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Responses to drought, heat, and combined stress were compared in tobacco (Nicotiana tabacum L.) plants ectopically expressing the cytokinin oxidase/dehydrogenase CKX1 gene of Arabidopsis thaliana L. under the control of either the predominantly root-expressed WRKY6 promoter or the constitutive 35S promoter, and in the wild type. WRKY6:CKX1 plants exhibited high CKX activity in the roots under control conditions. Under stress, the activity of the WRKY6 promoter was down-regulated and the concomitantly reduced cytokinin degradation coincided with raised bioactive cytokinin levels during the early phase of the stress response, which might contribute to enhanced stress tolerance of this genotype. Constitutive expression of CKX1 resulted in an enlarged root system, a stunted, dwarf shoot phenotype, and a low basal level of expression of the dehydration marker gene ERD10B. The high drought tolerance of this genotype was associated with a relatively moderate drop in leaf water potential and a significant decrease in leaf osmotic potential. Basal expression of the proline biosynthetic gene P5CSA was raised. Both wild-type and WRKY6:CKX1 plants responded to heat stress by transient elevation of stomatal conductance, which correlated with an enhanced abscisic acid catabolism. 35S:CKX1 transgenic plants exhibited a small and delayed stomatal response. Nevertheless, they maintained a lower leaf temperature than the other genotypes. Heat shock applied to drought-stressed plants exaggerated the negative stress effects, probably due to the additional water loss caused by a transient stimulation of transpiration. The results indicate that modulation of cytokinin levels may positively affect plant responses to abiotic stress through a variety of physiological mechanisms.

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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.,

Nitric oxide (NO) is an important signalling molecule with diverse physiological functions in plants. In plant cell, it is synthesised in several metabolic ways either enzymatically or nonenzymatically. Due to its high reactivity, it could be also cytotoxic in dependence on concentration. Such effects could be also mediated by NO-derived compounds. However, the role of NO in photosynthetic apparatus arrangement and in photosynthetic performance is poorly understood as indicated by a number of studies in this field with often conflicting results. This review brings a short survey of the role of exogenous NO in photosynthesis under physiological and stressful conditions, particularly of its effect on parameters of chlorophyll fluorescence.