Ways of combined application of nitrogen and potassium fertilizers to enhance the photosynthetic capacity of summer maize

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 field experiment was conducted in Qinyang City, Henan Province, China, using a completely randomized block design with a two-factor (nitrogen and potassium) arrangement. The experiment comprised four nitrogen levels (N 0, 90, 180, and 270 kg/hm², denoted as N0, N90, N180, and N270) and three potassium levels (K₂O 0, 60, and 120 kg/hm², denoted as K0, K60, and K120), resulting in 12 treatment combinations. Measurements included nutrient uptake at various growth stages of summer maize, photosynthetic parameters and canopy temperature at the large trumpet and grain-filling stages, chloroplast ultrastructure, and photosynthetic enzyme activities, as well as grain yield at maturity. Photosynthetic nitrogen use efficiency (PNUE), photosynthetic potassium use efficiency (PKUE), and fertilizer use efficiency were subsequently 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 index, 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 the canopy temperature showed a decreasing trend. 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 net photosynthetic rate (Pn), intercellular CO₂ concentration (Ci), transpiration rate (Tr), and stomatal conductance(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 maximum quantum yield of PSⅡ photochemistry (Fv/Fm), actual (effective) quantum yield of PSⅡ photochemistry (ΦPSⅡ), photochemical quenching coefficient (qP,), and electron transport rate (ETR) were significantly increased, while the non-photochemical quenching coefficient (NPQ) decreased. Potassium application had a stronger effect on improving apparent quantum efficiency, light compensation point, and light saturation point (AQE, LCP, and LSP), whereas nitrogen application was more effective in increasing maximum net photosynthetic rate (Pn_max). Additionally, the NK co-application significantly enhanced the activities of ribulose-1,5-bisphosphate carboxylase/oxygenase, ADP-glucose pyrophosphorylase, and sucrose phosphate synthase (RuBisCO, AGPase, and SPS), promoting carbon fixation and assimilation. The NK co-application 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 K-only application. 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 N 180 kg/hm² and K₂O 120 kg/hm².