Which one of the following plant hormones promotes stomatal closure during drought stress?
Abscisic acid
Auxin primarily regulates cell elongation and tropic growth; cytokinin promotes cell division and delays senescence; gibberellin promotes stem elongation, seed germination and flowering — none of these primarily controls stomatal closure under stress.
Abscisic acid (ABA) is known as the "stress hormone" of plants: under drought conditions it accumulates in leaves and triggers guard cells to lose turgor pressure, closing the stomata to reduce water loss through transpiration.
Hence, the answer is Abscisic acid.
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