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

Twenty-four herbaceous perennial species were treated at 5C for 0 or 15 weeks. Critical photoperiods for flower initiation and development with and without a cold treatment were determined. Photoperiods were 10, 12, 14, 16, and 24 h of continuous light and 9 h plus a 4-h night interruption. Continuous photoperiodic treatments consisted of 9-h natural days extended with light from incandescent lamps. Response to cold and photoperiod varied by species; Scabiosa caucasica `Butterfly Blue' flowered without a cold treatment under all photoperiods after 8 to 10 weeks of forcing, but plant height increased from 14 to 62 cm as daylength increased. Rudbeckia fulgida `Goldsturm' flowered without cold after 13 to 15 weeks of forcing, but only under 16 hours of continuous light and night interruption treatments. Heuchera sanguinea `Bressingham Hybrids' did not flower without cold under any photoperiod but did flower under all photoperiods with cold. The only Lavendula angustifolia `Munstead Dwarf' plants that flowered without cold were those under 24-h continuous light; ≈60% flowered. After cold, some lavender plants flowered under all photoperiods, and the flowering percentage increased with increasing daylength.

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

To determine the flowering requirements of Rudbeckia fulgida Ait. `Goldsturm', plants were grown under 9-hour photoperiods until maturity, then forced at 20 °C under one of seven photoperiods following 0 or 15 weeks of 5 °C. Photoperiods consisted of a 9-hour day that was extended with incandescent lamps to 10, 12, 13, 14, 16, or 24 hours; an additional treatment was a 9-hour day with a 4-hour night interruption (NI). Noncooled `Goldsturm' remained vegetative under photoperiods ≤13 hours, and essentially all plants flowered under photoperiods ≥14 hours or with a 4-hour NI. Flowering percentages for cooled plants were 6, 56, or ≥84 under 10-, 12-, or ≥13-hour daylengths and NI, respectively. Critical photoperiods were ≈14 or 13 hours for noncooled or cooled plants, respectively, and base photoperiods shifted from 13 to 14 hours before cold treatment to 10 to 12 hours following cold treatment. Within cold treatments, plants under photoperiods ≥14 hours or NI reached visible inflorescence and flowered at the same time and developed the same number of inflorescences. Fifteen weeks of cold hastened flowering by 25 to 30 days and reduced nodes developed before the first inflorescence by 28% to 37%. Cold treatment provided little or no improvement in other measured characteristics, such as flowering percentage and uniformity, flower number, plant height, and vigor.

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S.R. Adams, P. Hadley, and S. Pearson

The effects of temperature and sowing date on the time to first flowering were investigated in Petunia ×hybrida Vilm `Express Blush Pink' sown on three separate dates (8 Feb., 1 Mar. and 22 Mar. 1993) and grown in glasshouse compartments set to provide six air temperature regimes (minimum temperatures of 4, 10, 14, 18, 22, and 26 °C). Flowering was hastened under high temperatures and sowing later in the season (22 Mar.). To determine the extent to which this seasonal effect was due to photoperiod, a second experiment was conducted where plants were grown under controlled daylengths (8, 11, 14, and 17 h·d-1) within six temperature-controlled glasshouse compartments (set to provide minimum temperatures of 6, 10, 14, 18, 22, and 26 °C). The rate of progress to first flowering increased linearly with lengthening photoperiod up to a critical photoperiod of 14.4 h·d-1, while further increases in daylength had no further affect in hastening flowering. The rate of progress to flowering increased linearly with increasing temperature, however, the optimum temperature, at which the rate of progress to flowering was maximal, was lower under short days compared to long days. Furthermore, the rate of progress to flowering increased linearly with increasing photosynthetic photon flux (PPF). Data from both experiments were analyzed to construct a model to predict the effects of temperature, photoperiod, and PPF on time of flowering in petunia. This model accurately (r 2 = 0.88) predicted the flowering times of a different set of plants sown on three dates and grown under six temperature regimes (6, 10, 14, 18, 22, and 26 °C).

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

Phlox paniculata Lyon ex Pursh `Eva Cullum' plants were grown under seven photoperiods following 0 or 15 weeks of 5 °C to determine the effects of photoperiod and cold treatment on flowering. Photoperiods were a 9-hour day extended with incandescent lamps to 10, 12, 13, 14, 16, or 24 hours; an additional treatment was a 9-hour day with a 4-hour night interruption (NI). Noncooled plants remained vegetative under photoperiods ≤13 hours; as the photoperiod increased from 14 to 24 hours, flowering percentage increased from 20 to 89. Flowering of noncooled plants took 73 to 93 days. Flowering percentage was 19, 50, or 100 when cooled plants were held under photoperiods of 10, 12, or ≥13 hours or NI, respectively. Time to flower in cooled plants progressively decreased from 114 to 64 days as the photoperiod increased from 10 to 24 hours. Reproductive cooled plants had at least three times more flowers, were at least 50% taller, were more vigorous, and developed seven or eight more nodes than did noncooled plants. Photoperiod had no effect on height of flowering plants.

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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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Amanda Garris, Lindsay Clark, Chris Owens, Steven McKay, James Luby, Kathy Mathiason, and Anne Fennell

. vinifera (55% by pedigree) with wild species native to the south-central United States, Vitis rupestris , and Vitis aestivalis var. lincecumii . The V. riparia parent has a longer critical photoperiod (≈13.5 h) that confers earlier growth cessation

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Allan M. Armitage

Hamelia patens Jacq. (Texas firebush) is a long-day plant for flower initiation and flower development; however, flower development is more sensitive to photoperiod than is flower initiation. The critical photoperiod for flower development at 25C is between 12 and 16 hours. Flowering was delayed under low light conditions, and plant dry weight was heavier and flowering time was earlier for plants grown at a constant 25 or 30C than at 20C. A greenhouse environment with a 16-hour photoperiod and moderately high temperature (25C) would be appropriate for production of H. patens.

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D. Zhang, A.M. Armitage, J.M. Affolter, and M.A. Dirr

Achillea millefolium `Summer Pastels' is a qualitative long-day plant with a critical photoperiod between 12 and 16 hours at 18C. Plants grown under a 16-hour photoperiod flowered after 27 days, while those under 8 hours remained vegetative. Shoot dry weight was not affected by photoperiod. Low temperature (10C) delayed the time of flower bud formation and anthesis by ≈20 days. Low irradiance (100 μmol·m–2·s–1) delayed flowering and resulted in lower shoot dry weight, while moderate shading (200 μmol·m–2·s–1) did not significantly affect flowering time and growth compared with high irradiance levels (300 μmol·m–2·s–1).

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Changhoo Chun, Ayumi Watanabe, Toyoki Kozai, Hyeon-Hye Kim, and Junya Fuse

Spinach (Spinacia oleracea L. cv. Dimple) was chosen to determine whether bolting (i.e., elongation of flower stalks) could be controlled by manipulating the photoperiod during transplant production in a closed system using artificial light. Plants grown under various photoperiods during transplant production were transferred and cultured under natural short photoperiods and artificial long photoperiods. Vegetative growth at transplanting tended to be greater with the longer photoperiod because of the increased integrated photosynthetic photon flux. Bolting initiation reacted qualitatively to a long photoperiod, and the critical photoperiod for bolting initiation was longer than 13 h and shorter than 15 h. The plants grown under a longer photoperiod during transplant production had longer flower stalks at harvest. The long photoperiod and/or high temperature after transplanting therefore promoted flower stalk elongation. Growing plants under a photoperiod that was shorter than the critical photoperiod during transplant production reduced elongation of the flower stalks, thus there was no loss of market value even though the plants were cultured under a long photoperiod and high temperature for 2 weeks after transplanting.

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Anne Fennell and Emily Hoover

The grape species Vitis labruscana Bailey and V. riparia Michx. were subjected to a decreasing photoperiod at constant moderate temperatures to determine whether acclimation occurred in response to a shortening photoperiod. Cane growth, periderm development, killing temperature of the primary bud, and bud dormancy were measured in vines receiving a natural photoperiod (ND), a simulated long photoperiod of 15 hours (LD), and shorter photoperiods of 14, 13, or 12 hours (SD). The LD treatment was effective at maintaining growth and inhibiting periderm development and the onset of bud dormancy in V. labruscana. Cane growth rate with all SD treatments decreased as compared to the LD regime. A significant increase in periderm development occurred with the 12-hour SD treatment. Similarly, the onset of bud dormancy was promoted by the 12-hour SD in V. labruscana. The primary bud killing temperature was 1C lower in V. labruscana under the 12-hour SD than under the LD treatment. In contrast, the LD treatment neither maintained growth nor fully inhibited periderm development and the onset of dormancy in V. riparia. The decrease in the cane growth rate upon exposure to SD was significantly greater in V. riparia than V. labruscana. Periderm development was observed in both the SD and its respective LD-treated V. riparia vines. However, the rate of periderm development was significantly greater in the SD vines than in the LD vines. The onset of bud dormancy was promoted by 13-hour SD in V. riparia. Similarly, the primary bud killing temperature was 2 to 3C lower in V. riparia upon exposure to SD. Vitis riparia has a longer critical photoperiod than V. labruscana and appears to be more sensitive to changes in light intensity or light quality. While the change in freezing tolerance in response to short photoperiods is small, the photoperiod response at a longer critical photoperiod, when combined with lower temperatures, will promote an earlier and possibly more rapid cold acclimation in V. riparia than in V. labruscana.