Photosynthetic physiological characteristics of endangered Tsoongiodendron odorum seedlings by high irradiance in summer

The gas exchange characteristics, chlorophyll fluorescence and photosynthetic oxygen release of Tsoongiodendron odorum seedling (3-year-old, pot-cultured) were measured in summer using a portable photosynthesis system, a PAM chlorophyll fluorometer and a leaf discs oxygen electrode, respectively. The results showed that the light compensation point was 10.1mol m-2s-1, the light saturation point was 800 molm-2s-1, the light compensation point was 50.72 mol m-2s-1, and the saturation point was 1200 molm-2s-1.The diurnal variations of net photosynthetic rate (Pn) presented as a bimodal shape. The first peak was 9.0 molm-2s-1 which appeared at 08:00. The second peak was 4.6molm-2s-1 appearing at 16:00. There was a distinct noon?break phenomenon at 12:00. Both PSⅡphotochemical efficiency (Fv/Fm) and actual photochemical efficiency (PSⅡ) were low during 10:00 to 14:00 when photosynthetic active radiation were high. The Fv/Fm and PSⅡ decreased to the lowest at 12:00, which indicated that the activity of PSⅡ decreased and the photosynthetic?apparatus was severely inhibited by high light. The vulnerable PSⅡ might be a major reason that Tsoongiodendron odorum cannot resist highlight.