The impact of UV-B irradiation applied at different phases of somatic embryo development in Norway spruce on polyamine metabolism

KEY MESSAGE : Higher polyamine levels of fully developed embryos had positive effects on their ability to tolerate UV-B irradiation when compared with induced responses of early embryos. The aim of this work was to test the hypothesis that the higher levels of polyamines (PAs) might be involved in the response of Norway spruce somatic embryos to UV-B irradiation. We compare here the effects of 0.1, 0.6 and 6 W m⁻² h⁻¹ UV-B irradiation on polyamine metabolism in early and fully developed Norway spruce somatic embryos. The impact of UV-B treatment on irradiated embryogenic suspensor mass (ESM, consisting of early somatic embryos) and matured somatic embryos was assessed by measuring changes in the content of PAs and the activities of enzymes involved in their biosynthesis. Under control conditions, developmental stages of embryos are characterized not only by clear differences in their histological structure, but also by the levels of free PAs, which are several fold higher in fully developed embryos than those of early embryos. The decrease in the PA content and the decline in PA biosynthetic enzyme activities in irradiated ESMs were dependent on the doses of UV-B irradiation applied and the length of time after the exposure. The viability of ESM and its histological structure changed depending on the dose applied. The effect was much more pronounced in ESM treated with higher UV-B doses (0.6 W m⁻² h⁻¹), where the embryos were seriously damaged or killed, and irradiation with 6 W m⁻² h⁻¹ was lethal to the culture. No marked differences in PA contents were observed between control and UV-B irradiated fully developed embryos. The effect of UV-B irradiation on fully developed embryos was marginal when compared with that on proliferating tissue. The increase in malondialdehyde (MDA) levels in irradiated ESM was correlated with the decrease in their PA contents. Neither significant increases in MDA levels nor significant changes in PA content were observed in the fully developed embryos after irradiation; this may indicate that the plants’ defence mechanisms are particularly active in these tissues. The accumulation of higher levels of PAs in fully developed somatic embryos may be causally linked to their better tolerance to UV-B irradiation.