The plant hormone abscisic acid (ABA) is the major player in mediating the adaptation of the plant to stress. ABA mediates the conversion of the apical meristem into a dormant bud. ABA in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. ABA plays a role in seed maturation, at least in some species, and also enforces a period ofseed dormancy. ABA also promotes abscission of leaves and fruits. ABA is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration.