Lakso (1994), in a summary of many studies conducted over a 30-year period, demonstrates clearly that as total light interception increases, apple yield per unit of land increases. The implication is that areas of inherently high light levels will have higher yields than areas of lower light levels. Interception levels above 50 percent of available light result in more variability in the data, indicating other factors become more important. Another major factor affecting yield is how light is distributed through the canopy and how much of the canopy is above the critical level of light needed to saturate photosynthesis, cause flower initiation, and foster cell division for early fruit growth. Shaded areas of the canopy (less than 30 percent full sun) result in smaller fruit size and increased preharvest drop. With peach, shade has its greatest effect on fruit weight and quality during Stage III of development.
164 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT Vegetative Response
Leaves that develop in the sun have higher nitrogen levels, higher specific leaf area, more palisade layers, smaller leaf area, more cupping or curling, and less chlorophyll. Generally, shoot growth is greater in areas with high light and a long season. Trees under these conditions have greater precocity. Marini, Sowers, and Marini (1991) report that minimum light threshold for peach shoot growth is lower than that required for flower initiation, which is the opposite of the requirements for apple.
Apples are considered day neutral, and it does not appear that flower initiation, fruit growth rate, fruit size, or fruit color are phytochrome mediated. However, apples contain phytochrome, and fruit set and preharvest fruit drop can be influenced by changes in the red:far-red (R:FR) ratio mediated through phytochrome. Studies show that flower initiation does not occur in canopy areas receiving less than 30 percent full sun for apple and 20 percent full sun for peach and cherry.
Another direct response to light is fruit color. Anthocyanin formation in apple requires light, and a single leaf lying on the shoulder of an apple prevents red color formation. The poorest-colored and smallest fruit come from the most shaded portions of the canopy. On most cultivars, 30 to 50 percent full sun is needed to ensure adequate red color development and good fruit size. There is evidence that some highly colored 'Delicious' strains can be fully colored with as little as 9 percent full sun. However, soluble solids and starch levels in fruit receiving low light levels are far inferior to those in fruit from well-illuminated portions of the canopy. Fruit size and shape are always best in sections of the canopy that receive 30 percent full sun or above.
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