PENETRATION OF UV-B RADIATION IN FOLIAGE - EVIDENCE THAT THE EPIDERMIS BEHAVES AS A NONUNIFORM FILTER

In some plants, particularly herbaceous species, a considerable propor tion of incident ultraviolet-B radiation (UV-B, 280-320 nm) penetrates into the leaf mesophyll where it is potentially damaging to nucleic a cids and the photosynthetic machinery. We used optical techniques to l ook at the spatial variation in UV-B penetration through the epidermis of foliage of two herbaceous species (Chenopodium album and Smilacina stellata) and a conifer (Picea pungens). Measurements of UV-B penetra tion in intact foliage with a fibre-optic microprobe revealed that 300 nm radiation reached 161+/-36 mum (mean+/-SD) into leaves of C. album , 154+/-40 mum in S. stellata and 17+/-2 mum in P. pungens, with epide rmal transmittance being 39+/-14%, 55+/-19% and 0%, respectively. A th in polymer film was developed which fluoresced blue when irradiated by UV-B. Fresh epidermal leaf peels were placed over the film and irradi ated with UV-B, and microscopic examination of the film from below all owed us to determine the spatial pattern of UV-B penetration through t he epidermis. In herbaceous species, film fluorescence below cell wall s, but not epidermal and guard cell protoplasts indicated that UV-B tr ansmittance was much greater through anticlinal cell wall regions than protoplasts. Ultraviolet-B transmittance through large areas of epide rmal cells could be induced by plasmolysis. Epidermal transmittance wa s also relatively high through stomal pores (and what appear to be nuc lei in Smilacina), but relatively low through stomatal guard cells. Re sults from the fluorescing film technique were substantiated by direct measurements of UV-B transmittance through epidermal peels with a fib re-optic microprobe run paradermally along the bottom or inner side of irradiated peels. In Smilacina, we estimate that UV-B epidermal trans mittance was up to 90% through anticlinal cell wall regions, but <10% through protoplast areas. In contrast to herbaceous species, we did no t detect any UV-B transmittance through the epidermis of P. pungens wi th either the fluorescing film or the fibre-optic microprobe technique . The epidermis appears to be a much more spatially uniform UV-B filte r in conifers than in these herbaceous species.