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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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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.