Effect of Substrate Moisture Content on the Growth of an Exotic Species, Myriophyllum aquaticum

In this study, we investigated how substrate moisture content affects the growth performance and adaptive responses of Myriophyllum aquaticum. Through a controlled simulation experiment, we systematically analyzed the morphological characteristics and physiological responses of plants under five moisture levels: 0–15%, 15–30%, 30–45%, 45–60%, and 60–75%. The results indicate that optimal growth of M. aquaticum occurred at a substrate moisture content of 60–75%, with significant increases in plant height, branching ability, and biomass. A drought acclimation response was triggered at moisture levels ≤45%, characterized by shortened root length, increased total senescent internode length, biomass allocation shift toward aboveground parts, decreased chlorophyll a content, and elevated accumulation of malondialdehyde. Plants died at moisture levels ≤15%. However, they survived at 15–30% moisture, although their biomass continued to decline. A key finding was that under conditions where the sediment surface lacked water but the substrate moisture remained at 60–75%, plants achieved efficient water utilization and canopy reconstruction through rapid root extension and stem node proliferation, and the relative growth rate was significantly higher than that of the drought group (≤45% moisture). This strong adaptive capacity under specific water conditions, combined with its dehydration tolerance, suggests that M. aquaticum could potentially have a competitive advantage over native submerged plants that rely on stable water bodies, particularly in hydrologically fluctuating habitats. This study revealed that morpho-physiological plasticity driven by water gradients may be a key mechanism contributing to the invasive potential of M. aquaticum, providing new insights into its possible expansion potential in zones with fluctuating water levels.