Epidermal transmittance and phenolic composition in leaves of atrazine-tolerant and atrazine-sensitive cultivars of Brassica napus grown under enhanced UV-B radiation

Experiments were conducted on the atrazine-tolerant mutant Stallion and the atrazine-sensitive cv. Paroll of Brassica napus L., which were grown under either visible light or with the addition of UV-B radiation (280-320 nm) f or 15 days. The mutant has been shown to be sensitive to high levels of vis ible light as compared to the atrazine-sensitive cultivar and therefore we wished to determine plant response to UV-B radiation with respect to potent ial pigment changes, certain anatomical features, radiation penetration and partial photosynthesis. With regard to pigment changes, we were particular ly interested in whether the compositional shift in flavonol pigments under enhanced UV-B radiation, previously suggested to favour increased antioxid ant activity, is confined to the adaxial epidermis, which generally receive s most UV-B radiation or whether the pigment shift is also inducible in the abaxial epidermis. As was to be expected, the penetration of UV-B radiation (310 nm),ras lon e r in the UV-B-exposed plants, which was correlated with an increased amount of UV-screening pigments in the adaxial and abaxial epidermal layers. The main flavonoid glycosides showed the largest shift from kaempferol to querc etin as aglycone moiety in the adaxial epidermal layer, However, in the aba xial epidermal laver the hydroxy-cinnamic acid (HCA) derivatives and kaempf erol glycosides were predominant. Penetration of 430 nm light was higher af ter UV-B exposure, and probably contributed to the fact that photosynthetic efficiency of photosystem II was unchanged or higher after UV-B exposure. UV-B radiation decreased leaf area in the atrazine-tolerant mutant only, Bo th cultivars showed an increased leaf thickness after UV-B exposure due to cell elongation mainly of the palisade tissue, This was especially evident in the mutant.