☀️ Photoperiodism
Learn photoperiodism — short-day, long-day and day-neutral plant responses for CUET Agriculture. Garner-Allard discovery and phytochrome system.
Photoperiodism
Photoperiodism is the response of plants to the relative length of day and night — known as the photoperiod. This phenomenon was discovered by Garner and Allard in 1920 while working with tobacco at the U.S. Department of Agriculture. They noticed that certain tobacco varieties would only flower when days were short, regardless of other conditions. This discovery revolutionized our understanding of how plants "know" when to flower.
Classification of Plants by Photoperiod
Based on their flowering response to day length, plants are classified into three main categories:
| Category | Day Length Requirement | Examples |
|---|---|---|
| Short-Day Plants (SDP) | Flower when day length is less than the critical period | Rice, Soybean, Tobacco, Chrysanthemum, Cotton, Sugarcane |
| Long-Day Plants (LDP) | Flower when day length exceeds the critical period | Wheat, Barley, Oat, Sugar beet, Spinach, Radish |
| Day-Neutral Plants (DNP) | Flower regardless of day length | Maize, Tomato, Sunflower, Cucumber, Cotton (some varieties) |
IMPORTANT
For CUET, remember these key associations:
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Photoperiodism
Photoperiodism is the response of plants to the relative length of day and night — known as the photoperiod. This phenomenon was discovered by Garner and Allard in 1920 while working with tobacco at the U.S. Department of Agriculture. They noticed that certain tobacco varieties would only flower when days were short, regardless of other conditions. This discovery revolutionized our understanding of how plants "know" when to flower.
Classification of Plants by Photoperiod
Based on their flowering response to day length, plants are classified into three main categories:
| Category | Day Length Requirement | Examples |
|---|---|---|
| Short-Day Plants (SDP) | Flower when day length is less than the critical period | Rice, Soybean, Tobacco, Chrysanthemum, Cotton, Sugarcane |
| Long-Day Plants (LDP) | Flower when day length exceeds the critical period | Wheat, Barley, Oat, Sugar beet, Spinach, Radish |
| Day-Neutral Plants (DNP) | Flower regardless of day length | Maize, Tomato, Sunflower, Cucumber, Cotton (some varieties) |
IMPORTANT
For CUET, remember these key associations:
- Rice = Short-Day Plant (SDP)
- Wheat = Long-Day Plant (LDP)
- Maize = Day-Neutral Plant (DNP)
Critical Concepts in Photoperiodism
- Critical day length: This is the specific photoperiod (number of hours of light) that determines whether a plant will flower or remain vegetative. For SDPs, day length must be shorter than this critical value; for LDPs, it must be longer.
- Phytochrome: This is the pigment responsible for detecting light and dark periods in plants. It exists in two interconvertible forms — Pr (red-absorbing, inactive form) and Pfr (far-red-absorbing, active form). During daylight, Pr converts to Pfr; in darkness, Pfr slowly reverts to Pr.
- It is actually the dark period (night length) that controls flowering, not the light period. If you interrupt the dark period with even a brief flash of light, short-day plants will not flower — proving that it's the uninterrupted darkness that matters.
Experiment proving night length controls flowering
In a classic experiment, scientists grew short-day plants under two conditions: (1) Long nights were interrupted with a brief flash of light — the plants **did not flower**. (2) Long days were interrupted with a brief dark period — the plants **still flowered normally**. This proved that the **continuous dark period** (night length) is the true trigger for flowering, not the day length itself. This concept is sometimes called the **"critical night length"** hypothesis.Key Points to Remember
- Photoperiodism discovered by Garner and Allard (1920)
- Rice = SDP; Wheat = LDP; Maize = DNP
- It is the dark period (night length) that actually controls flowering — not day length
- Phytochrome (Pr ↔ Pfr) is the light-detecting pigment
- Interrupting the dark period with light prevents flowering in SDPs
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Photoperiodism — Definition | Response of plants to relative length of day and night (photoperiod) |
| Discovered By | Garner and Allard in 1920 (working with tobacco at USDA) |
| Short-Day Plants (SDP) | Flower when day length is less than critical period. Examples: Rice, Soybean, Tobacco, Chrysanthemum, Cotton, Sugarcane |
| Long-Day Plants (LDP) | Flower when day length exceeds critical period. Examples: Wheat, Barley, Oat, Sugar beet, Spinach, Radish |
| Day-Neutral Plants (DNP) | Flower regardless of day length. Examples: Maize, Tomato, Sunflower, Cucumber |
| Key Associations for CUET | Rice = SDP; Wheat = LDP; Maize = DNP |
| Critical Day Length | The specific photoperiod (hours of light) that determines flowering vs. vegetative growth |
| Phytochrome | Pigment responsible for detecting light/dark in plants. Two forms: Pr (red-absorbing, inactive) ↔ Pfr (far-red-absorbing, active). Daylight: Pr → Pfr; Darkness: Pfr → Pr. |
| What Actually Controls Flowering? | The dark period (night length), NOT the light period |
| Night-Break Experiment | Interrupting long dark period with brief light flash → SDPs do not flower. Interrupting long day with brief dark → SDPs still flower. Proves continuous darkness is the trigger. |
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