XVI International Botanical Congess
This study examined the effect of electromagnetic radiation on concentrations usnic acid in intact thalli of Cladina uncialis and C. mitis. Thalli of C. uncialis were exposed to bands of light in a growth chamber at two intensities for 7 days. Usnic acid amounts were determined by densitometric analysis. All bands of high intensity light significantly increased usnic acid concentrations over control levels. Because concentrations were significantly higher with more intense UV-A and visible light than with intense UV-A/B and visible light it appears that usnic acid may be photo-degraded by UV-B and its synthesis induced by UV-A. Usnic acid levels in natural populations of C. mitis were significantly lower in spring and summer when UV-B flux is the greatest, supporting the idea that usnic acid is photo-degraded by UV-B.