| Zinc (Zn) and iron (Fe) deficiencies are widespread nutritional disorders. Insufficient dietary intakes of Zn and Fe and limited dietary diversity are thought to be responsible for human micronutrient deficiencies, especially in developing countries where cereals constitute the major part of the diet.Nitrogen (N) supply is an important factor for improving grain Zn and Fe concentrations of wheat (Triticum aestivum L.) and maize (Zea mays L.). There are various physiological steps involved in the route of Zn from soil to grain, including root uptake, root-to-shoot translocation and remobilization of Zn. However, there was little information regarding how different N levels affected these physiological processes. In this study, a series of field experiments were conducted on winter wheat and summer maize to1) examine the effects of different N supplies on root uptake, root-to-shoot translocation, and remobilization of Zn from vegetative tissues into developing grain tissues,2) to quantify Zn, Fe, manganese (Mn) and copper (Cu) requirements (g of nutrient requirement in plant dry matter per Mg grain yield) in response to increased grain yields. The main results were summarized as follows:1. Results from a4-year experiment of winter wheat and summer maize, respectively showed that compared with no N application (the control), grain Zn, Fe and Cu concentrations of wheat increased1.1-10.2,1.6-5.8and0.5-1.1mg kg-1, respectively and their corresponding increasing percentages were4.3-41.4%,6.1-22.6and16.7-34.3%with N supplies. Compared with the control, grain Zn, Fe, Mn and Cu concentrations of maize increased0.2-3.1,1.3-3.6,0.0-0.6and about0.5mg kg-1, respectively and their corresponding increasing percentages were1.3-20.8%,9.3-25.8%,0.4-19.7%and about44.7%, respectively with N supplies. However, compared with the control, grain phosphorus (P) concentration of wheat and maize decreased0.22-0.73and0.18-0.20g kg-1respectively and their corresponding decreasing percentages were5.9-19.7%and8.4-9.2%with N supplies. Grain Mn concentration of wheat decreased2.6-4.9mg kg-1with its decreasing percentage being7.9-14.7%with N supplies. Grain potassium (K), magnesium (Mg) and calcium (Ca) concentrations of wheat and maize were less affected by different N supplies.2. Increasing N supply improved root length (RL), root surface area (RSA) and root dry weight (RDW) of wheat and maize, thus resulting in an improved ability to take up Zn from the soil. During the growing period of wheat,88-96%of RL,85-94%of RSA,80-92%of RDW and85-96%of root Zn uptake were recovered from the upper30cm soil layer. Additionally, root Zn concentration in the upper0-20cm or0-30cm depth of soil was significantly higher than in the30-60cm layer of soil. These results suggest a good spatial matching between root spatial distribution and root Zn uptake, especially in the upper30cm plough layer.3. For wheat, the percentages of shoot Zn content to total Zn content (the sum of shoot Zn content and root content) were55-79%,61-87%and83-90%at joining, anthesis and maturity, respectively. During the growing period of maize, the percentage of shoot Zn content to total Zn content was up to89-98%. These results indicate a strong Zn transport from root to shoot in wheat, especially in maize, which was not a limiting factor for grain Zn accumulation. Total Zn content, shoot Zn content and the ratios of shoot to root Zn concentrations were improved with increasing N supply, indicating that N supply improved both root uptake and root-to-shoot translocation of Zn.4.67-100of wheat grain Zn accumulation was provided by Zn remobilization from pre-anthesis vegetative tissues with N supplies. There was significant positive correlation between straw Zn and N remobilization from vegetative tissues to grains during grain filling stage (R2=0.59***). These results suggest N supplies improved Zn remobilization from vegetative tissues to grain. However,74-99%of maize grain Zn accumulation was provided by post-silking root Zn uptake (0.95-2.32mg plant-1). Moreover, the post-silking root Zn uptake and its contribution to grain Zn accumulation were increased with increasing N supply.5. Nitrogen supply increased the concentrations of total and the portions of Zn and Fe unextractable with a Tris-HCl buffer (pH7.5), but decreased the concentrations of Tris-HCl-extractable Zn and Fe of wheat grain tissues. Size-exclusion chromatography coupled with inductively-coupled plasma mass spectrometry (SEC-ICP-MS) was used to determine Zn and Fe speciation in the soluble extracts of grain tissues. Within the soluble fraction, Zn and Fe bound to low molecular weight compounds, likely to be Zn-nicotianamine (NA) or Fe-NA and Fe-deoxymugineic acid (Fe-DMA) were decreased by4-37%and5-12%, respectively, by high N treatment. Zn and Fe bound to soluble high molecular weight or soluble phytate fractions were less affected. These results suggest that NA and/or DMA are only responsible for Zn and Fe transport into grain where Zn and Fe are then sequestered in other forms.6. Based on a four-year experiment of winter wheat(n=104), with increasing grain yield (<6,6-7.5,>7.5Mg ha-1), the Zn requirement per Mg grain yield decreased from36.5to28.9g, while Fe and Mn increased from130.2to151.2g and49.6to55.0g, respectively. Cu requirements per Mg grain yield were5.0,6.3and5.0g, respectively. The decrease in Zn requirement was attributed to a decrease in Zn concentrations of grain (from28.1to22.9mg kg-1) and straw (from12.5to10.5mg kg-1).7. Based on a four-year experiment of summer maize (n-149), with increasing grain yield (<7.5,7.5-9,9-10.5and>10.5Mg ha-1), the requirements per Mg grain yield for Zn, Fe and Cu decreased from36.3to18.0g,68.6to57.4g and5.6to4.9g, respectively, while Mn increased from17.8to19.0g. The decrease in Zn requirement was attributed to the increase in Zn harvest index (from41to60%) and the decrease in Zn concentrations of grain (from17.4to12.2mg kg-1) and especially straw (from24.4to10.7mg kg-1... |
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