ABSORPTION OF UV-B TO BLUE-LIGHT RADIATION BY LEAF CUTICLES OF SELECTED CROP PLANTS

Plants have protective pigments absorbing destructive shortwave radiat ion. These pigments have been found in the epidermis and mesophyll of leaves. We studied the absorption characteristics of the leaf cuticle, the outermost part of the epidermis that is directly exposed to radia tion. Adaxial leaf cuticles of apple, pear, sour cherry, strawberry, c auliflower, sugarbeet, and 13 other plant species were tested. The UV- B absorption was highest in Citrus aurantium and Citrus maxima (<3 % t ransmittance) and lowest in sugarbeet and peach (>64 % transmittance). The absorption maxima are at wavelenghts below 320 nm. Significant ab sorption was also determined at 500 nm, which correlated with cuticle thickness of the plant species (r(2)=0.72). The absorption in the rang e of 250 to 350 nm is caused by pigments with a high extinction coeffi cient. This absorption is species dependent and the patterns were desi gnated to three different types. The highest absorption was found in e vergreen species. The extraction of cuticular waxes had little effect on absorption. The specific absorption of shortwave radiation by plant cuticles is probably caused by pigments covalently bound to cutin. It is known for some plant species that cuticles can contain the phenoli cs p-coumaric acid, ferulic acid, and vanillic acid. Mixtures of these phenolics had spectra similar to cuticles. For most species absorptio n of shortwave radiation by the cuticle alone does not give complete p rotection.