🌈 Two Forms of Phytochrome
Phytochrome exists in two interchangeable forms:
- P660 (Inactive Form):
- Absorbs red light (660 nm).
- Found during the night or darkness.
- P730 (Active Form):
- Absorbs far-red light (730 nm).
- Found during the daytime or in sunlight.
🔁 How Conversion Works:
- In daylight, red light changes P660 → P730.
- In darkness, P730 slowly converts back to P660.
This back-and-forth tells the plant how long it’s been dark — it acts like a biological stopwatch!
🌹 Effect on Flowering
- In short-day plants: Too much P730 (daylight form) stops flowering.
- In long-day plants: More P730 encourages flowering.
That’s why just a short flash of red light in the night can disrupt short-day plants — it increases P730 and tells the plant, “It’s not night yet!”
🧬 Florigen – The Flowering Hormone
So what actually causes a plant to flower?
🌱 Florigen is a hormone made in the leaves of the plant once all the correct light and dark signals are received.
How it works:
- Light + darkness + phytochrome signals → activate a biological clock.
- The clock tells the leaves: “Okay, it’s time to flower.”
- Leaves make florigen.
- Florigen travels through the phloem (the plant’s transport tubes) to the buds.
- Buds receive florigen → they start forming flowers.
📊 Summary Table: Photoperiodic Response
| Type of Plant | What Triggers Flowering | Examples |
| Short-day | Night longer than critical length | Tobacco, chrysanthemum, strawberry |
| Long-day | Night shorter than critical length | Henbane, cabbage, barley |
| Day-neutral | Not affected by light/dark length | Tomato, cucumber, maize |
🧠 Real-Life Analogy to Understand Better:
Imagine three types of people:
- Early birds (Long-day plants): Wake up and work when there is a lot of light.
- Night owls (Short-day plants): Only function well when it’s dark for a long time.
- Flexible workers (Day-neutral): Don’t care about time — they just work when they feel ready!
🪴 In Conclusion:
- Photoperiodism helps plants know when to flower by measuring day and night length.
- A protein called phytochrome helps detect light changes.
- Florigen hormone is produced in the leaves and triggers flowering.
- Different plants have different needs: some need short days, some long days, and some don’t care at all!
Let me know when you’re ready with the next chunk, and I’ll explain it just as clearly and deeply!