Lecture-15 Photoperiodism and role of Phytochrome in flowering, Vernalization
Physiology of Flowering, Photoperiodism and role of Phytochrome in flowering:-
Photoperiodism:- The flowering in plants in response to the relative lengths of light and dark periods, is called photoperiodism. Garner and Allard introduced the terms photoperiod and photoperiodism and classified plants into the photoperiodic groups we use today.
i. Long Day Plants:- A plant that flowers only after being exposed to light periods longer than a certain critical length, as in summer. Here light period is critical for floweing. Examples:- Spinach, lettuce, and some varieties of wheat.
ii. Short Day Plants:- A plant that flowers only after being exposed to dark periods longer than a certain critical length, as in winter. Here dark period is critical for floweing. Examples: - Chrysanthemum, rice, soybean, onion.
iii. Day Neutral Plants:- A plant that flowers regardless of the length of the period of light it is exposed to, is called day neutral plant. Example:- Maize, Cucumber
Role of Phytochrome in Flowering:-
Phytochrome:- It is a photoreceptor, a pigment that plants, and some bacteria and fungi, use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum.
a. Phytochromes role in Short-day Plants:-
> Short Day Plants are those that require less than 10 hours of daylight and more than 12 hours of darkness to begin flowering.
> Pr is changed to Pfr form in many SDPs when the dark cycle is broken with a brief exposure (approximately 1 hr) to red light.
> Flowering is inhibited because of the accumulation of Pfr.
> Pfr is changed to Pr and the plant produces flowers if far-red light is supplied for a short time following red light treatment.
> During the winter months, far-red light is received on the earth’s surface in greater quantities than portions of red light reaching the ground. This changes a large portion of the Pfr form into Pr, causing SDPs to flower.
> In the summer, however, the ratio is reversed because more sunlight reaches the soil, preventing SDPs from flowering.
b. Phytochromes role in Long-day Plants:-
> Long Day Plants are those that require more than 14-16 hours of daylight and 8–10-hour dark periods to begin flowering.
> LDPs will not flower if the photo-period is less than 14 hours of light and more than 8 hours of darkness.
> Light is critical for flowering in LDPs.
> The role of phytochrome in LDPs is more complicated, and a blue-light photoreceptor is also necessary for flowering control.
> During the summer, more red light reaches the earth’s surface, compared to portions of far-red light reaching the ground. This changes a large portion of the Pr form into the Pfr form, causing LDPs to flower.
> However, in the winter, the ratio is reversed because more far-red light reaches the land, keeping LDPs from flowering.
Vernalization:- It is the artificial exposure of plants or seeds to low temperatures in order to stimulate flowering or to enhance seed production. Gibberellin is a hormone that replaces vernalization. The metabolically active apical meristems are the sites of perception of temperature to initiate flowering. The younger leaves are more susceptible to the process of vernalization. The shoot apex of mature stems or embryo of seeds receives low temperature stimulus.
Mechanism of Vernalization:- Through vernalization, there is an advancement in the process of blooming as a result of the delayed period of low temperatures, for instance, that which is attained in winter. To describe the mechanism of vernalization, there are two main hypotheses –
a. Phasic development theory
b. Hormonal theories
a. Phasic Development Theory:- As per this hypothesis, there is organization of stages in the plant’s improvement. Each stage is under the impact of environmental elements such as light, temperature etc. Here, in turn, there are two main stages –
i.Thermostage:- It depends on temperature, wherein vernalization accelerates thermostat. Thermostage is the vegetative phase requiring low heat, aeration and enough dampness
ii. Photostage:- It necessitates high temperature. Here, vernalin assists in producing florigen.
b. Hormonal theories:- As per this hypothesis, the freezing treatment propels the development of a floral hormone referred to as vernalin. Such a hormone is imparted to various parts of the plant. The vernalin hormone diffuses from the vernalized plants to the unvernalized plants, prompting blooming.
Photoperiodism versus Vernalization:-
i. Definition:- Photoperiodism is defined as the period of day and night for flowering, while vernalization requires cold temperature treatment for flowering.
ii. Hormones:- Florigen hormone is responsible for photoperiodism whereas vernalin hormone is responsible for vernalization.
iii. Pigments:- Phytochrome pigment is involved in photoperiodism whereas no pigment is involved in vernalization.
iv. Stimulus:- In photoperiodism, the stimulus is recognized by leaves, whereas in vernalization, the stimulus is recognized by meristematic cells of shoot tip and embryonal cells.
v. Flowering:- In photoperiodism, flowers produce after specific photoperiod, whereas, in vernalization, flowering is received after previous cold treatment.
