Varying Durations of High Temperature Affects Flowering Time and Flowering Shoot Morphology in Pointsettia

Two cultivars of poinsettia (Euphorbia pulcherrima Willd. Ex Klotzsch) were grown in greenhouses in Overton, TX, to determine the effects of varying periods of elevated temperature on flowering time and morphology.  Plants were potted in early September and pinched to 5 nodes two weeks later for natural season flowering.  Plants were exposed to either 0, 1, 2, 3, 4, 5, 6, 7, or 8 weeks of a high temperature treatment consisting of a daily average temperature of 27 °C starting at the time of the pinch. For the rest of the forcing period, an average daily temperature of 22 °C was maintained.  Dates of first bract color, visible bud, and anthesis were recorded for each plant.  Anthesis of ‘Tikal Red’ plants was delayed by 2 to 7 weeks of high temperature treatment, but not by 1 or 8 weeks of treatment when compared to plants receiving no high temperature treatment.  Days to visible bud from transplant behaved in a similar pattern, but days to first bract color was delayed by heat treatment in a linear fashion.  Data indicate that floral initiation is delayed for ‘Tikal Red’, but floral development is hastened by high temperature treatment.  However, anthesis, days to visible bud, and days to first bract color of ‘Prestige Red’ plants was strongly delayed in all aspects by the high temperature treatment as the duration of treatment increased.  At anthesis, number of leaves, transitional bracts, fully expanded and pigmented bracts, and height was recorded for the distal flowering shoot on each plant.  Effects of high temperature on plant morphology were not related to timing of floral developmental events.  Increasing duration of high temperature treatment resulted in taller plants with increased leaf number and node length in both cultivars. However, for ‘Tikal Red’, six or more weeks of high temperature resulted in more transitional bracts and 8 weeks of treatment reduced the number of fully developed bracts when compared to untreated plants.  For ‘Prestige Red’, numbers of both bract types were unaffected by high temperature treatment.