THE EFFECT OF UV-B RADIATION ON UV-ABSORBING SECONDARY METABOLITES INBIRCH SEEDLINGS GROWN UNDER SIMULATED FOREST SOIL-CONDITIONS

Birch (Betula pendula Roth) seedlings were grown under enhanced u.v.-B radiation and simulated forest-soil conditions, after which individua l secondary metabolites were determined in the leaves. It was found th at not all of the u.v.-absorbing secondary metabolites of the seedling s responded to supplementary u.v.-B radiation. Under increased u.v.-B radiation, significant increases in concentration were observed only f or the major flavonoid, quercitrin, the minor flavonoid, myricetin-3-g alactoside, and for chlorogenic acid. On the other hand, ,4'dihydroxyp ropiophenone-3-beta-D-glucopyranoside decreased under u.v.-B irradiati on. The concentration of phenolic compounds in the leaves changed duri ng the growing season (between two harvests) but this change was not r elated to u.v.-B enhancement. A low availability of mineral nutrients did not impair the capacity of the seedlings to accumulate u.v.-protec ting phenolic compounds under increased u.v.-B radiation. The growth c onditions used might have affected the intracellular concentrations of secondary metabolites, and thus furnished the birch seedlings with an increased tolerance of u.v.-B radiation. These findings point to the significance of certain phenolic components in the protection of decid uous trees against u.v.-B radiation.