Physiological factors affecting flower and fruit abscission of ‘Hass’ avocado

‘Hass’ avocado (Persea americana Mill.) is characterized by distinct periods of excessive flower and fruit drop. Determining the cause(s) of these abscission events is critical for the development of strategies to increase fruit set and yield. Therefore, the objectives of this research were to determine pollination and fertilization rates of abscising flowers and to quantify the viability of developing ovules (seeds) and hormone concentrations of abscising versus persisting fruit through early and June drop of ‘Hass’ avocado. Experiments were conducted during sequential off- and on-crop years. Pollination, pollen germination, pollen tube growth and ovule viability of abscising flowers were determined by microscopic analysis. During early and June drop, cellular deterioration of developing ovules of abscising and persisting fruit was visualized with Evan’s blue stain. Abscisic acid (ABA), indole-3-acetic acid (IAA), and isopentenyladenine (IPA) concentrations were determined by radio-immuno assay (RIA) during the on-crop year only. On average ≥ 70% of abscising ‘Hass’ flowers were pollinated but due to the failure of the pollen grains to germinate and produce pollen tubes, fertilization never occurred and the ovule deteriorated. Fruit abscission during early and June drop was also due primarily to a lack of fertilization. Fruit abscission was associated initially with deterioration of the nucellus within the developing ovule in early drop, and subsequently with deterioration of the integument (seed coat) in June drop; both occurred more frequently in abscising than persisting fruit. Abscising fruit had greater ABA concentrations during early drop, June drop and fall fruit drop compared to persisting fruit; IAA and IPA were greater in abscising fruit during June drop, but not early drop or fall fruit drop. Only 25% of the total number of fruit that abscised per crop year were fertilized. Taken together, these results provide strong evidence that the majority of flowers and fruit of ‘Hass’ avocado abscise due to a lack of pollen germination and subsequent fertilization. In addition, the results suggest that ABA accumulation is related to ovule (seed) abortion and reduced fruit growth in abscising fruit.