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

With the increasing threat of destruction to diminishing seagrass resources, 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 increased irradiance. Leaf tissue was exposed to the following light treatments: 100, 500 and 1000 mu mol m(-2) s(-1) for 10, 20, 40, 60 and 120 min. Exposure to the higher irradiances resulted in significant photoinhibitory responses for both fluorescence and oxygen evolution measurements. There was evidence of both photoinhibitory responses; photoprotection and photodamage. Photoprotective processes appear to be operating 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 included an elevated initial fluorescence, accompanied by a reduction in the variable/maximum fluorescence ratio, maximum fluorescence and variable fluorescence. Photodamage appeared to be involved after 120 min exposure at 1000 mu mol m(-2) s(-1), where all fluorescence parameters including initial fluorescence were quenched and the maximum oxygen evolution rate saturated at a lower irradiance. A direct linear relationship between variable/maximum fluorescence ratio and the light saturated photosynthetic rate was demonstrated for H. ovalis. The oxygen evolution data corroborate the evidence of the stress responses indicated with 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 of chlorophyll a fluorescence to assess the relative photoinhibitory response of H. ovalis to short-term exposure to increased irradiance.