Introduction Boxwood - one of the oldest plants used in landscaping and ornamental horticulture. There is no doubt that over a long period boxwood acquired specifi c qualities conducive to survival in a variety of adverse environmental conditions. This may explain its high resistance to extreme summer and winter temperatures and unpretentiousness to soil growing conditions. Above convinces us that it can be used as a benchmark in assessing the problems of plants survival in the context of global warming. Material and methods Activity of H2O2 decomposition was determined in extracts from 2 year old boxwood leaves using the  laboratory oxygen meter YSI 5300A (USA). The water content was determined by weighing the leaves samples during dehydration and recovery.  The photosynthetic system II (FSII) activity was determined by using fl uorometer PAM 2100 (FRG). Boxwood leaves were dehydrated and rehydrated at 25oC. During dehydration and rehydration the water content, activity of PSII and activity of H2O2 decomposition of leaves extracts were determined. The extraction of crushed leaves was provided in 2 ml of 0.2M Tris buffer at pH 7.4 during 30 minutes, centrifuged for 15 minutes at 4000 g. The activity of H2O2 decomposing was determined in the supernatant. The reaction of H2O2 decomposition was initiated by adding 1.5 ml Tris buffer and 60 μcl of 0.05% H2O2 to 40 μcl liters of supernatant. Results and discussions The data indicate that the activity of H2O2 decomposition of the extracts from boxwood leaves was lowest (4.9 * 10 -3 U/mg wet weight) when water content in leaves was reduced from 75 % to 5 %. Concomitantly the Yield of FS-II was reduced from 0.7 to 0.1. With the accumulation of water content in leaves up to 55 -70 % the Yield of FS-II was increased from 0.1 up to 0.6 and the activity of H2O2 decomposition of increases from (4.9 * 10-3 to 1.25 * 10-2 U/mg of wet weight). It can be noted that the activity of H2O2 decomposing and FS-II activity decreases or increases in proportion to the water content in the leaves. This indicates that water is one of the decisive factors determining the activity of FS-II. Conclusions 1. The water content in the boxwood leaves is one of the most important factors determining the activity of FS-II. 2. Reduced activity of FS-II is accompanied by a corresponding decrease in the activity of decomposition H2O2, which is obviously due to the decrease in the level of reactive oxygen species at lower levels of photosynthesis.