Photoperiodism is the response of the plant to the relative length and timings of light and dark conditions.
Garner and Allard (1920) were the first to use the term Photoperiodism. They observed that Maryland Mammoth variety of tobacco failed to produce flowers during summer but when grown in green house during winter the plant flowered profusely. They further subjected Maryland Mammoth tobacco plant to short day lengths during summer by placing the plant in darkness after exposure to a day lengths that would be equivalent to a winter day. Plants treated in this way produced flowers.
Furthermore, they found that the plant can be kept in vegetative state during winter months by lengthening the days with artificial additional light. This variety of tobacco is called a short-day plant because it flowers only under short days. Later on it was found that some plants require longer photoperiods to induce flowering while others produce flowers under both long and short-day conditions. Still others respond to photoperiods situated somewhere between short and long-photoperiods. Some plants require a long photoperiod followed by short photoperiods to flower, while others are induced to flower when short photoperiods are followed by long photoperiods.
The plants fall into the following photoperiodic classes with respect to their flowering behaviour. The classification given below is based on a 24 hour cycle of light and darkness.
1. Short-day plants (SDP): Flower when daylength is shorter than a certain ‘critical period’. Under photoperiods longer than a critical point these plants will not flower e.g., Coleus blumei. Xanthium strumarium, Cannabis sativus, Pharbitis nil, Kalanchoe blossfeldiana, Nicotiana tabacum (Maryland Mammoth variety), Glycine max etc.
2. Long-day plants (LDP): Flower when daylength is longer than a certain ‘Critical period’, Under photoperiods shorter than a critical point these plants will not flower e.g., Plantago lanceolata, Hyoscyamus niger, Spinacea oleracea (Spinach) Beta vulgaris.
3. Indeterminate (Day-neutral) (= Daylength Indifferent plants) flower over a wide range of daylengths from relatively short photoperiods to continuous illumination e.g., Lycopersicum esculentum (Tomato), Mirabilis (four-o-clock plant) Capsicum annum and Zea mays.