Epicuticular waxes on crop plants have been shown to influence crop development and yield by reducing evapotranspiration, insect damage and pathogen infestation, and influences the optical properties of the leaves and stalks. Part of the variability found in the reflectance of leaf tissues appears related to the epicuticular wax distribution and wax abundance on the leaf. This study further characterizes the effects of ultraviolet (UV) radiation on epicuticular wax formation and the effect of the epicuticular waxes on leaf and canopy reflectances.

Two laboratory approaches were made to determine the influence of epicuticular waxes on the optical properties of Sorghum bicolor mutants. The first method measured the optical properties of stalk and leaf samples of eight sorghum varieties before and after wax removal to determine the interaction of wax amount and wax structure on the optical properties of sorghum tissues. The second method related UV and wax amount to stalk and leaf reflectance to establish whether higher UV levels produced more wax that in turn produced a higher reflectance in the leaves and stalks.

Field measurements of the bi-directional canopy reflectance were made in the ultraviolet-A (UVA), UVB, red (600-750 nm) and far-red (700-950nm) regions. Comparisons were made between data in the late vegetative and soft dough stages of three sorghum varieties and in sky conditions varying from clear to overcast.

This study further characterizes the effects of epicuticular waxes on canopy and leaf reflectance, in particular, the leaf characteristics that cause UVB leaf reflectance, and assists in understanding how Sorghum bicolor thrives in low latitude-high UVB irradiance regions.