Objective To investigate the effects of chilling on leaf photosynthesis and chlorophyll fluorescence parameters in mangrove Kandelia obovata seedlings, and the defense mechanism of K. obovata seedlings against chilling, so as to provide references for the breeding and introduction of the cold-resistant mangrove species.

Method Under low-temperature stress at 5℃ and 12℃, the changes of leaf photosynthetic parameters and chlorophyll fluorescence parameters of K. obovata seedlings were measured using portable photosynthesis analyzer (Li-6400XT) and portable pulse modulated chlorophyll fluorescence instrument (PAM-2500), respectively. Data processing, mapping and statistical analysis were carried out using the software EXCEL and SPSS13.0.

Result (1) The leaf photosynthetic rate (Pn), intercellular CO₂ concentration (Ci), stomatal conductance (Gs) and water use efficiency (WUE) of K. obovata seedlings were significantly affected by low temperature stress (P < 0.05). (2) At low temperature, the values of Pn, Gs and Ci decreased significantly, and the values of Pn and Gs showed a decreasing trend during 5 days of low temperature while the Ci showed a rising trend. Under the low-temperature stress at 5℃, the value of WUE was higher than CK at the first day, and then decreased to lower than CK. At 12℃, WUE was a little higher than CK during all the 5 days. (3) At 5℃, the variable fluorescence (Fv) and the maximum light energy conversion rate (Fv/Fm) were lower than CK, and showed a downward trend with the time duration of low temperature, and decreased significantly at low temperature at the fifth day (P < 0.05). At 12℃, the changes of Fv and Fv/Fm values were not significant.

Conclusion K. obovata seedling can survive under low-temperature stress at 12℃, and the growth and biomass of K. obovata seedlings will decrease. At the low temperature of 5℃, the survival threshold of K. obovata seedlings is 2 days (48 h). Under low-temperature stress which does not damage the potential activities of leaf Photosystem Ⅱ (PSⅡ) in K. obovata seedlings, the declining of Pn is mainly because of the restricted stomatal efficiency (WUE). During the process of photosynthesis, K. obovata seedlings will lower the declining of photosynthetic rate through increasing the leaf water use to improve its cold-resistance ability.