Light and Cooling Requirements for Vernalization of Two Hybrid Nobile Dendrobium Orchids

The nobile dendrobium (hybrids of Dendrobium nobile) must be exposed to moderate temperatures to induce flowering initiation. The objective of this study was to determine if light is required for vernalization and how light and cooling duration interactively affect flowering. Mature D. Sea Mary ‘Snow King’ and D. Red Emperor ‘Prince’ plants in 10-cm pots were subjected to 10°C to induce flower initiation beginning 15 Sept. 2008. Treatments were factorial consisting of two light intensities (300 - 350 μmol·m^–2·s^–1 photosynthetic photons and darkness) and four cooling durations (2, 4, 6, and 8 weeks). Following treatment, plants were moved to a greenhouse at 25°C day/20°C night. Time to anthesis, total flower count, flower size and flower longevity were determined to assess flower quality. For D. Red Emperor ‘Prince’, interaction between light level and cooling duration was significant for time to anthesis, flowering node percentage, total flower count, and aerial shoot formation. For all durations, only 45% of the plants cooled in darkness flowered, compared to 98% of those vernalized with light. For D. Sea Mary ‘Snow King’, the interaction between light level and cooling duration was not significant with one exception, flower number per flowering node. Regardless of light intensity, cooling for 2 weeks resulted in increased time to anthesis (54 vs. 47 days) and reduced total flower count (19 vs. 26) compared to 4, 6, or 8 weeks. For all cooling durations, when cooled in the dark, time to anthesis (52 vs. 46 days in light) and flower size (7.7 vs. 7.4 cm in light) increased while the percentage of nodes bearing flowers (48% vs. 75% in light) and total flower count (19 vs. 30 in light) decreased. For both cultivars, flower longevity increased by 10 days at 2 or 4 weeks cooling duration compared to 6 or 8 weeks. The results suggest that light is not required for flower induction in both clones, but may be needed for more complete flowering. However, for D. Sea Mary ‘Snow King’, vernalizing in darkness may delay flowering. Although increasing cooling duration shortens the time to reach flowering following cooling, it results in decreased flower longevity.