Effects of Heat Shock on Chlorophyll Content and Fluorescence Characteristics of Cycas multipinnata

In order to investigate the physiological responses of Cmultipinnata to heat stress and explore the ecological adaptive mechanisms of this species under heat stress conditions. Detached leaves were used to compare the effects on chlorophyll content and fluorescence parameter of different temperatures (25, 35, 45, 55℃). The results showed that the content of chlorophyll a, chlorophyll b and chlorophyll (a+b) first rising then falling with the increase of temperature. They were decreased to the lowest level after heat shock at 45℃ and then increased after heat shock at 55℃. However, chlorophyll a/b did not vary significantly among leaves treated at different temperatures. Fo, Fm, Fv/Fm, Y(NPQ), Y(NO), qP, NPQ, ETRmax, α and Ik did not vary significantly， but Y(Ⅱ) of leaves heated at 35℃ was significantly higher than that of leaves heated at 25-45℃. After leaves heated at 55℃, Ik did not change significantly, but Fo and Y(NO) increased significantly and other fluorescence parameters including Fm, Fv/Fm, qP, NPQ, ETRmax and α decreased significantly, compared with that of leaves heated at 25-45℃. The fraction of absorbed light allocated to antenna heat dissipation (D) increased with the increase of temperature. The fraction of absorbed light in photochemistry (P) reached to the highest at 35℃ but then decreased to the lowest at 55℃. The fraction of excess excitation energy (E) did not vary significantly at 25-45℃, but decreased significantly at 55℃. In conclusion, Cmultipinnata possessed relatively high tolerance to heat stress, and the photosynthetic activity was not affected at 25-45℃; however, the activity of PSⅡreaction center was decreased and the pathway of photosynthetic electron transport was inhibited at 55℃.