| Zinc(Zn)deficiency is a serious and worldwide threat to human health.A main cause of Zn deficiency is the low intake of Zn in the daily diet,especially for people who rely on cereals as staple foods.Although research has shown that phosphorus(P)fertilizer application significantly decreases the concentration of Zn in wheat and maize grain,detailed information on how P application affects Zn transport in wheat and maize is lacking.A long-term field experiment and a pot experiment are therefore conducted to determine how P application affects the movement of Zn from soil to grain under high-yield conditions.The main results were as follows:1.The responses of grain yield in wheat and maize to P application were consistent with a linear-plateau model,i.e.,yield increased as the P application rate increased to 50 and 12.5 kg P ha-1 for wheat and maize,respectively,but then plateaued at higher application rates.The soil Olsen-P concentration at which leaf area index(LAI)and photosynthetic rate(Pn)plateaued was about 17.7 mg kg-1 for wheat and 12.4 mg kg-1 for maize.The soil P application at which root dry weight,root surface area,and root length density plateaued was 50 kg ha-1 for wheat and 25 kg ha-1 for maize.2.P application significantly decreased the Zn concentration in roots and the specific root Zn uptake(total Zn accumulation g-1 root).Zn accumulation in wheat roots increased with increasing P application from 0 to 50 kg ha-1 but decreased as the P application rate exceeded 50 kg ha-1.Similarly,Zn accumulation in maize roots was higher when P was applied at 25 kg ha-1 than at lower or higher rates.3.The negative effects of P application on Zn accumulation in roots were associated with changes in root morphology(root dry weight,root surface area,root length density,and root volume)and in colonization of roots by arbuscular mycorrhizal(AM)fungi.For wheat,the accumulation of Zn in roots and shoots(flowering stage)and in grain(maturity stage)increased as AM colonization increased from 4%to 39%but then decreased at higher levels of AM colonization.In a pot experiment with maize,the decrease in AM colonization caused by P application partly(19%)explained the antagonism between P application and Zn accumulation in roots,especially in a Zn-deficient soil.P application did not affect the ratio of Zn concentration or Zn accumulation roots vs.shoots.4.P application decreased the concentration of Zn in wheat shoots and the concentrations of Zn in all maize tissues.In wheat,P application increased the accumulation of Zn in shoots at the jointing stage;after the flowering stage,Zn accumulation in shoots increased as the P application rate increased from 0 to 50 kg ha-1 but then decreased at higher rates of P application.In maize shoots,P application decreased the accumulation of Zn.5.P application significantly affected the contribution of post-anthesis Zn uptake and post-anthesis Zn remobilization to grain Zn accumulation in wheat and maize.For wheat,grain Zn remobilization from shoot to grain of post-anthesis contributed 47-70%of grain Zn accumulation.Share of grain Zn accumulation provided by remobilization decreased with increasing P application.For maize,Zn uptake of post-anthesis contributed 67-85%of grain Zn accumulation and P application decreased the share of grain Zn accumulation provided by Zn uptake.6.As the P application rate increased,the concentration and bioavailability of Zn in wheat and maize grain and flour decreased.Zn bioavailability was higher in standard flour,bread flour,and refined flour than in grain or coarse flour.P application also increased the ratio of P:Zn,P:Fe,P:Mn,and P:Cu and decreased the bioavailability of micronutrients in maize grain.In conclusion,’P induced Zn deficiency’ mainly results from the negative effects of P on uptake of Zn by roots and on Zn bioavailability in grain.P application decreases Zn bioavailability in grain by decreasing the AM colonization of roots and by increasing the phytic acid concentration in grain. |
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