Physiological and biochemical responses of the salt-marsh plant Spartina alterniflora to long-term wave exposure.

Ecosystem-based flood defence including salt-marsh as a key component is increasingly applied worldwide due to its multifunctionality and cost-effectiveness. While numerous experiments have explored the wave-attenuation effects of salt-marsh plants critical to flood protection, little is known about the physiological and biochemical responses of these species to continuous wave exposure.To address this knowledge gap, we developed a shallow-water wave simulator to expose individual Spartina alterniflora plants to waves in a greenhouse for 8 weeks. S. alterniflora individuals were partially submerged and experienced horizontal sinusoidal motion to mimic plant exposure to shallow water waves. A factorial experiment was used to test the effects of three wave heights (4.1 cm, 5.5 cm and a no-wave control) and two wave periods (2 s and 3 s) on the following key physiological and biochemical plant parameters: plant growth, antioxidant defence and photosynthetic capacity.Comparison of wave treatment and control groups supported our hypotheses that wave exposure leads to oxidative stress in plants and suppresses plant photosynthetic capacity and thereby growth. In response, the wave-exposed plants exhibited activated antioxidant enzymes. Comparison between the different wave treatment groups suggested the wave effects to be generally correlated positively with wave height and negatively with wave period, i.e. waves with greater height and frequency imposed more stress on plants. In addition, wave-exposed plants tended to allocate more biomass to their roots. Such allocation is favourable because it enhances root anchorage against the wave impact.Simulated wave exposure systems such as the one used here are an effective tool for studying the response of salt-marsh plants to long-term wave exposure, and so help inform ecosystem-based flood defence projects in terms of plant selection, suitable transplantation locations and timing, etc. Given the projected variability of the global wave environment due to climate change, understanding plant response to long-term wave exposure has important implications for salt-marsh conservation and its central role in natural flood defence.