Context and research question: Water and nitrogen (N) are primary limiting factors for maize productivity in arid and semi-arid regions. However, the interactive effects of water and N availability on the structure-function relationships governing plant hydraulics and photosynthesis remain poorly understood. Methods: A three-year field experiment was conducted to investigate how water-N interactions modify maize stem and leaf anatomy, regulate hydraulic and photosynthetic functions, and ultimately determine grain yield. The experiment comprised three irrigation lower limits (W85, W70, and W55) and four N application rates (N0, N90, N180, and N270). Results: Severe drought (W55) induced a conservative anatomical phenotype, characterized by narrower stem xylem vessels, reduced vessel numbers, and decreased leaf vein density, alongside a significant increase in vascular bundle density. Under adequate moisture (W85 and W70), N supplementation expanded the waterconducting tissues, substantially enhancing both stem-specific hydraulic conductivity and leaf hydraulic conductance. This efficient hydraulic network sustained rapid water transport and high stomatal conductance, thereby maximizing photosynthetic rates. However, under severe drought, the compensatory effect of N was nullified. Specifically, high N application (N270) under W55 stimulated transpirational demand that exceeded the restricted hydraulic supply, exacerbating physiological drought. This led to a sharp decline in midday leaf water potential (below -1.8 MPa) and carbon assimilation capacity, decoupling the positive relationship between hydraulic conductance and photosynthesis. Partial least squares structural equation modeling (PLS-SEM) confirmed that hydraulic conductivity acted as a central mediator, directly driving photosynthetic capacity (beta= 0.981) and grain yield (beta = 0.866). Conclusions: Adequate moisture combined with moderate N application (W85/W70 + N180) optimally balances hydraulic efficiency and transport safety, representing an effective water-N management strategy to maximize maize yield while preventing hydraulic failure under drought stress.