Objectives Combined nitrogen (N) and potassium (K) fertilization is a crucial nutrient management strategy for ensuring balanced nutrient supply, and enhancing photosynthetic efficiency and crop yield of summer maize in north China. We studied the effects of N and K co-application on leaf photosynthesis, light and temperature utilization, chloroplast structure, and photosynthetic enzyme activity during growth stages of summer maize.

Methods A two-factor completely randomized design field experiment was conducted in Qinyang city, Henan Province. The experiment included 12 treatments consisting of four nitrogen levels (N0, N90, N180, and N270 kg/hm²) and three potassium (K₂O) levels (K0, K60, and K120 kg/hm²). N and K uptake and accumulation were measured at various growth stages. At the large bell stage (LBS) and grain filling stage (FS), leaf photosynthesis, canopy temperature, chloroplast morphology and structure, and photosynthetic enzyme activities were monitored. At maturity, grain yield was recorded. Photosynthetic nitrogen use efficiency (PNUE), photosynthetic potassium use efficiency (PKUE), and fertilizer use efficiency were calculated.

Results Compared with the N0K0 treatment, combined N and K application increased summer maize yield by 55.6%, with average increases in aboveground N and K accumulation of 105.1% and 100.1%, respectively. These values were also significantly higher than those under single N or K application treatments. The NK co-application treatments increased chlorophyll content, leaf area, plant biomass, PNUE, and PKUE by 20.0%−25.1%, 40.6%−81.6%, 94.3%−112.7%, 39.2%−95.1%, and 54.0%−102.3%, respectively, and reduced canopy temperature by 0.59−1.14°C on average. Compared with single K or N application, NK co-application increased leaf thickness by 10.4%−16.1% and 3.9%−13.0%, respectively. In the N180K60 treatment, mesophyll chloroplasts were uniformly distributed along the cell walls, with clearly visible stroma and thylakoid lamellae. Combined N and K application significantly enhanced leaf photosynthetic rate and photosynthetic enzyme activity. Compared with K-only application, N addition increased Pn, Ci, Tr, and Gs by 18.3%−19.3%, 14.6%−15.2%, 18.0%−20.3%, and 31.7%−52.5%, respectively. Compared with N-only application, K addition increased these parameters by 17.1%−27.6%, 12.3%−45.1%, 14.6%−36.1%, and 32.1%−97.2%, respectively. Photosystem II parameters Fv/Fm, ΦPSII, qP, and ETR were significantly increased, while NPQ decreased. Potassium application had a stronger effect on improving AQE, LCP, and LSP, whereas nitrogen application was more effective in increasing Pn_max. Additionally, NK co-application significantly enhanced the activities of RuBisCO, AGPase, and SPS enzymes, promoting carbon fixation and assimilation. Potassium addition increased the activities of RuBisCO, AGPase, and SPS by 21.6%−41.3%, 27.7%−28.5%, and 11.6%−19.4%, respectively, compared with N-only treatments. Nitrogen addition increased these enzyme activities by 16.2%−34.0%, 12.9%−17.5%, and 11.3%−16.8%, respectively, compared with K-only treatments.

Conclusions Nitrogen fertilizer promoted maize leaf growth and chlorophyll synthesis, while potassium fertilizer enhanced CO₂ supply, strengthened photochemical quenching, facilitated photosynthetic product transport, increased maximum light utilization capacity, and raised the light saturation point. Both fertilizers increased leaf thickness and area, reduced canopy temperature, enhanced the activities of RuBisCO, AGPase, and SPS, improved PNUE and PKUE, ensured normal photosystem II function, and reduced light energy dissipation. An appropriate N:K ratio provided more stable and significant effects, thereby achieving higher yields. Under the experimental conditions, the recommended application rates for summer maize are N180 kg/hm² and K₂O120 kg/hm².