The rate of photosynthesis decreases in a wilted herbaceous plant because the

Step 1: Analyze the question The question asks why photosynthesis slows down in a wilted herbaceous plant. Wilting happens when the plant loses more water than it absorbs, so the cells lose turgor and the leaves droop.

Step 2: Explain the concept Carbon dioxide enters the leaf through the stomata. When a plant is short of water, the guard cells lose turgor and the stomata close. This reduces water loss, but it also stops CO₂ from entering the leaf. Since CO₂ is a raw material for photosynthesis, the rate drops.

The equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll)

Step 3: Check each option

• A. Enzymes denatured: Denaturation needs extreme temperature or pH. Wilting alone does not cause it, so this is not the main reason.
• B. Chlorophyll not broken down: If chlorophyll stayed intact, photosynthesis would continue normally. This is also illogical.
• C. Light reduced: Wilting does not reduce the light available to the plant. Drooping leaves may change their angle, but this is not the main cause.
• D. Less CO₂ entering the leaf: Stomata close during wilting, so less CO₂ gets in. This is correct.

Step 4: Answer The correct option is D.

Exam tip: Whenever you see "wilting" or "water stress" in a photosynthesis question, think "stomata close → less CO₂ enters." Remember: Wilt → stomata shut → CO₂ cut.