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 formation were measured in the marine seagrass Zostera capricorni (Aschers) in response to limiting (100 micromoles.m-2.s-1), saturating (350 micromoles.m-2.s-1) or photoinhibitory (1100 micromoles.m-2.s-1) irradiances. Synthesis of D1 was maximal at 350 micromoles.m-2.s-1 which was also the irradiance at which the rate of photosynthetic O2 evolution was maximal. Degradation of D1 was saturated at 350 micromoles.m-2.s-1. The rate of D1 synthesis at 1100 micromoles.m-2.s-1 was very similar to that at 350 micromoles.m-2.s-1 for the first 90 min but then declined. At limiting or saturating irradiance little change was observed in the ratio of variable to maximal fluorescence (Fv/Fm) measured after dark adaptation of the leaves, while significant photoinhibition occurred at 1100 micromoles.m-2.s-1. The proportion of zeaxanthin in the total xanthophyll pool increased with increasing irradiance, indicative of the presence of a photoprotective xanthophyll cycle in this seagrass. These results are consistent with a high level of regulatory D1 turnover in Zostera under non-photoinhibitory irradiance conditions as has been found previously for terrestrial plants.