A 3-DIMENSIONAL REPRESENTATION OF THE RELATIONSHIP BETWEEN PENETRATION OF UV-B RADIATION AND UV-SCREENING PIGMENTS IN LEAVES OF BRASSICA-NAPUS

A three-dimensional (3-D) representation of the penetration of u.v.-B (280-320 nm) radiation using fibre-optic microprobes, and of the spect rophotometric analysis of u.v.-screening pigments in leaves of Brassic a napus L. cv. Ceres is presented. In this way the results from radiat ion measurements were directly related to the distribution of u.v.-scr eening pigments. Brassica napus was grown in a greenhouse chamber with a photon flux density of 740 mu mol m(-2) s(-1) photosynthetically ac tive radiation (400-700 nm) with or without the addition of u.v.-B rad iation. The amount of u.v-B added was equivalent to 9 kJ m(-2) d(-1) b iologically effective u.v.-B. Growth parameters measured included leaf thickness and dry weight. Ultraviolet-B radiation stimulated the accu mulation of increased amounts of screening pigments, located mostly be neath the adaxial surface of the leaf. Leaf dry weight decreased by 15 % in u.v.-treated plants and, on a dry weight basis, chlorophyll conte nt increased by 19% in treated leaves. The chlorophyll profiles within leaves from both control and u.v.-treated plants showed an increase f rom the adaxial surface to a plateau in the middle of the leaf, at a l eaf depth of c. 150-320 mu m. Relatively large amounts of 285 nm radia tion penetrated into leaves of u.v.-B treated plants to sites located within the initial 40 mu m of the leaf. Thus, despite the presence of additional screening pigments, u.v.-B radiation was capable of reachin g potential targets within the epidermis and mesophyll.