S11 - Session P4 - Carbohydrate concentrations and determination of frost hardiness (DT50) with ion leakage test in apple tree (Malus × domestica Borkh.) shoots and buds during cold acclimation

Authors: Pauliina Palonen *, Maria Vaahtera, Iiris Lettojärvi, Jaana Luoranen, Tuuli Haikonen, Sanna Finni

Apple ( Malus × domestica Borkh.) is the most important cultivated fruit tree in Finland, and the production is constantly increasing. As cold winters limit the cultivation and the choice of cultivars, it would be useful to gain deeper understanding of factors affecting the adaptation of apple cultivars to Northern conditions. The aim of this study was to examine the relationship of carbohydrates in shoots and buds of apple cultivars to their frost hardiness. In addition, the usefulness of ion leakage test of buds and shoots in estimation of damaging temperature (DT 50 ) after controlled freezing tests was evaluated. Shoot samples of apple cultivars differing in winter hardiness, 'Aroma', 'Lobo' and 'Pirja', were collected from a commercial orchard in South-Western Finland (N 60 ° E 22 °) at three times: 14 Oct. 2019, 20 Nov. 2019 and 8 Jan. 2020. In addition, shoots of cv. Santana were collected from Åland Island (N 60 ° E 19 °) on 8 Jan. 2020. The accumulation of soluble carbohydrates during cold acclimation did not explain the differences in frost hardiness between the cultivars. However, in January, the concentration of non-soluble carbohydrate starch in both shoots and buds was higher in the susceptible cultivars 'Aroma' and 'Santana' as compared to the more winter hardy cultivars 'Pirja' and 'Lobo'. As soluble carbohydrates protect plant tissue from freezing injury, high starch concentration during cold acclimated state may explain the differences in frost hardiness between cultivars. Visual evaluation of injury after a controlled freezing test revealed differences between cultivars in frost hardiness in both shoots and buds, except for the early stages of cold acclimation. The resolution of the ion leakage test in the shoots was poor. However, the ion leakage test of the apple buds proved useful in estimation of injury caused by controlled freezing.