Histological and biochemical response of Norway spruce somatic embryos to UV-B irradiation

Plants respond to UV-B exposure by synthesizing a broad range of secondary metabolites, including ROS-scavenging antioxidants and UV-B screening phenylpropanoids. Our recent results indicated that the accumulation of higher levels of polyamines in fully developed somatic embryos of Norway spruce may be causally linked to better tolerance of UV-B irradiation. In the present work, we concentrated primarily on changes in phenolics and their localization in irradiated embryos. Somatic embryos, after 14 days of desiccation, were exposed to 6 W m(-2) h(-1) of 312 nm UV-B radiation. The accumulation of higher levels of spermidine and spermine (by about 20%) and total phenolics (about 25%) in these embryos indicates their participation in stress response. Histological localization of polyphenolic compounds was performed under a transmission light microscope. UV-B radiation elicited the accumulation of polyphenolics in the junction zone between root cap and hypocotyl and in the epidermal and subepidermal cells of hypocotyl and cotyledons. The deposition of polyphenolics in intact epidermal cells which were located in the neighbourhood of damaged cells may demonstrate that there is potential for transmission of a signal from injured cells to tissues protected against the direct effects of UV-B irradiation. UV-B irradiation evoked striking polyphenolic accumulation in specialized idioblastic cells localized beneath the epidermis of the somatic embryo hypocotyl and cotyledons. The fluorescence due to flavonoids, detected under confocal laser scanning microscope, increased dramatically after UV-B irradiation in the epidermis of the hypocotyl and cotyledons and in the surface of the root cap of spruce somatic embryos.