‘Photoperiodism’ is the physiological and behavioural reaction of organisms to changes in the length of the daylight period throughout the year. It was discovered in Nicotiana tabacum, a ‘short-day’ plant, which only flowers when the daily period of light is shorter than some critical length. By contrast, long-day plants such as Nicotiana sylvestris only flower when the period of darkness is shorter than some critical length. The plant measures the length of the dark period by coordinating its circadian clock with the action of a pigment called phytochrome (a photoreceptor found in plant cells which is sensitive to red and far red light). Phytochromes provide plants with time signals, making sure the plant’s internal time matches environmental time.
Tricking plants into believing it is the right time to flower is key to several industries, including the production of cut flowers. Chrysanthemums are a short-day plant, so will not naturally flower in summer. Chrysanthemum growers artificially darken their glasshouses for part of the day in summer to trigger flowering.