INFLUENCE OF SUNLIGHT LEVEL AND ROOTSTOCK ON APPLE FRUIT QUALITY

With 16-year-old 'Oregon Spur Delicious' and 8-year-old 'Golden Delicious' trees, sunlight levels (photosynthetically active radiation [PAR] in μmol.m².s and as % full sun) were determined at various locations in tree canopies and at each location fruit samples were collected for fruit quality evaluations. With 'Delicious', there were curvilinear increases in % red color, soluble solids and fruit weight as light level increased from 5% to approximately 50% full sun (860 μmol.m².s), above which there was little change. The regression equations for the curvilinear relationships showed that 50 to 61% (r² x 100) of the variation in these fruit traits was accounted for by differences in sunlight levels. Objective color measurements (Agtron) also showed a change from yellow to red background color with increasing light levels (r² = .30). Little variation in fruit firmness (r²=.15) or titratable acidity (r²=.03) was associated with light levels.

With 'Golden Delicious', positive curvilinear relationships with sunlight levels occurred for soluble solids, Agtron color (green to yellow), total carbohydrate, fructose and sucrose levels, and negative curvilinear relationships occurred with hue (green to yellow fruit color). Little change in these fruit traits occurred above 50% full sun (800 μ.m².s). There was little influence of light level on titratable acidity and fruit firmness. 'Golden Delicious' fruit samples were collected from trees on M.9, M.26 and Mark rootstocks. For the three rootstocks, the relationships of each fruit trait with sunlight levels were similar. When sunlight level was used as a covariant, there were no significant differences between adjusted rootstock means for any of the traits evaluated.