Stomata close as a response to water stress and one of the key hormones involved in this process is abscisic acid (ABA). When the ABA concentration is high, it signals the stomata to close, conserving water in the plant. Therefore, choice A is the best answer. Choices B, C, D, and E are not correct as they do not accurately describe the mechanism of stomatal closure.

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Stomata Close When

Question 121

Multiple Choice

Stomata close when

A) the abscisic acid level is high.
B) the concentration of CO₂ in the leaf is low.
C) the leaf is exposed to blue light.
D) K⁺ moves into the guard cells.
E) Cl^- moves into guard cells.

Correct Answer: