What is Photoperiodism?
Photoperiodism is a physiological mechanism of flowering in plants influenced by the relative length of day and night the term is coined by Garner and Allard, U.S Department of agriculture, in 1920.
They demonstrated that flowering can be induced in “Maryland Mammoth” a type of mutant Tobacco that did not flower in summer, by artificial darkening which reduces the day length and extended the vegetative phase in plants by providing extra light in winters.
Photoperiodism and Types of Plants
According to photoperiods plants are categorized into five groups as follows;
a. Short Day Plants
These plants require a day length, less than a critical length for flowering eg. Oryza sativa, wolfia microscopia.
Short day plants are further divided into three types-
i. Qualitative short-day plants – This type of plant require essentially short days and long nights for flowering also know as obligate short-day plants
ii. Quantitative Short-day plants- They are a facultative short-day plant
iii. Short long day plants- This type of plant demands long days followed by short days for flowering.
b. Long Day Plants
These plants need the day length, more than a critical length for flowering eg. Brassica campestris, sinapis alba.
Long day plants are further divided into three types-
i. Qualitative long day plant
ii. Quantitative long day plant
iii. Long Short-day plants
c. Day Neutral Plants
Flowering in these types of plants is independent of day length and temperature change. Eg. Zea mays, Pisum sativum.
d. Intermediate Day Length Plant
The flower in a narrow range (12-14 h) of day length eg. Saccharum sponneum.
e. Ambi-Photoperiodic Plant
These plants flowers in short days as well as long days eg. Setaria verticillate.
Mechanism of Photoperiodism
Several studies conclude that plant’s circadian rhythm and light-receiving molecule’s phytochrome interaction play a significant role in the activation of specific genes which induce photoperiodism.
Conversion of shoot apical meristem into floral meristem is induced by florigen which is synthesized in leaves after receiving photoperiod.
Two theories conceptualized the mechanism of photoperiodism.
1. Hourglass Theory: This is an ancient theory that has been now replaced by the models based on the latest discoveries, this theory assumed that flowering takes place due to the gradual accumulation of a specific chemical under the influence of daylight or dark phase.
2. Circadian Rhythm Theory: Circadian rhythm includes external coincidence and internal coincidence theories. Erwin bunning proposed the “external coincidence” model in 1936. He described photoperiodic photosensitivity (CRPP) eg. Long days in spring derives photosensitive phase results in physiological response.
This model explains the dual effect of light i.e entrainment of the rhythm of photosensitivity and as the stimulus.
Due to the relationship of coincidence light (stimulus) with internal sensitivity to perceive light, this theory is known as the external coincidence model.
Colin Pittendrigh and Dorothea Minis described the “internal coincidence” model in 1964.
This theory states that the function of light is to interact with circadian rhythm only.
Significance of Photoperiodism
• Induce flowering
• Vegetative growth regulation
• Increase in yield
• Annual can be grown twice
• Winter dormancy can be prevented