The Penetration of Global Solar Radiation into Apple Trees1

Abstract

Global radiation on cloudless days within apple trees varied considerably. At most times on any day absorption of global radiation was dependant on solar zenith angle and was greatest between 1 and 2m from the tree tops. Measurements of penetrating global radiation were similar whether made on E-W or N-W tracks and differed little in trees on M.26 and M.7 except deep in the canopy where energy levels were lower in the latter.Attenuation of infra-red radiation was less than visible resulting in higher levels of infra-red relative to visible deep in the canopy. Apples from trees on M.7 had less red color than those on M.26 but the amount of red color formed was positively correlated with global radiation. At the same global radiation fruit color was better on M.26 than on M.7. This may, in part, be a rootstock or tree training effect. An estimated minimum energy of 250 cal cm -2 day -1 was necessary for the initiation and maintenance of red color in apples.