Flowering in response to day length was identified for sunflower (Helianthus annuus L. `Pacino Gold'). Germination and seedling development occurred at 20 °C and long days (ld, 16 hours) following direct seeding into 10-cm pots. Sixteen days after seeding, plants were placed at ld or short days (sd, 8 hours), 20 °C and 8 mol·d-1·m-2. Flowering was recorded at the stage of reflexed petals after 48 sd. At the time of flowering in sd, flower buds were of minute size under ld. Plants started at ld, and moved to sd after 1, 2, or 3 weeks, flowered at similar times as those grown under uninterrupted sd conditions. Four initial weeks of ld delayed flower development with 7 days, compared to a continuous sd environment. On the other hand, 2 to 3 weeks of initial sd followed by ld hastened flowering with 5 to 10 days. With increasing number of early ld from 1 to 4 weeks, plant height at flowering doubled from 20 to 40 cm. Average plant height in continuous sd was 18 cm. Plants grown exclusively or moved to ld after 1 to 4 weeks of sd were similar in height to plants finished at sd with 4 initial weeks of ld. Combinations of sd and ld may be used to manage height and rate of development in the sunflower `Pacino Gold'.
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