Identification of Maximum Sensitivity of Developing Apple Fruits to Naphthaleneacetic Acid1

Naphthaleneacetic acid (NAA) thinning sprays applied at appropriate intervals and dosage levels to 3 apple cultivars, verified the existence of a critical stage during which developing fruitlets were maximally sensitive. Average fruit diameter accurately identified NAA-sensitivity, but the index varied among cultivars. Polynomial regression analyses revealed a direct relationship between diameters of individual fruit samples and thinning response for each experimental unit. NAA-induced fruit abscission was usually maximal 2 to 3 days prior to the onset of cytokinesis in the endosperm. Cell wall formation did not invariably signify an end to the induction of significant abscission.