Objectives We investigated the iron (Fe) concentration of wheat grain and flour in the major wheat production regions of China, trying to find out the main factors affecting the Fe concentration and to provide a basis for wheat production with high Fe nutrition.

Methods During 2020-2021 and 2021-2022 wheat growing seasons, 424 wheat and soil samples were collected from 17 provinces and autonomous regions of China, the Fe concentration in wheat grain, flour and bran were analyzed, and their relationship to wheat yield, yield components and soil properties were discussed.

Results The average Fe concentration of the wheat grain, flour and bran was 38.8, 13.9 and 86.7 mg/kg, respectively. 90.3% of grain and 93.9% of flour samples did not meet the recommended Fe concentration (50 mg/kg for grain, and 21 mg/kg for flour). Among the influence factors, higher soil organic matter (37.4 g/kg) and total N concentrations (1.90 g/kg) increased soil Fe availability, that promoted the Fe uptake and transfer to grain and finally increased Fe concentration in the spring wheat grain and flour to be as high as 41.4 and 15.4 mg/kg, respectively; The harvest index, grain number per spike and 1000-grain weight in the moderate Fe group were reduced by 13.1%, 32.2% and 31.3%, respectively, compared to the low Fe group. In dryland wheat region, the soil organic matter (19.7 g/kg) and available Fe concentrations (8.9 mg/kg) were the lowest among the four wheat regions, this was not conducive to the uptake and accumulation of Fe in wheat, therefore the Fe concentration is lowered in the grain and flour, with average respectively being 39.4 and 14.3 mg/kg. In wheat-maize regions, the yield was significantly higher than that of other wheat regions, consequently the Fe concentrations of grain and flour were decreased due to the "dilution effect" of yield increase, and it was the lowest respectively as 36.6 and 12.7 mg/kg, compared to other regions. In rice-wheat region, the soil available Fe concentration (133.4 mg/kg) is obviously higher and the soil pH (6.6) is remarkably lower than that of others, and this led the wheat grain Fe concentration to be the highest among all the regions, and easier to distribute and accumulate in the flour, with finally the Fe concentration in grain and flour respectively being 41.3 and 15.6 mg/kg; The spike number and grain number per spike in the moderate Fe group were increased by 31.5% and 15.7%, and the 1000-grain weight was decreased by 14.7%, respectively.

Conclusions Suitable soil pH, high organic matter, total nitrogen and available Fe content play vital roles for high Fe nutrition of wheat grain and flour. In addition, comprehensive measurements should be considered to synchronously improve the spike number, grain number per spike, and 1000-grain weight, to envied the possible dilution effect.