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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.

The effect of the brassinosteroids (BS) 24-epibrassiniolide and 24-epicastasterone on the thermoresistance of wheat coleoptile (Triticum aestivum L.) and their generation of the superoxide anion radical and antioxidant enzymes activity were investigated. The treatment of coleoptiles with 10 nM solutions of BS caused a transient increase in O2·- generation and a subsequent increase in the activity of superoxide dismutase and catalase and an improvement in heat resistance. Pretreatment of coleoptiles with the NADPH oxidase inhibitor imidazole leveled the increase in production of the superoxide anion radical and prevented an increase in the activity of antioxidant enzymes and the development of cell thermostability. The investigated effects of BS were also depressed by the pretreatment of coleoptile segments with extracellular calcium chelator EGTA and inhibitor of ADP-ribosyl cyclase nicotinamide. A conclusion was made about the participation of calcium ions and reactive oxygen species generated by the action of NADPH oxidase in the implementation of the stress-protective effect of the BS in the cells of wheat coleoptile.

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