INTRODUCTION We have been working on a number of plants native to New Zealand to bring our knowledge to a stage at which some of them could be of use in the fl oricultural industry, either as cut fl owers or fl owering pot plants. Candidates receiving the greatest attention to date have been members of the Pacifi c genus Metrosideros, especially the New Zealand Christmas tree or pohutukawa (M. excelsa), and the colourful native legumes Sophora (kowhai) and Clianthus (kowhai ngutukaka or kaka beak). More recently, fl owering in Phormium (fl ax) has been studied, and we are extending our fi ndings in Metrosideros to plants in the closely related Eucalyptus group. There are three parts to our research on pohutukawa: overcoming juvenility in micropropagated plants (i.e., making the plants become competent to fl ower as soon as possible), understanding the environmental signals that trigger fl owering, and working out the conditions needed to control the timing of fl owering and the effects on fl ower quality (Clemens et al., 2002). Working on overcoming juvenility has led to a theoretical breakthrough that could have far-reaching implications to the way we prune and train trees (Sismilich et al., 2003a, b). However, here we want to briefl y review the results for controlling the triggers for fl owering, and the accelerating and braking of fl oral development so that quality blooms can be produced to a set schedule.

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 mumol m(-2) s(-1) under conditions known to be inductive for flowering (10 h photoperiod at 17/14degreesC), before transfer to a common forcing greenhouse (mean 18degreesC, 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 mumol m(-2) s(-1) had the highest, proportion of floral primordia (50%). The lowest proportion of floral primordia (6%) occurred in the 174 mumol 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 mumol 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.

Changes in carbon isotope composition(δ13C) and leaf morphology associated withvegetative phase change were monitored in Metrosiderosexcelsa Sol. ex Gaertn. (family Myrtaceae). Plants of threeontogenetic states were used: juvenile seedlings, micropropagated plants in arejuvenated state, and reproductively mature plants bearing leaves with adultcharacteristics. The effects of temperature regime (32/24 °C,24/16 °C, and 16/8 °C day/night) and plantarchitecture (branched and single-stemmed plants) were studied in two separateexperiments. Although both juvenile and rejuvenated plants exhibited juvenileleaf morphology at the start of the experiments, there was no differencebetweenleaf δ13C in these plants and that in adultplantsat this time (mean ca. −27%). Vegetative phase change occurred injuvenileand rejuvenated plants grown at 24/16 °C, and there was acorresponding increase in leaf δ13C (from ca.−27% to −23%) in these two groups of plants. Leafδ13C in adult plants remained relatively constant(ca. −26%) at 24/16 °C. There was little change in leafδ13C in all plant states maintained at 32/24°C or 16/8 °C, and vegetative phase change didnot occur in juvenile and rejuvenated plants grown under these two temperatureregimes. Rejuvenated plants grown in a greenhouse also exhibited a progressivedevelopment of adult leaf morphology, accompanied by an increase in leafδ13C, an effect that was more pronounced insingle-stemmed (from −26.4% to ca. −24%) than in branched plants. Itis suggested that increasing δ13C in juvenile andrejuvenated plants undergoing phase change is a result of reduced sink strengthin single-stemmed plants, and to a lesser extent within each branch of branchedplants, causing reduced stomatal conductance and photosynthesis.

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.