Objective This study aims to compare the photosynthetic responses and carbon assimilation of the thermophilic mangrove plant Bruguiera gymnorrhiza (L.) Savigny from different regions under chilling stress.

Method The 5-year-old seedlings were collected from three different provenances Futian (FT) in Shenzhen, Yunxiao (YX) in Fujian and Okinawa (UR) in Japan (the average temperature of the coldest month was 14.1, 13.3, and 16.1 ℃, respectively). The seedlings were treated under normal temperature (20 ℃) and chilling temperature (10 ℃). For each treatment, the chlorophyll fluorescence properties and gas exchange were monitored.

Result (1) Compared with normal temperature treatment, after 3 days of chill temperature and cold stress treatment, the maximum light energy conversion efficiency of photosystem II (PSII) (Fv/Fm) in FT, UR, and YX decreased by 39.04%, 25.69%, and 22.83%, respectively. The maximum light energy conversion efficiency of photosystem I (PSI) (Pm) decreased by 21.80%, 20.19% and 42.84%, respectively. Under low temperature treatment, the proportion effective photochemical quantum yield of PSII (Y(II)) of UR and YX all decreased, and the ratio of non-regulatory energy dissipation (Y(NO)) of the three types of B. gymnorrhiza all increased, and the proportion of YX increased the most. The effective photochemical quantum yield of PSI (Y(I)) of B. gymnorhiza from the three regions decreased significantly, and the proportion of non-regulated energy dissipation (Y(NA)) of UR and YX decreased slightly. (2) After 4 days of cold damage treatment, net photosynthetic rate (Pn) of FT, UR and YX decreased by 92.12%, 97.50% and 86.44%, respectively. Transpiration rate (Tr) and stomatal conductance (Gs) also decreased and Pn, Tr and Gs were positively correlated. The stomatal limitation value (LS) of the three types of B. gymnorhiza decreased significantly, while the intercellular CO₂ concentration (Ci) increased significantly. (3) Fv/Fm, Pn and water use efficiency were significantly positively correlated with each other.

Conclusion Regardless of the photoreaction of photosynthesis or the carbon reaction, B. gymnorhiza from the three provenances show high sensitivity to short-term chilling. PSII was significantly inhibited, while PSI was relatively stable. Mesophyll damages limited the carbon assimilation more than stomata traits. In summary, FT highly suffers from chilling, while YX is less affected. Hence, cold tolerance of mangroves is closely related to the average temperature at the region sites.