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AGRONOMIC MODELING

Potential Effect of Soybean Heliotropism on Ultraviolet-B Irradiance and Dose

^a Dep. of Agronomy, Purdue Univ., West Lafayette, IN, 47907-1150 USA

rgrant{at}purdue.edu

Assessing the potential for decreases in crop yield due to stratospheric ozone loss and concomitant increases in ultraviolet-B (UVB) radiation requires knowledge of the characteristics of UVB irradiance above and within crops and the sensitivity of crops to the detrimental effects of the irradiance. Evaluations of the tolerance of many crops to increases in UVB irradiance have been made. The tolerance or intolerance of a crop to UVB irradiance is in part a result of physical characteristics of the plant. This study evaluated both the distribution of irradiance and the effects of plant leaf heliotropic movement of soybean [Glycine max (L.) Merr.] cultivars on the irradiance received on the leaf surface. Measurements of the above-canopy UVB irradiance on planes having a wide range of slope and aspect were made under cloud-free skies in a soybean field during the summer of 1990. Results showed that the relative irradiance was strongly dependent on the leaf inclination angle and could be modeled by a simple function having a relative irradiance accuracy of 0.14 (root mean square error); with greater model error at small incidence angles to the sun than at large incidence angles. The developed model was then used to evaluate the influence of heliotropy on the UVB dose received on the terminal leaflet. These results suggest that heliotropy in the leaflet acts to reduce irradiance. Since the more tolerant soybean cultivars appear to orient leaflets away from the sun, results suggest that the classification of UVB tolerance may be due in part to the capacity for paraheliotropic response reducing the UVB radiation received.

Abbreviations: F_d, diffuse fraction • g, anisotropy factor • I, irradiance • I_s, relative irradiance • MBE, mean bias error • RMSE, root mean squared error • UVB, ultraviolet-B • , plane slope • ß, plane aspect • , relative azimuth of the plane to the sun • , zenith angle • _can, hemispherical canopy reflectance • , azimuth angle • , scattering angle. Subscripts: d, sky • k, solar disk