Abstract: Temperature is widely considered to be one of the most important factors affecting the spring phenophases, such as leaf flushing and flowering. However, different viewpoints still exist as to the effect of the low temperature. Xishuangbanna (in China), which is located in the northern part of the tropics, has a lower temperature than the equatorial tropical regions. Since 1959, the Xishuangbanna Tropical Botanical Garden (XTBG) has introduced more than 40 000 tropical plant species for conservation from all over the world. The low temperature of the garden is a key factor significantly affecting the growth of these plants. The extreme chilliness that occurred in 1974 was a threat to the plants and also a test of their cold resistance. In this study, we compared the spring phenology (leaf flushing and flowering) of 43 introduced plants in 1974 with that of plants in other years and evaluated their cold resistance by different origins (Asian tropics, American tropics and African tropics). In total, 81% of the plants showed advanced leaf flushing because of both the extreme chilliness and the rapid temperature rebound afterwards. The chilliness also caused non-flowering in 35% of the plants. Successful flushing after chilliness represents a good vegetative cold resistance for all the species; however, the progenitive phenophase was more adversely affected by the chilliness. According to the performance of their spring phenology, species from Asian tropics showed the best cold resistance with the highest species percentage of flowering (83%) and the fewest shifts in phenotypic durations after the chilliness. Species from the American tropics were next highest in flowering and phenotypic durations, then African species. Hence, species from the Asian tropics should be considered first for ex situ conservation in Xishuangbanna, and African species must be carefully introduced.

Key words: extreme chilliness, flowering