| Nearly 50%of the cereal-grown areas in the world have soils with low plant availability of zinc?Zn?,which resulted in not only a limited crop yield,but also an increased risk of Zn deficiency in human body for those who take cereals as major staple food.Application of Zn fertilizer represents a short-term and effective strategy to close yield gap and improve crop and human Zn nutrition.Therefore,it is highly important to evaluate the effects of Zn application on Zn nutrition in wheat and maize crops and further establish the beneficial management practices.The intensive production system has greatly improved crop yields of wheat and maize,which may challenge the previously established recommendations on Zn fertilizer application rate and Zn diagnose index for soils and plant tissues.Based on a high-yielding wheat-maize rotation system on a calcareous soil in North China Plain,a long-term field experiment was conducted to evaluate the effect of Zn application on yield and grain Zn concentration of wheat and maize.Results of the study was used to establish the critical levels of soil DTPA-Zn and shoot Zn concentration to achieve high crop yields and high grain Zn concentrations.On that basis,further studies were carried out to investigate Zn transportation from soil to grain and its bioavailability in grain as affected by different Zn application strategies.The main results are summarized below:1.For winter wheat,soil Zn application greatly improved the DTPA-Zn concentration in soil,wheat yield,as well as Zn uptake and grain Zn concentration.To achieve high biomass accumulation,the critical shoot Zn concentrations at elongation,anthesis,early filling and late filling stages of wheat were 36.8,31.6,26.9 and 29.5 mg kg-1,respectively.To achieve high yield,shoot Zn concentration should be greater than 29.4 mg kg-1 and DTPA-Zn in the soil should be greater than 2.0 mg kg-1.To reach the Zn biofortification "target value"(45 mg kg-1 Zn in grain),shoot Zn concentration should be greater than 31.6 mg kg-1,and DTPA-Zn in the soil should be greater than 4.1 mg kg-1.These values were greater than the critical deficiency concentrations reported previously.2.For summer maize,Zn biofortification and high yield can be simultaneously achieved through soil application of Zn fertilizer,depending on the available Zn concentration in soil.To obtain the high biomass accumulation during V6,V12,VT and R3 stages of maize,the critical shoot Zn concentration should be 56.1,48.3,32.7 and 25.3 mg kg-1,respectively.The optimal soil DTPA-Zn concentration was 4.7 and 7.6 mg kg-1 to achieve the target high yield and high grain Zn concentration,respectively.The optimal shoot Zn concentration was 33.5 and 55.9 mg kg-1 to achieve the target high yield and high grain Zn concentration,respectively.We also found that Zn application significantly decreased the length of barren ear tips and thus increased kernel number and grain yield.3.Root dry weight,root length density and root surface area of wheat were increased by moderate rates(0-50 kg ha-1 ZnSO4·7H2O)of Zn application,but were slightly reduced by excessive rates.Zn application to the root zone?0-30 cm?where most roots distributed has increased plant Zn uptake.The percentage of Zn translocated from roots to shoots decreased while post-anthesis shoot Zn accumulation increased with increasing Zn application rates.The contributions of Zn remobilization from pre-anthesis and Zn uptake during post-anthesis to the accumulation of Zn in grain depended on soil Zn availability.The critical soil DTPA-Zn value that determined whether remobilization or uptake was more important was 7.2 mg kg-1 in the study soil.4.Similar to those of wheat,root dry weight,total root length and surface area of maize were increased by moderate rates(0-50 kg ha-1 ZnSO4·7H2O)of Zn application but were slightly reduced by excessive rates.Zn accumulation rate increased with Zn application rates.The highest Zn accumulation rate was observed at V6-V12 and VT-R3 stages.The percentage of Zn translocation from roots to shoots and the percentage of Zn accumulation in grain relative to shoot decreased with Zn application rates.The grain filling of inferior grains of maize was improved by Zn application,as demonstrated by the increasing thousand kernel weight and Zn concentration.5.Soil Zn application increased both the concentration and bioavailability of Zn in wheat grain and flours.Although Zn concentrations in wheat whole flour and coarse flour were high,Zn bioavailability and the calculated health impact?DALYs saved and reduction in the health burden?were lower in wheat whole flour and coarse flour than in standard flour and refined flour because of the extremely high concentrations of phytic acid?PA?in the bran.In addition,Zn biofortification did not influence the bioavailability of Fe,Mn,and Cu.These results indicate that application of Zn fertilizer can substantially increase the Zn bioavailability and health impact of wheat flour.6.In the pot experiment,overuse of Zn fertilizer resulted in Zn toxicity,which reduced wheat biomass,plant height and leaf SPAD value while increased the leaf SOD activity.Soil DTPA-Zn and shoot Zn critical toxicity concentration based on the biomass accumulation at seedling stage was 43.0 and 221.9 mg kg-1,respectively. |
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