ULTRAVIOLET AND PHOTOSYNTHETICALLY ACTIVE BANDS - PLANE SURFACE IRRADIANCE AT CORN CANOPY BASE

The potential increase in ultraviolet (UV) irradiance and potential decrease in productivity of agricultural crops due to stratospheric ozone loss requires knowledge of the characteristics of UV irradiance above and within crops. Little is currently known about the distribution of UV radiation within crops. The objectives of this study were to describe the UV radiation environment with respect to the photosynthetically active radiation (PAR) environment at the base of a corn canopy and relate the differences in measured irradiances to differences in canopy characteristics and differences in the proportion of direct and diffuse radiation in the two wavelength bands. Measurements of the near UV (wavelength band approximately 305-350 nm) and PAR irradiance on plane surfaces (such as leaves) oriented in a wide range of view zeniths and azimuths were made at the base of a corn canopy in September 1989 at West Lafayette, IN (solar zenith angle 54.6 +/- 5-degrees). Comparisons between the transmitted energy in near UV (NUV) and PAR wavelengths through the corn canopy were made using normalized paired irradiance measurements for a plane surface in 737 different view zenith and azimuth orientations. The NUV irradiance decreased more rapidly than the PAR irradiance as either the incidence angle (between the sun and the plane surface normal) or the view zenith angle increased. Results suggest that the corn canopy transmittance of PAR and NUV are similar but the canopy reflectance of PAR is greater than that of NUV.