Which plant hormone is primarily responsible for closing stomata during water-stress conditions in plants?

Correct answer: Abscisic acid (ABA)

Correct answer: B, abscisic acid (ABA). Stomata are tiny pores, usually present on leaves, through which carbon dioxide enters and water vapour leaves. During water shortage, excessive opening of stomata would increase water loss by transpiration. Water stress promotes the formation and transport of ABA, especially from roots and leaves. ABA acts on guard cells, causing ion and water loss from them; the guard cells become less turgid, so the stomatal pore closes. This helps the plant conserve water, although it can also reduce carbon-dioxide entry and therefore photosynthesis. A is wrong because auxin mainly regulates growth and tropic responses, not the main drought signal for stomatal closure. C is wrong because gibberellins generally promote stem elongation, seed germination, and related growth processes. D is wrong because cytokinins mainly promote cell division and delay leaf ageing. Memory cue: ABA is the “anti-water-loss” or stress hormone; remember “ABA = Abandon the opening” of stomata.