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

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 unknown whether the pollination site affects the frost hardiness (FH) of the progenies and thus 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 orchard, 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 to − 79 °C by relative electrolyte leakage (REL), and from − 29 to − 46 °C by chlorophyll fluorescence (CF). However, when FH was assessed based on the growth of shoots and roots in the greenhouse after three weeks, the differences between the progenies and orchards disappeared. The FH was then between − 10 and − 15 °C based on shoot growth, between − 6 and − 12 °C based on root growth, and between − 8 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 orchard on FH of roots. In summary, it seems to be possible to use seed produced in southern Ukraine in practical reforestation actions in southern Finland.