Canopy leaf display effects on absorbed, transmitted, and reflected solar radiation

Abstract Diurnal measurements of reflected, transmitted, and absorbed solar radiation, over an 80–280° solar azimuth range, were compared for wheat, corn, cotton, and sorghum canopies to study the effects of crop leaf display on light penetration into plant stands. The diurnal transmitted radiation (400–700 nm) differed for these four crops, apparently due to inherent canopy architecture and stand structure differences. Diurnal reflectance curves for visible (630–690 nm) and near infrared (760–800 nm) were flat except for the infrared reflectance curve of wheat. Thus, transmitted radiation contained most of the spectral information about canopy architecture of the four crops. Measurements several times during the day may be needed to reliably estimate daily absorbed photosynthetically active radiation (APAR) for generally erectophile crops such as wheat whereas for planophile crops such as cotton, whose leaves are closely and compactly spaced in the canopy, measurements close to solar noon may characterize the canopy. These results suggest that APAR estimates used in crop growth models may be in error, depending on canopy characteristics if based solely on reflectance and transmittance measurements near solar noon.