Identifying the Temporal Sensitivity of Poinsettia Flowering to High Temperatures

The objective of this study was to identify the specific weeks and night lengths when poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) flowering is most sensitive to high temperatures. One experiment was conducted in greenhouses under natural daylength (ND) conditions at lat. 34.7 & DEG;N starting on 4 Sep, and a second experiment was conducted in growth chambers with an initial night length (NL) of 11 hours 01 minutes that was increased by 2 min/d to simulate ND conditions through September and October. Each week, one group of plants was moved from a moderatetemperature environment [22 & DEG;C average daily temperature (ADT)] to a high-temperature environment 28 & DEG;C ADT. Each group of plants spent 1 week in the high-temperature environment before returning to the moderate-temperature environment. The temperature treatments lasted for 7 or 8 weeks for the growth chamber and greenhouse studies, respectively. Additional groups of plants were kept in either the moderate- or hightemperature environment for the entire treatment period as controls. Four cultivars were used in the greenhouse study: Advent Red, Freedom Red, Prestige Red, and Tikal Red; only Prestige Red was used in the growth chamber study. Advent Red was identified as the most heat-tolerant cultivar followed by Tikal Red, Freedom Red, and Prestige Red. 'Advent Red's' period of sensitivity to high temperatures ranged from 4 Sep to 1 Oct. 'Tikal Red's' period of sensitivity to high temperature ranged from 11 Sep to 8 Oct. 'Freedom Red' had a longer period of high-temperature sensitivity: from 11 Sep to 22 Oct. 'Prestige Red' had the longest period of sensitivity to high temperatures encompassing 4 Sep to 29 Oct, and 11 hours 01 minute to 12 hours 37 minutes NL for the greenhouse and growth chamber studies, respectively. Within periods of sensitivity to high temperatures, time to visible bud and anthesis were most affected by high temperatures in earlier weeks, and final bract color development and time to first bract color were more affected by high temperatures during the latter weeks. As cultivars varied in their duration of sensitivity to high temperatures, duration, as well as magnitude of response to high temperature, should be considered in future breeding projects.