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Jean-Jacques B. Dubois*, Frank A. Blazich, and C. David Raper

Research by the authors has demonstrated the effect of day/night temperature difference (Tdiff) on plant growth is as substantive as the effect of daily average temperature (DAT). Dependence of plant primary productivity on temperature cannot be assessed with fewer than two data per 24 hours. Thus, the same experimental approach was applied to time to anthesis in Delphinium cultorum Voss `Magic Fountains' and Stokesia laevis L. `White Parasols', and to survival in D. cultorum. Two hundred and seventy seedlings of D. cultorum and 72 plantlets of S. laevis were grown for 56 days in growth chambers under eighteen 12 hour day/12 hour night combinations of six day and six night temperatures (10, 15, 20, 25, 30, or 35 °C). Ninety plants of D. cultorum were harvested after 13, 34, or 56 days, and 36 plants of S. laevis after 34 or 56 days. For each event of interest (anthesis or death), one datum per plant was recorded, consisting of time elapsed when either the event occurred, or the plant was harvested, whichever came first. Each datum was paired with an indicator of whether the plant was harvested prior to the event being observed. Data were analyzed using time—to—event data analysis procedures. Several parametric distributions fitted the data equally well, and both day and night temperature had strong effects on time to anthesis and survival time. However, in contrast with biomass production, DAT was quite sufficient to account for timing of these developmental events in relation to temperature. Addition of Tdiff contributed marginally to the fit to the data, but the magnitude of the effect was considerably smaller. Within the range of temperatures likely to be encountered in cultivation, the effect was negligible.

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Jens J. Brøndum and Royal D. Heins

Effects of temperature and photoperiod on growth rates and morphological development of Dahlia pinnata Cav. `Royal Dahlietta Yellow' were determined by growing plants under 45 combinations of day and night temperatures (DT and NT, respectively, and photoperiod. DT and NT ranged from 10 to 30C and photoperiods from 10 to 24 hours·day-1. Photoperiod influenced vegetative development more than reproductive development as plants flowered in all photoperiods. Lateral shoot count and length decreased and tuberous root weight increased as photoperiod decreased from 16 to 10 hours. Temperature interacted with photoperiod to greatly increase tuberous root formation as temperature decreased from 25 to 15C. Increasing temperature from 20 to 30C increased the number of nodes below the first flower. Flower count and diameter decreased as average daily temperature increased. Nonlinear regression analysis was used to estimate the maximum rate and the minimum, optimum, and maximum temperatures for leaf-pair unfolding rate (0.29 leaf pair/day, 5.5, 24.6, and 34.9C, respectively), flower development rate from pinch to visible bud (0.07 flower/day, 2.4, 22.4, and 31.1C, respectively), and flower development rate from visible bud to flower (0.054 flowers/day, 5.2, 24.4, and 31.1C, respectively). The results collectively indicate a relatively narrow set of conditions for optimal `Royal Dahlietta Yellow' dahlia flowering, with optimal defined as fast-developing plants with many large flower buds and satisfactory plant height. These conditions were a 12- to 14-hour photoperiod and ≈ 20C.

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Meriam Karlsson and Jeffrey Werner

The growth of Cyclamen persicum Mill. `Laser Scarlet' and `Sierra Scarlet' was evaluated for plants grown at day/night temperature differences of +9, +3, 0, –3 or –9°C. The day temperature was maintained for the duration of the 16-hr photoperiod and the day and night temperatures were selected to provide an average daily temperature of 16°C. The plants were grown at the specific temperatures starting 15 weeks from seeding until flowering. Total daily irradiance was 10 mol/day per m2. There was no significant difference in time to flower for plants of `Laser' (115 10.3 days from transplant). Flower buds appeared earlier above the foliage for `Sierra' plants grown at negative differences of 3 or 9°C (113 11.4 days) compared to plants grown at constant 16C (124 9.7 days). At flowering, plants grown with a positive difference of 9°C were significantly taller (22 1.9 cm for `Laser' and 24 2.0 cm for `Sierra') than the plants at 16C (19 1.9 cm for `Laser' and 21 2.1 cm for `Sierra'). Plants of `Laser' grown at +3C difference were also taller (21 2.1 cm) than the control plants at 16°C. Plant dry weight was larger for plants of both `Laser' and `Sierra' grown with +9°C. There were no differences in flower number or flower size among plants within each cultivar grown at the different temperature conditions.

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Meriam Karlsson

The growth of Primula vulgaris Huds. `Dania Lemon Yellow' and `Blue Danova' was evaluated for plants grown at day/night temperature differences of 9, 3, 0, –3 or –9°C. The day temperature was maintained for the duration of the 16-hour photoperiod and the day and night temperatures were selected to provide an average daily temperature of 16°C. The plants were grown at the specific temperatures starting 8 weeks from seeding until flowering. Total daily irradiance was 12 mol·d–1·m–2. Time for visible flower bud, flower color and first open flower was recorded. Plant height and flower bud number were determined at the termination of the experiment. `Dania Lemon Yellow' plants grown with a positive or negative difference of 9°C were significantly (P < 0.05) later in reaching a visible bud stage. There were no differences however, in the number of days required for flower color or first open flower for `Dania Lemon Yellow'. Plants of `Blue Danova' showed a significant difference only in the number of days required for flowering. The plants grown with a positive or zero difference between day and night required on average 2 more days to reach the stage of first open flower. There were no significant differences in plant height or flower bud number in `Dania Lemon Yellow' or `Blue Danova'.

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Matthew G. Blanchard and Erik S. Runkle

Volatile energy costs and lower profit margins have motivated many greenhouse growers in temperate climates to improve the energy efficiency of crop production. We performed experiments with dahlia (Dahlia ×hybrida Cav. ‘Figaro Mix’), French marigold (Tagetes patula L. ‘Janie Flame’), and zinnia (Zinnia elegans Jacq. ‘Magellan Pink’) to quantify the effects of constant and fluctuating temperatures on growth and flowering during the finish stage. Plants were grown in glass-glazed greenhouses with a day/night (16 h/8 h) temperature of 20/14, 18/18, 16/22 (means of 18 °C), 24/18, 22/22, or 20/26 °C (means of 22 °C) with a 16-h photoperiod and under a photosynthetic daily light integral of 11 to 19 mol·m−2·d−1. Flowering times of dahlia, French marigold, and zinnia (Year 2 only) were similar among treatments with the same mean daily air temperature (MDT). All species grown at 20/14 °C were 10% to 41% taller than those grown at 16/22 °C. Crop timing data and computer software that estimates energy consumption for heating (Virtual Grower) were then used to estimate energy consumption for greenhouse heating on a per-crop basis. Energy costs to produce these crops in Charlotte, NC, Grand Rapids, MI, and Minneapolis, MN, for a finish date of 15 Apr. or 15 May and grown at the same MDT were estimated to be 3% to 42% lower at a +6 °C day/night temperature difference (DIF) compared with a 0 °C DIF and 2% to 90% higher at a −6 °C DIF versus a 0 °C DIF. This information could be used by greenhouse growers to reduce energy inputs for heating on a per-crop basis.

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M.M. Peet

The environmental and physiological causes of cracking or splitting of soft fruits and citrus as they ripen are not well understood. This paper explores factors contributing to radial cracking in tomatoes, gives suggestions for prevention of cracking, and suggests directions for future research. Fruit cracking occurs when there is a rapid net influx of water and solutes into the fruit at the same time that ripening or other factors reduce the strength and elasticity of the tomato skin. In the field, high soil moisture tensions suddenly lowered by irrigation or rains are the most frequent cause of fruit cracking. Low soil moisture tensions reduce the tensile strength of the skin and increase root pressure. In addition, during rain or overhead irrigation, water penetrates into the fruit through minute cracks or through the corky tissue around the stem scar. Increases in fruit temperature raise gas and hydrostatic pressures of the pulp on the skin, resulting in immediate cracking in ripe fruit or delayed cracking in green fruit. The delayed cracking occurs later in the ripening process when minute cracks expand to become visible. High light intensity may have a role in increasing cracking apart from its association with high temperatures. Under high light conditions, fruit soluble solids and fruit growth rates are higher. Both of these factors are sometimes associated with increased cracking. Anatomical characteristics of crack-susceptible cultivars are: 1) large fruit size, 2) low skin tensile strength and/or low skin extensibility at the turning to the pink stage of ripeness, 3) thin skin, 4) thin pericarp, 5) shallow cutin penetration, 6) few fruits per plant, and 7) fruit not shaded by foliage. Following cultural practices that result in uniform and relatively slow fruit growth offers some protection against fruit cracking. These practices include maintenance of constant soil moisture and good Ca nutrition, along with keeping irrigation on the low side. Cultural practices that reduce diurnal fruit temperature changes also may reduce cracking. In the field, these practices include maintaining vegetative cover. Greenhouse growers should maintain minimal day/night temperature differences and increase temperatures gradually from nighttime to daytime levels. For both field and greenhouse tomato growers, harvesting before the pink stage of ripeness and selection of crack-resistant cultivars probably offers the best protection against cracking. Areas for future research include developing environmental models to predict cracking and exploring the use of Ca and gibberellic acid (GA) sprays to prevent cracking.

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Karen K. Tanino and Ruojing Wang

changes in net carbon accumulation (see Öquist and Huner, 2003 for a review). Thus, conceivably, the high day/night temperature difference may be accumulating sucrose, which may then induce cytokinin levels with subsequent meristem transition to

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Juan Pablo Fernández-Trujillo, Gene E. Lester, Noelia Dos-Santos, Juan Antonio Martínez, Juan Esteva, John L. Jifon, and Plácido Varó

-skinned cultivars, and wide day–night temperature differences seem to precede fruit cracking ( Peet, 1992 ). Among vegetables, fruit cracking has been investigated extensively in tomato ( Matas et al., 2004 ; Moctezuma et al., 2003 ; Peet, 1992 ; Peet and Willits

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Britney Hunter, Dan Drost, Brent Black, and Ruby Ward

tunnels and inside the high tunnels, although the day–night temperature differences were not as extreme (data not shown). Without regular monitoring, low tunnel temperatures can quickly go from the optimal range for tomato to excessive, causing blossom

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Lu Zhang, Xiuming Hao, Yonggeng Li, and Gaoming Jiang

effects of day temperature, night temperature, and day–night temperature difference (DIF) on plant height and internode length ( De Koning, 1992 ; Papadopoulos and Hao, 2000a ). Plant height and internode length of tomatoes are usually increased with