LEAF ABSCISSION IN COLEUS

Summary The anatomical and histological development of the abscission zone is described in relation to the development of the whole leaf. The initiation of the abscission zone coincides with the change‐over from exponential to restricted growth in the blade. The response of explants from leaves at all stages of abscission zone development to a wide range of concentrations of indolyl acetic acid (IAA) applied at the distal end shows that concentrations of 10−9 M and lower accelerate, and higher concentrations retard abscission. Intact plants and excised abscission zones respond similarly. Delayed application of IAA to debladed petioles shows that it can reverse the abscission process for about 4 days after deblading. There is no evidence for an IAA‐accelerated phase in abscission. The effect of IAA is directly on the abscission zone.