PHOTOSYNTHETIC RESPONSES OF THE SEAGRASS HALOPHILA-OVALIS (R BR) HOOKF TO HIGH IRRADIANCE STRESS, USING CHLOROPHYLL-A FLUORESCENCE

With the increasing threat of destruction to diminishing seagrass reso urces, there is a need for an accurate stress monitoring procedure to be developed for species of this community. In this study chlorophyll a fluorescence (20 degrees C) was used to monitor the stress responses of the seagrass Halophila ovalis (R.Br.) Hook. f. when exposed to inc reased irradiance. Leaf tissue was exposed to the following light trea tments: 100, 500 and 1000 mu mol m(-2) s(-1) for 10, 20, 40, 60 and 12 0 min. Exposure to the higher irradiances resulted in significant phot oinhibitory responses for both fluorescence and oxygen evolution measu rements. There was evidence of both photoinhibitory responses; photopr otection and photodamage. Photoprotective processes appear to be opera ting in all 500 mu mol m(-2) s(-1) treatments, and in the 1000 mu mol m(-2) s(-1) treatments for at least the first 60 min. Responses includ ed an elevated initial fluorescence, accompanied by a reduction in the variable/maximum fluorescence ratio, maximum fluorescence and variabl e fluorescence. Photodamage appeared to be involved after 120 min expo sure at 1000 mu mol m(-2) s(-1), where all fluorescence parameters inc luding initial fluorescence were quenched and the maximum oxygen evolu tion rate saturated at a lower irradiance. A direct linear relationshi p between variable/maximum fluorescence ratio and the light saturated photosynthetic rate was demonstrated for H. ovalis. The oxygen evoluti on data corroborate the evidence of the stress responses indicated wit h the fluorescence results. Generally, the longer the exposure period or higher the irradiance, the greater the damage to the tissue and the associated stress symptoms. These results demonstrate the potential o f chlorophyll a fluorescence to assess the relative photoinhibitory re sponse of H. ovalis to short-term exposure to increased irradiance.