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At present hydrogen sulfide (H 2 S) is considered as one of the signal mediators in plant cells. However, its role in formation of plant resistance to low temperatures and, in particular, in regulation of secondary metabolism under stress conditions remains poorly understood. The influence of H 2 S donor sodium hydrosulfide (NaHS) on phenylalanine ammonia-lyase (PAL) activity and content of flavonoids in wheat seedlings at normal temperature (21 °C) and under cold hardening conditions (7 days at 3 °C) was studied. After 2 days of the hardening temperature, a transient increase in PAL activity was noted. Also, activity of the enzyme was increased by treatment of plants with 0.1 or 0.5 mM NaHS under normal temperature conditions and especially at the background of cold hardening. By themselves, the cold hardening and the action of H 2 S donor caused an increase in total content of flavonoids and amount of anthocyanins. With the combination of hypothermia and treatment of seedlings with NaHS, this effect enlarged and the total content of flavonoids increased by 3.8, and anthocyanins increased by 1.8 times in comparison to the control. Treatment with the H 2 S donor caused a decrease in content of the lipid peroxidation product malonic dialdehyde in seedlings after the action of hardening temperature, and especially after their freezing at-5 °C. Also, under the influence of NaHS, survival of hardened and unhardened seedlings after cryostress increased. It was concluded that one of the mechanisms of the positive influence of the H 2 S donor on resistance of wheat seedlings to hypothermia is the PAL-dependent accumulation of flavonoid compounds, which have a high antioxidant activity, and a decrease in effects of secondary oxidative stress. K e y w o r d s: hydrogen sulfide (H 2 S), phenylalanine ammonia-lyase, flavonoids, lipid peroxidation, frost resistan ce, Triticum aestivum. A t present, the functions of hydrogen sulfide (H 2 S) as a signal mediator in mammalian cells have been well studied; its involvement in regulation of vascular tone, neuromodulation, cytoprotection, inflammation, and apoptosis was established [1-3]. The role of hydrogen sulfide in plant cells is investigated significantly weaker, but

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Activities of antioxidant enzymes and the osmolyte contents in seedlings of winter rye (Secale cereale L.), soft (Triticum aestivum L.) and durum (T. durum L.) wheat, and barley (Hordeum vulgare L.) grown at 20°C (control) or after 7 day cold hardening at 2°C and/or 5 hour freezing at -6°C were investi gated. It was found that nonhardened rye seedlings differed from those of other cereals by their ability to sur vival after freezing at -6°C and higher activity of guaiacol peroxidase (GPO) and high content of proline. Hardening induced the increase in the frost tolerance of all cereals under study, and the resistance of rye and soft wheat was found to be significantly higher than that of durum wheat and barley. Rye and soft wheat exhib ited more profound tolerance to oxidative damages as well, and it was expressed in lesser increase in the MDA content after freezing. In the course of hardening, detectable increase in the activities of GPO and catalase (CAT), as well as the contents of proline and soluble carbohydrates, was observed in seedlings of all cereals under study. In barley, the activity of superoxide dismutase (SOD) increased to the highest extent under these conditions. After freezing of both hardened and nonhardened seedlings, higher activities of all tested antiox idant enzymes were revealed in rye and soft wheat as compared to those in durum wheat and barley. In this case, hardened rye and soft wheat seedlings after freezing displayed increased content of proline. All these results lead to the conclusion that the high content of proline and activity of GPO observed in rye seedlings may determine their increased constitutive frost tolerance, whereas high tolerance of hardened soft wheat seedlings is primarily associated with accumulation of low molecular weight protectors, such as sugars and proline, and, to some extent, with the increased activity of antioxidant enzymes.Abbreviations: GPO-guiaicol peroxidase; CAT-catalase; SOD-superoxide dismutase; TBA-2 thiobarbituric acid.