Effects of photoperiod, temperature and bud size on flowering in Metrosideros excelsa (Myrtaceae)

The effects of photoperiod. temperature and bud size on floral initiation and development were examined in two cultivars (Scarlet Pimpernel and Vibrance) of Metrosideros excelsa (family Myrtaceae). Two complementary experiments were conducted. In the first, a factorial combination of two photoperiods (10 or 16 h) and two temperature regimes (12/9 degrees C or 17/14 degrees C) was applied for 0, 5, 10 or 15 weeks before plants were transferred to a forcing greenhouse at 24/17 degrees C. Plants transferred from the 17/14 degrees C and 10 h photoperiod treatment after 15 weeks had the highest proportion of flowering plants, with significantly more inflorescences per plant. In general, no flowering was observed in plants transferred after 0, 5 or 10 weeks. In a second experiment, carried out simultaneously with the first, the same cultivars were grown continuously in four greenhouses using a factorial combination of two photoperiod treatments (ambient daylength or 16 h) and two temperature treatments (ambient temperature or 24/17 degrees C) until plants reached anthesis. The ambient daylength/ambient temperature conditions produced a higher proportion of flowering plants, and a significantly greater number of inflorescences per plant. No flowering occurred under the 24/17 degrees C and 16 h regime. In both experiments, buds initially 2.0-3.0 mm in diameter had the highest probability of becoming floral; buds less than 2.0 mm in diameter were more likely to remain vegetative or unbroken. Rates of floral development varied significantly between treatments.