On the basis of Photoperiodism responses the plants are divided in to different types by Garner & Allard. 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.