Why Plants Sleep? The Mystery of Photoperiodism | NEET 2025 Biology

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🔑 Introduction

Have you ever wondered why some plants bloom only in summer while others wait for winter? 🌸 This fascinating behavior is due to photoperiodism, the response of plants to the length of day and night. For NEET aspirants, this topic is a must because it connects plant physiology, NCERT diagrams, and high-weightage questions. Today, let’s unravel why plants “sleep and wake up” with light and dark cycles.


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🌞 What is Photoperiodism?

Photoperiodism is the physiological reaction of plants to the relative lengths of light (day) and darkness (night).

Long-day plants: Flower when daylight is longer than a critical period (e.g., spinach, radish).

Short-day plants: Flower when daylight is shorter than a critical period (e.g., rice, chrysanthemum).

Day-neutral plants: Flower regardless of day length (e.g., tomato, cotton).


👉 Key NEET point: Critical photoperiod = specific duration of light/dark required for flowering.


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🌸 Mechanism of Photoperiodism

Plants sense day length using a pigment called phytochrome.

Phytochrome exists in two forms: Pr (red-light absorbing) and Pfr (far-red-light absorbing).

The ratio of Pr to Pfr helps plants decide when to flower.
👉 For NEET, remember: Phytochrome = Photoreceptor pigment in plants.



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🌍 Significance of Photoperiodism

1. Ensures flowering occurs in favorable seasons.


2. Helps in synchronizing pollination and seed dispersal.


3. Plays a role in agriculture and crop yield.



💡 Fun fact: You can “trick” plants to flower early by manipulating light cycles in greenhouses!


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📒 Quick Revision Notes for NEET

Photoperiodism = response to light/dark period.

Controlled by phytochrome.

Long-day, short-day, and day-neutral plants.

Basis for flowering in angiosperms.



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📝 NEET-Style MCQs

Q1. Photoperiodism in plants is mainly concerned with:
a) Growth of roots
b) Flowering ✅
c) Seed dormancy
d) Fruit ripening

Q2. The photoreceptor pigment in plants is:
Answer: Phytochrome ✅

Q3. Which of the following is a short-day plant?
Answer: Rice ✅


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🏆 Conclusion

Plants don’t sleep like us, but they respond smartly to day and night. This natural clock is called photoperiodism, and it ensures their survival and reproduction. For NEET aspirants, mastering this topic means securing sure-shot marks with minimal effort.

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