Implications of environmental conditions for health status and biomechanics of freshwater macrophytes in hydraulic laboratories

Submerged freshwater macrophytes are frequently used in hydraulic laboratories to study flow–plant interactions and the role of plants in aquatic ecosystems, but environmental conditions in flume facilities are often suboptimal for plants and can cause plant stress. Physiological responses of plants under stress can trigger modifications in plant biomechanics, which may affect plant–flow interactions and compromise experimental results. In the extreme, dead plants cannot be expected to reveal how live plants interact with flowing water, but stressed plants that are not visibly unhealthy may also affect experimental results. The present work aims to assess if and how environmental conditions typical of flume facilities can impact plant health status and induce variations in plant biomechanics. Using chlorophyll fluorescence analysis, a standard method for assessing plant health, we found that freshwater macrophytes can be significantly stressed under conditions typically found in hydraulic laboratories. Even though the abiotic factors investigated affected different species in different ways, exposure to tap water and low irradiance were the most stressful conditions for freshwater macrophytes. Biomechanical properties with a primary role in flow–plant physical interactions (e.g. flexural rigidity) changed significantly as a result of exposure to stressful conditions. In general, plant stress was associated with a reduction in flexural rigidity at the top of plant stems, suggesting a potential effect on plant hydrodynamics when leaves and petioles are considered. The maximum quantum yield of photosystem II, used as proxy of plant health status, was positively correlated with flexural rigidity of plant stems.