S18 - Session O5 - A comparison between relative fruit growth rate and fruit size distribution models to predict apple fruitlet abscission

Authors: Todd Einhorn *, Laura Hillmann, Stefano Musacchi, Sara Serra, Tom Kon, Terence Robinson, Luis Gonzalez Nieto, Mokhles Elsysy

Flower and fruitlet thinning of apple ( Malus x domestica Borkh.) is critically important to yield security and fruit quality targets. Thinning, however, is notoriously unpredictable. Precision crop load management has improved over recent years given advances in the pollen tube growth model and the carbohydrate balance model- both facilitate the selection and timing of thinning applications. After thinner application, a fruit growth rate (FGR) model can inform repeat use of PGRs before fruit become too difficult to thin. The FGR model requires repeated, non-destructive measures of fruit diameter using digital calipers. Fruit with a relative growth rate (RGR) less than 50% of the maximum RGR fruit are predicted to abscise. Execution of the FGR model, however, is time consuming and tedious. In support of nascent technologies to image and analyze fruitlet growth, we tested a user-friendly alternative to the FGR by destructively sampling and weighing fruitlets at two-day intervals. An empirical model based on fruit size distribution from random populations generated similar prediction responses of absolute fruit set than the current FGR model. The size distribution model was validated in several distinct geographical apple production regions of the U.S. to determine reproducibility. To explain the mechanisms leading to fruitlet abscission or retention, we assessed vascular tissues of fruitlet pedicels and the non-structural carbohydrate composition of fruitlets in the 10 th , 50 th and 90 th percentile of fruit mass at each sample date until fruit set predictions stabilized. The relationship between the physiological mechanisms assessed and apple fruitlet abscission will be presented.