effects of temperature acclimation on the photoinhibitory responses of Ulva rotundata Blid

The effect of acclimation to 25, 18, or 10 degrees C on the relationship between photoprotection and photodamage was tested in low-light-grown (80 micromoles.m-2.s-1) Ulva rotundata Blid. exposed to several higher irradiances at the acclimation temperature. Changes in chlorophyll fluorescence parameters (minimum fluorescence, F(0), and the ratio of variable to maximum fluorescence, F(V)/F(m), measured after 5 min darkness) were monitored during 5 h transfers to 350, 850, and 1700 micromoles.m-2.s-1, and during recovery after 1- or 5-h treatments. At all temperatures, rate of onset and final extent of photoinhibition, measured by a decrease in F(V)/F(m), increased with increasing irradiance. At a given photoinhibitory irradiance, rate of onset was most rapid at 10 degrees C, but the extent was temperature-independent. Recovery rates from mild light stress were similar at all temperatures, but recovery from the most extreme photoinhibitory treatment lagged 2 h at 10 degrees C. De-epoxidation of xanthophyll-cycle components proceeded faster and to a lower epoxidation status at 25 degrees C, but there was little difference in the pool size among the three growth conditions. Using chloramphenicol to inhibit chloroplast protein synthesis and dithiothreitol to inhibit violaxanthin de-epoxidation, it was shown that at the lowest light treatment given, the extent of photoinhibition could be attributed both to greater amounts of photodamage and to greater zeaxanthin-related photoprotection at 25 than at 10 degrees C. While these two mechanisms for high-light-induced loss of photosynthetic efficiency were operating at 10 degrees C, there was evidence for a relatively greater proportion of zeaxanthin-unrelated photoprotection at the low temperature. This photoprotective mechanism is related to a rapidly reversible increase in F(0) and is insentivite to both chloramphenicol and dithiothreitol.