A naturally occurring plant hormone that appears to be involved primarily in seed maturation, stress responses, and in regulating closure of leaf pores (stomata). It thus acts primarily to slow growth, in antagonism to the effects of growth-promoting plant hormones such as auxins and cytokinins. Levels of ABA increase greatly in maturing seeds to prevent premature germination, and germination can only occur when the ABA is removed or inactivated, e.g. by prolonged cold or light. ABA promotes the synthesis of proteins that enable the seeds to survive the dehydration associated with maturation. In leaves, ABA is produced in large amounts when the plant lacks sufficient water, promoting loss of turgor in guard cells, closure of stomata, and hence reducing further water losses. Levels of ABA increase suddenly in response to various forms of stress, including heating, waterlogging, and chilling. It was formerly believed to play a central role in abscission; hence the name.