Photoperiodism is the response of a plant to the relative length of light and dark periods to produce flower. The relative length of day and night to which a plant is exposed is called the photoeriod. Photoperiodism is a photobiological process – a process that involves absorption of light by a receptor (usually a pigment) that results in activation of that receptor which results in initiation of a chemical sequence of chemical reactions that leads to a general plant response.
The responses to photoperiod include seed germination, stem elongation, promotion of rooting in cuttings, formation of storage organs such as potato tubers and onset of dormancy. But most important photoperiodic response is perhaps initiation and further development of flowers.
Julien Tournois (1910) provided first practical evidence regarding the effects of the day length (photoperiod) on flowering. He found that hemp (Cannabis sativus) flower vigorously if planted early in the spring but remains in a vegetative state if planted in later spring or summer. George Kaleb (1918) working with house leek plants (Seinpervivuni fnnkii – a long day plant), found that flowering can be induced by artificial illumination in midwinter in a green house, although the normal time of flowering for this plant is June. Most physiologists of Kaleb’s days believed that role of light in the life of plants in nutritional. When the plants are exposed to continuous light, they produce more photosynthate which causes flowering. But, when hop (Houbin japonais) and hemp (Cannabis sativus) were exposed to low light intensities of light for longer periods, the plants did not flower. This observation confirmed that photoperiod (day length) is more important than intensity of light.
Garner & Allard’s Work
The first clearly hypothesis on photoperiodism was given by Garner and Allard. They observed that Biloxi soya-bean (Glycine max) plants flowered in September and October even if they were germinated over a 3-month span in May, June or August. They observed that a variation of 59 days in germination date during May and June caused a difference of only 11 days to the first open flowers. The number of days from germination to blossoming and the final height of plants decreased as the season continued. This suggest that in soya-bean plants there is a seasonal timing mechanism.
Later they studied the flowering pattern of tobacco plants growing in Maryland state of U.S.A. The seeds of commercial strain of tobacco plant (Marry-land narrow leaf) are germinated in beds during Feb-March and transplanted in fields during April-May when the day length is about 14 hours. The plants flower during late summer when day length is about 12 hours. Garner & Allard observed a mutant in the fields that was taller, have much larger leaves and did not flower throughout the season. They named it as Maryland mammoth and transferred it to green-house at the end of the season. The plant flowered in winter and produced seeds hen the days are shorter relatively. They also found that if the seeds are germinated in the winter in green-house and the seedlings are transferred to the fields during spring, small plants develop, flower and produce seeds. However, when the seeds are germinated in spring and transplanted to fields, the plants grew vegetatively.
Next Garner and Allard constructed dark chambers to cover plants grown in field during summer. They covered the plants after exposing them to day length equivalent to winter day (shorter duration of light) and found that the plants flowered in summer. They also built growth chambers in the green-house where the length of light period could be varied during winter with artificial light. They found that plants remain vegetative when the light period was increased during winter. After ruling out other factors, such as temperature, nutrition and light intensity, Garner & Allard concluded that the length of light and dark periods within a 24-hours cycle is a significant factor in promoting flowering. Garner and Allard called Maryland mammoth a short-day plant (SDP) as the short photoperiods promoted flowering. Later, Garner & Allard found a number of other plants in which flowering is promoted by short durations of light period.
Classification of Plants Based on Photoperiodism
As a result of their studies, Garner & Allard developed terms to describe photoperiodic responses of flowering. The plants are classified according to photoperiodic responses into following types:
The short-day flowering plants, long-day flowering plants and day-neutral plants.
Short-Day Flowering Plants
The plants that flower when the day length is less than a certain critical length termed as short-day plants (SDPs). These plants require a specific short duration of light period for speeding up flowering. The day length in excess of critical point will keep the short-day flowering plant vegetative:
The critical day length varies from species to species. The common examples of short-day plants are Maryland mammoth (Nicotiana tabacum), cocklebur (Xanthium pennsylvanicum), soyabean (Glycine max), cosmos (Cosmos bipinnata), etc.
Long-day Flowering Plant
The plants that flower when a critical day length is exceeded are called long-day plants (LDPs). These plants require exposure to longer durations of light periods before the flowering is initiated. The critical day length differs from species to species in LDPs as in short-day plants. Some examples of long-day plants are:
spinach (Spinacea oleracea), sugar beet (Beta vulgaris), black henbane (Hyoscymus niger), lettuce (Lactuca sativa). These plants remain vegetative if grown under short-day conditions.
Day-Neutral Flowering Plants
The plants whose flowering is not affected by either short or long durations of light are called day-neutral plants. These plants flower after a period of vegetative growth. Some examples of day-neutral plants are tomato (Lycopersicum esculentum), four-o’ clock (Mirabilis), certain varieties of pea (Pisum sativum), cucumber (Cucumis sativus) and maize (Zea mays).
Since, Garner & Allard’s work, the classification of plants according to photoperiodic responses has been expanded.
Short-Long-Day Flowering Plants
The plants that require exposure to a number of short days followed by long days for initiation of flowering are called short-long-day plants. These plants flowers in late spring or early summer when there is shift from short to long days. The common examples of short-long-day plants are winter rye (Secale cereal cv. Petkus) and candytuft (lberis durandii).
Long Short Day Flowering Plants
The plants in which flowering is induced after exposure to long-day conditions followed by short duration of light are called long-short-day plants. These plants flower in late summer or early fall, for example bnjophyllum daigremontianum and night-jasmine (Cestrum nocturnum).
Concept of Critical Day length
The particular duration of light (photoperiod) required to induce flowering is termed critical day length. It has been observed that photoperiodism refers not to absolute length of day and night, but each class of plants flower in response to a particular duration of light (photoperiod). For example, Maryland mammoth tobacco and Xanthium sp both are short-day plants, but Maryland mammoth flowers when it is exposed to a light period shorter than 12 hours, i.e., 12 hours of light and 12 hours of night; while Xanthium sp flowers when it is exposed to a light period shorter than 15.5 hours, i.e., 15.5 hours of light and 8.5 hours of dark. Thus, the critical day length of Maryland mammoth is 12 hours and for Xanthium sp it is 15.5 hours. This suggest that a short-day plant flowers when it is provided with photoperiod shorter than its critical day length.
Long-day plants, on the other hand flower when they are provided with photoperiods longer than their critical day length. For example, the critical day length for black henbane (Hyoscymus niger) is 11 hours, i.e., 11 hours of light and 13 hours of dark. It induces flower when provided with a light period more than 11 hours duration.