Changes in leaf spectral reflectance, transmittance, and absorbance to UV/visible light of pecan leaves throughout a growing season are illustrated in Fig. 18.3. The changes in leaf optical properties were relatively small in UV-B (280 nm - 320 nm) and UV-A (320 nm - 400 nm) spectral regions compared to that of the visible spectral region (400 nm - 760 nm). Throughout the growing season, pecan leaves reflected 4% - 8%, transmitted 0% -1%, and absorbed up to 96% of UV radiation (280 nm - 400 nm). In the visible light region, leaf absorbance to green light (at 555 nm) and to red light (at 680 nm) increased dramatically during leaf development from April to July. This trend seems to hold true for all other species studied. Since the leaf optical properties remained relatively steady within the UV region, this indicated we could make the species comparison at a single wavelength. Since our focus was primarily on UV-B, we chose 300 nm for the comparison of the leaf optical properties amongst the 35 species (Table 18.1). The species were ranked from the highest to the lowest based on their reflectance (Fig. 18.4). As shown in Table 18.1 and Fig. 18.4, leaf reflectance to the 300 nm UV-B was generally low across the species, ranging between 4.77% - 8.61% with a mean value of 6.55%, +1% of standard deviation. The leaf transmittance to the 300 nm was rather low for all the species, ranging between 0% - 0.17%. The leaf absorbance to the 300 nm UV-B was above 90% across the species, ranging between 91.34% -95.23% with a mean value of 93.44%, +1% of standard deviation. These findings are generally in agreement with the results reported by Gausman et al. (1975), Robberecht et al. (1980), Cen and Bornman (1993), and Yang et al. (1995). It is generally agreed that leaves absorb over 90% of incident UV-B; leaf surface
reflection, as a first line of defense against UV-B, is less than 10% in most species, and there is negligible transmission of UV-B through leaves.
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