Abstract
Abscisic acid (ABA) inhibited sucker development and promoted callus formation from cambium exposed at cut ends of excised roots from two clones of trembling aspen, Populus tremuloides Michx. Clonal differences in response to ABA inhibition of sucker outgrowth appeared to be related to the developmental stage of the preexisting shoot primordia. When ABA treatment was followed about a month later by gibberellic acid, ABA inhibition was partially reversed. This is one of few investigations in which both promotive and inhibitory effects of ABA have been demonstrated simultaneously on the same plant part.