8.3.5 Photoperiodism and Flowering

Introduction

Plants need to flower at the right time of year to ensure successful reproduction. They use environmental cues, especially the length of day and night, to determine when to flower. This response to the duration of light and dark periods is called photoperiodism. Understanding photoperiodism helps us understand how plants adapt to their environment and how we can control flowering in agriculture and horticulture.

Definition

Photoperiodism: Photoperiodism is the response of plants to the relative lengths of light and dark periods (day and night) that controls various physiological processes, especially flowering.

In simple words: Photoperiodism is how plants use the length of day and night to decide when to flower.

Core Concept

• Plants use the length of day and night to determine when to flower.
• This response is called photoperiodism.
• Based on their photoperiodic response, plants are classified into:
• The critical day length is the specific day length that triggers flowering.
• Phytochrome is the pigment that detects light and dark periods.

Detailed Explanation

1. Classification of Plants Based on Photoperiodism

a. Short-Day Plants:

• Definition: Short-day plants are plants that flower when the day length is shorter than a critical length (i.e., when the night length is longer than a critical length).
• Examples: Chrysanthemum, soybean, poinsettia, strawberry.
• Flowering season: Autumn or winter (when nights are long).
• Note: Short-day plants are also called long-night plants because they actually respond to the length of the night.

b. Long-Day Plants:

• Definition: Long-day plants are plants that flower when the day length is longer than a critical length (i.e., when the night length is shorter than a critical length).
• Examples: Spinach, lettuce, radish, wheat, barley.
• Flowering season: Spring or summer (when days are long).
• Note: Long-day plants are also called short-night plants because they actually respond to the length of the night.

c. Day-Neutral Plants:

• Definition: Day-neutral plants are plants that flower regardless of the day length.
• Examples: Tomato, cucumber, sunflower, rice.
• Flowering season: Any time of year.
• Note: Their flowering is not controlled by photoperiod.

2. Critical Day Length

• Definition: The critical day length is the specific day length that triggers flowering in a plant.
• Each plant species has its own critical day length.
• If the day length is shorter than the critical day length, short-day plants flower.
• If the day length is longer than the critical day length, long-day plants flower.

3. Phytochrome

• Definition: Phytochrome is a pigment that detects light and dark periods in plants.
• It exists in two interconvertible forms:
• Conversion:
• Pr is converted to Pfr when exposed to red light.
• Pfr is converted to Pr when exposed to far-red light or in darkness.
• Role: The ratio of Pfr to Pr determines the plant's response to photoperiod.
• High Pfr/Pr ratio → Long-day plants flower.
• Low Pfr/Pr ratio → Short-day plants flower.

4. Mechanism of Photoperiodism

• Light perception: Phytochrome detects light and dark periods.
• Signal transduction: The ratio of Pfr to Pr is interpreted by the plant.
• Floral stimulus: A floral stimulus (also called florigen) is produced in the leaves.
• Transport: Florigen is transported to the shoot apical meristem.
• Flowering: The shoot apical meristem is triggered to produce flowers.

5. Importance of Photoperiodism

• Reproduction: Ensures that flowering occurs at the right time of year.
• Adaptation: Helps plants adapt to their environment.
• Agriculture: Used to control flowering in crops.
• Horticulture: Used to produce flowers at desired times.
• Research: Used to study plant growth and development.

Summary Table of Photoperiodism

Summary Table of Phytochrome

Intermediate Level

Classification of Plants in Detail

Critical Day Length in Detail

Phytochrome in Detail

Mechanism of Photoperiodism in Detail

Importance of Photoperiodism in Detail

Important Points

• 1. Plants need to flower at the right time of year to ensure successful reproduction.
• 2. They use environmental cues, especially the length of day and night, to determine when to flower.
• 3. This response to the duration of light and dark periods is called photoperiodism.
• 4. Understanding photoperiodism helps us understand how plants adapt to their environment and how we can control flowering in agriculture and horticulture.
• 5. Classification of Plants Based on Photoperiodism:
• - Short-day plants: Plants that flower when the day length is shorter than a critical length. Examples: Chrysanthemum, soybean, poinsettia, strawberry. Flowering season: Autumn or winter.
• - Long-day plants: Plants that flower when the day length is longer than a critical length. Examples: Spinach, lettuce, radish, wheat, barley. Flowering season: Spring or summer.
• - Day-neutral plants: Plants that flower regardless of the day length. Examples: Tomato, cucumber, sunflower, rice. Flowering season: Any time of year.
• 6. Critical Day Length:
• - The critical day length is the specific day length that triggers flowering in a plant.
• - Each plant species has its own critical day length.
• - If the day length is shorter than the critical day length, short-day plants flower.
• - If the day length is longer than the critical day length, long-day plants flower.
• 7. Phytochrome:
• - Phytochrome is a pigment that detects light and dark periods in plants.
• - It exists in two interconvertible forms:
• - Pr (P₆₆₀): Absorbs red light (660 nm).
• - Pfr (P₇₃₀): Absorbs far-red light (730 nm).
• - Conversion:
• - Pr is converted to Pfr when exposed to red light.
• - Pfr is converted to Pr when exposed to far-red light or in darkness.
• - Role: The ratio of Pfr to Pr determines the plant's response to photoperiod.
• - High Pfr/Pr ratio → Long-day plants flower.
• - Low Pfr/Pr ratio → Short-day plants flower.
• 8. Mechanism of Photoperiodism:
• - Light perception: Phytochrome detects light and dark periods.
• - Signal transduction: The ratio of Pfr to Pr is interpreted by the plant.
• - Floral stimulus: A floral stimulus (also called florigen) is produced in the leaves.
• - Transport: Florigen is transported to the shoot apical meristem.
• - Flowering: The shoot apical meristem is triggered to produce flowers.
• 9. Importance of Photoperiodism:
• - Reproduction: Ensures that flowering occurs at the right time of year.
• - Adaptation: Helps plants adapt to their environment.
• - Agriculture: Used to control flowering in crops.
• - Horticulture: Used to produce flowers at desired times.
• - Research: Used to study plant growth and development.

Remember

• - Photoperiodism = response to day and night length.
• - Short-day plants = flower when days are short.
• - Long-day plants = flower when days are long.
• - Day-neutral plants = flower anytime.
• - Critical day length = specific day length that triggers flowering.
• - Phytochrome = pigment that detects light and dark.
• - Florigen = floral stimulus.
• - Examples: Chrysanthemum, spinach, tomato.

Common Mistakes

• 1. Confusing short-day and long-day plants – Short-day = flower when days are short; Long-day = flower when days are long.
• 2. Thinking short-day plants respond to short days – Short-day plants actually respond to long nights (they are also called long-night plants).
• 3. Thinking long-day plants respond to long days – Long-day plants actually respond to short nights (they are also called short-night plants).
• 4. Confusing day-neutral plants with short-day or long-day plants – Day-neutral plants flower regardless of day length.
• 5. Confusing phytochrome forms – Pr = absorbs red light; Pfr = absorbs far-red light.
• 6. Thinking phytochrome is only in leaves – Phytochrome is present in many plant tissues.
• 7. Confusing the role of florigen – Florigen is the floral stimulus produced in leaves.
• 8. Thinking photoperiodism only controls flowering – Photoperiodism also controls other processes (e.g., seed germination, dormancy).
• 9. Confusing critical day length – Each species has its own critical day length.
• 10. Thinking photoperiodism is not important – Photoperiodism is essential for plant reproduction and adaptation.
• 11. Confusing the examples – Chrysanthemum (short-day), spinach (long-day), tomato (day-neutral).
• 12. Thinking all plants are photoperiodic – Some plants are day-neutral.

Quick Revision

• Plants need to flower at the right time of year to ensure successful reproduction.
• They use environmental cues, especially the length of day and night, to determine when to flower.
• This response to the duration of light and dark periods is called photoperiodism.
• Understanding photoperiodism helps us understand how plants adapt to their environment and how we can control flowering in agriculture and horticulture.
• Classification of Plants Based on Photoperiodism:
• Short-day plants: Plants that flower when the day length is shorter than a critical length. Examples: Chrysanthemum, soybean, poinsettia, strawberry. Flowering season: Autumn or winter.
• Long-day plants: Plants that flower when the day length is longer than a critical length. Examples: Spinach, lettuce, radish, wheat, barley. Flowering season: Spring or summer.
• Day-neutral plants: Plants that flower regardless of the day length. Examples: Tomato, cucumber, sunflower, rice. Flowering season: Any time of year.
• Critical Day Length:
• The critical day length is the specific day length that triggers flowering in a plant.
• Each plant species has its own critical day length.
• If the day length is shorter than the critical day length, short-day plants flower.
• If the day length is longer than the critical day length, long-day plants flower.
• Phytochrome:
• Phytochrome is a pigment that detects light and dark periods in plants.
• It exists in two interconvertible forms:
• Pr (P₆₆₀): Absorbs red light (660 nm).
• Pfr (P₇₃₀): Absorbs far-red light (730 nm).
• Conversion:
• Pr is converted to Pfr when exposed to red light.
• Pfr is converted to Pr when exposed to far-red light or in darkness.
• Role: The ratio of Pfr to Pr determines the plant's response to photoperiod.
• High Pfr/Pr ratio → Long-day plants flower.
• Low Pfr/Pr ratio → Short-day plants flower.
• Mechanism of Photoperiodism:
• Light perception: Phytochrome detects light and dark periods.
• Signal transduction: The ratio of Pfr to Pr is interpreted by the plant.
• Floral stimulus: A floral stimulus (also called florigen) is produced in the leaves.
• Transport: Florigen is transported to the shoot apical meristem.
• Flowering: The shoot apical meristem is triggered to produce flowers.
• Importance of Photoperiodism:
• Reproduction: Ensures that flowering occurs at the right time of year.
• Adaptation: Helps plants adapt to their environment.
• Agriculture: Used to control flowering in crops.
• Horticulture: Used to produce flowers at desired times.
• Research: Used to study plant growth and development.

60-Second Revision

• Photoperiodism = response to day and night length.
• Short-day plants = flower when days are short.
• Long-day plants = flower when days are long.
• Day-neutral plants = flower anytime.
• Critical day length = specific day length that triggers flowering.
• Phytochrome = pigment that detects light and dark.
• Florigen = floral stimulus.
• Examples: Chrysanthemum, spinach, tomato.
• Warning: Short-day plants respond to long nights; long-day plants respond to short nights. Day-neutral plants flower regardless of day length. Phytochrome has two forms: Pr (absorbs red light) and Pfr (absorbs far-red light). Florigen is produced in leaves and transported to the shoot apical meristem. Photoperiodism is essential for plant reproduction and adaptation.