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Manipulating light quality is a potential alternative method of regulating plant height in the commercial production of ornamental crops. In particular, end-of-day (EOD) lighting with a high red (R; 600–700 nm) to far-red (FR; 700–800 nm) ratio (R:FR) can suppress extension growth, whereas a low R:FR can promote it. We investigated the effects of the R:FR and duration of EOD lighting in regulating extension growth and flowering of two poinsettia cultivars, White Glitter and Marble Star. Plants were grown at 20 °C under 9-hour days with or without EOD lighting provided by two types of light-emitting diode bulbs: R+white+FR (subsequently referred to as R+FR) and FR only. The R:FR ratios were 0.73 and 0.04, respectively, and the photon flux density between 400 and 800 nm was adjusted to 2 to 3 μmol·m⁻²·s^–1 at plant canopy. The six EOD lighting treatments were R+FR or FR for 2 or 4 hours, 2 hours of R+FR followed by 2 hours of FR, and 4 hours of R+FR followed by 2 hours of FR. We also investigated the impact of a 4-hour moderate-intensity (13 μmol·m⁻²·s^–1) EOD FR treatment in the second replication. EOD lighting generally increased poinsettia extension growth, with the greatest promotion under the longest lighting periods. There were no differences in days to first bract color and days to anthesis when the 9-hour day was extended by 2 hours, but flowering was delayed under 4- or 6-hour EOD treatments, except for the 2-hour R+FR + 2-hour FR and 4-hour FR treatments. Four hours of moderate-intensity EOD FR greatly promoted extension growth and delayed or prevented bract coloration in both cultivars. We conclude that EOD lighting promotes extension growth of poinsettia, and specifically, EOD FR at a low intensity (2–3 μmol·m⁻²·s^–1) is not perceived as long-day signal, whereas a higher intensity (13 μmol·m⁻²·s^–1) of FR delays flowering.

Photoperiodic lighting from lamps with a moderate ratio of red [R (600–700 nm)] to far-red [FR (700–800 nm)] light effectively promotes flowering of long-day plants (LDPs). Because of spectral controllability, long life span, and energy efficiency, light-emitting diodes (LEDs) have emerged as an alternative to conventional light sources, such as incandescent (INC) and high-pressure sodium (HPS) lamps. We conducted a coordinated trial with five commercial greenhouse growers to investigate the efficacy of R + white (W) + FR LEDs, with an R:FR of 0.82, to regulate flowering of daylength-sensitive ornamental crops. The trial was also performed in two replicate greenhouses at Michigan State University (MSU). Ageratum (Ageratum houstonianum), calibrachoa (Calibrachoa ×hybrida), dahlia (Dahlia ×hybrida), dianthus (Dianthus chinensis), petunia (Petunia ×hybrida), snapdragon (Antirrhinum majus), and verbena (Verbena ×hybrida) were grown under natural short days (SDs) with 4-hour night-interruption (NI) lighting provided by the R + W + FR LEDs or conventional lamps typically used by each grower. Two companies used HPS lamps, whereas the other sites used INC lamps. In addition, a natural SD treatment, a truncated 9-hour SD treatment, or a compact fluorescent lamp (CFL) NI treatment was provided at three different sites. With few exceptions, time to flower and flowering percentage of the bedding plant crops tested were similar under the R + W + FR LEDs to that under the conventional lamps at all sites. At MSU, ageratum, dianthus, petunia, snapdragon, and verbena flowered earlier under NI lighting treatments than under 9-hour SDs. In addition, plant height and visible flower bud or inflorescence number at flowering were similar under the R + W + FR LEDs and INC lamps for most crops. Therefore, we conclude that the R + W + FR LEDs are as effective as lamps traditionally used in greenhouses at controlling flowering of photoperiodic plants.

In protected cultivation of short-day (SD) plants, flowering can be inhibited by lighting from incandescent (INC) lamps during the night. INC lamps are being phased out of production and replaced by light-emitting diodes (LEDs), but an effective spectrum to control flowering has not been thoroughly examined. We quantified how the red [R (600 to 700 nm)] to far red [FR (700 to 800 nm)] ratio (R:FR) of photoperiodic lighting from LEDs influenced flowering and extension growth of SD plants. Chrysanthemum (Chrysanthemum ×morifolium), dahlia (Dahlia hortensis), and african marigold (Tagetes erecta) were grown at 20 °C under a 9-hour day with or without a 4-hour night interruption (NI) treatment by INC lamps or LEDs with seven different R:FR ranging from all R to all FR. Flowering in the most sensitive species, chrysanthemum, was not inhibited by an R:FR of 0.28 or lower, whereas an R:FR of 0.66 or above reduced flowering percentage. Flowering in dahlia was incomplete under the FR-only NI and under SDs, but time to flower was similar under the remaining NI treatments. The least sensitive species, african marigold, flowered under all treatments, but flowering was most rapid under the FR-only NI and under SDs. For all species, stem length increased quadratically as the R:FR of the NI increased, reaching a maximum at R:FR of ≈0.66. We conclude that in these SD plants, a moderate to high R:FR (0.66 or greater) is most effective at interrupting the long night, blue light is not needed to interrupt the night, and FR light alone does not regulate flowering.

Coreopsis grandiflora `Sunray' has been reported to flower under long days (LD) following vernalization or short days (SD). The objectives of this study were to characterize the effective duration of vernalization and SD and to determine if photoperiod during vernalization influences flowering. Vegetative cuttings taken from stockplants developed from one seedling were rooted for 2 weeks and grown for 5 weeks. Plants were provided with a 9-hour photoperiod for 2, 4, 6, or 8 weeks or were vernalized at 5 °C under a 16-hour photoperiod for 2, 4, 6 or 8 weeks or under a 9-hour photoperiod for 2 or 8 weeks. Following treatments, plants were grown in a greenhouse at 20 °C under a 16-hour photoperiod. Control plants were grown under constant 9- or 16-hour photoperiod. Leaf development, days to first visible bud (DVB), days to first open flower (DFLW), and height and total number of flower buds at FLW were recorded. No plants flowered under continuous SD. Under continuous LD, two plants flowered on axillary shoots but only after 95 days. All vernalized and SD-treated plants flowered on both terminal and axillary shoots. Photoperiod during vernalization did not affect subsequent flowering. DFLW decreased from 56 to 42 and from 50 to 42 after 2 to 8 weeks of vernalization and SD treatments, respectively. Following 2, 4, 6, and 8 weeks of vernalization, plants had 116, 116, 132, and 204 flower buds, respectively. Plant height at FLW of all SD-treated and vernalized plants was similar. Thus, 2 weeks of 9-hour SD or vernalization at 5 °C followed by LD was sufficient for flowering of our clone of C.`Sunray', although longer durations hastened flowering and increased flower bud number.

The production value of potted orchids has increased by 155% in the past decade, and they are now the second-most valuable potted flowering plant in the United States. Scheduling orchids to flower on specific dates requires knowledge of the environmental parameters that regulate flower induction. However, the flowering requirements of the vast majority of orchid species and hybrids have not been well described. Odontioda is a cool-growing, epiphytic genus originating from the Andes Mountains of South America, and several hybrids are commercially grown for their bright-colored flowers and compact habit. We quantified the promotion of inflorescence initiation and time from visible inflorescence (VI) to anthesis at constant and fluctuating day/night temperatures. Odontioda George McMahon `Fortuna' and Lovely Penguin `Emperor' were grown at constant temperatures of 14, 17, 20, 23, 26, or 29 °C, and day/night (12 h/12 h) temperatures of 20/14, 23/17, 26/14, 26/20, 29/23, or 29/17 °C. Plants were grown in glass greenhouses under a 12-h photoperiod, and shading was provided so that the maximum instantaneous irradiance was ≤300 μmol·m^-2·s^-1. After 6 weeks at the various temperature setpoints, heat stress symptoms were observed on plants grown at 26, 29, 26/14, 26/20, 29/23, and 29/17 °C. After 14 weeks, ≥60% of both hybrids had VI when grown at 14, 17, 20, or 20/14 °C. Data for time from VI to anthesis were converted to a rate and a thermal-time model relating temperature with inflorescence development was developed. This information could be used by commercial orchid growers to schedule flowering Odontioda orchids for specific market dates.

Environments with a low red (R, 600 to 700 nm) to far-red (FR, 700 to 800 nm) ratio (e.g., with high plant density) promote stem elongation, and a high R: FR suppresses it. While FR light promotes stem extension, it is also required for rapid, uniform flowering of many long-day plants. We investigated how a new FR filter [creating a FR-deficient (FRd) environment] influenced plug growth and subsequent flowering of pansy (Viola ×wittrockiana `Crystal Bowl Yellow'), petunia (Petunia ×hybrida `Carpet Pink'), impatiens (Impatiens wallerana `Accent Rose'), snapdragon (Antirrhinum majus `Liberty Scarlet'), and tomato (Solanum lycopersicon `Beefmaster'). One-week-old seedlings were placed under three filter treatments with 16-h photoperiods: the FRd filter, a neutral-density filter (N) that transmitted a similar PPF, and transferring plugs from the N to the FRd filter when leaves of each species began to touch (7 to 11 days later). The predicted phytochrome photoequilibria under the FRd and N filters was 0.80 and 0.72, respectively. After 25 to 35 days at 20 °C, node number and stem (or petiole for pansy) length were collected. Twenty plants of each species and filter treatment were then transferred to 4-inch pots and grown under natural photoperiods (14 to 15 h) at 20 °C until flowering. Compared to plants continually under the N filter, stem length under the FRd filter was significantly reduced in impatiens (by 11%), pansy (by 18%), petunia (by 34%), snapdragon (by 5%), and tomato (by 24%). Flowering of plants from plugs under the FRd filter was delayed by 2 to 3 days for snapdragon, petunia, and pansy. Filter treatment of plugs had no significant effect on flower number or plant height at flower.

Flowering of the herbaceous perennial Aquilegia is generally considered to require vernalization after seedlings are mature, whereas photoperiod has little or no effect. We performed experiments to determine the flowering responses for two Aquilegia ×hybrida varieties, one of which reportedly has reduced cold requirements. Seedlings of Aquilegia ‘Origami Blue and White’ and ‘Winky Double Red and White’ with three to five leaves were either placed directly into a 5 °C cooler with low-intensity lighting for 9 hours/day or transplanted to 13-cm containers and grown (bulked) for 0, 3, or 6 weeks at 20 °C under 9-hour short days (SDs) or 16-h long days (LDs). Plants were then cooled at 5 °C for 0, 5, or 10 weeks and placed in a common forcing environment at 20 °C under LDs. Flowering response of the two cultivars differed markedly. All Aquilegia ‘Origami Blue and White’ plants placed directly into the forcing environment flowered and in a mean of 93 days. Flowering percentage of plants cooled in the plug tray decreased with increasing duration of cold treatment, and only 15% flowered after a 10-week cold treatment. All plants bulked for 3 or 6 weeks before cold treatment flowered after 25 to 36 days in the forcing environment. Adding bulking and forcing time together, time to flowering of ‘Origami Blue and White’ was complete and most rapid (62 days) when plants were bulked for 3 weeks under SDs and then forced under LDs. In contrast, no ‘Winky Double Red and White’ plants flowered without cold treatment, and 6 weeks of bulking followed by 10 weeks of cold was required for 100% flowering. These results indicate that ‘Origami Blue and White’ has a relatively short juvenile phase and flowering was promoted by SD bulking or cold treatment, whereas ‘Winky Double Red and White’ has a longer juvenile phase and requires cold for flowering.

The vegetatively propagated `Fire Kiss' clone of the hybrid Zygopetalum Redvale orchid has appealing potted-plant characteristics, including fragrant flowers that are waxy lime-green and dark maroon with a broad, three-lobed, magenta and white labellum. We performed experiments to quantify how temperature influenced leaf unfolding and expansion, time from visible inflorescence to flower, and longevity of individual flowers and inflorescences. Plants were grown in controlled-environment chambers with constant temperature set points of 14, 17, 20, 23, 26, and 29 °C and an irradiance of 150 μmol·m^-2·s^-1 for 9 h·d^-1. As actual temperature increased from 14 to 25 °C, the time to produce one leaf decreased from 46 to 19 days. Individual plants were also transferred from a greenhouse to the chambers on the date that an inflorescence was first visible or the first flower of an inflorescence opened. Time from visible inflorescence to open flower decreased from 73 days at 14 °C to 30 days at 26 °C. As temperature increased from 14 to 29 °C, flower and inflorescence longevity decreased from 37 and 38 days to 13 and 15 days, respectively. Data were converted to rates, and thermal time models were developed to predict time to flower and senescence at different temperatures. The base temperature was estimated at 6.2 °C for leaf unfolding, 3.5 °C for time to flower, and 3.7 °C for flower longevity. These models could be used by greenhouse growers to more accurately schedule Zygopetalum flowering crops for particular market dates.

In 2003, commercial greenhouse growers in the United States imported 724 million nonrooted cuttings valued at $53 million. During transit and storage, cuttings can be exposed to environmental stresses (e.g., low or high temperature), which can consequently decrease quality, rooting, and subsequent plant performance. We performed experiments to quantify how temperature and storage duration of cuttings influence root initiation, root number, lateral branch count and length, and time to flower of Tiny Tunia `Violet Ice' petunia (Petunia × hybrida hort. Vilm. -Andr.). Dry or wet cuttings were harvested and packaged into perforated bags within small, ventilated boxes and then into traditional shipping boxes. The boxes were placed in environmental chambers with temperature setpoints of 0, 5, 10, 15, 20, 25, or 30 °C for 0, 1, 2, 3, 4, or 5 d. Cuttings were then rooted in a propagation house at 26 °C with a vapor pressure deficit of 0.3 kPa under ambient photo-periods. The visual quality rating of dry packaged cuttings decreased with increasing temperature and shipping duration. After 2 d at ≥25 °C, cuttings were horticulturally unacceptable due to water stress and chlorophyll degradation and they never fully recovered. Dry- or wet-packaged cuttings held at temperatures of 0 to 30 °C formed significantly fewer roots and lateral branches as duration increased from 1–5 d. Although cuttings held for 5 d at 0 °C produced 60% fewer lateral branches, they subsequently flowered 5 d earlier than plants held at 0 °C for 1 d. Therefore, exposure to temperatures >15 °C for ≥3 d can reduce petunia cutting quality, delay rooting, and decrease plant size at flowering.

Prohexadione-Ca (ProCa) is a relatively new plant growth regulator (PGR) that inhibits internode length in rice, small grains, and fruit trees. However, little is known about its efficacy and potential phytotoxicity on floriculture crops and how it compares to other commercially available PGR chemicals. The effects of two foliar spray applications (2 weeks apart) of ProCa (500, 1000, or 2000 ppm), paclobutrazol (30 ppm), or a tank mix of daminozide plus chlormequat (2500 and 1000 ppm, respectively) were quantified on Dianthus barbatus L. `Interspecific Dynasty Red', Ageratina altissima R. King & H. Robinson (Eupatorium rugosum) `Chocolate', Lilium longiflorum Thunb. `Fangio', and Buddleia davidii Franch. `Mixed.' All plants were forced in a glass-glazed greenhouse with a constant temperature setpoint of 20 °C under a 16-h photoperiod. Two weeks after the second spray application of ProCa at 500, 1000, or 2000 ppm, plant height of Dianthus and Lilium was shorter than control plants by 56%, 60%, and 65% and by 6%, 26%, and 28%, respectively. However, ProCa bleached and reduced the size of Dianthus flowers. ProCa at 2000 ppm and daminozide plus chlormequat were effective at controlling the height of Eupatorium (64% and 53% reduction, respectively); however, leaves of Eupatorium were discolored and showed symptoms of phytotoxicity 1 week after the first ProCa application. Only daminozide plus chlormequat were effective on Buddleia. ProCa is an effective PGR for most of the crops we tested; however, its discoloration of red flowers and foliage may limit its application for commercial use.