Frost hardiness (FH) is one of the limiting factors for the successful growth of woody plants in the boreal zone. To cultivate the plants in cold conditions, they need to be tested before they are launched to the farmers and forest owners. Appropriate preconditioning for the different progenies of the plus tree of forest seedlings within the same species, and different horticultural woody species and cultivars are not known well. To answer those questions, this study designed and implemented experiments for Scots pine (Pinus sylvestris L.) progenies, for three horticultural species, i.e., apple (Malus domestica Borkh.), blueberry (Vaccinium corymbosum L.), blackcurrant (Ribes nigrum L.), and for pear cultivars (Pyrus communis L.). The study is composed of three parts with the following aims: i) to determine whether the pollination site affects the FH of the Scots pine progenies, ii) to determine the proper late-autumn preconditioning before running the frost hardiness tests of different apple, blueberry, and blackcurrant cultivars, and iii) to determine the effects of a short warm spell in mid-winter on the FH of pear cultivars. One of the important aims of the thesis was to assess and compare the different FH testing methods with the help of the experiments executed here.
The first part of the study consisted of the progenies of Scots pine plus-tree seed orchards in Finland and Ukraine, in addition to the progenies from natural stands in Finland, with three seed lots from each site. FH was examined twice during cold acclimation in controlled conditions. The second part concerned the effects of different preconditioning temperatures (+3, −3, −7, and −10 °C) and their durations (one or three weeks) on the FH of two apple cultivars, three blueberry cultivars, and three blackcurrant cultivars. The third part concerned the effects of short term warm spells in mid-winter on the FH of three pear cultivars that were preconditioned in natural conditions, then dehardened in a growth chamber at +5 °C for either 3-4 days or 16 days, and then rehardened at −7 °C for 5-7 days.
It was found that the freezing test temperature had a strong effect on the physiology and growth of different organs of the plus-tree progenies of Scots pine, but no consistent differences were found in FH among the progenies. The proper preconditioning temperature for the development of the maximum frost hardiness of the aboveground parts in late autumn is three weeks at −3 °C for apple and blueberry, though a shorter time for blackcurrant would be enough. The frost hardiness of the pear cultivars responded to temperature changes in mid-winter, but less than expected, and the responses were similar in all cultivars. In addition, the FH estimates of the stem by electrical impedance spectroscopy (EIS) and relative electrolyte leakage (REL) were quite similar, but these methods overestimated FH when compared to the FH by visual damage scoring. DTA results had a small variation compared to other methods but the use of DTA is limited due to the low occurrence rate of the low temperature exotherm (LTE) in several species (e.g., blackcurrant).