Possible hormonal modes of action of three apple thinning agents

Thinning-experiments were carried out with carbaryl, ethephon (CEPA) and the amide of naphthaleneacetic acid (NAAm) in the apple cultivars ‘Golden Delicious’ and ‘King of the Pippins’. In order to get more information on the possible hormonal modes of action of these substances, fruit treated with thinning-agents were analysed for ethylene (C₂H₄), diffusable and extractable indoleacetic acid (IAA), abscisic acid (ABA) and gibberellin-like substances (GA_s), and compared with water-treated controls.It is thought that the observed reduction in IAA transport is a major factor in the mode of action of all 3 thinning-agents investigated. Auxin transport may be affected directly by the chemicals or via a shift in the mobilizing ability of the treated fruits. The role of other hormones investigated is briefly discussed.