LIGHT ENVIRONMENT AND TIME OF HARVEST AFFECT DELICIOUS APPLE FRUIT-QUALITY CHARACTERISTICS

Photosynthetic photon flux density (PPFD), measured at various canopy positions throughout the growing season in 1989 and 1990, was used to explain variation in fruit characteristics of 'Delicious' apples (Malus domestica Borkh.) harvested from these positions at 135, 145, 155, and 165 days after full bloom (DAFB). Hours above an average PPFD threshold of 250-mu-mol.m-2.s-1 (HR250) explained an average of only 2% more variation in fruit characteristics than other PPFD threshold levels or total cumulative photosynthetic photon density (PPD) in each year. Percent of red surface had a positive linear relationship with HR250 on all harvest dates in both years; intercepts increased on each successive harvest. The slopes and R2 were highest at 135 DAFB and decreased on each successive harvest. Intensity of red pigmentation and soluble solids concentration also increased linearly with HR250, with equivalent slopes and increasing intercepts on each successive harvest. Fruit weight, flesh firmness, length:diameter ratio, and starch index were not consistently affected by any measure of canopy light levels. Except for intensity of redness, relationships developed between fruit characteristics and cumulative late-season PPD during the final 10 weeks before harvest (CPPD(LS)) had trends similar to the models for HR250 for all harvests in both years. Models developed with instantaneous light measurements were similar to those developed with the more detailed cumulative light measurements.