With the aim to contribute to elucidation of the role of phytohormones in plant responses to stresses the endogenous contents of abscisic acid (ABA) and cytokinins (CK) were followed in French bean, maize, sugar beet, and tobacco during water stress and subsequent rehydration. The effects of pre-treatments with exogenous ABA or benzyladenine (BA) before imposition of water stress were also evaluated. The content of ABA increased by water stress, and with the exception of bean plants increased content of ABA remained also after rehydration. In all plant species the ABA content was further increased by ABA pre-treatment, but in bean and maize it decreased by BA pre-treatment. The highest total content of CK was observed in bean and the lowest in maize during water stress. In their spectrum, the storage CK were dominant in bean, and inactive CK in tobacco while in sugar beet and maize all groups were present in comparable amounts. In all plant species, the contents of CK increased during water stress and with exception of bean they decreased back after rehydration. ABA pre-treatment further increased contents of CK in water-stressed bean and tobacco. BA pretreatment increased contents of CK in sugar beet and tobacco after rehydration.Additional key words: benzyladenine, isopentenyladenine, Phaseolus vulgaris, relative water content, zeatin, zeatin riboside.

The aim of this research was to determine whether exogenous abscisic acid (ABA) applied immediately after ex vitro transfer of in vitro grown plants can improve their acclimatization. Tobacco (Nicotiana tabacum L.) plantlets were transferred into pots with Perlite initially moistened either by water or 50 μM ABA solution and they were grown under low (LI) or high (HI) irradiance of 150 and 700 μmol m -2 s -1 , respectively. Endogenous content of ABA in tobacco leaves increased considerably after ABA application and even more in plants grown under HI. Stomatal conductance, transpiration rate and net photosynthetic rate decreased considerably 1 d after ex vitro transfer and increased thereafter. The gas exchange parameters were further decreased by ABA application and so wilting of these plants was limited. Chlorophyll (a+b) and β-carotene contents were higher in ABA-treated plants, but the content of xanthophyll cycle pigments was not increased. However, the degree of xanthophyll cycle pigments deepoxidation was decreased what also suggested less stress in ABA-treated plants. No dramatic changes in most chlorophyll a fluorescence parameters after ex vitro transfer suggested that the plants did not suffer from restriction of electron transport or photosystem damage.

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With the aim to contribute to the elucidation of the role of phytohormones in response of plants to adverse environmental conditions, seedlings of Phaseolus vulgaris, Nicotiana tabacum, Beta vulgaris, and Zea mays were supplied with water, 100 μM abscisic acid (ABA), or 10 μM N 6 -benzyladenine (BA) immediately before imposition of water stress (WS). In all four species, contents of chlorophylls (Chls) and carotenoids were markedly decreased during WS and after rehydration only in plants pre-treated with water but not in those pre-treated with ABA or BA. Contents of pigments of xanthophyll cycle increased during WS more in plants pre-treated with ABA or BA than in those pre-treated with water, but the degree of their de-epoxidation was highest in the later. Similarly, the efficiency of photosystem 2, determined as variable to maximal Chl fluorescence ratio, was not markedly decreased in bean plants pre-treated with ABA or BA in contrast to those pre-treated with water. The imposed WS was not severe enough to damage chloroplast ultrastructure. However, different changes in a size of starch inclusions were observed. In bean plants, the amount of starch increased considerably in plants pre-treated with water, while it decreased in BA pre-treated plants and no change was found in ABA pre-treated ones. The starch content declined under WS in sugar beet and tobacco plants but only moderate changes were found in ABA or BA pre-treated plants. Thus the application of BA and especially of ABA reduced the negative effects of subsequent WS.

Ex vitro transfer is often stressful for in vitro grown plantlets. Water stress and photoinhibition, often accompanying the acclimatization of in vitro grown plantlets to ex vitro conditions, are probably the main factors promoting production of reactive oxygen species (ROS) and in consequence oxidative stress. The extent of the damaging effects of ROS depends on the effectiveness of the antioxidative systems which include low molecular mass antioxidants (ascorbate, glutathione, tocopherols, carotenoids, phenols) and antioxidative enzymes (superoxide dismutase, ascorbate peroxidase, catalase, glutathione reductase, monodehydroascorbate reductase, dehydroascorbate reductase). This review is focused on ROS production and development of antioxidative system during in vitro growth and their further changes during ex vitro transfer.