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This study investigated photosynthetic and oxidative stress-related responses to moderate (500 μmol photons m −2  s −1 ) and high irradiances (1500 μmol photons m −2  s −1 ) in two tropical intertidal seagrasses,  Halophila ovalis  and  Thalassia hemprichii . Exposure to high irradiance for 3 h resulted in a large decline in the maximum quantum yield of photosystem II (Fv/Fm) and in the effective quantum yield of photosystem II (ΦPSII) but induced non-photochemical quenching (NPQ) and chloroplast clumping in  H. ovalis .  Thalassia hemprichii , however, showed only a minor decrease in Fv/Fm and ΦPSII, and no change in either NPQ or the intracellular arrangement of chloroplasts. The irradiance did not affect reactive oxygen species (ROS) or superoxide dismutase (SOD) activity in either seagrass, whereas high irradiance enhanced ascorbate peroxidase (APX) activity in  H. ovalis . The results from a subsequent experiment using an oxidative stress inducer, methyl viologen (MV), also showed a higher degree of Fv/Fm inhibition in  H. ovalis . These results show that  H. ovalis  is much more physiologically responsive to high irradiances and oxidative stress than  T. hemprichii  and that its photoprotective mechanisms involve NPQ, APX and reduction of the amount of light absorbed through the clumping of its chloroplasts.

botanica marina

Photosynthetic and antioxidant responses of the tropical intertidal seagrasses  Halophila ovalis  and  Thalassia hemprichii  to moderate and high irradiances