soil,did not affect yield but significantly increased grain Zn concentration by up to 46%.This was due to,during heading to filling,EDTA application significantly increased the relatively available Zn fractions like the exchangeable Zn,Zn weakly bound to organic matter and Zn bound to carbonate.While the unavailable Zn fractions like the Zn bound to armorphrus iron oxide,Zn bound to crystal iron oxide,and Zn in residual mineral significantly decreased by78%,10-31%,and 6%,respectively.These resulted in the significant increase of available Zn in both rhizosphere and nonrhizosphere soil by 185-377%during heading to filling.Meanwhile,the straw Zn uptake was increased by 69-120%,and after anthesis this part of Zn was largely remobilized to grain,contributing 61%of the grain Zn uptake.（2）Under the condition of dryland calcareous soil,the grain Zn concentration of different wheat cultivars was not related to yield,but related to Zn uptake and its distribution,and the relations were dependent on yield level.The cultivars with consistent yield and grain Zn performances over experimental years were identified as four low-yield low-Zn,four low-yield high-Zn,seven high-yield low-Zn,and four high-yield high-Zn cultivars.At low-yield levels,grain Zn concentration was closely related to shoot Zn uptake.While at high-yield levels,both shoot Zn uptake and Zn harvest index were closely related to grain Zn concentration.For the four high-yield high-Zn cultivars,the average yield,grain Zn concentration,shoot Zn uptake,and Zn harvest index were 7287 kg ha^-1,20.8 mg kg^-1,190 g ha^-1,and 79%,respectively.（3）For the high-yield wheat cultivars grown in dryland,the minimum straw Zn concentration in vegetative parts at maturity was around 1.5 mg kg^-1,above which the Zn in straw could be remobilized into grain.For the 19 high-yield cultivars identified from 123tested varieties,they exhibited the same yield of around 7000 kg ha^-1,but the grain Zn concentration ranged from 9.3 to 26.7 mg kg^-1 and the straw Zn concentration varied from 1.2to 10.5 mg kg^-1.The cultivars with lower straw Zn at maturity have higher Zn remobilization abilities.Assumed that the straw Zn of these high-yield cultivars could be decreased to the minimum,their grain Zn concentrations would increase by 2.1-7.5 mg kg^-1,from the current levels to higher attainable 14.5-31.3 mg kg^-1.（4）The quantitative model of grain Zn concentration with shoot Zn uptake and yield was established for high-yield cultivars.With this model,the required shoot Zn uptake was at least 288 g ha^-1,to realize high yield of 7000 kg ha^-1 and high grain Zn of 40 mg kg^-1 in dryland low-Zn calcareous soil.This level was higher than the observed highest shoot Zn uptake of 223 g ha^-1.It indicates that the soil available Zn supply in the studied area,can not meet the Zn uptake requirement for high grain Zn above 40 mg kg^-1 of high-yield cultivars.Increasing soil Zn availability is necessary to increase the shoot Zn uptake of the cultivars with straw Zn close to 1.5 mg kg^-1 at maturity.For dryland low-Zn calcaresous soil,the most suitable approach to realize the Zn biofortification target of high-yield wheat cultivars should be:applying soil Zn mobilization agents like EDTA to promote the soil Zn transformation from unavailable to available fractions and enhance the soil available Zn supply and straw Zn uptake,combined with the high-yield cultivars with low straw Zn concentration at maturity or high Zn remobilization abilities,to realize high yield and high grain Zn simulatnesously.