Small increases of epidermal transmittance for UV in fava bean leaves were found under the 13% reduction in PAR and with UV-B exclusion. These results are consistent with increases in leaf UV-absorbing compounds with increased UV-B exposure (Mazza et al., 2000; Searles et al., 2001; Coleman and Day, 2004). Over a wide range of solar radiation regimes, we found that only small differences in epidermal transmittance occurred over large ranges of PAR (500 - 2,000 ^mol m s ), and that this relationship held even for leaves developed under full UV-B and UV-A exclusion (Fig. 13.4). These findings suggest that the primary effect on changes in leaf epidermal transmittance due to global dimming will likely occur for leaves developed in shaded environments (< 200 ^mol m s ) where reductions in ambient PAR and UV are related to large changes in epidermal transmittance.
We also observed that changes in levels of whole-leaf UV-absorbing compounds were not linearly correlated with measurements of UV-A transmittance. Rather, this relationship was curvilinear whereby leaves with low UV-A transmittance exhibited a range of whole-leaf absorbance values (Fig. 13.5). In addition, the relationship more closely fit a power function (y = bxa), rather than exponential (y = beax), indicating that the distribution of the absorbing compounds did not approximate a random distribution of particles as in the Beer-Lambert law of light absorbance through particles suspended in a medium (Kostinski, 2001). Krause et al. (2003) also reported constant levels of UV-A transmittance over a wide range of whole-leaf absorbance values in leaves from tropical tree crowns. The non-random distribution of UV-absorbing compounds in the epidermis (Day et al., 1993) may help explain the broad range of whole-leaf absorbance values for a given quantity of epidermal transmittance.
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