Red drop occurs due to :

Options

A. Low `CO_(2)` in atmosphere
B. High `CO_(2)` in atmosphere
C. Supply of light intensity below 680 nm
D. Supply of monochromatic light of 700 nm

Answer

**Step-by-Step Solution:**

1. **Understanding Red Drop**: Red drop refers to a phenomenon observed in photosynthesis where the efficiency of photosynthesis decreases when plants are exposed to light of certain wavelengths, particularly far-red light.

2. **Historical Experiment**: The concept of red drop was introduced by scientist Robert Emerson. He conducted experiments on the cholera plant to study the effects of different light wavelengths on photosynthesis.

3. **Light Wavelengths**: In his experiments, Emerson used red light with a wavelength of 680 nanometers (nm) and observed an increase in the rate of photosynthesis. However, when he switched to far-red light with a wavelength of 700 nm, he noted a decrease in photosynthesis efficiency.

4. **Electron Excitation**: The increase in photosynthesis at 680 nm is due to the excitation of electrons, which are crucial for the photosynthetic process. In contrast, the far-red light (700 nm) causes damage to these electrons, leading to a drop in photosynthesis efficiency.

5. **Analyzing the Options**:
- **Option A**: Low carbon dioxide in the atmosphere – Not related to red drop.
- **Option B**: High carbon dioxide in the atmosphere – Also not related to red drop.
- **Option C**: Supply of light intensity below 680 nm – This is normal photosynthesis, not related to red drop.
- **Option D**: Supply of monochromatic light of 700 nm – This is the correct answer as it directly relates to the phenomenon of red drop.

6. **Conclusion**: The correct answer is **Option D**: Red drop occurs due to the supply of monochromatic light of 700 nanometers.

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