In order to avoid UV-B radiation, plants have developed several mechanisms for UV-B exclusion. Plants are thought to employ a variety of UV-B-protective mechanisms, including increases in UV-6-absorptive pigments, UV-6-reflective properties, and leaf thickness. Thicker leaves may decrease the internal influence of UV-B radiation. This additional leaf thickness in field-grown silver birch has been associated with a slight increase in the thickness of the upper epidermis, spongy parenchyma and spongy intercellular space. In addition, optical structures in the leaf, such as epidermal wax and leaf hairs, scatter and reflect UV-B radiation, but in general, the reflectance of the UV-B radiation reaching leaf surface is only about 10%. Apparently, the most efficient mechanism of exclusion is the accumulation of UV-B-screening phenolics in the epidermal cells of leaves. Consequently, the penetration of UV-B radiation through the epidermis has been shown to be nearly zero in conifer needles, 3-12% in the leaves of deciduous trees and grasses, and 18-41% in the leaves of herbaceous plants.
One of the most common responses of field-grown plants to elevated UV-B radiation is an increase in UV-B-absorbing phenolics in the leaves. In fact, accumulation of certain phenolic filters with UV-B levels above the ambient level has been found to be a continuation of the response within the ambient range. UV-B radiation stimulates the expression of genes that encode phenylalanine ammonialyase (PAL) and chalcone synthase (CHS), which are the key regulatory enzymes in the phenylpropanoid and flavonoid pathways.
Recently, it was also found that UV light selectively induces several primary metabolic activities that are directly or indirectly required for flavonoid formation. This implies complex regulation in the different branches of the phenylpropanoid biosynthesis pathway during UV-B stress. Elevated UV-B radiation significantly increased the concentrations of UV-B-absorbing flavonoids, such as quercetin-3-arabinoside, quercetin-3-glucose + glucuronide and kaempferol- 3-rhamnoside, and a few phenolic acids in silver birch leaves. In Arabidopsis thaliana, the transparent testa-4 (tt4) mutant, which has reduced flavonoids and normal levels of sinapate esters, is more sensitive to UV-B than the wild type when grown under high UV-B irradiance. The tt5 and tt6 mutants, which have reduced levels of UV-absorptive leaf flavonoids and the monocyclic sinapic acid ester phenolic compounds, are highly sensitive to the damaging effects of UV-B radiation. These demonstrated that both flavonoids and other phenolic compounds play important roles in vivo in plant UV-B protection. In Arabidopsis UV damage and heat induce a common stress response in plants that leads to tissue death and reduced chloroplast function.
