Two components of onset and recovery during photoinhibition of Ulva rotundata.

Short-term (up to 5 h) transfers of shade-adapted (100 μmol · m(-2) · s(-1)) clonal tissue of the marine macroalga Ulva rotundata Blid. (Chlorophyta) to higher irradiances (1700, 850, and 350 μmol · m(-2) · s(-1)) led to photoinhibition of room-temperature chlorophyll fluorescence and O2 evolution. The ratio of variable to maximum (Fv/Fm) and variable (Fv) fluorescence, and quantum yield (ϕ) declined with increasing irradiance and duration of exposure. This decline could be resolved into two components, consistent with the separation of photoinhibition into energy-dissipative processes (photoprotection) and damage to photosystem II (PSII) by excess excitation. The first component, a rapid decrease in Fv/Fm and in Fv, corresponds to an increase in initial (Fo) fluorescence and is highly sensitive to 1 mM chloramphenicol. This component is rapidly reversible under dim (40 μmol · m(-2) · s(-1)) light, but is less reversible with increasing duration of exposure, and may reflect damage to PSII. The second (after 1 h exposure) component, a slower decline in Fv/Fm and Fv with declining Fo, appears to be associated with the photoprotective interconversion of violaxanthin to zeaxanthin and is sensitive to dithiothreitol. The accumulation of zeaxanthin in U. rotundata is very slow, and may account for the predominance of increases in Fo at high irradiances.