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Meriam G. Karlsson

The rate of leaf unfolding as a function of temperature was determined for Begonia × hiemalis Fotsch under long-day (16 hours of light) conditions before flower initiation. Irradiance was maintained at 280 ± 20 μmol·m–2·s–1 (16.1 mol·m–2·day–l). The two cultivars Hilda and Ballet had similar rates of leaf unfolding in the range from 13 to 28C. The rate increased to a maximum of 0.116 leaves/day at 21C and then decreased at higher temperature. The following quadratic function (where T is the temperature in °C) was selected to describe initial long-day leaf unfolding rate in B. × hiemalis: leaves/day = -0.2083 + 0.03145 × T – 0.0007631 × T2, (r2 = 0.97). The leaf unfolding response to temperature varied for plants of `Hilda' and `Ballet' during short days (10 hours of light) following the initial long-day period. Plants of `Ballet' continued to unfold leaves at a similar rate as under initial long photoperiods, while the leaf unfolding rate for `Hilda' decreased to half the rate observed under long days.

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Meriam G. Karlsson and Jeffrey W. Werner

The potential photoperiodic effects and interactions with temperature were identified for flowering of german primrose (Primula obconica). The german primrose `Libre Light Salmon' was grown at long days (LD, 16 hours) or short days (SD, 8 hours) and 61 or 68 °F (16 or 20 °C). Visible bud (VB, 2-mm flower buds) averaged 90 days from seeding for plants grown at 61 °F independent of photoperiod or at 68 °F under LD. At 68 °F and SD, VB was delayed and flowering (horizontal petals) had not been observed at termination of the study (146 days from seeding). Flowering averaged 111 days at LD and 68 °F, 122 days at LD and 61 °F, and 133 days at SD and 61 °F. When plants within each temperature were shifted at weekly intervals from one photoperiod to the other, increasing duration of initial SD resulted in slower VB and at 68 °F more than 8 weeks resulted in no flowering. Changing to SD from initial LD did not affect VB or flowering at either 61 °F or 68°F. These results suggest flowering of german primrose is faster under LD than SD at the recommended production temperatures of 65 to 68 °F (18 to 20 °C).

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Heidi B. Rader and Meriam G. Karlsson

A high tunnel environment was evaluated for production of leaf (`Two Star') and romaine (`Parris Island Cos') lettuce (Lactuca sativa) in a northern location (lat. 64°49'N). Ten plantings were made 1 week apart from May to August. Averaged over the season, the air temperature was 1.5 °C and the soil 0.8 °C higher in the high tunnel than the field. `Two Star' planted on 3 Aug. and harvested on 16 Sept. produced higher yield (P < 0.001) in the tunnel. Head weight was 195 ± 12 g in the tunnel and 99 ± 8 g in the field. For the 13 July-planted `Two Star' lettuce, the field produced significantly (P < 0.001) more at 202 ± 21 g/head than the 135 ± 29 g/head in the tunnel. The three consecutive field plantings of 1, 8, and 15 June resulted in higher `Parris Island Cos' yields than corresponding plantings in the high tunnel. Head weights for harvests on 11, 18, and 25 July were 457 ± 60, 476 ± 65, and 478 ± 25 g under field conditions and 354 ± 46, 331 ± 52, and 312 ± 14 g in the high tunnel. `Two Star' was observed less prone to bolting than `Parris Island Cos'. Although a high tunnel did not generally support increased productivity in this study, the added protection resulted in high quality lettuce with limited necessary preparation and marketing loss in comparison to the field-grown lettuce.

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Meriam G. Karlsson and Jeffrey W. Werner

Plants of Begonia x tuberhybrida `NonStop Orange', `Clips Orange' and `Musical Orange' were exposed to 1, 2, 3 or 4 weeks of short days initiated at 4 stages of plant development (immediately upon germination, 5 weeks after germination, 10 weeks after germination and 15 weeks after germination). Prior to and succeeding short days, plants were exposed to a day length of 16 hours at 100 μmol·m-2s-1. Short days were 9 hours at an irradiance level of 180 μmol·m-2s-1 to give the same total daily irradiance (5.8 mol· m-2day-1) as long day conditions. The temperature was maintained at 21° ± 4°C during the day and 18° ± 2°C during night. The observed growth and development responses were similar among the studied cultivars. During the period of 4 to 8 weeks after germination, the seedling height increased at an average rate of 0.7 mm day-1 for plants grown under long days and 0.3 mm day-1 for short day plants. The photoperiodic conditions did not affect the number of emerging leaves. The root development was more proliferate on plants allowed to develop under long days compared to plants exposed to short days during early development.

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Meriam G. Karlsson and Jeffrey W. Werner

Plants of Begonia x tuberhybrida `NonStop Orange', `Clips Orange' and `Musical Orange' were exposed to 1, 2, 3 or 4 weeks of short days initiated at 4 stages of plant development (immediately upon germination, 5 weeks after germination, 10 weeks after germination and 15 weeks after germination). Prior to and succeeding short days, plants were exposed to a day length of 16 hours at 100 μmol·m-2s-1. Short days were 9 hours at an irradiance level of 180 μmol·m-2s-1 to give the same total daily irradiance (5.8 mol· m-2day-1) as long day conditions. The temperature was maintained at 21° ± 4°C during the day and 18° ± 2°C during night. The observed growth and development responses were similar among the studied cultivars. During the period of 4 to 8 weeks after germination, the seedling height increased at an average rate of 0.7 mm day-1 for plants grown under long days and 0.3 mm day-1 for short day plants. The photoperiodic conditions did not affect the number of emerging leaves. The root development was more proliferate on plants allowed to develop under long days compared to plants exposed to short days during early development.

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Meriam G. Karlsson and Janice T. Hanscom

The progression of flower initiation was documented in Dendranthema X grandiflorum (Ramat) Kitamura `Bright Golden Anne'. Rooted cuttings were planted and grown under 16 hours photoperiod (360 μmol·s-1m-2) and a constant 20C. After 7 days, the plants were pinched, the temperature reduced to 5, 10 or 15C and the day length shortened to 10 hours (13 mol·day-1m-2). Scanning electron microscopy was used to determine the transition from vegetative to reproductive meristem and to document the flower formation process. Shoot apices from three randomly selected plants were dissected weekly from each temperature until plants had developed floret primordia to completely cover the apical dome. Delayed floral development in the low temperature grown plants was a combination of a later flower initiation event and a slower progression of flower development. Required time for formation of 3-4 rows with floret primordia was about 21 days at 15C, 32 days at 10C and 70 days at 5C.

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Meriam G. Karlsson and David A. Gilbertz

The persistence of five growth regulators applied as spray or soil drenches was studied in vegetative Dendranthema X grandiflorum (Ramat) Kitamura `Bright Golden Anne' (BGA) and `Deep Luv' (DL). Spray treatments applied as 3 ml per plant consisted of 0.038 mg uniconazole (UC), 15 mg daminozide (DM), or 0.075 mg of either paclobutrazol (PB), flurprimidol (FP), or ancymidol (AC). Drenches of 0.025 mg UC, or 0.05 mg of either PB, FP or AC were applied at 60 ml per 500 cm3 pot. Growth (cm per week) in BGA was suppressed 60-73% compared to controls the week after application in all treatments except DM, which suppressed growth only 43%. By week 8, growth was similar to controls in all treatments except FP. With DL, all treatments suppressed growth 34-75% compared to controls the second week following application, except for no response to DM and PB sprays. Only plants treated with FP or UC drenches had not returned to similar growth as the control plants by week 10.

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Meriam G. Karlsson and Jan T. Hanscom

Seeds of cyclamen `Laser Scarlet' were germinated at 20C in darkness. Four weeks after germination, the seedlings were moved to a greenhouse at 20C and 16 hours daylength and 4 weeks later transplanted into 10 cm (520 cm3) pots. Plants were grown under 8 or 16 hours daylength in combination with 3.0, 7.5 or 12.0 mol·day-1m-2 for 9 weeks after transplant. The instantaneous irradiance was adjusted based on photoperiod to provide the desired total daily irradiance levels. After the 9 weeks photoperiod and irradiance treatment, plants were allowed to develop and flower at 16 hours daylength, 7.5 mol·day-1m-2 (130 μmol·s-1m-2) and 15C. There was a trend for an increased number of leaves for cyclamen grown at higher total daily irradiance levels at either 8 or 16 hours photoperiod. The largest number of leaves (14 ± 2.2 leaves) after the 9 weeks was observed for cyclamens grown at 16 hours photoperiod and 12 mol·day-1m-2 (210 μmol·s-1m-2). The plants grown at the longer day length and highest irradiance level also, accumulated most dry weight (92 ± 18.7 mg) during the 9 weeks of photoperiod and it-radiance treatment.

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Meriam G. Karlsson and Jeffrey W. Werner

Plants of Begonia ×tuberhybrida Voss. `Nonstop', `Clips', and `Musical' were exposed to 0, 1, 2, 3, or 4 weeks of short days (SD, 9-hour daylength) initiated at 0, 4, or 8 weeks following germination. Plants grown under long days (LD, 16-hour daylength) flowered 68 (`Musical'), 78 (`Clips'), or 83 days (`Nonstop') after germination. Exposure to SD delayed flowering in all three cultivars. The delay in `Nonstop' and `Clips' was independent of plant age at time of SD exposure. One to 4 weeks of SD delayed flowering 11 to 14 days in `Nonstop'. In `Clips', the delay in flowering increased linearly from 7 to 19 days with increasing duration of SD. Flowering was delayed up to 15 days in `Musical' when SD were begun 0 or 4 weeks after germination. Exposure to SD during the final 4 weeks of development did not affect flowering in `Musical'. Exposure to SD did not affect shoot, leaf, or flower number in any of the cultivars. The root/shoot dry-weight ratio within cultivars was independent of daylength.

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Meriam G. Karlsson and Royal D. Heins

The relative progression of lateral shoot elongation from pinch to flower of chrysanthemum [Dendranthema grandiflora (Ramat.) Kitamura `Bright Golden Anne'] plants grown under 2 to 22 mol·day-1·m-2 photosynthetic photon flux and 10 to 20C was modeled using Richards function. Parameters for the function were determined by first transforming data of shoot length and time from pinch (start of short photoperiods) to flower to a relative scale of 0.0 to 1.0 by dividing all intermediate shoot lengths and measurement dates by final shoot length and number of days to flower, respectively. Data used for parameter estimation originated with plants grown at a daily average of ≤20C, since those grown at a daily average above 20C exhibited delayed morphological flower induction and reached 50% of the final shoot length earlier in development. Relative shoot elongation was described by Richards function in the following form: Relative shoot length = SF × {1 + [(SF/SO)N-1] e-SF Kt}-1/N where t (relative time) = 0.0 to 1.0, SF (maximum relative shoot length) = 1.018, SO (relative shoot length at t = o) = 0.0131, N (model parameter related to the shape of the curve) =0.3923, and K (model parameter related to mean relative growth rate) = 5.8138.