At high oxygen concentration, the rate of photosynthesis decreases due to

Options

A. Warburg effect
B. Pasteur effect
C. Emerson effect
D. Richmond Lang effect.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks why the rate of photosynthesis decreases at high oxygen concentrations. We need to identify the correct biological phenomenon that explains this decrease.

2. **Identifying the Options**: The options provided are:
- Warburg effect
- Pasteur effect
- Emerson effect
- Richmond-Lange effect

3. **Analyzing Each Option**:
- **Warburg Effect**: This effect describes the phenomenon where the enzyme ribulose bisphosphate carboxylase/oxygenase (Rubisco) has a higher affinity for oxygen than for carbon dioxide at high oxygen concentrations. This leads to a decrease in the efficiency of photosynthesis because Rubisco will bind with oxygen instead of carbon dioxide, resulting in a lower rate of photosynthesis.

- **Pasteur Effect**: This effect refers to the inhibition of fermentation by oxygen. It is not related to photosynthesis and therefore can be ruled out.

- **Emerson Effect**: This effect involves the increase in the rate of photosynthesis when chloroplasts are exposed to light of specific wavelengths (680 nm or more). It does not explain the decrease in photosynthesis at high oxygen levels.

- **Richmond-Lange Effect**: This effect pertains to leaf senescence and is not relevant to the question regarding oxygen concentration and photosynthesis.

4. **Conclusion**: Based on the analysis, the correct answer is the **Warburg Effect**. At high oxygen concentrations, Rubisco preferentially binds to oxygen, which decreases the rate of photosynthesis.

### Final Answer:
The rate of photosynthesis decreases at high oxygen concentrations due to the **Warburg effect**.

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