Boosting maize yield under water reduction: synergistic effects of increased planting density and sustained nitrogen input on photo–physiological characteristics

Ensuring an adequate grain yield to meet global food demand is a critical challenge for agricultural production. Close planting is an important agronomic practice for increasing crop yield, but excessively high density can reduce photosynthesis per plant and limit resource use efficiency. This study investigates whether increased planting density can sustain maize grain yield by enhancing photo–physiological traits under reduced irrigation while maintaining nitrogen input. A long-term split-split plot field experiment was conducted in 2016 in the arid irrigated region of northwestern China. Treatments included two irrigation quotas (the local conventional irrigation, W2, 4050 m 3 ha −1 ; reduced irrigation by 20 %, W1, 3240 m 3 ha −1 ), two nitrogen application rates (conventional rate, N2, 360 kg ha −1 ; a25 % reduced rate, N1, 270 kg ha −1 ), and three planting densities (75000 plants ha −1 , D1; +30 %, 97500 plants ha −1 , D2; +60 %, 120000 plants ha −1 , D3). Measurements in 2020–2021 included grain yield, gas exchange parameters, chlorophyll content, photosynthetic enzyme activity, and relative expression of functional genes. Maize yield declined under reduced water and nitrogen inputs, but a 30 % increase in planting density (D2) compensated for yield losses. Under reduced irrigation with maintained nitrogen input, D2 increased grain yield and biomass by 12.9–15.4 % and 4.0–13.3 %, respectively. Grain yield was associated with improvements in net photosynthetic rate (+12.7–28.6 %), transpiration rate (+2.2–9.0 %), chlorophyll a content (+4.0–28.8 %), chlorophyll a /b ratio (+12.8–28.3 %), phosphoenolpyruvate carboxylase activity (+6.5–10.2 %), and relative expression of the pepc gene (+11.3–19.9 %). Overall, increasing planting density by 30 % while maintaining nitrogen input alleviated the decline in photosynthetic performance per plant under reduced irrigation, thereby sustaining population-level maize yield in arid irrigated systems. • Increased density by 30 %(D2) compensated maize yield loss under water and N reduction. • D2 increased gas exchange parameters in water reduction and N remained unchanged(N2). • N2D2 enhanced chlorophyll content and chlorophyll a /b under water reduction(W1). • W1N2D2 promoted photosynthetic key enzyme activity and gene expression. • W1N2D2 increased maize yield by improving the photo–physiological characteristics.