The influence of temperature on seed germination and seedling growth of an exotic mangrove species Laguncularia racemosa

Laguncularia racemosa (Combretaceae, Lr), one of the fast-growing exotic mangrove species, was introduced for afforestation along the southeast coast of China in the last two decades. Compared with another widely used exotic Mangrove species, Sonneratia apetala, information about Lr is still limited. Due to the lack of research on the adaptive mechanism and potential invasion of Lr, whether this exotic species will cause biological invasion in mangrove wetland is still controversial. The present study aims to evaluate the effect of low temperature on seed germination and early growth of Lr as these two factors have been considered as important indicators for the adaptive and invasive abilities of an exotic plant species. Results showed that Lr seeds did not germinate under 5°C, and the time achieved 70% germination percentage was related to temperature, which was 34, 26, 20 and 10 days at 10, 15, 20 and 25°C, respectively. The healthy seedlings germinated under 25°C were selected, their growth and physiological responses, including photosynthetic parameters, activity of peroxidase (POD) and content of MDA, were monitored under three different temperature, namely, 25, 10 and 5°C at 6, 12, 24, 72 and 120 hours. Both POD activity and MDA content were higher under lower temperature and increased with treatment time. Among photosynthetic parameters, net photosynthetic rate, transpiration rate, stomata conductivity and water use efficiency were positively correlated with temperature but relationships with treatment time were negative. The seedlings under low temperature stresses (5 and 10°C) after 24, 72 and 120 hours were then transported to 25°C for recovery in the next 48 hours. Photosynthetic data showed that previous exposure of seedlings at low temperature was good for seedling growth. These findings indicated that an exotic mangrove species (Lr) could germinate and grow at low temperature (10°C), it could also recover after exposure to even lower temperature like 5°C, suggesting that Lr could spread to high latitudes, thus the invasive potential of Lr and related mechanisms need further research.