SCATTERING OF ULTRAVIOLET AND PHOTOSYNTHETICALLY ACTIVE RADIATION BY SORGHUM-BICOLOR - INFLUENCE OF EPICUTICULAR WAX

Near-isogenic mutants of Sorghum bicolor with genetic alterations affe cting epicuticular wax (EW) structure but having similar canopy archit ecture provided a model system to examine the influence of EW on plant radiation scattering. Differences in canopies with two different shea th EW amounts showed differences in angular reflectance and transmitta nce. The differences varied with waveband of radiation. Canopy ultravi olet-B (UVB) and photosynthetically active radiation (PAR) backward re flectance in the principal solar plane were higher by wild-type plants (N-15) bearing reflective stalk EW filaments than mutant plants (bm-1 5) lacking stalk EW filaments. Between panicle emergence to anthesis t he backward PAR reflectance increased more in the N-15 than bm-15 cano py. We suspect that the increase was a result of reflections from stal k facets emerging above the surface plane of the canopy foliage and ex posing reflective EW. As panicles emerged above the foliage, canopy UV B and PAR forward reflectance by bm-15 increased while forward reflect ance by N-15 decreased. The increased forward reflectance from bm-15 m ay be because of high specular reflectance from the microscopically sm ooth bm-15 stalk surfaces. Based on comparisons of probability distrib utions, significant differences in PAR and UVB canopy transmittance we re detected between N-15 and bm-15. The median UVB transmittance was g reater in the bm-15 canopy than the N-15 canopy, while the median PAR transmittance was the same for the two canopies. The greater transmitt ance in the N-15 canopy corresponded with lower EW load of the sheaths , but the difference between canopies was within the experimental erro r. Distinct influences of the stalk EW on canopy reflectance and trans mittance were difficult to assess because of the relatively low propor tion of surface area containing EW, the experimental errors associated with UVB irradiance field measurements.