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We induced a mutation in Pyropia tenera using gamma irradiation to obtain strains with improved tolerance to high temperatures. We selected a mutant Pt1k with improved heat tolerance and different blade colour at high temperatures (20°C) after an irradiation dose of 1 kGy. We observed physiological changes and responses in the wild-type (WT) and Pt1k blades under high-temperature stress. The selected Pt1k mutant blades were dark green, which was different from the natural red of the WT. At 12°C, Pt1k blade growth rate and biomass were four times higher than those of the WT, and phycocyanin (PC) content also increased. When cultured at 20 and 25°C, blade shapes and growth rates of Pt1k were maintained; however, WT blades were considerably decayed and growth rate was inhibited. When cultured at 25°C for 3 weeks, chlorophyll a and PC contents were higher in Pt1k than in WT strains, whereas the phycoerythrin (PE) level was constant. Changes in hydrogen peroxide (H 2 O 2 ) and malondialdehyde (MDA) content after 3 weeks at 25°C Pt1k showed a slow increase; however, activities in the WT strain were highly increased. Under these conditions, activities of the antioxidant enzymes (superoxide dismutase and ascorbate peroxidase) of the two strains similarly increased when subjected to high-temperature stress for 1 week; however, the WT enzymes decreased rapidly during long-term exposure. Conversely, the Pt1k mutant had constant activities even when exposed to longterm high temperatures. These results indicate that the isolated mutant Pt1k had improved growth and tolerance to higher temperatures than the WT. In conlusion, we suggest that antioxidant enzyme activity, which is more abundant in the mutant Pt1k than in WT at high temperatures, is involved in the tolerance mechanism against temperature.

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