Freezing tolerance of Porphyra yezoensis (Bangiales, Rhodophyta) gametophyte assessed by chlorophyll fluorescence

In January and February 2010, heavy sea ice formed along the coast of the Bohai Sea and the northern Yellow Sea, China. Intertidal organisms were subjected to serious freezing stress. In this study, we investigated the freezing tolerance of the upper intertidal economic seaweed Porphyra yezoensis. The maximum photochemical efficiency of PS II (Fv/Fm) in undehydrated thalli remained high after 24 h at −2°C and that in dehydrated thalli decreased in a proportion to thallial water loss. Fv/Fm dropped sharply after 24 h at −20°C, regardless of absolute cellular water content (AWC). The Fv/Fm in frozen thalli recovered rapidly at 0–20°C. A wide range of water loss in the thalli enhanced their tolerance to freezing. Fv/Fm values in undehydrated thalli dropped sharply after 3 d at −2°C or 10 d at −20°C while those in dehydrated thalli (20–53% AWCs) remained at high levels after 9 d at −2°C or 30 d at −20°C. These results indicate that P. yezoensis has high freezing tolerance by means of dehydration during the ebb tide and rapid recovery of Fv/Fm from freezing. A strategy of P. yezoensis industry to avoid heavy loss during freezing season is discussed based on these findings.