Poinsettia is an SD plant that forms a determinate inflorescence under inductive photoperiods (Ecke et al., 2004). The inflorescences described in this manuscript are defined as follows: the primary cyathium ends the apical stem and is the first cyathium to reach anthesis; the secondary cyathia consist of three cyathia that subtend the primary cyathium; and one primary bract subtends each of the three secondary cyathia and displays a whorl of three bracts. Stem bracts refer to bracts that develop on the stem below the whorl of primary bracts. Primary and stem bracts display a range in the area of the bract containing red pigmentation, and the area of pigmentation typically increases with the proximity of the bract to the shoot apex.
Several of the foundational studies on poinsettia flowering used anatomical observations by dissecting apical meristems from plants under inductive photoperiods (Goddard, 1960; Larson and Langhans 1962a, 1962b; Miller and Kiplinger, 1962). In each of these studies, the vegetative meristem was shown to persist for several days after the start of SD until a morphological transformation occurred that physically distinguished the meristem from one that was vegetative. The measurements and descriptions slightly vary among these studies; however, in general, the meristem becomes reduced in height, with a horizontally flat surface, followed by the differentiation of the primordia of the three primary bracts and the primary cyathium. The time from the start of an inductive photoperiod until the transition of the meristem from vegetative to reproductive development is defined as flower initiation, i.e., the sum of the various physiological and biochemical responses to inductive photoperiods that elicit a morphological change at the shoot apex. Flower development is defined as the sum of all floral development events downstream of the first observable change at the apex.
To evaluate the effects of NL on flower initiation, several studies have subjected poinsettias to various photoperiods at moderate temperatures and dissected shoot tips to determine when flower initiation has occurred (Larson and Langhans, 1962b; Miller and Kiplinger, 1962; Wang, 2001; Wieland, 1998). For example, Larson and Langhans (1962b) reported that flower initiation of ‘Barbara Ecke Supreme’ occurred after 14, 16, 18, and 30 d when subjected to NLs of 16, 15, 14, and 12 h, respectively. Miller and Kiplinger (1962) found similar results for ‘Barbara Ecke Supreme’: flower initiation occurred at 14, 16, 27, and 65 d when subjected to NLs of 14, 13, 12, and 11 h. Grueber and Wilkins (1994) subjected poinsettia cultivars ‘Brilliant’ and ‘V-14’ to 16-h NLs or natural day photoperiods starting on 3 Sept. (lat. 44°57′N) and reported that flower initiation occurred within 6 to 10 d under the 16-h NL treatment, whereas flower initiation occurred within 19 to 24 d under the natural day photoperiods. These results demonstrate that flower initiation occurs more rapidly at NLs between 13 and 16 h than at NLs of 11 to 12 h; therefore, when black cloth is used to artificially extend the NL to 13 h or more, poinsettias will initiate flowers faster than when provided natural NLs during the fall.
The photoperiod requirements for flower initiation and flower development can be different for the same plant. For example, chrysanthemums (Dendranthemum ×grandiflorum) are SD plants that will initiate flowers under photoperiods that are insufficiently short for flower development (Cockshull, 1976). Similarly, poinsettias maintained under long-day conditions will initiate a flower bud when a cultivar-specific node number has been achieved, but the bud will fail to develop into a functional flower under those same long-day conditions (Evans et al., 1992). In both species, flower initiation may occur with photoperiods under which flower development otherwise fails. Furthermore, different phases of flower development of poinsettia have varying degrees of sensitivity to the photoperiod. For example, Miller and Kiplinger (1962) reported that the time from flower initiation to a macroscopically visible flower bud, termed visible bud, increased from 17 to 23 to 38 d as the NL decreased from 13 to 12 to 11 h, respectively, for poinsettia ‘Barbara Ecke Supreme’. In contrast, no differences were observed when plants were placed under 15-h NL until visible bud, followed by a shift to NLs of 11, 12, and 13 h until anthesis. These results suggested that the early phase of flower development was sensitive to NL from 11 to 13 h, whereas the development of the inflorescence from visible bud to anthesis was not. Thus, flower initiation and flower development may respond differently to a given NL, and different stages of flower development may also respond differently to NL.
Previous research studying the effects of NL on flower development resulted in conflicting outcomes. For example, Grueber and Wilkins (1994) reported that poinsettias forced to flower under 16-h NLs developed faster than poinsettias under natural NLs (lat. 44°57′N) beginning on 3 Sept. from flower initiation to visible bud; however, the inverse was true for flower development from visible bud to anthesis. Miller and Kiplinger (1962) found that NLs more than 13 h decreased the number of days from flower initiation to visible bud; however, no differences were observed among NL treatments (11, 12, 13, 14, and 15 h) from visible bud to anthesis. Wieland (1998) found no differences in the number of days from flower initiation to visible bud or anthesis with a NL of 15 h and natural NLs beginning on 17 Sept. or 6 Oct. (lat. 29°40′N).
Previous studies of poinsettia flower development have not evaluated bract color development beyond recording the date of first color. Bract color development is the primary marketable trait of poinsettia; therefore, understanding how the photoperiod affects color development has significant value for commercial growers, and it cannot be assumed that bract and cyathia development respond similarly to NL.
The objective of this research was to evaluate the effect of NL on the bract color and flower development of a modern poinsettia cultivar and, specifically, to determine whether naturally occurring NLs in October and November are optimal for flower development (i.e., 12-h to 13-h NL compared with a constant 14-h NL).
Alden, M. & Faust, J.E. 2021 Unravelling the role of temperature and photoperiod on poinsettia heat delay J. Amer. Soc. Hort. Sci. 56 1097 1103 https://doi.org/10.21273/HORTSCI15874-21
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