Effect of irradiance during floral induction on floral initiation and subsequent development in buds of different size in Metrosideros excelsa (Myrtaceae)

The response of floral development in two cultivars of Metrosideros excelsa (Myrtaceae) to irradiance applied during floral induction was studied to test the hypothesis that floral initiation and flowering would be linearly and positively correlated with this environmental factor. Plants were grown for 20 weeks at 174, 567, 961 or 1,355 µmol m-2 s-1 under conditions known to be inductive for flowering (10 h photoperiod at 17/14°C), before transfer to a common forcing greenhouse (mean 188C, 16 h photoperiod). Floral development was examined histologically in buds that were initially in three size ranges (<1.6.mm, 1.6-2.0.mm and >2.0.mm diameter) collected 13, 20 and 23 weeks after the start of the experiment. Floral primordia were first observed after 20 weeks in the highest three irradiance treatments. After 23 weeks, buds that were originally 1.6-2.0.mm in diameter in plants treated at 567 µmol m-2 s-1 had the highest proportion of floral primordia (50%). The lowest proportion of floral primordia (6%) occurred in the 174 µmol m-2 s-1 treatment, and only in the buds that were initially the largest. Floral meristem size after 23 weeks showed a unimodal (non-linear) response to irradiance, a pattern reflected in the number of inflorescences that reached anthesis after 31 weeks, both peaking at close to 567 µmol m-2 s-1. Some buds that initially contained floral primordia underwent reversion to vegetative growth, possibly as a result of unfavourably high temperatures during the time when organogenesis would have occurred. Most inflorescences occurred in buds that were initially in the intermediate and largest bud size ranges. Carbohydrate concentrations showed a positive linear response to irradiance, whereas chlorophyll showed a significant negative response. The unimodal response of flowering to irradiance is in contrast to many other published studies, and highlights the potentially damaging effects of high irradiance, and the importance of making assessments of the effects of environmental factors over a wide range of levels.