LIGHT RESPONSE OF D1 TURNOVER AND PHOTOSYSTEM-II EFFICIENCY IN THE SEAGRASS ZOSTERA-CAPRICORNI

Biochemical and biophysical parameters, including D1-protein turnover, chlorophyll fluorescence, oxygen evolution activity and zeaxanthin fo rmation were measured in the marine seagrass Zostera capricorni (Asche rs) in response to limiting (100 mu mol . m(-2). s(-1)), saturating (3 50 mu mol . m(-2). s(-1)) or photoinhibitory (1100 mu mol . m(-2). s(- 1)) irradiances. Synthesis of D1 was maximal at 350 mu mol . m(-2). s( -1) which was also the irradiance at which the rate of photosynthetic O-2 evolution was maximal. Degradation of D1 was saturated at 350 mu m ol . m(-2). s(-1). The rate of D1 synthesis at 1100 mu mol . m(-2)s(-1 ) was very similar to that at 350 mu mol . m(-2).(s-1) for the first 9 0 min but then declined. At limiting or saturating irradiance little c hange was observed in the ratio of variable to maximal fluorescence (F v/Fm) measured after dark adaptation of the leaves, while significant photoinhibition occurred at 1100 mu mol . m(-2). s(-1). The proportion of zeaxanthin in the total xanthophyll pool increased with increasing irradiance, indicative of the presence of a photoprotective xanthophy ll cycle in this seagrass. These results are consistent with a high le vel of regulatory D1 turnover in Zostera under non-photoinhibitory irr adiance conditions, as has been found previously for terrestrial plant s.