LEAF REFLECTANCE AND TRANSMITTANCE IN SOYBEAN AND CORN

Leaves have a major influence on canopy reflectance when they constitute the main spatial component in a vegetative canopy. Near normal-incidence, directional-hemispherical reflectance and transmittance of in situ individual leaves of soybean (Glycine max., Merr.) and corn (Zea mays, L.) were characterized as a function of wavelengths and growth. Spectral properties were measured in seven wavebands with an integrating sphere and prototype radiometer unit. Individual leaves periodically were monitored from emergence and unfolding through 47 d in soybean and 77 d in corn. Visible reflectance and transmittance decreased in soybean as leaves expanded, but increased after full leaf expansion. An opposite pattern was observed with soybean near-infrared radiation (NIR) reflectance. Spectral properties varied little in mid- and upper-canopy corn leaves with the exception of the green spectral region. Near constant values were attributed to the fact that corn leaves are fully expanded by the time they have fully emerged. Reflectance and transmittance properties of adaxial and abaxial surfaces differed by as much as an absolute 5% in soybeans while there were essentially no differences in corn. Differences in surface reflectance and transmittance in soybean were attributed to the dorsiventral morphology of soybean leaves. Reflectance and transmittance from adaxial and abaxial leaf surfaces may have to be considered in modeling soybean canopies while one surface should suffice to describe light interaction with corn canopy leaves.