Poinsettia ‘Prestige Red’ was exposed to high-temperature (28 °C) treatments during the first 6 h, the last 6 h, or the entire 12 h of the night or day period within a 24-h cycle (12-h night/12-h day) and the entire 24-h day/night period (control). The plants were in a moderate-temperature (22 °C) environment at all other times for a duration of 2 weeks. The days from the start of the experiment to visible bud (A), first color (B), and anthesis (C) are reported for the eight temperature treatments. Different letters represent significant differences between treatment means (P < 0.05) using Fisher’s least significant difference test.
Poinsettia ‘Advent Red’ plants exposed to high-temperature (26, 28, 30, 32, and 34 °C) treatments during the last 6 h of the night and first 2 h in the morning (8 h total), followed by a moderate temperature (24 °C) for the remainder of the 24-h cycle. Control plants were exposed to 24 °C for 24 h/d. Plants were treated during the first 2 weeks after flower initiation (12-h photoperiod). Average time (in days) from the start of the experiment to first color (A), visible bud (B), and anthesis (C) are reported for the six temperature treatments. Different letters represent significant differences between treatment means (P < 0.05) using Tukey’s honestly significant difference test.
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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).
Poinsettia is a short-day plant that typically undergoes floral initiation between mid-September and early October when grown under natural day length conditions in the contiguous United States (lat. 24.5–49.4 °N). High temperatures, particularly during initiation and early stages of floral development, delay flowering; this phenomenon is known in the floriculture industry as heat delay (Ecke et al. 2004). The poinsettia wholesale market typically occurs for approximately 4 weeks from early November through early December, which is a fairly narrow window. Therefore, flowering delayed by 1 or more weeks can dramatically impact the ability of a business to deliver flowering plants on time, resulting in a substantial negative economic impact.
Several studies have indicated that delayed flowering of poinsettia is primarily driven by high night temperatures (NTs) (Berghage et al. 1987a; Kofranek and Hackett 1965; Langhans and Miller 1959; Larson and Langhans 1960; Roberts and Struckmeyer 1938). Berghage et al. (1987a) provided factorial day temperature (DT)/NT combinations of 29, 26, 23, 20, 17 and 14 °C under a 14-h NL on poinsettia ‘Annette Hegg Dark Red’ and found that NT ≥26 °C caused significant delays to cyathia and bract development, whereas high DTs had no significant impact. Schnelle (2008) provided data that appeared to contradict the conclusions drawn from Berghage et al. (1987). Cultivars Red Velvet, Prestige Red, and Barbara Ecke Supreme were grown using the following four DT/NT treatments, 23/19, 26/22, 24/24, and 29/24 °C, which provided three average daily temperatures (ADT) of 21, 24 and 27 °C. The experiment was conducted with a 12-h NL. The 29/24 °C (27 °C ADT) treatment significantly delayed time to visible bud, first color, and anthesis compared with the other treatments. If NT was the driving factor that caused heat delay, then the 24/24 °C DT/NT treatment should have been equally delayed compared with the 29/24 °C treatment, which was not. It was concluded that ADT, not NT, is responsible for heat-delayed flowering of poinsettia. This apparent contradiction was resolved by Alden and Faust (2021), who researched the interaction between photoperiod and temperature. Three DTs (20, 24, and 28 °C), four NTs (16, 20, 24, and 28 °C), and five NLs (14, 13, 12, 11, and 10 h) were provided in factorial treatment combinations for 17 d. When the heat-sensitive cultivar Prestige Red was grown at a 12-h NL, there was a linear decrease in progress to flower as DT increased; however, increasing DT did not delay flowering at a 13-h or 14-h NL. At a 13-h or 14-h NL, increasing NT delayed flowering, particularly when NT increased from 24 to 28 °C. At a 12-h NL, increasing NT from 24 to 28 °C also significantly delayed flowering. These studies indicated that under 13-h to 14-h NLs, high NT is the primary cause of heat delay, as was observed by Berghage et al. (1987); however, at a 12-h NL, high ADT was most impactful because both high DTs and high NTs contribute to heat delay, as was observed by Schnelle (2008). Under natural-day photoperiods, poinsettia begins floral initiation when NL is in the range of 11 h 0 min to 11 h 45 min (Larson and Langhans 1962). Thus, under natural-day photoperiods, it is expected that either high DTs or NTs can contribute to heat delay, whereas when poinsettia flower initiation is performed under black curtain systems delivering ≥13-h NLs, NT is the most important parameter to consider.
The diurnal sensitivity to high temperatures of poinsettia has not been studied, but research has examined chrysanthemum (Chrysanthemum morifolium and C. seticuspe). Nakano et al. (2020) grew chrysanthemum under various light/dark periods (16 h/16 h, 8 h/16 h, 8 h/24 h, 11 h/13 h, 8 h/16 h, 14 h/12 h, and 9 h/15 h light/dark) and subjected the plants to 4-h heat pulses at 30 °C at varying times within the light/dark periods. It was found that high-temperature treatments that occurred at the start of the light period had no significant impact on time to anthesis, while the maximum delay to anthesis was observed in high-temperature treatments occurring 10 to 16 h into the dark period. High-temperature treatments applied earlier or later in the dark period caused delays to anthesis, but to a lesser extent than treatments applied 10 to 16 h into the dark period. These results suggest that heat sensitivity varies in circadian patterns. Additionally, these results indicate that sensitivity to high temperatures in chrysanthemum gradually escalates when a dark period is perceived and then rapidly decreases when a light period is perceived. However, because only one heat pulse was applied at the start of the light period, it is not possible to determine if sensitivity to high temperatures also varies within the light period. For chrysanthemum, high temperatures appear to delay flowering by decreasing the gene expression for capitulum development rather than delaying flower initiation (Nakano et al. 2015).
If poinsettia, like chrysanthemum, has periods of high sensitivity to high temperatures within a dark or light period, then growers could prioritize cooling during the critical times, leading to improved avoidance of heat delay. This would also improve our ability to predict flowering times based on temperature data. Therefore, the objectives of this study were to determine the relative sensitivity of poinsettia flowering to high temperatures within a day period or night period during a 24-h cycle and more precisely identify the temperatures that cause heat delay.
One-hundred ‘Prestige Red’ cuttings were propagated in a foam medium (Oasis Rootcubes Plus Wedge; Smithers-Oasis, Kent, OH, USA) for 4 weeks inside a greenhouse with day/night temperatures of 28.5 ± 1.3/26.3 ± 0.3 under night interruption lighting (2200 HR–0200 HR) consisting of light-emitting diode (LED) bulbs that delivered 1.2 ± 0.2 µmol·m−2·s−1 and natural light levels with photosynthetic photon flux density (PPFD) of 808.6 ± 100.5 µmol·m−2·s−1. Cuttings were then transplanted into 1.33-L pots containing a peat-based growing medium (Fafard 3B; Sun Gro, Anderson, SC, USA) and moved to a greenhouse that provided night interruption lighting (2200 HR–0200 HR) by metal halide lamps that delivered 50.0 ± 5.5 µmol·m−2·s−1 until the start of the experiment. One week after transplanting, plants were pinched to five nodes; after an additional 2 weeks, lateral shoots were removed so that two uniform shoots remained on each plant. The most uniform 48 plants were then selected to begin the experiment. The plants were continuously fertigated with Peters Excel Cal-Mag Special (15N–5P2O5–15K2O; ICL, St. Louis, MO, USA) at 150 mg·L−1 N for the duration of the experiment.
One growth chamber provided a constant moderate-temperature treatment of 22 °C (22.6 ± 1.2 °C) and the other provided a constant high-temperature treatment of 28 °C (28.4 ± 0.9 °C). Both growth chambers had LED fixtures (RAZR 97 W LED; Fluence Bioengineering, Austin, TX, USA) providing 175 ± 25 µmol·m−2·s−1 at canopy level throughout the 12-h photoperiod. The photoperiod started at 1200 HR and ended at 2400 HR. High-temperature treatments were applied by moving groups of six plants per treatment from the moderate-temperature growth chamber to the high-temperature growth chamber at specific times within the 24-h cycle for a duration of either 6 or 12 h (Table 1). The four 6-h high-temperature treatments were applied from 2400 HR to 0600 HR, 0600 HR to 1200 HR, 1200 HR to 1800 HR, or 1800 HR to 2400 HR (i.e., high temperatures were applied for either the first 6 h or the last 6 h of either the night or day). Two 12-h high-temperature treatments were applied for the entire duration of the night or day period. Finally, a group of plants was kept in either the moderate-temperature or high-temperature growth chamber for the entire 24-h cycle to serve as a moderate-temperature or high-temperature control group. These eight temperature treatments were delivered for 2 weeks; after that time, all plants were moved to a greenhouse to finish flowering at moderate temperatures (21.3 ± 3.0 °C ADT) under a 12-h NL maintained with a black curtain system. An average daily light integral (DLI) of 16.1 ± 4.6 mol·m−2·d−1 was recorded in the greenhouse. Treatments were completely randomized within both the growth chamber and greenhouse environments. This experiment was repeated twice; the first replicate began on 11 Aug 2021, and the second replicate began on 14 Jan 2022.
Poinsettia ‘Advent Red’ cuttings were propagated and grown as in Expt. 1. Six growth chambers were used to provide six constant temperature treatments, 24 °C (24.5 ± 0.5 °C), 26 °C (26.4 ± 0.8 °C), 28 °C (28.6 ± 0.2 °C), 30 °C (29.9 ± 0.5 °C), 32 °C (32.1 ± 0.9 °C), and 34 °C (33.6 ± 1.5 °C), and were outfitted with LED lamps (Phillips TLL 630 DRW_VISN2; Ball Horticultural Co., Chicago, IL, USA) that provided a PPFD of 175 ± 25 µmol·m−2·s−1 at canopy level throughout the 12-h photoperiod for a DLI of 15 mol·m−2·d−1. High-temperature treatments were applied for 8 h/d by moving groups of six plants per treatment from the 24 °C growth chamber to five growth chambers during the last 6 h of the dark period and first 2 h of the light period, while one group remained at 24 °C for 24 h/d (control). After the 8-h temperature treatments, plants were returned to the 24 °C growth room. The vapor pressure deficit values for each treatment were 24 °C (0.9 ± 0.3 kPa), 26 °C (1.1 ± 0.3 kPa), 28 °C (1.2 ± 0.4 kPa), 30 °C (2.4 ± 0.5 kPa), 32 °C (2.5 ± 0.4 kPa), and 34 °C (3.3 ± 0.6 kPa). After the 2-week treatments, all plants were moved to a greenhouse (25.2 °C ± 3.7 and PPFD 198 ± 167 µmol·m−2·s−1) (Rep. 1) or a 24 °C growth chamber (Rep. 2). Both environments included a 12-h photoperiod for a DLI of 17 mol·m−2·d−1 (Rep. 1) and 14 mol·m−2·d−1 (Rep. 2). The first replicate of the experiment started in Jul 2023, and the second replicate started in Aug 2023.
Dates of first color (when 100% of one bract had red pigmentation), visible bud (when the primary bracts unfolded to reveal the primary cyathium), and anthesis (when pollen was visible on one stamen within the primary cyathium) were recorded. Additionally, the three primary bracts and five stem bracts subtending the primary bracts were rated using a qualitative scale from 0 to 4 at the time of anthesis: 0 = no red pigmentation; 1 = 1% to 25% red; 2 = 26% to 75% red; 3 = 76% to 99% red color; and 4 = 100% red. A statistical data analysis was performed using JMP Pro (version 16.0; SAS Institute Inc., Cary, NC, USA). Analyses of variance (ANOVA) were conducted to evaluate the significance of each main effect and their interactions on each of the flowering responses. Least square means were calculated for each treatment for time from the start of the experiment to visible bud, first color, anthesis, and color rating of the upper five bracts (the three primary bracts and two upper stem bracts). Significance of treatment means was calculated using Fisher’s least significant difference test (P < 0.05) to minimize type two statistical errors and to ensure that the significance of individual high-temperature treatment groups was detected.
Time to visible bud, first color, and anthesis were significantly affected by temperature, but the bract color rating measured at the time of anthesis was not (Table 2). No significant differences occurred between the two replications of the experiment; therefore, the data were pooled. Delays in flowering are reported relative to the moderate-temperature (22 °C) control group. Time to visible bud was significantly delayed by all high-temperature treatments (Fig. 1A). The most significant delay, 12 d, was the result of high-temperature (28 °C) treatments applied during the last 6 h of the night, the 12-h night period, and the entire 24-h period (high-temperature control). These were followed by the 12-h day high-temperature treatment that resulted in a 9-d delay. A 6-d delay to visible bud was observed in the high-temperature treatment applied during the first 6 h of the day. Finally, high-temperature treatments applied during the first 6 h of the night and the last 6 h of the day caused a 3-d delay.
Citation: HortScience 60, 5; 10.21273/HORTSCI18419-25
The most significant delay to first color, 11 d, was the result of high temperatures applied during the last 6 h of the night, the 12-h night period, and the entire 24-h period (Fig. 1B). High temperatures applied during the first 6 h of the day and the entire 12-h day caused a 5-d delay. There was no significant difference in time to first color when the high-temperature treatment was applied during the first 6 h of the night and the last 6 h of the day and when moderate-temperature (22 °C) control treatment was applied.
Time to anthesis was significantly delayed by all high-temperature treatments (Fig. 1C). The largest delays of 11 d, 10 d, and 10 d resulted from high temperatures applied during the last 6 h of the night, the 12-h night period, and the entire 24-h period, respectively. Delays of 7 d and 6 d were observed in high-temperature treatments applied during the first 6 h of the day and the 12-h day period, respectively. High-temperatures applied during the first 6 h of the night and the last 6 h of the day caused a 3-d delay in the time to anthesis.
Plant exposure to temperatures ≥26 °C for 8 h/d, which were delivered during last 6 h of the dark period and the first 2 h of the light period, caused a significant delay in time to first color, visible bud, and anthesis compared with plants grown at 24 °C for 24 h/d (Fig. 2A–2C). The 26 °C treatment for 8 h/d caused delays of 14 d, 11 d, and 10 d to time to first color, visible bud, and anthesis, respectively, compared with control plants provided 24 °C for 24 h/d, and the delay to first color and anthesis were not significantly different between plants receiving 26 °C and 34 °C for 8 h/d.
Citation: HortScience 60, 5; 10.21273/HORTSCI18419-25
This current research refines the temperatures and time of high-temperature exposure that are detrimental to poinsettia flowering. A temperature of 26 °C was sufficiently high to cause flowering delay in plants exposed for 8 h/d, specifically the last 6 h of the dark period and the first 2 h of the light period. These temperature treatments were provided when the poinsettia is at its most sensitive period, namely, during the first 2 weeks of flower initiation when NL is 12 h/d. Poinsettia is less sensitive to high temperatures during later stages of flower development and during longer NLs (Alden and Faust 2021, 2022).
The time of the 24-h day when poinsettia is most sensitive to high temperature is similar to the response reported for chrysanthemum. Nakano et al. (2015) reported a delay in flower development for more than 25 d when plants were exposed to high temperature (30 °C) between midnight and dawn for 8 h. High temperatures were applied for 14 d under short-day conditions (8-h photoperiod). High temperatures applied for 4 h or 8 h after dusk affected flower development to a lesser extent.
Nakano et al. (2015) reported the molecular mechanism involved in the delayed flower development after high-temperature treatment (30 °C) applied between midnight and dawn for 8 h in chrysanthemum. High temperatures decreased the expression of the flowering locus T-like3 (CsFTL3) gene required for flower development. Chrysanthemum requires repeated short-days to express CsFTL3 at high levels. If the gene is not present at high levels, then flower development does not occur; therefore, it appears that high temperature delays flowering by reducing CsFTL3 gene expression.
Previous research has shown that diurnal fluctuations of light sensitivity of photoperiodic plants are regulated by circadian rhythms (Claes and Lang 1947; Coulter and Hamner 1964; Harder and Bode 1943). The diurnal fluctuation of sensitivity to high temperatures observed in this study indicated that this process may also be regulated by circadian rhythms. Nakano et al. (2020) found that high-temperature treatments applied relatively late into the night (10–16 h from the start of the night) caused more delay to anthesis in chrysanthemum compared with high-temperature treatments applied earlier in the night. These results are consistent with those found in the current study; however, Nakano et al. (2020) also reported that high temperatures applied at the start of a day period did not cause significant delays to anthesis. This observation contrasts with the results of this study, which found high temperatures applied during the first 6 h of the day significantly delayed time to anthesis.
No significant effect of temperature on bract color rating was measured at the time of anthesis. This may be the result of high temperatures applied for only the first 2 weeks after the inductive 12-h NL was provided. In a previous study, Millar et al. (2023) demonstrated that ‘Prestige Red’ started to initiate flowers the week of 18 to 24 Sep. High temperatures provided at this time delayed anthesis but did not affect final bract color, presumably because no bract development processes occur during the first week of flower initiation. Plants receiving high temperatures for 8 weeks, from 4 Sep through 29 Oct, responded similarly; in other words, anthesis was delayed but final bract color was normal because bract development occurred only after the high temperature ceased. Interestingly, plants receiving high temperatures after flower initiation during any single week from 25 Sep to 29 Oct experienced delayed flowering and reduced bract color development because the high temperatures occurred after flower initiation and during a time when bract development was sensitive. A specific gene for anthocyanin accumulation has been reported for poinsettia (Gu et al. 2018). The red color of poinsettia bracts depends on the accumulation of anthocyanins (Slatnar et al. 2013). The dihydroflavonol 4-reductase (DFR) gene was identified as a promoter of anthocyanin accumulation in poinsettia bracts under short-day conditions. The effect of high temperatures on the expression of the DFR gene requires further study to obtain a better understanding of the molecular mechanisms involved in anthocyanin accumulation in plants grown under high temperatures during specific times of the season.
Millar et al. (2023) demonstrated that the most critical time for temperature management in poinsettia production varies between cultivars but, in general, extends from mid-September to mid-October, which encompasses the period of flowering initiation for the range of cultivars categorized as early, mid, and late season. The results of the current study further narrow this window of critical temperature management. Within the overall period of sensitivity to high temperature identified for a particular cultivar, the later portion of the night to the early portion of the day is the most critical time period within a given day for temperature management. Therefore, growers should prioritize maintaining lower temperatures during these times. This may involve the use of exhaust fans and cooling pads that are often activated much less often during the latter hours of the night and early morning compared with the mid-day and early hours of the evening.
These results also have implications for black cloth management of NL because black cloth can be pulled over poinsettia plants before sunset or can be removed after sunrise to increase NL and induce flowering. Heat tends to builds under the black curtains when the sun is delivering light to the greenhouse. Our results suggest that there may be more risk associated with black cloth pulled during sunrise because poinsettia is more sensitive to high temperatures at the end of the night.
Poinsettia ‘Prestige Red’ was exposed to high-temperature (28 °C) treatments during the first 6 h, the last 6 h, or the entire 12 h of the night or day period within a 24-h cycle (12-h night/12-h day) and the entire 24-h day/night period (control). The plants were in a moderate-temperature (22 °C) environment at all other times for a duration of 2 weeks. The days from the start of the experiment to visible bud (A), first color (B), and anthesis (C) are reported for the eight temperature treatments. Different letters represent significant differences between treatment means (P < 0.05) using Fisher’s least significant difference test.
Poinsettia ‘Advent Red’ plants exposed to high-temperature (26, 28, 30, 32, and 34 °C) treatments during the last 6 h of the night and first 2 h in the morning (8 h total), followed by a moderate temperature (24 °C) for the remainder of the 24-h cycle. Control plants were exposed to 24 °C for 24 h/d. Plants were treated during the first 2 weeks after flower initiation (12-h photoperiod). Average time (in days) from the start of the experiment to first color (A), visible bud (B), and anthesis (C) are reported for the six temperature treatments. Different letters represent significant differences between treatment means (P < 0.05) using Tukey’s honestly significant difference test.
Contributor Notes
Poinsettia ‘Prestige Red’ was exposed to high-temperature (28 °C) treatments during the first 6 h, the last 6 h, or the entire 12 h of the night or day period within a 24-h cycle (12-h night/12-h day) and the entire 24-h day/night period (control). The plants were in a moderate-temperature (22 °C) environment at all other times for a duration of 2 weeks. The days from the start of the experiment to visible bud (A), first color (B), and anthesis (C) are reported for the eight temperature treatments. Different letters represent significant differences between treatment means (P < 0.05) using Fisher’s least significant difference test.
Poinsettia ‘Advent Red’ plants exposed to high-temperature (26, 28, 30, 32, and 34 °C) treatments during the last 6 h of the night and first 2 h in the morning (8 h total), followed by a moderate temperature (24 °C) for the remainder of the 24-h cycle. Control plants were exposed to 24 °C for 24 h/d. Plants were treated during the first 2 weeks after flower initiation (12-h photoperiod). Average time (in days) from the start of the experiment to first color (A), visible bud (B), and anthesis (C) are reported for the six temperature treatments. Different letters represent significant differences between treatment means (P < 0.05) using Tukey’s honestly significant difference test.
