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Plantlets grown in vitro might be easily impaired by sudden changes in environmental conditions after ex vitro transfer. They usually need several weeks under shade and gradually decreasing air humidity to acclimate to the new conditions and to correct all abnormalities in their anatomy and physiology induced by special conditions of in vitro culture. For plant survival, the most important changes include development of cuticle, epicuticular waxes, and effective stomatal regulation of transpiration leading to stabilization of water status. For plant growth, changes in photosynthetic parameters (chlorophyll content, chloroplast ultrastructure, efficiency of photosystem 2, net photosynthetic rate) ensuring fully autotrophic growth with the rate corresponding to naturally grown plants are the most important. Acclimation can be speeded up by hardening of plantlets in vitro or after transplantation by decreasing the transpiration rate by antitranspirants including abscisic acid, or by increasing photosynthetic rate by elevated CO 2 concentration. INTRODUCTION Within the last four decades, plant micropropagation has developed from a laboratory curiosity to a real industry. Its use in horticulture, agriculture and forestry is currently expanding worldwide. Micropropagation of many species can be achieved through the establishment of explants, their initial growth in vitro being followed by transplanting into the greenhouse or field. During in vitro cultivation, plantlets grow under constant temperature, very high air humidity, low irradiance, very low air turbulence, variable and often insufficient CO 2 concentration, water potential dependent on medium composition, sugars as carbon source, growth regulators in nutrient medium, ethylene and other volatiles, etc. The conditions are very dependent on the vessel and closure types (e.g., Solárová et al., 1996). Acclimation to these conditions leads to formation of plantlets with morphology, anatomy and physiology different from naturally grown plants (for review see, e.g.,

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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.