The Timing and Duration of High Temperature Exposure Impacts Flowering Time of Poinsettia

High temperature delay in poinsettia floral initiation termed ‘heat delay’ has been recorded following 28 days of exposure to 28°/24°C day/night temperatures around the time of floral initiation.  The role of the relative timing and duration of high temperature exposure on heat delay in poinsettia ‘Autumn Red’ is investigated.  Two experiments were carried out under natural day conditions at 29°40'N latitude.  Plants were exposed to high temperatures (28°/24°C day/night) for 14 or 21 days as designated by treatment and grown at moderate temperatures (24°/21°C day/night) for the remainder of the experiment. Both experiments included control groups exposed to 0 and 28 days of high temperatures. High temperature treatments were imposed at 7 day intervals beginning 21 days before predicted floral initiation through 14 days before predicted anthesis.  The calendar dates representing the predicted initiation and anthesis dates for ‘Autumn Red’ with natural day lengths at 29°40’N latitude were determined in previous experiments.  Data were collected on the number of days from pinching (1 Sept. 2006 in Expt. 1 and 1 Sept. 2007 in Expt. 2) to floral initiation and anthesis.  Plants in the high temperature control groups reached floral initiation 11 or 12 days later and anthesis 11 or 14 days later than the low temperature control plants in Expts. 1 and 2, respectively.  In Expt. 1, plants exposed to 14 days of high temperature beginning 14, 7, or 0 days before predicted floral initiation reached floral initiation 5, 5, and 1 days and anthesis 4, 6, and 1 days later the low temperature control plants, respectively.  In Expt. 2, plants exposed to 14 days of high temperatures beginning 21, 14, 7, or 0 days before predicted floral initiation reached floral initiation 0, 4, 5, and 0 days and anthesis 0, 5, 6, and 1 days later the low temperature control plants, respectively. Plants exposed to 21 days of high temperatures beginning 14 or 7 days before predicted floral initiation reached floral initiation 10 and 14 days and anthesis 11 and 12 days later than low temperature control plants, respectively which are 3 or 0 days before high temperature control plants, respectively. These data indicate that the timing and duration of high temperature exposure determines the magnitude of heat delay of poinsettia.