Poinsettia (Euphorbia pulcherrima) is a popular potted flowering crop, especially for the Christmas holiday, because of its colorful and showy bracts. Approximately one-third of the global poinsettia market is in the United States, whereas the remaining market is mostly in Europe (Taylor et al., 2011). Poinsettia was the second most valuable potted flowering plant after orchids in 2015, accounting for $140 million in wholesale sales and representing 17% of the total value of potted flowering plants sold in the United States (U.S. Department of Agriculture, 2016). Coordinated control of flowering of poinsettia is crucial so that commercial crops are marketable for predetermined dates for the Christmas holiday. It is a short-day (SD) plant (SDP) with a critical photoperiod of ≈12 h and 20 min (night length of 11 h and 40 min), and thus is naturally induced to flower in the northern hemisphere beginning in late September (Ecke et al., 2004).
Height control is one of the major challenges in commercial greenhouse production of poinsettias. High-quality poinsettia requires the plants to be marketed at specific target heights, which is typically 36 to 41 cm for plants grown in 15-cm containers (Taylor et al., 2011). Excessively tall poinsettias can develop branches that are more susceptible to breakage and increase transportation costs (Clifford et al., 2004). Therefore, plant growth retardants are commonly used to suppress stem extension to produce more compact plants. Chlormequat chloride, daminozide, uniconazole, paclobutrazol, and flurprimidol, used as a substrate drench and/or foliar spray, are effective at inhibiting stem elongation (Currey and Lopez, 2011; Latimer and Whipker, 2013; Lopez and Runkle, 2007).
For ornamentals, although the promotion of stem growth is usually not desired in commercial production, it can be beneficial in some situations, such as in cut flower production or when plants are shorter than desired. Gibberellic acid is commonly applied by commercial growers to increase plant height and promote leaf expansion of poinsettia when an excessive concentration of a plant growth retardant was previously applied (Lopez and Runkle, 2007); however, the use of growth-regulating chemicals is restricted to various degrees by different governments (Rajapakse et al., 1999). Different strategies can be used as an alternative way to promote extension growth, such as a warmer day than night temperature, greater planting density (i.e., less spacing), and avoidance of phosphorus deficiency (Ecke et al., 2004; Runkle, 2005).
Light quality manipulation is an alternative strategy to manage plant height, especially the ratio of R (600–700 nm) and FR (700–800 nm) radiation. R and FR radiation are perceived by phytochrome photoreceptors, which have two forms: an R light–absorbing (Pr) form and an FR light–absorbing (Pfr) form (Butler et al., 1959). The interconversion between these two forms of each phytochrome in plants is known as photoreversibility and can at least partially regulate seed germination, extension growth, and flowering (Borthwick and Downs, 1964; Borthwick et al., 1952; Schopfer et al., 1982). Spectral filters have been used experimentally to determine the effects of different light qualities on plant height. For example, poinsettias were grown under cladding materials of neutral density (control) (R:FR = 1.07) or FR, R, and blue (400–500 nm) plastic filters, creating an FR-deficient, R-deficient, and blue-deficient environment with an R:FR of 1.74, 0.04, and 1.05, respectively, under sunlight (Clifford et al., 2004). The internode lengths were increased by 71% and 9% under R- and blue-deficient filters, respectively, and decreased by 20% under the FR-deficient filter compared with the control.
EOD lighting also can be used to regulate extension growth by manipulating the R:FR ratio. Generally, plants elongate under a low R:FR ratio. For example, chrysanthemum (Chrysanthemum ×morifolium) was grown under a 9-h SD with EOD lighting for 30 min at a photosynthetic photon flux density (PPFD) of 1 to 3 μmol·m−2·s−1 with either a high (2.4) or low (0.4) R:FR (Lund et al., 2007). Plant height after 3 weeks increased by 50% to 75% with EOD lighting at the low R:FR compared with the high ratio. In a separate study, impatiens (Impatiens walleriana), geranium (Pelargonium ×hortorum), and petunia (Petunia ×hybrida) were grown under an 8.5-h day with 30 min of EOD lighting at 20 μmol·m−2·s−1 with an R:FR of 0.9 or 8.4 (Randall, 2014). Stem length of impatiens, geranium, and petunia was promoted by 29%, 20%, and 44%, respectively, after 21 d under the lower R:FR compared with the higher ratio.
EOD lighting also can be used to extend the natural photoperiod and create long days to regulate flowering of photoperiodic crops. The threshold irradiance to regulate flowering of several herbaceous perennials ranged from 0.05 to 0.4 μmol·m−2·s−1, whereas the saturation irradiance ranged from 0.2 to 1.0 μmol·m−2·s−1, depending on the species (Whitman et al., 1998). Generally, an intensity of 1 to 3 μmol·m−2·s−1 is recommended to accelerate flowering of long-day plants (LDPs) and inhibit flowering of SDPs (Runkle, 2015).
R light is typically effective at inhibiting flowering of SDPs. For example, the flowering of chrysanthemum, dahlia (Dahlia hortensis), and marigold (Tagetes erecta) was delayed by 42, 11, and 10 to 20 d when grown under 4-h night interruption (NI) provided by light-emitting diodes (LEDs) with an R:FR ≥0.66 at a photon flux density of 1.3 to 1.6 μmol·m−2·s−1 compared with SD control (Craig and Runkle, 2013). The inclusion of FR with R radiation, delivered as EOD or NI lighting, had little to no effect on regulating flowering of SDPs, but did promote flowering of some LDPs. However, a low intensity of FR alone as an NI is generally not perceived as a long-day signal by LDPs and SDPs. For example, the SDP marigold flowered 10 to 19 d earlier under SDs or NI with only FR compared with NI with a combination of R and FR (Craig and Runkle, 2013).
R/FR photoreversibility is one of the characteristic features for phytochrome responses and was first discovered in a lettuce (Lactuca sativa) seed germination experiment (Borthwick et al., 1952). R light promoted lettuce seed germination, whereas subsequent FR inhibited it. This kind of photoreaction response was also established on flower initiation of some model plants. For example, a short period of R light as an NI prevented the flower initiation of the SDPs soybean (Glycine max) and pigweed (Amaranthus caudatus) and induced the flowering of the LDP henbane (Hyoscyamus niger), whereas subsequent FR irradiation reversed the effects of R light (Downs, 1956). Although there are three action modes for phytochrome responses, photoreversibility is a characteristic of low-fluence response (Li et al., 2011). This suggests that phytochrome-mediated flowering responses could be affected by the last NI lighting treatment and potentially show photoreversibility in at least some light-sensitive plants.
The objective of this study was to investigate whether EOD FR would promote extension growth without influencing flowering of poinsettia. We also wondered whether flowering could be inhibited by delivering FR after R, given the phytochrome photoreversibility response. We postulated that a low intensity of FR as EOD lighting would promote extension growth but not regulate flowering and thus could be used to increase plant height without influencing crop scheduling. We also speculated that ending EOD lighting with FR would at least partly reverse the effects of R light at inhibiting flowering.
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