SUMMARY
The problem of natural forests conservation is relevant in the Ukrainian steppe zone where the forest ecosystems develop in the unfavorable conditions of geographic mismatch; therefore, they suffer as a result of any environmental changes. The hypothesis of susceptibility of tree leaves’ metabolism even to slight changes of climatic parameters was tested. The study was conducted in more than 75 years-old natural mixed forest located on the right bank of the Samara river. The chlorophyll (Chl) content, and catalase (CAT), benzidine-peroxidase (BPOD) and guaiacol-peroxidase (GPOD) activities in leaves of autochthonous maple species (Acer platanoides L.) and adventive acacia species (Robinia pseudoacacia L.) were investigated on the lower (52 m above see level, a.s.l.), middle (74 m a.s.l.) and upper (96 m a.s.l.) coastal slope altitude. In maple leaves decreasing chlorophyll amount on the middle and upper plots (8.8% and 19.5% compared with the lower plot) together with Chla/Chlb ratio decrease (from 4.7 on the lower to 4.4 on the middle and 4.0 on the upper plots) was found. Chlorophyll content in acacia leaves decreased only on the upper plot (by 8.5% compared to the lower one), and Chla/Chlb ratio on the lower and upper plots reached 5.1. In maple leaves the increase in altitude was accompanied by the decrease in BPOD activity (by 26% and 63% on the middle and upper plots, accordingly), and GPOD and CAT (accordingly, by 31% and 37% on the upper plots compared to the lower plots). High correlation coefficients of light, temperature and humidity with activity of CAT (respectively, r = –0.98; r = –0.85; r = 0.83), BPOD (r = –0.96; r = –0.93; r = 0.90) and GPOD (r = –0.98; r = –0.82; r = 0.82) were estimated in A. platanoides leaves. Sharp decline in GPOD activity in R. pseudoacacia leaves was revealed on the upper plot (by 95% compared to the lower one), whereas BPOD activity increased by 47% on the middle plot but decreased by 74% on the upper one compared to the lower plot, and CAT activity on the middle and upper plots exceeded 4.3-fold and 1.8-fold the activity on the lower plot. In R. pseudoacacia leaves high correlation of light, temperature and humidity was found with GPOD activity (respectively, r = –0.99; r = –0.82; r = 0.82), whereas correlation with the light level was significant only for BPOD (r = –0.84), and no significant correlation was recorded for CAT activity. Results of the study confirm high sensibility of photosynthetic and antioxidant processes in leaves of both tree species to microclimate changes, which is defined as a conventional enhancement of aridity traits when moving upwards on the slope. It is suggested that the level of correlation between enzyme activity and microclimate and lighting parameters can serve as a marker of tree species adaptation to the slope altitude-associated changes of the local environmental factors in the natural forest.