TISSUE LOCALIZATION OF UV-B-SCREENING PIGMENTS AND OF CHALCONE SYNTHASE MESSENGER-RNA IN NEEDLES OF SCOTS PINE-SEEDLINGS

Epidermal tissue was isolated from Scots pine (Pinus sylvestris L.) ne edles by enzymatic digestion in order to study tissue distribution of u.v.-B-screening pigments. Up to 90% of the needle content of a group of diacylated flavonol glycosides that were structurally closely relat ed was found in the epidermal layer. Among these metabolites, 3 '',6 ' '-di-para-coumaroyl-isoquercitrin and 3 '',6 ''-di-para-coumaroyl-astr agalin were the main u.v.-B-induced compounds in cotyledons and primar y needles, respectively. However, catechin and astragalin (kaempferol 3-glucoside), two non-acylated flavonoid metabolites, were only observ ed in total needle extracts, and at levels independent of u.v.-B treat ment. According to this metabolite distribution, the mRNA of chalcone synthase, the key enzyme to flavonoids, was found in epidermal and mes ophyll as well as vascular tissues. The major alkali-extractable wall- bound phenolic metabolites, astragalin, 4-coumaric acid, and ferulic a cid, a minor component of the cell wall, were also found exclusively i n the epidermal layer. These compounds were not stimulated by u.v.-B i rradiation within the experimental period. Staining of needle cross se ctions and epidermal layer preparations with Naturstoffreagenz A confi rmed the specific localization of wall-bound astragalin in the outer w all of the epidermal layer. Model calculations of u.v.-B absorptions a t 300 nm of soluble and cell-wall-bound metabolites of the epidermal l ayer revealed an almost complete shielding of the mesophyll tissue fro m u.v.-B radiation.