Fractional interception of photosynthetically active radiation (FIPAR) (i.e. light interception) by tree fruit canopies determines, in large part, fruit quantity and quality, and thus profitability. This study aimed to elucidate the diurnal pattern in FIPAR and determine the optimum time at which the measured FIPAR best represents mean daily FIPAR in sweet cherry trees trained to the planar UFO (Upright Fruiting Offshoots) architecture. Two experiments were conducted to assess the influence of canopy development stages and of canopy height to row spacing ratios (H/S ratio) on diurnal FIPAR pattern and optimum measurement time. For different canopy development stages, three adjacent north-to-south sample blocks were selected. For different H/S ratios, three levels of 0.75, 1.00, and 1.25 were used, and 9 north-to-south sample block for each level were selected. Daily FIPAR was determined from measurements taken within ±6 h of solar noon. Gaussian process regression with four parameters was applied to obtain the diurnal pattern with coefficients of determination R2 ranging from 0.97 to 1.00. The diurnal pattern was symmetric with the lowest point around solar noon. Mean daily FIPAR increased during the season, as well as with increasing H/S ratio. For both experiments, there was no substantial difference in optimum measurement time among stages and among H/S ratios, with maximum difference of 0.3 h and 0.4 h respectively. We recommend an optimal measurement time window of −2.5 h to −2.0 h with reference to solar noon for the planar UFO architecture to estimate mean daily FIPAR with ±10% tolerance interval.
