Question 34

A plant in dry soil wilts and its rate of photosynthesis drops. What is the main reason for that drop at school level?

The pores in its leaves close, so the gas it needs cannot get inCorrect answer
The plant has used up all the water that the reaction consumes
The leaves lose their green pigment when they are short of water
The roots stop absorbing mineral ions, which the reaction needs

Explanation

Short of water, a plant shuts the pores in its leaf surface to hold on to what it has left, and with those pores shut the gas the reaction needs can no longer diffuse in. The tidy-looking alternative, that the water consumed by the reaction has run out, is not the school explanation, because the amount used up in the reaction is tiny beside the amount a leaf loses through open pores.

Explanation

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