Abscisic Acid, Auxin, and Ethylene in Explant Abscission

Experiments with explants of Phaseolus vulgaris L., cv. Canadian Wonder, show that abscission and the associated rise in carboxymethyl-cellulase activity in the separation zone are initiated by a peak in ethylene production during senescence of pulvinar tissue distal to the zone. Distal applications of abscisic acid (ABA) induce an earlier peak in ethylene production, increase cellulase activity, and promote abscission. ABA is more effective in these ways if treatment is delayed from 0 to 24 h after excision. With increasing concentra tions of ABA the maximum rate of ethylene production is achieved sooner. Indol-3yl acetic acid (IAA) and ABA are antagonistic in this system and have opposing effects. IAA retards the time of peak ethylene-production and delays abscission. Expiants may be retained for long periods without abscinding if incubated in an ethylene-free atmosphere : the addition of ethylene for any one 24-h period (except the first 24 h after excision) will induce abscission. The initial period of insensitivity to ethylene is extended by distal appli cations of IAA. Ethylene-induced abscission can be inhibited by IAA applied up to 72 h after excision provided the ethylene is not applied first. It is proposed that abscission in the explant is controlled at two levels : (1) an auxin-dependent stage determining the duration of insensitivity to ethylene ; (2) the timing of a rise in ethylene production in senescing tissue distal to the separation zone. An auxin-ethylene balance-mechanism at the separation zone is discussed.