In 1920, Garner and and Allard discovered the phenomenon of Photoperiodism in Maryland Mammoth plant. Photoperiodism is the effect of photoperiods or daily duration of light hours and dark periods on the growth and development of plants especially flowering. Plants can be divided into various categories depending on the basis of photoperiodic response to flowering

i. Long Day Plants are the plants in which flowering takes place only when the photoperiod provided to them is longer than critical photoperiod. These plants are also known as short night plants as they require darkness below critical level. If light period in these plants is interrupted by dark period, flowering is inhibited. The physiological active form in these plants is Pfr that activates flowering and hence flowering requires higher Pfr level. Long day plants will traditionally not flower during the winter and autumn when night lengths are long. Examples are Henbane, wheat, oat, sugar beet, radish, barley etc.

ii. Short Day Plants are the plants in which flowering takes place only when photoperiod provided is shorter than the critical period. The physiological active form in these plants is Pr. In short-day plants, Pfr inhibits flowering and hence flowering requires low levels of Pfr. These plants flower in late spring and summers. Examples are xanthium, chrysanthemum, dahlia, rice, sugarcane, potato, tobacco etc.

iii. Day-neutral Plants are the plants that can flower in any photoperiod. Instead, they begin to flower at certain stage of development or in response to environmental stimuli.. Examples are cucumber, pea, tomato, maize, cotton etc.
iv. Short Long Day Plants are plants that require short photoperiods for flowering and long photoperiods for blossoming i.e, petkus variety of rye.

v. Long Short Day Plants are plants that require long photoperiods for flowering and short photoperiods for blossoming e.g, bryophyllum, cestrum etc.

vi. Intermediate Plants are plants that flower only within a definite range of light hours e.g., wild kidney bean etc.

Importance of Photoperiodism
• It helps in determining the session in which particular plant will flower. This helps in laying out gardens, orchards and planning crop pattern.
• The knowledge of this phenomenon is of great practical importance in hybridisation experiments.
• Photoperiodism knowledge helps commercial growers to flower a plant throughout the year under green house conditions
• also helps in keeping plants in vegetative growth period in order to obtain higher yield of tubers, rhizomes etc.

Significance of night length and day length
In principle, by measuring the duration either light or darkness, a plant could perceive a critical day length. However, plants do not measure day length directly- they measure the dark period‟s (night) duration. The effect of photoperiod was studied by Carl Hamner and James Bonner in cocklebur (Zanthium), a short day plant. They found that interruption of the day with the darkness had little or no effect but interruption of night with light inhibitedthe flowering in cocklebur. The light interruption of the dark period is referred to as the night break. Thus, it is also possible to speak of the critical night, the night length that must be exceeded for flowering of SDPS. In LDPS, the duration of darkness is also important. In short photoperiod condition flowering is promoted by interruption of night with flash of light. In the LDP, night break treatment of longer duration promotes flowering.

Biological clocks in plants
Circadian rhythms After every 24 hours pattern changes are seen in plants this is known as circardian rhythm. This circadian rhythm is the best-known and most general rhythm in living creatures. It lasts about one day.

Plant clock Plants blossom in spring, or only in autumn and other species blossoming the year round as long as it is not too cold. Many plants need a signal for forming their flowers; they are called long day or short day plants. Early spring flowers and autumn flowers need a short day; summer flowers need a long day. In reality the length of the night is decisive: when short day plants receive a light signal in the night no flowers appear. Chrysanthemums blossoming in late summer form their flowers during the time of long days and short nights.
Flower clock The flowering plants bud in summer season so that they are able to flower in autumn seasons. If they bud in autumn they cannot flower. The same is true for opening the flower or producing a smell at a certain time of the day. They must produce the smell or open at the time when insects are active.