Frost hardiness of Finnish plus-tree progenies of Scots pine from seed orchards in Finland and Ukraine

Abstract To avoid the surrounding pollination, seed orchards of Finnish plus trees have been established in Ukraine, i.e. far from their natural distribution. However, it is not known whether the pollination site affects the frost hardiness (FH) of the progenies, and therefore their use in forest regeneration in Finland. Using whole-plant freezing tests, we examined the FH of plus-tree seed orchard progenies in Finland and Ukraine, with three lots of seeds from each origin, twice during cold acclimation in controlled conditions. The FH of needles, as assessed immediately after the freezing tests, was higher in the Finnish than Ukrainian seed orchard progenies. The variation in the FH of needles was high among the progenies, ranging from − 44°C to − 79°C by relative electrolyte leakage (REL), and from − 29°C to − 46°C by chlorophyll fluorescence (CF). When the FH was assessed according to the growth of shoots and roots after growing the seedlings for three weeks in the greenhouse the differences between the progenies/origins disappeared which was due to root damage. The FH was then between − 10°C and − 15°C based on shoot growth, between − 6°C and − 12°C based on root growth, and between − 8°C and − 16°C based on the number of new root tips. The results imply an epigenetic effect on FH of needles but no effect of progeny or origin on FH of roots. To summarize the results, it seems to be possible to use seed produced in southern Ukraine in practical reforestation actions in southern Finland.