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-Essential signaling processes such as changes in calcium mobilization, protein phosphorylation and gene expression are known to be modulated by hydrogen peroxide (H 2 O 2 ). A lot of silver maple trees in the city of Osijek (Croatia) were noticed to have bleached leaves by early summer as well as during the whole vegetation season. In this study we aimed to investigate the processes that regulate H 2 O 2 levels in healthy (green) and prematurely aged (bleached) leaves. For that purpose, photosynthetic performance and antioxidative response of green and bleached silver maple leaves were studied. Bleached leaves had higher hydrogen peroxide level, a three-fold level of total soluble proteins as well as a lower level of ascorbic acid. Concentrations of chlorophyll a, chlorophyll b, total chlorophylls and total carotenoids as well as maximum quantum yield of photosystem II were lower in bleached leaves. This indicated their impaired photosynthetic performance. Further more, bleached leaves were characterized by lower specific activities of the main antioxidative enzymes, which influenced their reactive oxygen species scavenging capability. The higher level of H 2 O 2 content in bleached leaves as the consequence of reduced antioxidative enzyme specific activities as well as ascorbic acid level could be the reason for the down-regulation of photosynthetic performance and premature aging of those leaves.

Olive is one of the most important cultivated Mediterranean plants. In order to determine the differences in frost resistance of two, two-year-old olive cultivars (Olea europaea cv. Leccino and cv. Oblica) growing on different types of nutrient substrates (soil and coconut fibres), the trees were exposed to low temperature (-5 °C) in the dark. It was shown that low temperature caused an increase in H2O2 concentration, level of lipid peroxidation and carbonyl protein content in both cultivars and on both nutrient substrates, respectively. The CAT and APX activities significantly varied depending on the cultivar, the nutrient substrate type and the time of exposure to low temperature. Cv. Oblica and cv. Leccino growing on coconut fibres showed a better antioxidative response to low temperature probably due to the higher nitrogen and phosphorus concentration established in this type of nutrient substrate. That positive antioxidative response determined on coconut fibres was more pronounced in leaves of cv. Leccino.

Research with the objective of finding efficient (non residual) sterilants for the purpose of greater automatization in establishing aseptic in vitro tissue culture was conducted on cherry rootstocks CAB 6P and Gisela 6. Two ways of sterilization were examined (NaOCl and ozone O 3 ) through survival rate on three types of explants; buds with primordial leaves, buds without primordial leaves and nodal non lignified segment with axillary buds. Ozone resulted with the high rate of survival (from 57 to 93%) regardless of the type of explants and the rootstock variety. By introducing a complete bud without destruction significantly resulted with the higher rate of survival (from 90 to 97%). Results indicate the potential for ozone sterilization of the starting material not only as an ostensible but the perforated (sub-cuticular transpiration) agent.