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Mark S. Strefeler

The influence of temperature and genotype on plant height, internode length, and morphological development of 20 cultivars of Pelargonium ×hortorum Bailey were determined by growing plants under one of three day–night temperature regimes (18/18C, 18/24C, and 24/18C). Temperature regime influenced internode length and plant height regardless of plant genotype. Internode length and plant height increased as the day–night temperature differential (DIF) increased from –6 to 6C. Average internode length increased from 5.3 ± 0.2 mm for –6C DIF to 6.3 ± 0.2 mm for +6C DIF. Genotypes differed for average internode length (4.2 to 8.7 mm) and plant height (54 to 95 mm). Node count increased as average daily temperature (ADT) increased. Node counts were 11.2 at 18/18C (ADT = 18), 11.9 at 24/18C (ADT = 20.3), and 12.1 at 18/24C (ADT = 21.8). Genotype × temperature interactions were not significant for the recorded traits. This study demonstrates that DIF is an effective height control strategy, regardless of geranium genotype, and that DIF combined with the selection of genetically short cultivars may eliminate the need for chemical height control in the commercial production of geraniums.

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D.G. Mortley, J.H. Hill, C.K. Bonsi, W.A. Hill, and C.E. Morris

Growth chamber studies were conducted to determine if inverse day/night temperature could control canopy height of sweetpotato without adversely affecting storage root yield. Four 15-cm-long vine cuttings of TU-82-155 sweetpotato were grown in rectangular nutrient film technique hydroponic troughs for 120 days. Two troughs were placed into each of six reach-in growth chambers and subjected to 24/18, 26/20, 28/22, 18/24, 20/26, and 22/28 °C, respectively. Growth chamber conditions included a 12/12-h photoperiod, 70% RH, and photosynthetic photon flux of 1000 μmol·m-2·s-1 at canopy level. Total and edible storage root yields were reduced by 50% among plants grown under cool days/warm nights regimes. Harvest index was similar among treatments except for the low value obtained at 22/28 °C. Canopy height was positively correlated with the change in temperature, and for every 2 °C decrease there was a 3.1 centimeter decrease in canopy height. Inverse day/night temperature effectively controlled canopy height but at the expense of storage root production.

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Cheryl L. Reese and John E. Erwin

The interaction between day/night temperature (DT/NT) and irradiance during the photoperiod prior to the inductive night on Pharbitis nil (L.) cv. Violet flower induction was studied. Plants exposed to 12 or 18 °C NT did not flower regardless of DT. When NT was 24 or 30 °C, percent flowering plants increased progressively as DT increased from 12 to 30 °C. Percent flowering plants and total flower bud number per plant was greatest when seedlings were induced with a 24 or 30 °C DT/30 °C NT regime. DT/NT did not affect the node number to first flower. Irradiance did not affect flowering. Temperature effects on P. nil flowering could be described as a function of average daily temperature, where flowering increased as temperature rose from 22 to 30 °C.

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Asiah A. Malek, Frank A. Blazich, Stuart L. Warren, and James E. Shelton

Seedlings of flame azalea [Rhododendron calendulaceum (Michx.) Torr] were grown for 12 weeks under long-day conditions with days at 18, 22, 26, or 30C for 9 hours in factorial combination with nights at 14, 18, 22, or 26C for 15 hours. Total plant dry weight, top dry weight, leaf area, and dry weights of leaves, stems, and roots were influenced by day and night temperatures and their interactions. Dry matter production was lowest with nights at 14C. Root, leaf, top, and total dry weights were maximized with days at 26C in combination with nights at 18 to 26C. Stem dry weight was maximized with days at 26 to 30C and nights at 22C. Leaf area was largest with days at 18 and 26C in combination with nights at 18 or 26C. Within the optimal, day/night temperature range of 26 C/18-26C for total plant dry weight, there was no evidence that alternating temperatures enhanced growth. Shoot: root ratios (top dry weight: root dry weight) were highest with days at 18 and 30C. Leaf area ratio (total leaf area: total plant dry weight) was highest and specific leaf area (total leaf area: leaf dry weight) was largest when days and nights were at 18C and were lower at higher temperatures. Regardless of day/night temperature, leaf weight ratio (leaf dry weight: total plant dry weight) was higher than either the stem weight ratio (stem dry weight: total plant dry weight) or root weight ratio (root dry weight: total plant dry weight). Net leaf photosynthetic rate increased with day temperatures up to 30C.

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Chin Chin Lee, Ted E. Bilderback, and Judith F. Thomas

Photoperiod treatments of 10, 12, 14, and 16 hours and a field control were used to determine the photoperiodic response of Heptacodium miconioides Rehd. The F values for vegetative growth responses under various photoperiods exhibited a highly significant linear effect. Leaf count, area, and weight, shoot length, and stem weight were lower for plants exposed to the 10- or 12-hour photoperiod than those of plants grown under the 14- or 16-hour photoperiod or in the field. Plants under the 10- or 12-hour photoperiod became dormant after 5 weeks of treatment. The growth responses for the 10- and 12-hour photoperiods were similar. There also were no differences in growth responses of plants from the 14- and 16-hour photoperiods or from the field. A favorable photoperiod for growth of Heptacodium must exceed 12 hours; thus, it can be classified as a long-day plant in reference to vegetative growth. Leaf tissues under the 10- and 12-hour photoperiods were significantly thicker than those under the 14- and 16-hour periods or under field conditions due to longer cells of the palisade mesophyll layer. Plants grown in the field and under the 14- or 16-hour photoperiods were the only ones that initiated inflorescences. With days at 30C, leaf and stem dimensions were larger than those at 22C. Nights at 18C resulted in a larger leaf area, leaf weight, and stem weight than at 26C. There was a significant effect on total leaf thickness due to day × night temperature interaction.

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Richard J. McAvoy

Poinsettias, Euphorbia pulcherrima Willd. cvs Lilo and Diva Starlight, were exposed to either warm day-cool night or cool day-warm night greenhouse temperature regimes. Day time temperatures were imposed between 900 to 1600 HR. Within each temperature regime, poinsettias were grown single stem or pinched and drenched with either 0.04 or 0.08 mg a.i. uniconazole per 1.6 1 pot or grown as untreated controls. Light levels (PAR) and potting medium and plant canopy temperatures were continuously monitored.

Over the course of the study, the day-night temperature differential (DIF), in the plant canopy, averaged 4.2C in the warm day regime and -1.4C in the cool day regime. The average daily temperature was lower (16.9C) in the warm day regime than in the cool day regime (18.7C).

DIF treatment significantly affected final leaf area, leaf and total plant dry weight, leaf area ratio and specific leaf weight, The DIF treatment by cultivar interaction was significant for final poinsettia leaf area, stem, leaf and total plant dry weight, break number and average break length. Uniconazole significantly affected final plant height, stem and total plant dry weight, break number, average break length and specific leaf weight. Uniconazole by DIF treatment effects were not significant,

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Asiah A. Malek, Frank A. Blazich, Stuart L. Warren, and James E. Shelton

Seedlings of mountain laurel (Kalmia latifolia L.) were grown for 16 weeks under long-day conditions with days at 18, 22, 26, or 30C for 9 hours in factorial combination with nights at 14, 18, 22, or 26C for 15 hours. Total plant dry weight, top dry weight, and dry weights of leaves, stems, and roots were influenced by day and night temperatures. The night optimum for all dry weight categories was 22C. Dry matter production was lowest with nights at 14C. Total plant dry weight and dry weights of tops, leaves, and stems were maximized with days at 26C, but for roots the optimum was 22C. Dry weight accumulation was lower with days at 18 or 30C. Responses of leaf area were similar to that of total plant dry weight, with optimum days and nights at 26 and 22C, respectively. Within the optimal day/night temperature range of 22-26/22C for dry weights, there was no evidence that alternating temperatures enhanced growth. Shoot: root ratios (top dry weight: root dry weight) increased with day temperatures up to 30C and were highest with nights at 14 or 26C. Leaf weight ratio (leaf dry weight: total plant dry weight) decreased with increasing night temperature, and increased curvilinearly in response to day temperature with the minimum at 26C. Stem weight ratio (stem dry weight: total plant dry weight) increased with increasing day or night temperature. Root weight ratio (root dry weight: total plant dry weight) was highest with nights at 18 or 22C and decreased with days >22C. Net leaf photosynthetic rate was maximized with days at 26C.

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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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F. Todd Lasseigne, Stuart L. Warren, Frank A. Blazich, and Thomas G. Ranney

maintain appropriate day/night temperatures. Plants exposed to the same day and night temperatures were also moved daily to different areas of the chamber to simulate transient mechanical perturbations. Relative humidity (RH) was greater than 70%, and CO 2

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Woei-Jiun Guo and Nean Lee

In this study, effects of leaf age (20 to 240 days), plant age (4, 8, and 14 months after deflasking), and various day/night temperature regimes (16 to 33 °C) on photosynthesis of Phalaenopsis amabilis L. Blume var. formosa Shimadzu (Phal. TS97) leaves were investigated. The diurnal net CO2 uptake in Phal. TS97 leaves was measured and integrated to obtain total net CO2 uptake, which represents photosynthetic efficiency in plants performing crassulacean acid metabolism (CAM). Under all conditions, Phal. TS97 leaves performed typical CAM photosynthesis and reached their highest net CO2 uptake rate, ≈6 μmol·m-2·s-1, after 3 to 4 hours in the dark under a 12-hour photoperiod. When grown under 30 °C day/25 °C night temperature, the total net CO2 uptake of leaf increased with maturation and was highest at 80 days old, 20 days after full expansion. The CAM photosynthetic capacity of mature leaves remained high after maturation and began to decline at a leaf age of 240 days. The trend was consistent with malate fixation but the highest nocturnal malate concentration was observed in 100-day-old leaves. Young leaves or leaves from small juvenile plants had higher daytime CO2 fixation compared to mature leaves or large plants, suggesting that Phal. TS97 leaves progressed from C3-CAM to CAM during the course of maturation. The second newly matured leaf from the top had the highest net CO2 fixation when the newest leaf was 8 cm in length. Although plant age did not influence total CO2 uptake in the leaf, photosynthetic efficiency of leaves in small younger plants was more sensitive to high light intensity, 340 μmol·m-2·s-1 photosynthetic photon flux. The day/night temperature of 32/28 and 29/25 °C resulted in the highest total net CAM CO2 fixation in vegetative Phal. TS97 plants than higher (33/29 °C) and lower temperatures (21/16 °C).