SUMMARYA three-dimensional (3-D) representation of the penetration of u.v.-B (280-320 nm) radiation using fibre-optic microprobes, and of the spectrophotometric analysis of u.v.-screening pigments in leaves of Bras.ii.ca napus L. cv. Ceres is presented. In this way the results from radiation measurements were directly related to the distribution of u.v.-screening pigments. Brassica napus was grown in a greenhouse chamber with a photon flux density of 740 /imo\ m"^ s"^ photosynthetically active radiation (400-700 nm) with or without the addition of u.v.-B radiation. The amount of u.v-B added was equi\-alent to 9 kj m"' d"' biologically effective u.v.-B. Growth parameters measured included leaf thickness and dry weight. Ultraviolet-B radiation stimulated the accumulation of increased amounts of screening pigments, located mostly beneath the adaxial surface of tbe leaf, l^eaf dry weight decreased by 15 "o in u.v.-treated plants and, on a dry weight basis, chlorophyll content increased by 19 "o in treated leaves. The chlorophyll profiles within leaves from both control and u.v.-treated plants showed an increase from the adaxial surface to a plateau in the middle of the leaf, at a leaf depth of c. 150-320/im. Relatively large amounts of 285 nm radiation penetrated into leaves of u.v.-B treated plants to sites located within the initial 40 //m of the leaf. Thus, despite the presence of additional screening pigments, u.v.-B radiation was capable of reaching potential targets within the epidermis and mesophyll.

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