(Dedicated to Professor Klaus Hahlbrock on the occasion of his 60th birthday) SUMM.^RY Epidermal tissue was isolated from Scots pine {Pinus sylvestris L.) needles by enzymatic digestion in order to study tissue distribution of u.v,-B-screening pigments. Up to 90 "o ofthe needle content of a group of diacylated flavonoi glycosides tbat were structurally closely related was found in the epidermal layer. Among these metabolites. 3",6"-di-para-coumaroyl-isoquerfitrin and 3",6"-di-para-coum3roy]-astragal in were the main u.v.-B-induced compounds in cot\ ledons and primary needles, respectively. However, catechin and astragalin (kaempferol 3-glucoside), two non-acylated fla\'onoid metabolites, were only observed in total needle extracts, and at levels independent of u.v.-B treatment. According to this metabolite distribution, tbe mRNA of chalcone syntbase, the key enzyme to flavonoids, was found in epidermal and mesophyll as well as vascular tissues. The major alkaliextractable wall-hound phenolic metabolites, astragalin, 4-coumaric acid, and ferulic acid, a minor component of tbe cell wall, were also found exclusively in the epidermal layer. These compounds were not stimulated by u.v.-B irradiation within the experimental period. Staining of needle cross sections and epidermal layer preparations with NaturstofTreagenz A confirmed the specific localization of wall-bound astragalin in the outer wall of the epidermal layer. Model calculations of u.v.-B absorptions at 300 nm of soluble and cell-wall-bound metabolites of the epidermal layer revealed an almost complete shielding of the mesophyll tissue from u.v.-B radiation.
Diacylated flavonol glycosides are the response of Scots pine seedlings to UV‐B irradiation. The two induced compounds 1 and 2 both have their absorption maximum at 315 nm (ϵ = 50300) in the range of UV‐B radiation (290–320 nm), are compartmented in the epidermis, and offer much better protection than the monoacylated analogues at the same molarity.
Increased levels of both ozone and ultraviolet‐B radiation (UV‐B) are typical for high‐altitude sites. Here we report on interactive effects of both stresses on Scots pine (Pinus sylvestris L.). Scots pine seedlings were exposed in a phytotron to a simulated outdoor climate. To assess effects of ozone and of UV‐B radiation on the plants, ozone was added at ambient or at twice‐ambient levels, and UV‐B was eliminated or added at a low ambient level. Visible needle damage became apparent at enhanced ozone concentrations after 10 d of exposure and increased continuously. The accumulated exposure over a threshold value of 40 nL L−1 ozone (AOT40) revealed that an ozone exposure dose of 10 μL L−1 h was necessary for the development of needle injury, regardless of the UV‐B conditions. Cinnamyl alcohol dehydrogenase (CAD) and stilbene synthase (STS) have been reported to be induced by ozone. The mRNAs of these stress‐related enzymes were induced only under twice‐ambient ozone concentrations. Ozone‐induced transient STS transcript levels reached their maximal values between day 1 and day 5, and were more pronounced in the presence of UV‐B. Similarly, pinosylvin, as well as pinosylvin methyl ether contents, showed moderate transient increases under these conditions. In contrast, CAD mRNA content and CAD enzyme activity increased continuously under enhanced ozone concentrations. Additional UV‐B resulted in suppressed transcript level and enzymatic activity of CAD. The data indicate that in conifer needles, at the level of gene expression, there is an interaction between responses to UV‐B and ozone.
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