In summary, the exposure of Norway spruce EC to UV-B radiation at different stages of maturation had the strongest effect on cell viability of embryos at early stages of development and resulted in the inhibition of somatic embryo development and/or the substantial decrease in the number of embryos. The extent of cell damage was dependent on the UV-B dose applied, as well as the embryo developmental stage, and might be related to differentiation of the outermost cell layers and formation of protoderm. Developmentally more advanced embryos were superior to early embryos in terms of a more efficient stress defense response to UV-B exposure. The response of early embryos was characterized by an increase in MDA, a marked decrease in PA contents and a decline in phenolics. The reduced ability to activate the defense system seems to be responsible not only for the severe cell damage and decrease in viability but also for the inhibition of embryo development. The significant reduction in Spd, which has been shown to be crucial in the somatic embryo development of several coniferous species, seems to be causally linked to the limited development of embryos. Furthermore, decreased levels of cell wall-bound phenolics arising from the pronounced decrease in cell wall-bound ferulic acid might correspond to failure of somatic embryos to reach more advanced stages of development.
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