Daily fluctuation in fruit detachment force of 'Valencia' orange is related to time of day, temperature, relative humidity, fruit weight, and juice percentage

Throughout the course of our research addressing topics related to abscission, we noted differences in fruit detachment force (FDF) that were dependent upon the time of day measurements were taken. We tested the hypothesis that diurnal fluctuations in FDF exist in mature citrus fruit (Citrus sinensis cv. Valencia). FDF was measured at 8 AM, 11 AM, 2 PM, and 5 PM for several consecutive days each month in Jan., Apr., and May 2007. Fruit weight and juice content were determined for samples harvested at 8 AM and 2 PM. Hourly averages for temperature and relative humidity were recorded by the nearby University of Florida's Florida Automated Weather Network (FAWN). Daily fluctuation in FDF changed 4% to 23% within a period from 8 AM to 5 PM. Fluctuations in FDF were correlated with time of day, temperature, relative humidity, juice percentage, and fruit weight. Efficacy of abscission agents was increased when applying at 1 PM when daily FDF was low. Abscission agent-associated fruit and leaf drop responded in a similar manner to application time. These results demonstrate diurnal fluctuations in FDF exist in mature citrus fruit, and that these daily changes can be used to manage fruit response to abscission agents.