Light-harvesting nitrogen depletion, more than carboxylation limitation, constrains photosynthetic carbon gain under combined water and nitrogen stress in drip-irrigated maize

Context Water and nitrogen are the two primary resources limiting maize productivity in arid and semi-arid irrigated regions, yet their interactive effects on the internal partitioning of leaf nitrogen among photosynthetic functional pools, and the consequences of such partitioning for photosystem II (PSII) structural integrity, electron transport efficiency, and ultimately net photosynthesis and grain yield, remain inadequately resolved. Methods A two-year field experiment (2023–2024) was conducted on drip-irrigated spring maize ( Zea mays L.) under three irrigation lower limits (W1: 85% θ f , W2: 70% θ f , W3: 55% θ f , where θ f is the field capacity) and four nitrogen rates (N1-N4: 0, 90, 180, 270 kg N ha −1 ). Leaf nitrogen was partitioned into Rubisco (N R ), PEPC (N P ), bioenergetics (N B ), light-harvesting (N LH ), and structural (N S ) pools using A-C i curve fitting coupled with the C4 nitrogen allocation model. Photochemical performance was assessed through OJIP transients and JIP-test parameters. Causal pathways were quantified via partial least squares structural equation modelling (PLS-SEM), random forest regression, and LMG relative importance decomposition. Results Severe water and nitrogen deficits (W3N1) reduced A by 51.2%, with the limitation shifting from stomatal to non-stomatal between W2 and W3: C i declined from W1 to W2 but rose again under W3 as A decreased further. The OJIP analysis revealed concurrent oxygen-evolving complex (OEC) inactivation on the PSII donor side and QA - →QB - electron transport blockage on the acceptor side as primary damage sites. The performance index (PI abs ) declined by 57.4%, far exceeding the 11% reduction in F v /F m , indicating that stress-induced impairment extended beyond reaction centers into the downstream electron transport chain. Among the five nitrogen pools, N LH showed the greatest reduction under severe drought, whereas N S was preferentially maintained, providing the structural basis for the photochemical bottleneck. Multi-model analyses consistently demonstrated that the light-reaction pathway exerted a substantially stronger influence on A than the carboxylation pathway, with PI abs and N LH identified as the dominant determinants of photosynthetic performance. Nitrogen application reached saturation at 180 kg N ha −1 , and further addition failed to improve photosynthesis while reducing photosynthetic nitrogen use efficiency (PNUE), indicating that impairment of PSII donor-side function under severe water deficit prevents further nitrogen from being expressed as photochemical capacity. Implications Photosynthetic limitation under water-nitrogen stress in drip-irrigated maize arises predominantly from light-reaction damage (OEC inactivation and light-harvesting nitrogen depletion), while carboxylation limitation contributed a smaller but significant component. The W2N3 preserved PSII function and matched full-irrigation yield, supporting precision management. Nitrogen allocation efficiency, not total nitrogen, governs photosynthesis under stress, highlighting PSII protection as central to improving resource use efficiency.