Seasonal accumulation of ultraviolet-B screening pigments in needles of Norway spruce (Picea abies ( L.) Karst.)

Conifer needles are highly effective in screening ultraviolet- B radiation (280-320 nm). This ability is mainly attributed to the presence of flavonoi ds and hydroxycinnamic acids in the epidermal tissue. In two field cabinet experiments with two different clones of Norway spruce we assessed the seas onal accumulation of UV-B screening pigments under near-ambient, and close- to-zero UV-B irradiation. At the beginning of needle development, i. e. in June, kaempferol 3-O-glucoside was the dominant UV-B screening pigment. It was replaced during needle differentiation by the more effective diacylated flavonol glucosides, particulary kaempferol 3-O-(3", 6"-O-di-p-coumaroyl)- glucoside, which reached highest concentrations in July. In addition to th e soluble pool of diacylated flavonol glucoside derivatives, a cell wall-bo und UV-B screen in the epidermal cell walls was formed during needle differ entiation, consisting mainly of p-coumaric acid and kaempferol 3-O- glucosi de. An effect of UV-B radiation on the accumulation of diacylated flavonol glucosides was only observed in 1996 with clone 2, when the concentrations of kaempferol 3-O-( 3", 6"-O-di-p-coumaroyl) glucoside were significantly h igher in July and August under field, and near-ambient than under close-to- zero UV-B irradiance. For wall-bound p-coumaric acid and kaempferol 3-O- gl ucoside UV-B radiation enhanced the concentrations of these compounds by ap proximately 20% in relation to the concentrations in close-to-zero UV-B-tre ated plants in both field cabinet experiments.