Differences in the response of grain zinc concentration to nitrogen application between high- and low-zinc winter wheat cultivars and the underlying mechanisms

Objectives The breeding of high-yielding and high-zinc (Zn) wheat cultivars is an important approach to meet the dual demands of grain supply and Zn nutritional quality for residents. We investigated the differences in grain Zn content response to nitrogen application between high- and low-Zn wheat cultivars, as well as the underlying mechanisms.

Methods A pot experiment was conducted using a calcareous soil with a pH of 8.55 and available Zn content of 0.58 mg/kg. Four high-yielding wheat cultivars with comparable yield performance but contrasting grain Zn concentrations‌ were selected as experimental materials. Two treatments were set: no nitrogen application and nitrogen application at a rate of 200 kg/hm². Rhizosphere soil, non-rhizosphere soil, and plant samples were collected at the jointing, anthesis, grain-filling, and maturity stages. We determined‌ soil pH, fractions of different Zn forms, and available Zn content; ‌analyzed‌ biomass and Zn concentrations in roots, stems and leaves, spikes, and grains; and calculated key indices including the rhizosphere Zn activation index, root Zn uptake efficiency, root-to-shoot and stem-leaf-to-spike Zn transfer coefficients, ‌and‌ Zn harvest index at maturity, ‌along with their responses to N fertilization‌.

Results After nitrogen application, the grain Zn content of high-Zn cultivars increased from 29.1 mg/kg to 42.8 mg/kg (a 48% increase), while that of low-Zn cultivars rose only from 21.1 mg/kg to 23.4 mg/kg (an 11% increase). Compared with no nitrogen application, the high-Zn cultivars exhibited 101%, 151%, and 108% higher Zn uptake in the shoot at anthesis, grain-filling, and maturity stages, respectively. Root Zn acquisition efficiency increased by 26.1% and 53.1% at anthesis and grain-filling stages, and the root-to-shoot Zn transfer index increased by 78.6% during grain-filling—all these increments were statistically significantly larger than those observed in low-Zn cultivars‌. N application reduced rhizosphere soil pH, with decreases of 0.08−0.12 in high-Zn cultivars and 0.07−0.09 in low-Zn cultivars. The rhizosphere Zn activation index of high-Zn cultivars increased by 17.1%, 33.5%, and 26.0% at jointing, anthesis, and grain-filling stages, whereas that in L_Zn1 cultivar increased only by 8.8% at anthesis, and even decreased by 9.2% during grain-filling. The available Zn content in the rhizosphere of high-Zn cultivars increased by 30.9%−67.4%, far exceeding the 3.1%−26.0% increase in low-Zn cultivars. In addition, after nitrogen application, the rhizosphere ammonium and nitrate nitrogen content in high-Zn cultivars increased by 1.96−3.65-fold and 24.7−30.9-fold, respectively, both greater increases than in low-Zn cultivars.

Conclusions On Zn-deficient calcareous soils, nitrogen application significantly enhances grain Zn content in wheat, and high-Zn wheat cultivars are more responsive to nitrogen fertilizer. The primary mechanism lies in the more pronounced rhizosphere acidification in high-Zn cultivars after nitrogen application, which enhances Zn mobilization capacity—especially during the anthesis and grain-filling periods—greatly improving the supply of bioavailable Zn. Meanwhile, the greater improvements in root Zn acquisition and root-to-shoot translocation capacity lead to significantly higher Zn accumulation in the shoot and ultimately in the grain.