The degree of participation of oleuropein, rutin, and cinaroside in the processes of formation of winter hardiness and frost resistance in four varieties of European olives Olea europaea L. and subspecies O. europaea subsp. сuspidata (Wall. and G. Don) Cif. was determined in natural and laboratory conditions. To assess the level of stress in the leaves, the proline content was determined, the maximum of which was noted in the weakly resistant subspecies O. europaea subsp. сuspidata. To identify the peculiarities of the accumulation of phenolic substances in natural conditions, the changes in their content in leaves and the average 10-day values of equivalent effective temperatures were compared. In laboratory conditions, the content of phenolic compounds was studied under different cooling modes: 0°C, 6 h → –8°C, 8 h (option 1); 0°С, 6 h → –2°С, 6 h → –8°С, 8 h (option 2); –8°C, 8 h (option 3); –8°C, 12 h (option 4). It was found that keeping shoots at 0°C for 6 h had a positive effect on O. europaea’s winter hardiness, while 6 h of exposure at –2°C led to the development of stress. In the first variant of the experiment, the oleuropein content increased in the resistant variety Nikitskaya. In varieties with an average degree of resistance (Razzo and Ascolano), activation of the biosynthesis of flavonoids and oleuropein was observed. In the weakly resistant Coreggiolo variety, phenolic substances were actively consumed, and trace amounts of oleuropein and no changes in the content of rutin and cynaroside in the subspecies O. europaea subsp. cuspidata, probably, were associated with his less close family ties with O. europaea. It was revealed that, in genotypes with low frost resistance, activation of the synthesis of phenolic compounds occurred only under the influence of a stress factor (–2°C), which did not allow them to adapt in a timely manner, whereas these processes began at earlier stages of cold adaptation in relatively resistant varieties. For the varieties Coreggiolo, Razzo and Ascoliano, prolonged exposure to negative temperatures was a critical factor, while it was lethal for O. europaea subsp. cuspidata (option 4). Differences in the synthesis of phenolic compounds identified under these conditions were are associated not only with the degree of frost resistance but also with the variety specificity of O. europaea genotypes. The obtained data suggested that oleuropein, rutin, and cynaroside are elements of the mechanisms of protection of olive plants from the negative effects of negative temperatures. Apparently, the studied compounds play the role of cryoprotectors and antioxidants and participate in the formation of winter hardiness. The conditions for activating the biosynthesis of phenolic compounds were of great importance. In the highly resistant variety Nikitskaya, their accumulation occurred under the influence of temperatures close to 0°С and directly during the initial damaging temperatures in the unstable varieties Coreggiolo and subspecies O. europaea subsp. сuspidata.
