Coupling photosynthetic physiology and C₄ enzyme regulation enhances grain yield of intercropped maize under no-tillage and moderate irrigation in oasis regions

Background The Hexi Oasis irrigation area is endowed with abundant light and heat resources, making it suitable for intercropped maize systems. However, after the traditional “high-water-consumption–high-yield” pathway under conventional tillage and full irrigation was disrupted, studies on how photosynthetic physiology and the expression of key C₄ enzymes synergistically compensate for yield losses under limited water supply remain scarce. Methods To address this gap, a field experiment was conducted in 2024 to systematically evaluate the coupled effects of tillage (no-tillage, NT; conventional tillage, CT), cropping pattern (intercropping, IM; monocropping, SM), and three irrigation regimes (low, I1; medium, I2; high, I3) on maize grain yield, photosynthetic physiology, and key enzyme regulation. Results The results showed that the NT × IM × I2 combination achieved a grain yield of 12,400 kg·hm⁻² at 14% moisture in 7 m² yield plots, representing significant increases of 10.5% and 27.2% compared with CTIMI2 and NTSMI2, respectively, while reducing irrigation by 8.3% relative to I3. During the silking–grain filling stage, this treatment maintained the highest SPAD, Pn, Gs, and Y(II), along with the lowest Ci and Y(NO). Enzyme activities of phosphoenolpyruvate carboxylase, ribulose-1,5-bisphosphate carboxylase/oxygenase, and pyruvate phosphate dikinase increased by 6–11%, 8–10%, and 9–14%, respectively, with corresponding gene expression upregulated by 31–80%. Structural equation modeling indicated that the standardized path coefficient and explained variance of the photosynthesis–enzyme coupling on yield reached 0.977 and 94.8%, respectively. Conclusion In summary, no-tillage combined with moderate irrigation enhanced intercropped maize yield stability under limited water supply through a dual mechanism of “photosynthetic performance maintenance + C₄ enzyme activity/transcription enhancement.”