2017 ASHS Annual Conference

Chrysanthemum (Chrysanthemum morifolium) is a popular ornamental flowering plant grown in summer months for fall markets in the United States of America. As a photoperiodic short-day plant, its production in greenhouses during summer requires skotoperiod extensions with opaque black cloth to induce flowering. Each sequential day of covering and uncovering crops requires extensive labor or expensive automated systems. Proportions of red:far-red light regulate flowering in many crops, because of the effect on phytochrome interconversions. Using narrow bandwidth red and far-red light emitting diodes (LEDs) to induce flowering, instead of physical darkening, could reduce production costs and increase profits for growers. Inducing flowering in short-day plants grown under long days has not received much attention. We hypothesized that night interruption with light with a low red:far-red ratio can cause the rapid conversion of far-red absorbing phytochrome to red-absorbing phytochrome. This conversion typically occurs slowly at night and is needed for flowering of short-day plant. By speeding up this conversion using dark interruption with light rich in far-red, we hoped to induce flowering of chrysanthemums. Since the red:far-red ratio can induce shade responses, we also documented the morphological and developmental effects of varying ratios of red:far-red light. Chrysanthemums were grown in a growth chamber under a 14-hour photoperiod and exposed to different ratios of red (peak at 630 nm) and far-red (peak at 730 nm) light during the 10-hour skotoperiod. Night interruption with red and far-red light was applied for 200 minutes at either the beginning, middle, or end of the skotoperiod. Night interruption high in far-red light increased plant height (P = 0.001), without influencing the number of nodes or branches. Internode length increased with higher proportions of far-red light (P = 0.017). Plant height differences were evident early and persisted throughout the growing cycle. The low red:far-red ratio likely induced shade-avoidance responses. Night interruption timing had no significant effect on any morphological or developmental parameters of chrysanthemum. No flower induction occurred in any treatment. These results further our understanding of morphological and developmental effects resulting from night interruption with different red:far-red ratios, but we did not find any proof that far-red light can induce flowering of short-day plants growing under long days.