Photoperiodism

Different plants take dramatically different amounts of time to shift from vegetative growth into flowering - a tree may take years, while an annual herb flowers within months - and this timing is regulated by an internal biological clock responding to two external physiological cues: light period (photoperiodism) and temperature (vernalization, covered in the next section). Photoperiodism specifically is the physiological change in flowering behaviour caused by the relative length of light and darkness a plant experiences - the photoperiod. The term was coined in 1920 by Garner and Allard, who first observed the effect in the 'Biloxi' variety of soybean and the 'Maryland Mammoth' variety of tobacco; each species has its own critical day length needed to trigger flowering (12 hours for Maryland Mammoth tobacco, about 15.05 hours for cocklebur). Based on their photoperiodic requirements, plants fall into six categories (Figure 15.24): long day plants (need day length above a critical value to flower, e.g. pea, barley, oats), short long day plants (long-day plants that also need an early exposure to short days, e.g. wheat, rye), short day plants (need day length below a critical value to flower, e.g. tobacco, cocklebur, soybean, rice, chrysanthemum), long short day plants (short-day plants that also need an early exposure to long days, e.g. some Bryophyllum sp …