The productivity of a crop begins to decline when the leaves begin to wilt mainly because

Options

A. flaccid mesophyll cells are incapable of photosynthesis
B. stomata close, preventing `CO_2` from entering the leaf
C. photolysis, the water-splitting step of photosynthesis, cannot occur when there is a water deficiency
D. accumulation of `CO_2` in the leaf inhibits enzymes required for photosynthesis

Answer

**Step-by-Step Solution:**

1. **Understanding the Problem**: The question asks why the productivity of a crop declines when the leaves begin to wilt. This indicates a relationship between leaf condition and the plant's ability to perform photosynthesis.

2. **Identifying the Cause of Wilting**: Leaves wilt primarily due to excessive loss of water. This loss of water leads to a condition known as "flaccidity" in plant cells, which affects their function.

3. **Impact on Stomata**: When leaves wilt, the stomata (tiny openings on the leaf surface) tend to close. This closing is a protective mechanism to prevent further water loss.

4. **Consequences of Stomata Closure**: The closure of stomata prevents carbon dioxide (CO2) from entering the leaf. Since CO2 is a crucial component for photosynthesis, its absence directly affects the plant's ability to produce food.

5. **Photosynthesis and Crop Productivity**: Photosynthesis is the process through which plants convert light energy into chemical energy (food). If stomata are closed and CO2 cannot enter, the photosynthesis process is halted, leading to a decrease in food production.

6. **Conclusion**: As a result of the inability to perform photosynthesis due to the closure of stomata, the overall productivity of the crop declines. Therefore, the correct answer to the question is that the stomata close, preventing carbon dioxide from entering the leaf.

**Final Answer**: The productivity of a crop begins to decline when the leaves begin to wilt mainly because stomata close, preventing carbon dioxide from entering the leaf.

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