plant growth substances
(Science: plant biology) substances that, at low concentration, influence plant growth and differentiation. Formerly referred to as plant hormones or phytohormones, these terms are now suspect because some aspects of the hormone concept, notably action at a distance from the site of synthesis, do not necessarily apply in plants. Also called plant growth regulators.
The major classes are absicisic acid, auxin, cytokinin, ethylene and gibberellin, others include steroid and phenol derivatives.