Duration of sunlight needed for flowering - Photoperiodism

Photoperiodism is the process in most of the plants. In this process, plants are exposed either more  in the day or night for a specific  duration of light in  day to induce flowering.

The dark period is also  important for the flowering. On the other hand we can say that  the flowering depends on the duration of the light and the darkness exposures. This specific response to the light and dark is known as photoperiodism.

The term Photoperiod has been derived from the word ‘Photo’ means ‘light’ and ‘period’ means ‘length of time’. Photoperiodism is the typical movement in plants under the influence of the availability of light to induce flowering. Plants respond to light according to the  intensity and quality of light  to convert from vegetative to flowering.

Garner and Allard first studied Photoperiodism on plants called  tobacco mutant for their experiment. They observed that this tobacco mutant flowered at different times at different places. After controlling other factors like temperature, nutrition, etc., they concluded that it was the length of the day which affected flowering.  Most of the plants produce flowering only when  they get to light for less or more than a certain period called ‘critical photoperiod.

Depending on the duration of plants can be divided into the following categories:

Short Day Plants 

These plants flower only under day lengths shorter than the critical period.They do not flower when the day is long and the night is short.They do not flower if the dark period is mediated by red light Examples -Nicotiana tabacum, Glycine max , Coffea arabica Chrysanthemum.

Long Day Plants 

These plants flower only under day length more than the critical period.  They require a short period of darkness. They require prolongation of the light period for a brief exposure to light during the dark periods that bring about flowering.They are also known as short night plants. Examples: Beta vulgaris  Raphanus sativus, Spinacia oleracea 

Day Neutral Plants 

These plants do not require any specific day or night period for flowering. They can blossom throughout the year.

Examples: Helianthus annuus Lycopersicon esculentum Pisum sativum  Capsicum annuum 

 

Intermediate Plants 

These plants flower

within a definite range of light hours. They cannot flower above and below this range.

Example: Wild Kidney Bean.

Role of Phytochrome in Photoperiodism

Phytochrome is a blue-green pigment and helps in photoperiodic induction. Phytochromes exist in two forms: active form and inactive form. The active form of phytochrome  absorbs red light  up to 660nm , and inactive form of  phytochrome  absorbs  light of 730nm. The active form of phytochrome  absorbs red light  up to 660nm , and inactive form of  phytochrome  absorbs  light of 730nm. the daytime, the active  and inactive form of phytochrome interchange till they reach an equilibrium. The  biologically active form of phytochrome  becomes functional and active  during the day because more red light is present. This active phytochrome  stimulates flowering or seed germination.During the night time, the active phytochrome breaks down into the inactive phytochrome and this conversion is  determined by the length of the night period. During day time, the complete process of Phytochrome is again reversed.