Ben-Gurion University of the Negev

1.2.1.3. UV protection
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Anthocyanins generally have two absorption maxima, one in the UV (between 270-290
nm) and the other in the visible spectrum (500-550 nm) (Markham, 1982). Acylation of
anthocyanins with aromatic organic acids increases their UV absorbance by addition of
another absorption maximum in the 310-320 nm. The strong UV absorption of
anthocyanins has led to the hypothesis that they may protect leaves from the harmful
effects of UV radiation (Lee and Lowry, 1980; Coley and Kursar, 1996). Several studies
have supported this hypothesis: a positive correlation exists between leaf anthocyanin
content and UV-B radiation (Alexieva et al., 2001; Brandt et al., 1995; Lindoo and
Caldwell, 1978). Stapleton and Walbot (1994) demonstrated that the DNA in Zea mays
plants that contain flavonoids (primarily anthocyanins) was protected from damage
caused by UV radiation relative to the DNA in plants that were genetically deficient in
these compounds. It has also been reported that red Coleus varieties are less damaged
by both UV-B and UV-C radiation, when compared to green varieties (Burger and
Edwards, 1996). Therefore, anthocyanins have been implicated in UV-B protection (Lee
and Lowry, 1980). Moreover, some anthocyanin biosynthesis genes, i.e. chalcone
synthase (CHS) and xavanone 3-hydroxylase (F3H) are induced only upon UV-A
exposure (Zhou et al., 2007). However, a closer look at the anthocyanin spectral
absorbing properties and their contribution to total leaf UV-B absorbing capacity
renders their putative protection against UV-B radiation questionable. Molar absorption
coefficients for anthocyanins in the UV-B (280-320 nm) are ca. 3-5 times lower as
compared to corresponding values at their visible maxima (Giusti et al., 1999;
Harborne, 1976). Glycosylation per se does not appreciably alter the spectral
absorbance profile. However, acylation of anthocyanins by phenolic acids increases
UV-B absorptivity due to superimposed absorbance of the phenolic ring, resulting in
roughly equivalent absorption in both the visible and UV-B bands (Giusti et al., 1999;