Identifying the Diurnal Sensitivity of Poinsettia Flowering to High Temperatures

This study aimed to determine the relative sensitivity of poinsettia ( Euphorbia pulcherrima Willd. ex Klotzsch ‘Prestige Red’) flowering to high temperatures within a 24-hour cycle. For the first experiment, two growth chambers were used, one provided a moderate-temperature environment of 22 °C, and the other provided a high-temperature environment of 28 °C. The day length and night length in each chamber were each 12 hours. Plants were moved between chambers to provide different temperature treatments over a 2-week period. During this time, plants were exposed to high temperatures during the first 6 hours of the day, the last 6 hours of the day, the first 6 hours of the night, or the last 6 hours of the night. Additional treatments included plants that were exposed to the high-temperature environment for 12 hours of the night or day period and plants that were exposed to the moderate-temperature or high-temperature environments for the entire 24 hours. After the treatment period, all plants were moved to a 22 °C greenhouse and a 12-hour night length until flowering data were collected. The last 6 hours of the night had the highest relative sensitivity to high temperatures, followed by the first 6 hours of the day. A second experiment was conducted to provide temperatures at a broader range and higher resolution (e.g., 24, 26, 28, 30, 32 and 34 °C) while testing our hypothesis that poinsettia is highly sensitive to high temperatures during the last 6 hours of the night and the first 2 hours of the day. Plants exposed to high temperatures (≥26 °C) during the last 6 hours of the night and the first 2 hours of the day displayed a greater delay in flowering compared with plants that received 24 °C for 24 hours per day. This project demonstrated that heat delay of poinsettia is highly sensitive to temperatures ≥26 °C that occur during the last 6 hours of the night and the first 2 hours of the day during flower initiation (the first 2 weeks of inductive photoperiods) under natural day length conditions (12-hour night length).