Rhododendron mucronatum G. Don grown in subtropical Taiwan does not manifest endodormancy

Microscopic observation of bi-weekly sampled terminal buds revealed that most flower buds of R. mucronatum were formed between late-July and mid-August and differentiation of specific floral organs was completed before mid-October. Flower buds continued to enlarge through the winter. When flower buds on two-yearold plants from rooted cuttings were allowed to grow at day / night temperature of 15/13°C, 20/15°C, 25/20°C or 30/ 25°C, the 30/25°C treatment was observed to inhibit their growth; those subjected to the 15/13°C treatment grew the fastest during the early stages. Neither treatment, however, enabled attainment of anthesis by the end of the experiment. The 20/15°C treatment hastened flower bud growth and reduced the number of days to flowering. Bud growth was slower and flowering percentages were lower at 25/20°C, than at 20/15°C. When plants were transferred to 30/25°C after 2, 4, or 6 weeks in the 15/13°C treatment, the growth rate of flower buds increased greatly. Also, when the 15/13°C treatment was extended, the growth rate of flower buds increased, while the number of days needed for flowering after transferring from the low temperature treatment was reduced. The 15/13°C treatment simulated the winter temperatures in Taiwan. Our finding disclosed that bud growth was fastest, rather than ceasing, under this temperature regimen. Although growth rate decreased near the end of the experiment, it increased immediately and bloomed quickly when plants were transferred to higher temperatures. Thus, our observation indicated that R. mucronatum does not enter endodormancy during winter in subtropical Taiwan lowlands.