Date of Award


Document Type


Degree Name

Master of Science (MS)


Plant and Environmental Science

Committee Chair/Advisor

Dr. James E. Faust

Committee Member

Dr. William Bridges

Committee Member

Dr. Brent Pemberton


High temperatures can cause delayed flowering in poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch), a phenomenon known in the industry as heat delay. The poinsettia wholesale market is concentrated to a relatively brief period from early-November through early-December putting growers at risk to heat delay. This problem is compounded by the unusual weather patterns created by climate change. The objective of this thesis was to identify periods in which poinsettia flowering is sensitive to high temperatures. First, the weeks within the growing season when the plants were sensitive to high temperatures were identified (Chpt. 2). Then within those sensitive weeks, the hours within a 24-h cycle when the plants were most sensitive were identified (Chpt. 3). In Chapter 2, two experiments were conducted; one experiment was conducted in greenhouses under natural daylength (ND) conditions while the other was conducted in growth chambers with LED lighting that initially provided a 11 h 01 min night length (NL) that increased by 2 min each day to simulate ND conditions in September and October at 34°N.Lat. One group of plants was moved each week from a moderate-temperature environment (22 °C average daily temperature [ADT]) to a high-temperature environment (28 °C ADT); plants spent one week in the high-temperature environment before returning to the moderate-temperature environment and being replaced by the next weekly treatment group. This treatment period lasted for 7 or 8 weeks for the growth chamber and greenhouse studies, respectively, after which plants were finished in a 22 °C ADT greenhouse. An additional group of plants was kept in either the moderate- or high-temperature environment for the entire treatment period to serve as controls. Four cultivars were grown in the greenhouse experiment: Advent Red, Freedom Red, Prestige Red and Tikal Red, while only Prestige Red was grown in the growth chamber experiment. Advent Red was identified as the most heat tolerant cultivar followed by Tikal Red, Freedom Red and Prestige Red. Periods of sensitivity to high temperatures for Advent Red, Tikal Red, Freedom Red and Prestige Red were 4-Sept. to 1-Oct., 11-Sept. to 8-Oct., 11-Sept. to 22-Oct. and 4-Sept. to 29-Oct., respectively. From the growth chamber study, a period of temperature sensitivity was identified when night lengths were between 11 h 01 and 12 h 37. Within these periods of sensitivity to high temperature, time to visible bud and anthesis were most affected by high temperatures in earlier weeks while final bract color development and time to first color were more affected by high temperatures during the latter weeks. Across both experiments, Freedom Red, Tikal Red and Prestige Red experienced delays to anthesis >1 d per day of high-temperature treatment suggesting a residual impact of high temperatures. In Chapter 3, over 2 weeks Prestige Red plants were exposed to high-temperature treatments of 28 °C during four periods of a 24-h cycle (12-h night/ 12-h day): the first 6 h of the day, last 6 h of the day, the first 6 h of the night, or the last 6 h of the night. Additionally, plants were exposed to the high-temperature treatment during the entire 12-h night or 12-h day period within a 24-h cycle as well as a moderate- and high-temperature control for a duration of 2-weeks. Following the treatment period, all plants were moved to a glass greenhouse to finish flowering in moderate temperatures (22 °C ADT) under a 12-h NL. High-temperature treatments applied during the last 6 h of the night, the entire 12 h of the night and the entire 24-h period caused the most significant flowering delays. There were no significant differences between these treatments indicating that these delays were primarily due to high temperatures during the last 6 h of the night. High-temperature treatments applied during the first 6 h of the day and entire 12 h of the day were significantly less impactful. For most floral responses, there was no significant difference between these two treatments indicating that delays in the 12-h-day treatment were primarily caused by high temperatures during the first 6 h of the day. High-temperature treatments applied during the first 6 h of the night and last 6 h of the day resulted in comparatively minor flowering delay.

Included in

Horticulture Commons



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