Cold tolerance of Latvian local sweet cherries selected for agroforestry system

The climate in Latvia is influenced both by maritime and continental air intrusions. The cherries in continental regions used to be under the threat of prolonged low temperatures but in the maritime regions – of temperature fluctuations. The interest has arisen to grow local sweet cherries in the agroforestry system, and after initial evaluation, eight accessions with a large and healthy trunk, dominant apex, sparse branching, and stable fruit production were selected. The research aimed to evaluate the cold tolerance of the selected accessions compared to the cultivars ‘Bryanskaya Rozovaya’ (Russia) and ‘Meelika’ (Estonia). The annual and biannual shoots, spurs and flower buds were evaluated visually in the natural conditions and after artificial freezing at -20, -25 and -30°C in the middle of winter dormancy; and at -10, -15 and -20°C at the end of winter dormancy. They were evaluated visually on a scale from 0 (no visible changes of cross-section) to 5 (cross-section is completely brown); the percentage of damaged flower buds was calculated. Additionally, the analysis of relative electrolyte leakage (REL) was done for annual shoots. The accessions differed in their response to low temperatures and the results indicated various possible patterns of the acclimation and re-acclimation. In general, the accessions from Latvian continental regions had significantly less damaged flower buds, less browned spurs and lower REL of annual shoots during the middle of winter dormancy. One of them, ‘Smiltenes 9’ stands out with high cold tolerance of the shoots and spurs and acceptable intact flower bud proportion after the freezing at -30°C. In the March, at the end of dormancy, shoots of the accessions from Latvian continental regions still showed lower REL in natural conditions indicating the slower overcoming of dormancy. The accessions from maritime regions had a decrease of REL after the freezing at -15 or 
-20°C indicating re-acclimation to low temperature.