Growth-defense syndromes of a flooding-sensitive plant species in temperate freshwater marshes

Aims Exploring covariation patterns of plant growth and defense traits along environment gradients can provide key insights into plant strategies coping with environmental variability, including fluctuation in nutrient availability. However, previous studies have primarily focused on single or pairwise trade-offs among plant defense traits, leaving a critical gap in understanding how environmental stresses influence co-variation in plant defense traits in perspective of trait integration and source pools. Methods In this study, we conducted a field survey in a freshwater marsh in northeastern China to investigate plant defense traits of the flooding-sensitive plant species Deyeuxia angustifolia, as well as soil nutrient availability, along a water level gradient. We aimed to test for multivariate defense syndromes along the gradient, and determine whether these syndromes correlate with soil nutrient availability. Results We found that D. angustifolia populations clustered into two distinct syndromes along the water level gradient. The population exhibited a high-growth strategy with enhanced mechanical defenses (e.g., cellulose, lignin) under lower water levels, but chemical defense strategy dominated tannins and phenols under higher water levels. These syndromes were jointly shaped by water depth and soil nitrogen availability. Flooding stress directly upregulated chemical defenses and structural defenses, while indirectly modulating foliar carbon: nitrogen ratios via altering soil nutrient availability. Conclusions Our results exhibit environment-dependent defense syndromes and trade-offs between plant defense traits, reflecting plant adaptative strategies to hydrological heterogeneity. Moreover, this study provides critical insights into trait coordination mechanisms in freshwater marsh ecosystems.