Photoperiodism in the Glasshouse Carnation: The Effectiveness of Different Light Sources in Promoting Flower Initiation

Carnations (Dianthus) cultivar White Sim, were grown under photoperiodic cycles comprising 8 h of natural daylight followed by 16-h ‘nights’ during which different lighting treatments were given. Irradiation with tungsten-filament lamps throughout the night was the most effective treatment for the promotion of flower initiation. Irrespective of the timing or duration of the lighting, tungstenfilament lamps were more effective than ‘daylight’ fluorescent lamps. Promotion of flower initiation due to radiation from a far-red source was greatest when irradiation followed immediately after the period of natural daylight and was least when it immediately preceded the period of daylight. Promotion of flowering due to a red source was least when the effect of far-red was greatest. Red and far-red radiations were synergistic in their promotion of flower initiation when given simultaneously for a period of 4 h in the middle of the night. It is suggested that in the promotion of flower initiation in long-day plants, a requirement for a relatively long duration of irradiation is related physiologically to a requirement for a relatively high proportion of far-red radiant energy.