Effect of epicuticular wax on UV scattering of sorghum leaves and canopies

Sorghum bicolor is grown in equatorial regions that have naturally high ultraviolet-B (UVB) exposures. To determine whether the increased wax production on the sorghum leaves and sheaths protects the plant by increased scattered radiation from the plant surface, the effects of wax amount on UVB reflectances were examined in greenhouse and field experiments involving three isolines of sorghum - wild-type and two wax mutants. Reflectance of the wild-type sheath was found to be a result of the wax present while that on the mutant sheaths was not dependent on wax amount. Overhead UVB exposure corresponded with reduced sheath and increased leaf UVB reflectance for wild-type but negligible changes in both sheath and leaf reflectance for the two mutants. Although sheath reflectances of wild-type were twice that of the two mutants, the negligible difference in leaf reflectance between isolines resulted in negligible differences in the canopy bi-directional reflectance, even at high view angles. The UVA canopy reflectance factors of the three sorghum isolines were measured at 0.03 at viewing angles near nadir on clear sky days. Predicted reflectance factors were calculated using the SAIL model then compared with the measured reflectance factors to evaluate the effect of sky diffuse fraction on the measured differences.