Aims <p>Understanding the coordinated patterns of plant growth and defense traits along environmental gradients is crucial for deciphering plant adaptive strategies under varying conditions. However, previous studies have extensively documented examining single or pairwise trade-offs among defense traits, leaving a critical gap in understanding how environmental stresses influence co-variation in multiple defense traits from the perspectives of trait integration and resource pools.</p> Methods <p>A field study was conducted in a freshwater marsh to investigate plant defense traits of a flooding-sensitive plant species <i>Deyeuxia angustifolia</i> along natural water level gradients. We aimed to identify the adaptive defence strategies of <i>D. angustifolia</i> along the gradients, and assess their correlation with concomitant variations in soil nutrient availability across the hydrological sequence.</p> Results <p><i>Deyeuxia angustifolia</i> populations clustered into two distinct defense syndromes along the water level gradient. At lower water levels, a high-growth syndrome with enhanced mechanical defenses (e.g., cellulose, lignin) was observed, whereas at higher water levels, a syndrome dominated by chemical defenses (tannins and phenols) emerged. These syndromes were jointly shaped by water depth and soil nitrogen. Flooding stress directly regulated chemical defenses and structural defenses, while indirectly influenced foliar carbon: nitrogen ratios via altering soil nutrient conditions.</p> Conclusions <p>Our findings underscore environment-dependent defense syndromes and strategic trade-offs between plant defense traits, reflecting key adaptive strategies to hydrological heterogeneity. By characterizing the defense syndromes, this study provides critical insights into trait coordination mechanisms in freshwater marsh ecosystems.</p>