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Hiroshi Shimizu and Royal D. Heins

The effects of photoperiod and the difference between day temperature (DT) and night temperature (NT) (DIF) on stem elongation in Verbena bonariensis L. (tall verbena) were investigated. Plants were exposed to nine treatment combinations of -10, 0, or 10 °C DIF and 8-, 12-, or 16-hour photoperiods. Stem elongation was measured and analyzed by a noncontact computer-vision-based system. Total daily elongation increased as DIF increased; it also increased as photoperiod increased under positive DIF (DT > NT) and zero DIF (DT = NT), but not under negative DIF. Under positive DIF, daily elongation was 90% greater under the 16-hour photoperiod than under the 8-hour photoperiod. DIF affected elongation rate during the daily light span but not during the daily dark span. Total light-span elongation increased as DIF or photoperiod increased. Total dark-span elongation was not influenced by DIF or photoperiod. Elongation rates per hour in the light and dark were not significantly affected by photoperiod but increased in the light as DIF increased. Therefore, for a particular DIF, total elongation during 16-hour photoperiods (long days) was greater than that under 8-hour photoperiods (short days) because there were more hours of light under long days.

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Yaping Si and Royal D. Heins

Sweet pepper (Capsicum annuum `Resistant Giant no. 4') seedlings were grown for 6 weeks in 128-cell plug trays under 16 day/night temperature (DT/NT) regimes from 14 to 26 °C. Seedling stem length, internode length, stem diameter, leaf area, internode and leaf count, plant volume, shoot dry weight (DW), seedling index, and leaf unfolding rate (LUR) were primarily functions of average daily temperature (ADT); i.e., DT and NT had similar effects on each growth or development parameter. Compared to ADT, the difference (DIF, where DIF = DT - NT) between DT and NT had a smaller but still statistically significant effect on stem and internode length, leaf area, plant volume, stem diameter, and seedling index. DIF had no effect on internode and leaf count, shoot DW, and LUR. The root: shoot ratio and leaf reflectance were affected by DT and DIF. Positive DIF (DT higher than NT) caused darker-green leaf color than negative DIF. The node at which the first flower initiated was related to NT. The number of nodes to the first flower on pepper plugs grown at 26 C NT was 1.2 fewer than those of plants grown at 14 °C NT.

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Bin Liu and Royal D. Heins

Shoot elongation of `Stargazer' lily is rapid during the first 15 to 20 days after planting (1 to 2 cm·day–1 is common). Lower stem leaves are small, separated by long internodes. We determined if dipping `Stargazer' bulbs in uniconazole (5-, 10-, 20-, or 40-ppm solutions for 1 min) before planting would slow initial stem elongation, decrease final height, and improve appearance. Emergence, visible bud, anthesis dates, and flower bud count were recorded. Plant height was measured three times per week until anthesis. Uniconazole did not affect time to emergence, visible bud, anthesis, or flower bud count. Compared to the final height of 48 cm (untreated plants), height was reduced 7, 17, 22, and 30 cm (5%, 35%, 46%, and 62%) at anthesis for plants in the 5-, 10-, 20-, and 40-ppm treatments, respectively. The uniconazole bulb dips did not affect stem elongation rate for the first 10 days after treatment or from 45 days after treatment through anthesis (day 65). Relative to untreated plants, stem elongation rate of treated plants decreased linearly from 10 to about 35 days after treatment, with a maximum reduction of 55%, 75%, 85%, and 100% for plants in the 5-, 10-, 20-, and 40-ppm treatments, respectively.

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Robert D. Berghage and Royal D. Heins

Elongation characteristics of each internode on a lateral shoot of poinsettia (Euphorbia pulcherrima Klotz) `Annette Hegg Dark Red' were determined from pinching through anthesis for plants grown with 36 day/night temperature (DT/NT) combinations between 16 and 30C. The Richards function was used to describe the elongation of each internode. The first internode developing on a lateral shoot was longer and matured faster than subsequent internodes. The length of the first internode was a function of the difference between day and night temperatures (DIF = DT - NT). Subsequent internodes elongated uniformly in the absence of flower initiation. In the absence of flower initiation, the length of an internode, after the first, was a function of DIF. Internodes were shorter as proximity to the inflorescence increased. Internode length after the start of short days was a function of DIF and the visible bud index where visible bud index = [(days from pinching to the day an internode began to elongate - days from pinching to the day of the start of flower initiation)/the number of days from pinching to visible bud]. A poinsettia lateral shoot elongation model was developed based on the derived functions for internode elongation. The model predicted lateral shoot length within one standard deviation of the mean for plants grown in a separate validation study with 16 combinations of DT/NT. The model allows the prediction of lateral shoot length on any day from pinching through anthesis based on temperature, the number of nodes on the lateral shoot, the time each internode on the lateral shoot began elongating, and the visible bud index at the start of elongation of each node.

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James E. Faust and Royal D. Heins

Quantum sensors were placed at plant canopy height inside and outside a glass greenhouse. Photosynthetic photon flux (PPF) was measured during September for a 3-hour period near sunrise and sunset, which were determined from US Naval Observatory Circular #171. Under clear skies, the PPF at the canopy exceeded 0.25 μmol·m-2·s-1 for nearly 20 minutes before sunrise through 20 minutes after sunset. Under heavy overcast, the duration was only 5 minutes before sunrise through 5 minutes after sunset. The PPF at the canopy reached 0.25 μmol·m-2·s-1 approximately 12 minutes later in the morning and 12 minutes earlier in the evening than it did outside the greenhouse. The length of the dark period perceived by plants in a greenhouse on September 21st (assuming plants perceive light at 0.25 μmol·m-2·s-1) can range from 11:37 (hr:min) during cloudy conditions to 11:15 during clear ones, a difference of 22 minutes. At 43°N latitude, the maximum difference in date of flower initiation because of an extended period of heavily overcast versus clear weather on a crop such as poinsettias would be one week since the night length during September increases by 3 minutes per day. The actual difference from year to year is probably less because a seven-day duration of heavily overcast weather is unlikely.

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Mark P. Kaczperski and Royal D. Heins

Plug-grown Pelargonium × hortorum `Pinto Red' seedlings were grown under natural daylight (average of 4.7 mol/day) or with supplemental irradiance from high-pressure sodium lamps. Seedlings were grown under 8-, 16-, or 24-h photoperiods with supplemental irradiances of 2.5, 3.75, or 5.0 mol/day at each photoperiod. Supplemental irradiance was provided for 7, 14, 21, and 28 days beginning 7, 14, 21, 28, and 35 days after sowing. Seedlings were transplanted 63 days after sowing to 8-cm containers (121 plants/m2) and grown to flower. Leaf number at time of transplant was not affected by photoperiod, but increased as daily irradiance and weeks of supplemental irradiance increased. Seedlings were more responsive to supplemental irradiance applications beginning 28 and 35 days than at 7 to 21 days after sowing. Ninety-two percent of seedlings receiving 28 days of 5.0 mol/day supplemental irradiance under a 24-h photoperiod starting 35 days after sowing had initiated flower buds at time of transplant; 75% of those receiving 3.75 mol/day were initiated. Plants receiving less than 3 weeks of supplemental irradiance or with an irradiance period beginning less than 28 days after sowing had not initiated flowers at transplant.

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Charles L. Rohwer and Royal D. Heins

Experiments were performed on Hatiora gaertneri (Regel) Barthlott ‘Jan’ and ‘Rood’ and H. ×graeseri (Wedermann) Barthlott ‘Evita’ to determine their flowering responses to 1) daily light integral (DLI) before and during vernalization; 2) 0 to 6 weeks of short-day (SD) or long-day (LD) photoperiods before vernalization at 10, 12.5, or 15 °C; 3) propagation from April to July; 4) timing of leveling before or during inductive treatments; and 5) SD photoperiods before vernalization under darkness at 0 to 10 °C. ‘Jan’ grown under elevated DLI before vernalization and low DLI during vernalization flowered more prolifically than plants grown under low DLI before vernalization or high DLI during vernalization at 15 °C. Six weeks of SD photoperiods before vernalization increased the number of buds per flowering phylloclade after vernalization at 10 °C and increased flowering uniformity when vernalization duration was insufficient at 10 °C or vernalization temperature was 12.5 or 15 °C. For plants flowering in January, propagation the previous April produced better flowering than propagation in May, June, or July. Removal of apical phylloclades during prevernalization SD or during vernalization was deleterious to flowering. Vernalization in the dark produced marginal flowering, but SD treatment before vernalization increased the percentage of apical phylloclades flowering, buds per flowering apical phylloclade, and percentage of plants flowering after dark vernalization. ‘Evita’ flowered more poorly than either ‘Jan’ or ‘Rood’. Collectively, the most uniform flowering in January occurred when plants were exposed to a sequence of 4 to 6 weeks of SD, vernalization at 7.5 to 15 °C for 8 weeks, then growth under LD for 7 weeks.

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James E. Faust and Royal D. Heins

The effects of temperature and irradiance on flower initiation and development were quantified to provide a basis for an inflorescence development model. The percentage of leaf axils forming an inflorescence increased as the daily integrated PPF increased from 1 to 4 mol m-2 d-1, while the rate of inflorescence development was a linear function of temperature from 18 to 26C. The appearance of a visible flower bud in the leaf axil was correlated with leaf blade length of the subtending leaf. Mathematical functions were used to describe leaf blade length at the time of visible flower bud as a function of temperature and irradiance, and also to describe the influence of temperature on the rate of leaf extension. The time of visible flower bud in the leaf axil was then predicted by measuring the current length of the subtending leaf blade and estimating the time required for the leaf blade to extend to the length required for visible flower bud appearance. A phasic development scale was used to describe the developmental status of an inflorescence from visible flower bud to anthesis. A model was then created which predicted time to anthesis based upon temperature and the current stage of inflorescence development.

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Jens J. Brondum and Royal D. Heins

Dahlia “Royal Dahlietta Yellow” plants were grown in controlled temperature chambers under 25 different day and night temperature environments ranging from 10°C to 30°C. The day length was 12 hours with an average PPF level of 300 micromolm-2 s-1 at canopy level. Leaf unfolding rate, shoot elongation and flower development rate were determined and models developed. Leaf unfolding rate increased as temperature increased up to 25°C. Stem elongation increased as the difference between day and night temperature increased. Flower initiation was delayed at high (30°C) temperature and flower development rate increased as temperature increased from 10°C to 25°C. Plants are currently being grown under greenhouse conditions to provide data for validating the models.

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James E. Brown-Faust and Royal D. Heins

Saintpaulia ionantha `Utah' plants were grown in growth chambers at constant 15, 20, 25, and 30°C temperatures and daily photosynthetic irradiances of 1, 4, 7, and 10 mol1 m-2 day-1 delivered by 23, 92, 161, and 230 μmol m-2 s-1 for 12 hours. Models were developed describing leaf unfolding rate (LUR) and flower development rate (FDR) as a function of temperature and irradiance by recording the dates of leaf unfolding and flower opening over the course of the experiment and then calculating rates using regression. Both LUR and FDR increased as temperature increased from 15 to 25°C and then decreased. Both LUR and FDR increased as irradiance increased from 1 to 4 mol m-2 day-1. Increasing daily irradiance above 4 mol m-2 da y-1 did not significantly increase LUR or FDR. Model validation data are being collected from plants growing under 3 irradiance levels in greenhouses maintained at 15, 20, 25, and 30°C air temperatures.