Unit 5: Plant Response to Light and Temperature
Photoperiodism: SDP, LDP, and DNP
Photoperiodism is the physiological reaction of organisms to the length of night or a dark period. It is most commonly associated with the initiation of flowering in response to the relative lengths of day and night.
Classification based on Photoperiodic Response
- Short-Day Plants (SDP): These plants flower when the day length is shorter than a certain critical duration. They actually require a continuous long night period. Examples: Chrysanthemum, Soybean.
- Long-Day Plants (LDP): These plants flower when the day length exceeds a certain critical duration. Examples: Wheat, Radish.
- Day-Neutral Plants (DNP): These plants flower regardless of the day length. Flowering is often regulated by other factors like age or temperature. Examples: Tomato, Sunflower.
Phytochrome: Discovery and Structure
Phytochrome is a specialized protein pigment that acts as a photoreceptor, absorbing red and far-red light to regulate various developmental processes.
Discovery
The existence of phytochrome was first predicted based on the reversible effects of red and far-red light on seed germination and floral induction. It was later isolated as a blue-green pigment.
Structure
Phytochrome exists in two interconvertible forms:
- Pr Form: Absorbs red light (approx. 660 nm) and is physiologically inactive.
- Pfr Form: Absorbs far-red light (approx. 730 nm) and is the physiologically active form that triggers developmental responses.
Red and Far-red Light Responses
Photomorphogenesis is the development of form and structure in plants that is controlled by light. The Pr/Pfr ratio determines the plant's growth pattern.
- Red Light Effects: Promotes seed germination, inhibits stem elongation (leading to sturdier plants), and initiates the development of chloroplasts.
- Far-red Light Effects: Often reverses the effects of red light. In shade conditions, the higher ratio of far-red light triggers "shade avoidance," causing the plant to elongate its stem rapidly to reach for light.
Vernalization
Vernalization is the process by which flowering is promoted by a period of low temperature.
Many plants in temperate regions require a "cold treatment" to ensure they flower during the favorable conditions of spring and summer rather than mid-winter.
Key Features
- Site of Perception: The cold stimulus is primarily perceived by the shoot apical meristem.
- Application: Used in agriculture to shorten the vegetative phase and accelerate flowering in biennial plants like cabbage and carrots.
- Reversibility: The effects of vernalization can sometimes be reversed by high temperatures, a process known as de-vernalization.
Exam Tips
- Critical Night Length: For SDP, the uninterrupted dark period is more critical than the light period. A flash of light during the night can inhibit flowering in SDP.
- Pfr Active Form: Always remember that **Pfr** is the form that triggers biological responses like flowering and germination.
- SDP vs LDP: Be ready to identify plants as SDP or LDP based on their flowering season; most spring/summer flowers are LDP, while autumn flowers are often SDP.
Common Mistakes
- Thinking Photoperiodism is just about day length; it is primarily about the duration of the continuous dark period.
- Confusing Vernalization with Stratification; Vernalization is for flowering, while Stratification is the cold treatment of seeds to break dormancy.
Frequently Asked Questions (FAQ)
Q: What happens to phytochrome during the night?
A: Pfr slowly reverts back to the inactive Pr form in the dark, or it may be degraded by the cell.
Q: Can vernalization be replaced by chemicals?
A: Yes, in some plants, the application of **Gibberellins** can mimic the effects of cold treatment and induce flowering.