(ABA, dormin) A growth substance
that exerts numerous different, mainly inhibitory, effects on the growth and development of many species. It is a sesquiterpenoid, formula C15
, (see diagram) and, like gibberellin, may be synthesized from mevalonic acid. It is active, possibly in association with gibberellic acid, in the promotion of leaf and fruit abscission
and the control of dormancy
. It prevents cell elongation and shoot growth and also inhibits seed germination and some tropic responses. At physiological concentrations, ABA is not toxic to plants, which can, if necessary, remove its effects by converting it to an abscisyl glucoside by linking it to a glucose residue. A large proportion of ABA is synthesized in the chloroplasts. The rate of synthesis increases dramatically when the plant is under stress, especially from water shortage. ABA overrides the normal diurnal pattern of stomatal opening and closure and causes the stomata to close during the day. This response decreases water loss by transpiration in times of drought.