Scattering of ultraviolet and photosynthetically active radiation by sorghum bicolor: influence of epicuticular wax

Near-isogenic mutants of Sorghum bicolor with genetic alterations affecting epicuticular wax (EW) structure but having similar canopy architecture provided a model system to examine the influence of EW on plant radiation scattering. Differences in canopies with two different sheath EW amounts showed differences in angular reflectance and transmittance. The differences varied with waveband of radiation. Canopy ultraviolet-B (UVB) and photosynthetically active radiation (PAR) backward reflectance in the principal solar plane were higher by wild-type plants (N-15) bearing reflective stalk EW filaments than mutant plants (bm-15) lacking stalk EW filaments. Between panicle emergence to anthesis the backward PAR reflectance increased more in the N-15 than bm-15 canopy. We suspect that the increase was a result of reflections from stalk facets emerging above the surface plane of the canopy foliage and exposing reflective EW. As panicles emerged above the foliage, canopy UVB and PAR forward reflectance by bm-15 increased while forward reflectance by N-15 decreased. The increased forward reflectance from bm-15 may be because of high specular reflectance from the microscopically smooth bm-15 stalk surfaces. Based on comparisons of probability distributions, significant differences in PAR and UVB canopy transmittance were detected between N-15 and bm-15. The median UVB transmittance was greater in the bm-15 canopy than the N-15 canopy, while the median PAR transmittance was the same for the two canopies. The greater transmittance in the N-15 canopy corresponded with lower EW load of the sheaths, but the difference between canopies was within the experimental error. Distinct influences of the stalk EW on canopy reflectance and transmittance were difficult to assess because of the relatively low proportion of surface area containing EW, the experimental errors associated with UVB irradiance field measurements. The optical properties of the S. bicolor canopy varied by waveband. Results suggested that the canopies both had a higher UVB transmittance than PAR in the shaded fraction, due in part to the greater proportion of sky radiation in the global UVB over the PAR waveband and possibly in part as a result of differences in scattering by the canopy. Distinct differences were difficult to assess because of the experimental error and differences in diffuse fraction of UVB and PAR under clear sky conditions. The empirical probability distributions of canopy transmittance in both UVB and PAR were non-gaussian; fitting the β distribution better than the gaussian distribution. It is suggested that the irradiance in sunlit and shaded fractions not be described using statistics from the normal distribution.