Light response of D1 turnover and photosystem II efficiency in the seagrassZostera capricorni

Abstract

Biochemical and biophysical parameters, including D1-protein turnover, chlorophyll fluorescence, oxygen evolution activity and zeaxanthin formation were measured in the marine seagrassZostera capricorni (Aschers) in response to limiting (100 μmol·m⁻²·⁻¹), saturating (350 μmol·m⁻²·s⁻¹) or photoinhibitory (1100 μmol·m⁻²·s⁻¹) irradiances. Synthesis of D1 was maximal at 350 μmol·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 μmol·m⁻²·s⁻¹. The rate of D1 synthesis at 1100 μmol·m⁻²·s⁻¹ was very similar to that at 350 μmol·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 μmol·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 inZostera under non-photoinhibitory irradiance conditions, as has been found previously for terrestrial plants.