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Grace M. Pietsch, William H. Carlson, Royal D. Heins, and James E. Faust

The effects of day and night temperatures (15 to 35C) and three irradiance levels [50% of ambient, ambient, and ambient plus 12 mol·m-2·day-1 of supplemental photosynthetic photon flux (PPF)] on development of Catharanthus roseus `Grape Cooler' were determined. Time to flower decreased by 30 days and leaf-pair unfolding rate (LUR) increased linearly as average daily temperature increased from 18 to 35C. Flower size was greatest when plants were grown at 25C. Supplemental light decreased days to flower and increased flower size. Flowering occurred when nine leaf pairs were present on the plant. Using the inverse of the LUR curve, i.e., days per leaf pair, the number of days to flower could be predicted at any time during plant development based on plant leaf number.

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Erik S. Runkle, Royal D. Heins, Arthur C. Cameron, and William H. Carlson

In 1996 and 1997, eight cultivars of cold-treated field-grown Astilbe were grown in a 20 °C green-house with short days (SDs = 9-h natural days) or long days (LDs = 9-h natural days with night interruption with incandescent lamps from 2200 to 0200 hr) to determine how photoperiod influences flowering. Cultivars studied were Astilbe × arendsii Arends `Bridal Veil', `Cattleya', `Fanal', and `Spinell'; A. chinensis Franch. `Superba'; A. japonica A. Gray `Deutschland' and `Peach Blossom'; and A. thunbergii Miq. `Ostrich Plume'. Flowering percentage was highest (≥90%) for `Cattleya', `Deutschland', `Fanal', `Ostrich Plume', and `Peach Blossom', regardless of photoperiod. Photoperiod did not affect the time to visible inflorescence or flower number for any cultivar studied. The time from visible inflorescence to first flower took 27 to 36 days, irrespective of photoperiod. Time to flower varied by cultivar; `Deutschland' was the earliest to flower (31 to 41 days) and `Superba' was the last to flower (51 to 70 days). `Fanal' and `Ostrich Plume' flowered slightly but significantly faster (by 1 to 6 days) under LDs than SDs. For five cultivars, the inflorescence was taller under LDs than SDs. All cultivars reached visible inflorescence and flower significantly faster (by 1 to 15 days) in 1997 than in 1996.

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Erik S. Runkle, Royal D. Heins, Arthur C. Cameron, and William H. Carlson

`Snowcap' Shasta daisy [Leucanthemum ×superbum Bergmans ex. J. Ingram (syn: Chrysanthemum ×superbum, C. maximum)] was grown under various photoperiods and temperatures to determine their effects on flowering. In the first experiment, plants were held for 0 or 15 weeks at 5 °C and then were grown at 20 °C under the following photoperiods: 10, 12, 13, 14, 16, or 24 hours of continuous light or 9 hours with a 4-hour night interruption (NI) in the middle of the dark period. Without cold treatment, no plants flowered under photoperiods ≤14 hours and 65% to 95% flowered under longer photoperiods or NI. After 15 weeks at 5 °C, all plants flowered under all photoperiods and developed three to four or 10 to 11 inflorescences under photoperiods ≤14 or ≥16 hours, respectively. To determine the duration of cold treatment required for flowering under short photoperiods, a second experiment was conducted in which plants were treated for 0, 3, 6, 9, 12, or 15 weeks at 5 °C, and then grown at 20 °C under 9-hour days without or with a 4-hour NI. Under 9-hour photoperiods, 0%, 80%, or 100% of plants flowered after 0, 3, or ≥6 weeks at 5 °C, and time to flower decreased from 103 to 57 days as the time at 5 °C increased from 3 to 12 weeks. Plants that were under NI and received ≥3 weeks of cold flowered in 45 to 55 days. For complete and rapid flowering with a high flower count, we recommend cold-treating `Snowcap' for at least 6 weeks, then providing photoperiods ≥16 hours or a 4-hour NI during forcing.

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Sean C. Clifford, Erik S. Runkle, F. Allen Langton, Andrew Mead, Shirley A. Foster, Simon Pearson, and Royal D. Heins

Most commercial markets require growers of poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch.) to produce plants within strict height specifications. Plant growthretarding chemicals (PGRs) are commonly used to limit internode extension, but in some countries, growers are being pressured to reduce chemical use. Recently, a photoselective film was developed that specifically reduces the transmission of far-red light [(FR), 700 to 800 nm], offering an alternative strategy for height control. Two complementary trials, one in the United Kingdom and one in the United States, showed that plants grown under the FR film for 10 to 12 weeks were ≈20% shorter than control plants growing under neutral density (ND) films transmitting a similar photosynthetic photon flux as the FR film. In the United Kingdom trial, the FR filter delayed time to 50% bract color and first visible cyathia by 6.0 and 3.5 days, respectively, but did not influence time to final harvest. In the United States trial, plants under the FR film had an average of 25% more axillary branches than those under the ND film. In addition, the effects of reduced red [(R), 600 to 700 nm] and blue [(B), 400 to 500 nm] light on internode length, plant biomass, and axillary branching were determined using other photoselective plastics. Compared with plants under the ND film, internode length was 9% or 71% greater in plants grown under environments deficient in B or R, respectively. Our results indicate that poinsettia is highly sensitive to the R: FR ratio, and that spectral manipulation has potential for height control of commercial poinsettia crops.