Effects of low temperature stress on the antioxidant system and photosynthetic apparatus of Kappaphycus alvarezii (Rhodophyta, Solieriaceae)

To understand the effects of low temperature stress on Kappaphycus alvarezii and the responses of antioxidant systems and photosystem II (PSII), behaviour in K. alvarezii thalli exposed to low temperatures (20 degrees C, 17 degrees C and 14 degrees C) for 2 hours was evaluated. Compared with the control at 26 degrees C, activities of some antioxidant enzymes including superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) and the level of antioxidant substance (reduced glutathione) increased in K. alvarezii thalli when exposed to lowered temperatures (20 degrees C, 17 degrees C). Hydroxyl free radical (center dot OH) scavenging activity of K. alvarezii thalli also increased at 20 degrees C and 17 degrees C compared with the control. This indicated that the resistance to low temperature stress in the antioxidant system of K. alvarezii increased at lowered temperatures of 20 degrees C and 17 degrees C. However, at the lowest temperature (14 degrees C), no significant increases of this algal antioxidant were observed. Under low temperature stress, the maximum quantum yield of PSII photochemistry (F-V/F-M) and PSII actual photochemical efficiency (Phi(PSII)) decreased in K. alvarezii thalli, suggesting that the photosynthetic capacity declined. Components of the photosynthetic apparatus (such as the oxygen-evolving complex, light absorption antennas, reaction centres, electron acceptor sides and electron donor sides of PSII) were damaged by low temperature stress to varying degrees. In addition, it was found that low temperature stress led to decreases of both D1 protein and Rubisco LSU (Rubisco large subunit) protein levels. This work is a significant contribution towards understanding the basic mechanism involved in the resistance and the adaptation of K. alvarezii to low temperature stress conditions.