The effect of temperature and irradiance on the photosynthesis of a seagrass, Halodule uninervis, from Amami-Oshima Island, Kagoshima

The effect of temperature and irradiance on the photosynthesis of a tropical seagrass, Halodule uniner-vis, from Amami-Oshima Island, Kagoshima, Japan, close to the northern distributional limit in the western Pacific, was determined using a pulse amplitude modulation (PAM)-chlorophyll fluorometer and dissolved oxygen sensors. The response of oxygenic photosynthesis to irradiance (0-1000 mu mol photons m-2 s-1) at 15 and 24celcius revealed that the maximum net photosynthesis (NPmax) was 14.7 and 51.9 mu g O2 gww-1 min-1, re-spectively. The temperature response of gross photosynthesis (8-40celcius ) showed that the highest gross photo-synthetic rate occurred at 30.5celcius (TGPopt). In contrast, the temperature response of the maximum quantum yield (Fv /Fm) of photosystem II after 3 days of cultivation in darkness revealed that Fv/Fm gradually in-creased to the optimal temperature (TFv /Fm opt ) of 23.9celcius, then gradually declined at higher temperatures. The combined effects of 12-hour chronic exposure to temperature and irradiance at 15 and 24celcius, under 300 and 1000 mu mol photons m-2 s-1showed that the depression of Fv/Fm was more pronounced under low tempera-ture with high irradiance, indicating the occurrence of chronic low temperature-light stress.