The purpose of this thesis was to study the responses of the olive tree (Olea europaea L.) cultivar 'Istrska belica' in stressful winter conditions. The experiment was carried out in an olive grove in Dekani on 4 olive trees. The level of stress was estimated in two ways: by measuring fluorescence, which was used to estimate photochemical efficiency and by analyzing values of xanthophyll cycle pigments violaxanthin (V), antheraxanthin (A) in zeaxanthin (Z) extracted from olive leaves. We also monitored the air temperature and the intensity of photosynthetic radiation (PPFD). The measuring period (14. 2. 18 to 15. 3. 18) was divided into 3 sub-periods. Temperatures varied between 0 °C and 15 °C in the first period, followed by a cold period with temperatures between -7 °C and 5 °C, and a third, warm period with temperatures between 2 °C and 15 °C. In the first period, the photochemical efficiency was higher than in the second period. In the third period, both temperatures and photochemical efficiency increased. We observed that the daily courses of actual photochemical efficiency were more pronounced than the courses of potential photochemical efficiency. The more pronounced daily decrease in actual photochemical efficiency was due to the high temperatures and high intensity of photosynthetic radiation. In the ratio of pigments (A + Z) / (V + A + Z).There were no differences between sets of measurements on any measuring day, which may indicate poor conservation, since a lot of pheophytin was recorded in the samples. On the other hand, the constant high (A + Z) / (V + A + Z) ratio, with no apparent daily changes as recorded on all three days of leaf sampling, may reflect a stable acclimatization of the xanthophyll cycle to winter conditions. The results confirmed the hypothesis that photoinhibition occurred at low temperatures and high light intensity.
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