Two kinds of light are important—direct and diffused, or indirect. Direct light is uninterrupted as it falls directly on the leaf, while diffused light is reflected from clouds, particles in the air, or leaves. Shade of a single leaf reduces the light level well below the saturation level for photosynthesis. As light travels through a tree canopy, much of the visible range is absorbed, and thus the spectral balance in the lower portion of the canopy is relatively higher in infrared than the canopy periphery. Recent evidence suggests that some of the differences in growth response and leaf shape in the interior of the canopy may be due to these changes in spectrum. Lakso (1994) reports that light levels on the interior of the canopy will be higher on hazy or partly cloudy days than on clear days or very overcast days. Some fruit-growing areas, such as Italy, New Zealand, and the West Coast of the United States, have 30 to 40 percent more light than the eastern
United States or western Europe. Light also declines at more northerly latitudes and results in lower yields and smaller fruit size.
The sun provides a point source of light, and a classic work conducted by D. R. Henicke in the 1960s shows that a shell over the top and partially down the sides of a tree receives light in excess of saturation. An inner shell receives an adequate level and the shell in the bottom and center of the canopy receives inadequate light to saturate photosynthesis. In the old-fashioned, large seedling tree, the portion of the canopy receiving inadequate light may be as much as 30 to 40 percent of the total canopy. One of the greatest advantages of dwarf trees is that this interior shaded area is greatly reduced on each tree, and with many more trees per hectare, light interception and orchard efficiency are dramatically increased. Transmittance of light through an apple leaf is about 7 percent of photosynthetically active radiation, based on research by Jackson (1980). Reflectance is greater than transmittance over all visible wavelengths and is greatest in May.
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