A nonlinear synergy‐to‐antagonism shift between superabsorbent polymers and drip irrigation governs canopy persistence, grain‐filling strategy, and yield in drip‐irrigated winter wheat

Water scarcity destabilizes winter wheat ( Triticum aestivum L.) yield by disrupting the coordination between post‐anthesis carbon (source) persistence and grain (sink) filling. Superabsorbent polymers (SAPs) buffer root‐zone moisture, yet their interaction with drip irrigation may be nonlinear: excessive hydrogel expansion under high water supply can impair soil aeration. We hypothesized that SAP × irrigation responses shift from synergy to antagonism once a dose‐dependent soil physical threshold is exceeded. A 2‐year field experiment (2022–2024) tested three irrigation levels (deficit, moderate, and sufficient) and four SAP rates (0, 15, 30, and 45 kg ha −1 ; p0–p3). Grain yield responded unimodally to SAP rate across all regimes: p2 increased yield by 11.4%–21.4% relative to p0, whereas p3 reduced yield by 2–7 percentage points below the p2 peak. Under moderate irrigation, p2 optimized population stability and canopy physiological persistence, shifting the grain‐filling strategy from an “acceleration‐only” to a “sustained‐efficiency” model by extending the active filling duration and increasing late‐stage contribution. Conversely, under sufficient irrigation, p3 induced a threshold effect whereby reduced soil porosity and possible root‐zone hypoxia may have constrained gas exchange and impaired grain‐filling efficiency. Our findings demonstrate that SAP–irrigation synergy is governed by a trade‐off between moisture retention and soil aeration. A moderate SAP dose (30 kg ha −1 ) optimizes the soil–canopy–grain cascade, providing a framework for precision soil management to stabilize wheat yields in water‐limited regions.