Objectives Both nitrogen and zinc are essential nutrients for the growth, yield and quality of crops. We studied the root morphology and physiological characteristics of wheat under combined application of nitrogen and zinc, to deeply understand the mechanism of the yield and quality effects of N and Zn application.

Methods Field experiments were conducted in the farm of Henan University of Science and Technology from 2016 to 2018, using wheat cultivar of ‘Luomai 28’ as tested materials. A complete design of 2 factors and three levels was used. The three N levels were N 120, 180 and 240 kg/hm² (expressed as N120, N180, N240), and those for Zn (ZnSO₄·7H₂O) were 0, 20 and 40 kg/hm² in turn (expressed as Zn0, Zn20, Zn40). At the stages of jointing, booting, filling and maturity, soil pillars of 20 cm × 20 cm × 40 cm were dug up, and the roots inside were washed out for measurement of root dry weight, root length, root surface area, N metabolism related enzyme activities, and Zn and N contents. At maturity, yield and its components were investigated.

Results In the same N application level, the root dry weight, the root length, the root surface area, and the activities of nitrate reductase, glutamine synthetase, indoleacetic acid oxidase and the concentrations of N and Zn at main growth stages were all highest under Zn application level of 20 kg/hm², while ribonuclease activity was the lowest. In the same Zn application level, the root length, the root surface area, and the activities of nitrate reductase, glutamine synthetase, indoleacetic acid oxidase and the concentrations of N and Zn at main growth stages were all highest under the N application level of 180 kg/hm², while ribonuclease activity was the lowest. N and Zn had significant synergistic effect on yield, root dry weight, root length, root surface area, nitrate reductase activity, glutamine synthetase activity, indoleacetic acid oxidase activity, N content and Zn content. In the treatment of middle N and Zn combination, the number of panicle per hectare, grains per panicle and 1000-grain weight of wheat were the highest, and the yield increased by 3.5%–53.4% (2016–2017), 5.3%–54.5% (2017–2018). The grain yield was positively correlated with the root dry weight, the root length, the root surface area, and the activities of nitrate reductase, glutamine synthetase, indoleacetic acid oxidase and the concentrations of N and Zn. And the grain yield was negatively correlated with the activity of ribonuclease.

Conclusions N and Zn fertilizers significantly affect wheat root dry weight, root length, root surface area, nitrate reductase activity, glutamine synthetase activity, indoleacetate oxidase activity, ribonuclease activity, N and Zn contents. Appropriate levels of N and Zn fertilizer have synergistic effect in promoting the establishment of good root morphology, regulating the physiological activity of roots and the absorption of N and Zn nutrients, thus beneficial to the formation of high yield of wheat.The comprehensive analysis shows that the N180Zn20 treatment is the best combination under the experimental conditions.