| Zinc(Zn)is an important micronutrient in biological systemsand is receiving growing attention worldwide because of increasing reports about Zn deficiencies in human populations.Recent estimates indicate that over three billion of the world population is affected by Zn deficiency.Wheat is a major dietary source of calories,proteins,and bioavailable micronutrients(Zn,Fe and etc.).However,wheat is inherently low in Zn and rich in compoundssuch as phytate that limit Zn bioavailability,which rapidly results in Zn malnutrition in human beings whose diets are mainly based on wheat products.Foliar application of Zn is thought tobethe most effective method for Zn biofortification in wheat grain(Zn concentration exceeding 40 mg/kg).Also,including Zn in existing foliar application of macronutrients represent an effective approach to increase the likelihood of adoption of foliar Zn application by farmers.Studies have indicated that macronutrients(N,P or K)supply could affect Zn uptake,translocation,and retranslocation within wheat plants as well as Zn deposition in wheat grain.However,little is known about the interactive influences of foliar-applied Zn and N,P,or K on grain nutritional quality(especially Zn).In particular,no research has evaluated the effects of a combined application of foliar Zn and P on Zn availability in wheat.In order to guide reasonable foliar application of Zn fertilizer,it is also important to evaluate the influence of commonly used agronomic practices(e.g.soil N management,water management,straw returning and etc.)on grain Zn concentration.Thus,a series of field experiments were conducted at two sites that employing different wheat cropping systems [wheat-maize rotation with straw returning(Sanyuan)and single wheat plantation with no straw returning(Yangling)],as well as a solution culture experiment,to clarify the effectiveness and mechanism of foliar Zn combined with N,P,and K applications on grain Zn nutritional quality of winter wheat.The major results were summarized as follows:(1)A 2-yr(2009.10-2011.06)field experiment was conducted to examine the effect of water management practices(CP: conventional planting;SI: supplemental irrigation;RF: plastic film-mulched ridge and straw-mulched furrows)and soil N(0,120 and 240 kg N/hm2)on micronutrient concentration and bioavailability in wheat grown under single wheat plantation.The results indicate that fertilizer N(120 or 240 kg N/hm2)application significantly improved grain Zn,Fe,Cu,and protein concentration with a mean increase of 24.7%,39.2%,18.6%,23.9% in 2009-2010 and 8.9%,21.1%,10.5%,29.6% in 2010-2011,respectively,when compared with the unfertilized treatment.However,grain Mn concentration in both cropping seasons was significantly decreased(on average by 8.4%)by N fertilization.Compared to the CP,the RF significantly increased grain Zn by an average of 13.2%,grain Fe by 22.2%,grain Mn by 4.4%,and grain protein by 8.3%;the SI did not affect grain micronutrient concentrations,but significantly increased grain yield.Grain phytic acid(PA)concentration and PA/micronutrient ratios were on average 28.0% and 32.2% lower in the N fertilized treatment than in the unfertilized treatment.Decreasing the N application rate from 240 to 120 kg N/hm2 did not affect much of the grain nutritional quality,with the exception of a significant decrease in grain Zn concentration,and a significant increase in grain PA concentrations and the PA/Zn molar ratio.These data demonstrate the importance of monitoring grain Zn concentration,and perhaps the inclusion of supplemental Zn when soil N supply was reducedin this region.(2)A 4-yr(2010.10-2014.06)field experiment was conducted to evaluate the efficacy of concurrent foliar Zn,and commonly used N,P and K applications on grain nutritional quality(especially Zn)of wheat grown under two typical wheat cropping systems(WMs: wheat-maize rotation with straw returning;SW: single wheat plantation with no straw returning).On average,grain Zn concentration increased by 57%-140%,71%-144%,40%-106% and 52%-172% following foliar application of Zn,Zn+N,Zn+P and Zn+K,respectively.Comparing with foliar Zn only treatment,foliar Zn+N application further increased grain Zn concentration by 9.5% under WMs,and significant positive interactions were observed between foliarly-applied Zn and N(4.53 mg/kg);foliar Zn+K application further increasedgrain Zn concentration by 13.4% under SW,and significant positive interactions were observed between foliarly-applied Zn and K(8.14 mg/kg);however,foliar Zn+P application further decreased grain Zn concentration by 13.7% and 13.3% under WMs and SW,and significant negative interactions were observed between foliarly-applied Zn and P(-4.27 and-4.68 mg/kg).Grain phytic acid:Zn molar ratios were 52.0%,53.1%,43.4% and 63.5% lower in the foliar Zn,Zn+N,Zn+P and Zn+K treatments than in the control treatment.Generally,the combined foliar application of Zn with macronutrients had no adverse impact on grain yield and other nutritional quality,and in some cases slightly increased grain yield and macronutrient concentrations.Therefore,combined foliar application of Zn with N,P,or K can fortify wheat grain with Zn(exceeding 40 mg/kg)under different field management practices,and foliar Zn+N and Zn+K application performed better than foliar Zn+P application.(3)By analyzing Zn concentration in vegetative tissues(leaf,glume and stem)of field-grown(2010.10-2014.06)wheat under single wheat plantation,the influences of combined foliar application of Zn with N,P,or K on Zn distribution,and the relative contributions of Zn remobilization and post-anthesis shoot Zn uptaketo grain Zn accumulation were evaluated.The Zn concentration following foliar application was much greater in the leaves(2.4-7.7 fold)and glume(2.0-4.7 fold)than in the stem(0.2-1.0 fold),and therefore resulted in significant increase of Zn distribution in leaf and glume.Comparing with foliar Zn only treatment,foliar Zn+N and Zn+K applications increased leaf and glume Zn concentration,but foliar Zn+P application reduced leaf and glume Zn concentration.When Zn was not applied,the remobilization of pre-anthesis Zn stores accounted for 51-52% of Zn source,and soil N supply promoted Zn remobilization.However,when Zn was foliarly-applied,grain Zn accumulation was completely provided by post-anthesis shoot Zn uptake.Also,compared to the foliar Zn only treatment,the post-anthesis shoot Zn uptake and its mobilization to grain were 12 and 14 g/hm2 higher in the foliar Zn+N treatment,and 44 and 32 g/hm2 higher in the foliar Zn+K treatment,but were 37 and 18 g/hm2 lower in the foliar Zn+P treatment.It is therefore can be concluded that the increased grain Zn concentration in foliar Zn+N and Zn+K applications were resulted from increased post-anthsis Zn uptake and mobilization,while decreased grain Zn concentration in foliar Zn+P applications were resulted from decreased post-anthsis Zn uptake and mobilization.(4)Hydroponic experiment(P levels: 0,1000 and 2000 ?mol/L)and a 3-yr(2011.10-2014.06)field experiment(Soil N levels: 0,120 and 240 kg N/hm2)were conducted to evaluate the effect of foliar P combined with Zn or N on total,soluble and insoluble Zn concentrations in different wheat organs(grain,leaf,glume and stem).Results showed that increased total Zn concentration in different wheat organs following foliar Zn application were due mainly to the increase in soluble Zn concentration.Thus,the proportion of soluble Zn significantly increased after foliar Zn application.Also,grain Zn response to foliar Zn application were not affected by different root P supply levels under hydroponic experiment,but the grain,stem and glume soluble Zn concentration were significantly decreased by 30.4%,32.8% and 38.4% following foliar Zn+P application,which resulted in significant increase in the proportion of insoluble Zn.Under the field conditions,foliar application of P alone decreased soluble Zn concentrations in grain,stem,leaf and glume by 18.5%,12.6%,7.5% and 16.5%.Compared to foliar Zn only treatment,foliar Zn+P application resulted in 20.0%,25.6%,33.2% and 32.1% decrease in grain,stem,leaf and glume soluble Zn concentration,but 106% and 46.6% increase in leaf and glume insoluble Zn concentration,and 10%-26% and 10%-14% increase in leaf and glume proportion of insoluble Zn.Moreover,soil N application had little influence on Zn concentration,but the soluble Zn concentration and its proportion were higher in the foliar Zn+P+N treatment than in the foliar Zn+P treatment.Overall,the availability of Zn(through foliar application)in vegetative tissues(leaf and glume)were not inhibited by root P supply,but significantly decreased by foliar P application;this negative effect can be partially mitigated by foliar N application.(5)A 2-yr(2012.10-2014.06)field experiment was conducted at two sites with two typical wheat cropping systems(WMs: wheat-maize rotation with straw returning;SW: single wheat plantation without straw returning),to evaluate the effectiveness of different source of Zn fertilizer [ZnSO4 or Zn(Gly)2] and adding urea in maximizing grain Zn concentration under conventional and reduced soil N supply(15% and 30% under WMs,50% and 100% under SW).Results showed that,foliar application of Zn(Gly)2 significantly increased grain Zn concentration by 88% and 150% under WMs and SW.The Zn concentration and phytic acid: Zn molar ratios following foliar Zn(Gly)2 treatment was similar with that in the foliar ZnSO4 treatment.Adding urea in foliar ZnSO4 or Zn(Gly)2 further increased grain Zn concentration by 13.5% and 4.7% under different soil N levels at the site with WMs,and by 10.7% and 7.5% when soil N supply was reduced by 100% at the site with SW.Additionally,combined foliar application of urea with Zn fertilizers also increased grain protein concentration as well as Fe concentration and bioavailability.Significant decrement of Zn concentration with the reduction of soil N supply was observed when foliar ZnSO4 was applied.However,reducing the N supply by 15% under WMs and by 50% under SW did not decrease grain Zn concentration when foliar Zn(Gly)2 was applied;reducing the N supply by any rate under WMs and SW did not decrease grain Zn concentration when urea was added in the foliar ZnSO4 application.Therefore,foliar application of Zn(Gly)2 or ZnSO4 plus urea are recommended to maximize the grain Zn concentration under reduced soil Nsupply.Overall,the targeted levels of Zn in wheat grain for a better human nutrition(exceeding 40 mg/kg)can be achieved substantially by combined foliar application of Zn with N,P,or K under different field management practices.Combined foliar application of Zn with N(under WMs)or with K(under SW)increased leaf Zn uptake as well as the mobilization of post-anthesis shoot Zn uptake to grain,which thereby displayed significant synergistic effect on grain Zn concentration.Also,the decrease of grain Zn concentration caused by applying reduced soil N can be restored by adding N(urea)in foliar Zn application.Root P supply had little effect on grain Zn response to foliar Zn application.However,combined foliar application of Zn with P decreased leaf Zn uptake as well as the mobilization of post-anthesis shoot Zn uptake to grain,which thereby displayed significant antagonism effect on grain Zn concentration.The increase of insoluble Zn concentration and proportion in leaf and glume,which led to significant decrease of soluble Zn concentration(decrease in physiological Zn availability),might be the reason of significant decrease in shoot Zn uptake remobilization and Zn accumulation in grain.Thus,concurrent foliar applications of Zn with N or K could bepreferred agronomic approaches to fortify wheat grain with target Zn levels,while measures are needed to overcome the decrease of grain Zn concentration when Zn was foliarly-applied with P. |
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