Phytochrome is a light-sensitive pigment in plants that regulates photoperiodism, flowering, and other developmental processes. It exists in two interconvertible forms:
Pr (Inactive form): Absorbs red light (~660 nm).
Pfr (Active form): Absorbs far-red light (~730 nm).
Mechanism:
1. Light Absorption:
In daylight, Pr converts to Pfr, triggering growth and development.
During darkness, Pfr gradually reverts to Pr.
2. Signal Transduction:
Pfr acts as a signaling molecule, activating specific genes responsible for flowering and other responses.
3. Photoperiodic Control:
The Pfr/Pr ratio determines whether a plant flowers based on day length (short or long photoperiods).
Mechanism of Photoperiodism
Photoperiodism is the response of plants to light duration, regulated by phytochromes.
Short-Day Plants (SDP):
Requirement:
Long nights and short days for flowering.
Mechanism:
Flowering is inhibited if the dark period is interrupted by light.
Pfr levels must fall below a critical threshold during the dark period to initiate flowering.
Examples:
Rice, chrysanthemums.
Long-Day Plants (LDP):
Requirement:
Short nights and long days for flowering.
Mechanism:
Flowering occurs when Pfr levels remain high during extended light periods.
Darkness below a critical duration prevents Pfr from reverting to Pr.
Examples:
Spinach, lettuce.
Role of Gibberellins in Photoperiodism
Gibberellins are plant hormones that play a key role in promoting flowering and elongation in photoperiodic plants.
1. Flowering Induction:
In long-day plants, gibberellins work with phytochromes to enhance flowering under appropriate light conditions.
2. Stem Elongation:
Gibberellins stimulate elongation of flowering shoots, facilitating reproductive success.
3. Breaking Dormancy:
They help overcome the inhibitory effects of short photoperiods in some plants.
The interplay of phytochrome action, photoperiodism mechanisms, and gibberellins orchestrates the flowering and growth of plants, emphasizing the complexity of plant adaptation to light.