The influence of applied auxin on abscission in explants (excised cotyledonary nodes) of cotton was investigated.Proximal applications to the stem stump accelerated abscission with all concentrations applied. Distal applications to the petiole stumps retarded abscission with all concentrations applied.Simultaneous applications to petiole and stem stumps accelerated, retarded, or were without effect on abscission depending on the relative amounts applied. Relatively high concentrations proximal on stem stumps accelerated abscission whereas relatively high concentrations distal on petiole stumps retarded abscission. The abscission rates from various combinations resulted in a multiphasic abscission response curve as the amounts applied were increased.Consideration of these and related data in the literature indicates that many factors can have profound effects on the nature of abscission responses to auxin.Interest in auxin as a controlling factor in abscission developed rapidly after Laibach (18) found that abscission can be retarded by the application of auxin to debladed petioles. His discovery led to a number of important agricultural practices (1,30,31) and stimulated basic researches on the influence of auxin in the control of abscission. All but the most recent literature has been described in reviews (1,5,16). A comprehensive article describing the physiological factors in abscission, including other hormones as well as auxin, has recently appeared (2).The early investigations established that auxin commonly functions to retard abscission. Further investigations disclosed other influences of auxin. The onset of abscission was found to be correlated with the gradient, or balance, of auxin across the abscission zone, auxin distal to the abscission zone tending to retard abscission, and auxin proximal to the zone tending to accelerate abscission. This general concept is supported by the results of a number of workers with a variety of plant materials from experiments involving extraction, diffusion, applications, and metabolism of auxin (6,14,15,19,21,(26)(27)(28). However, a few experiments have been reported in which proximal applications of auxin have retarded abscission (24), and in some cases distal applications have accelerated abscission (10,12,32
MATERIALS AND METHODSExcised abscission zones (explants) were used in this investigation following a method similar to that described by Addicott et al. (4). Explants were cut from the cotyledonary nodes of cotton seedlings (Gossypium hirsutum L. cultivar Acala 4-42) grown in perlite in galvanized flats in the greenhouse. Seedlings were used when their first internodes had attained a length of 2 cm (21-25 days after planting). Cotyledons were debladed 24 hr before the nodes were excised as this led to more rapid abscission, reducing the time required for an experiment. After excision each explant consisted of two 5-mm petiole stumps, a 5-mm stem stump (first internode), and a 10-mm hypocotyl stump, and thus each contained two abscission zones. The ...