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Polyamines are stress plant metabolites with a wide range of physiological effects. Their effect on functioning of the stomatal apparatus of plants remains poorly understood. The aim of the study was to use inhibitory analysis to investigate the possible participation of different calcium pools and lipid signaling components in the implementation of the effect of putrescine and spermine on stomatal aperture of pea leaves (Pisum sativum L.). It was shown that the epidermis incubation in the presence of both polyamines within the concentration range of 0.25–5 mM caused a decrease in the stomatal aperture. This effect was observed within 1 h after the start of incubation and was most pronounced after 2.5 h exposure in a medium containing 1 mM putrescine or spermine. In the presence of the calcium channel blocker LaCl3, the effect of polyamines on the state of stomata was weak; also it was partially neutralized by the extracellular calcium chelator EGTA; however, polyamines’ effects were completely eliminated by the phospholipase C inhibitor neomycin. The effect of putrescine and spermine on the stomatal aperture was not detected with the presence of n-butanol, an inhibitor of phospholipase D-dependent phosphatidic acid formation, but not with its inactive isomer 2‑butanol. The obtained results point to the possible role of calcium entry into cytosol from intracellular compartments, and the importance of signaling intermediates formed with the participation of phospholipases C and D in the implementation of the stomatal effects of polyamines.

The role of jasmonic acid (JA) and jasmonate signaling in the regulation of plant adaptive responses to the action of stressors is reviewed. The synthesis of JA in plants and the main pathway for the transduction of jasmonate signals are briefly described. The effect of JA on the content of other signaling mediators (calcium ions, reactive oxygen species, nitric oxide, hydrogen sulfide, and carbon monoxide) in cells is surveyed. Data on the involvement of jasmonate signaling components (in particular, COI1 and JIN1/MYC2 proteins) in the physiological effects of signaling mediator gasotransmitters are summarized. Data on changes in the endogenous content of JA under the action of stressors and information on the effect of exogenous JA and its derivatives on the plant resistance are given. The spectrum of jasmonate-dependent defense responses of plants and the mechanisms of their induction are analyzed. Particular attention is given to the role of JA in the activation of the antioxidant system and the regulation of stomata under stress conditions.

To clarify a possible role of jasmonate signaling in regulation of the state of stomata under salt stress, the effect of the treatment of Arabidopsis thaliana L. plant leaves (wild type and mutants deficient in jasmonate signaling) with sodium chloride (50–200 mM) and/or methyl jasmonate (200 μM) was studied. A 2–3-h effect of NaCl induced stomatal closure in wild-type plants (Col-0). The most noticeable effect was observed under the influence of 100 mM sodium chloride. At the same time, NaCl treatment of the leaves of the jin1 mutant, deficient by the gene encoding the JIN1/MYC2 transcription factor, had hardly any effect on the state of the stomata. Only a slight decrease in stomatal aperture occurred under the influence of salt in the plants of the coi1 genotype (mutants by the gene encoding the COI1 protein, which is involved in the removal of jasmonate signaling transcriptional factors’ repressors). A 3-h treatment of leaves with 200 μM methyl jasmonate caused a noticeable stomatal closure in wild-type plants but not in jin1 and coi1 mutants. With the combined effect of methyl jasmonate and salt on the leaves of Arabidopsis of three genotypes, a trend towards additional decrease in the stomatal aperture was manifested only in wild-type plants. A conclusion was drawn on the role of jasmonate and its signal transduction system in the regulation of stomatal movements under salt stress.

The participation of reactive oxygen species (ROS) and nitric oxide (NO), and also enzymatic systems generating them, in the development of heat resistance of wheat (Triticumaestivum L.) seedlings, induced by the hydrogen sulfide (H2S) donor sodium hydrosulfide (NaHS), has been studied. It was found that 24-h pretreatment of seedlings with 0.1–1 mM NaHS increased their survival after the subsequent 10-min damaging heating at 45°C. The content of hydrogen peroxide and nitric oxide increased together with the nitrate reductase (NR) activity in the seedling roots within the first 4 h of their treatment with the H2S donor. The rise in the NO level significantly suppressed by the inhibitor of NR sodium tungstate but not the inhibitor of NO synthase (NG-nitro-L-arginine methyl ester, L-NAME). The hydrogen peroxide scavenger dimethylthiourea (DMTU) and the NADPH oxidase inhibitor imidazole abolished the increase in NR activity and NO content in the roots. However, the nitric oxide scavenger (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, PTIO) and the inhibitors of the NO-producing enzymes only weakly influenced the increase in H2O2 content caused by the root treatment with sodium hydrosulfide. The NaHS-induced rise in the seedling heat resistance was eliminated by both ROS antagonists (DMTU and imidazole) and NO antagonists (PTIO and tungstate). It is concluded that the boost in the wheat seedling heat resistance, which is caused by exogenous hydrogen sulfide, is mediated by the increased ROS generation, followed by NR activation, and resultant rise in the level of nitric oxide produced by this enzyme.

The JIN1/MYC2 transcription factor, which is currently considered to be the key in jasmonate signaling, is also involved in the transduction of abscisic acid signals and, probably, in the effects of other intermediaries involved in the formation of plants’ adaptive responses. Arabidopsisjin1 mutants were used to study its possible involvement in protective effects of hydrogen sulfide (H2S) against salt stress. The treatment of wild-type Arabidopsis plants (Col-0) with the donor of hydrogen sulfide (50 μmol NaHS) led to an increase in their salt resistance, which was expressed in a decrease in oxidative damages, a decrease in water deficit, and preservation of a pool of photosynthetic pigments under 150 mmol NaCl. The treatment of Col-0 plants with NaHS prevented the stress-induced decrease in the activity of antioxidant enzymes, i.e., superoxide dismutase and catalase, and contributed to the increase in the activity of guaiacol peroxidase. In addition, leaves’ proline content in wild-type plants treated with the H2S donor was lower and sugar was higher after salt stress than in untreated plants. The treatment of jin1 mutants did not contribute to their salt resistance and did not have a noticeable effect on the functioning of the studied protective systems. The obtained results make it possible to assume the involvement of the JIN1/MYC2 transcription factor in the effects of hydrogen sulfide and/or intermediaries of its signaling pathways involved in the formation of plants’ adaptive responses to salt stress.

The effect of priming winter rye (Secale cereale L., var. Pamiat’ Khudoerko) and wheat (Triticum aestivum L., var. Dosconala) seeds with nitrogen oxide (NO) donor sodium nitroprusside (SNP) on their frost resistance was studied. The NO donor at concentrations of 0.1–0.5 mM increased the cold-hardening ability of seedlings of both cereals (6 days at 2–4°С), leading to a significant increase in their survival after freezing at –6 and –8°С. Seed treatment with SNP contributed to an increase in the content of sugars, proline, anthocyanins, and UV-B absorbing flavonoids in cereal seedlings. An increase in the activity of superoxide dismutase and guaiacol peroxidase in them was also detected. After cold hardening, especially after freezing, the content of the lipid peroxidation product malondialdehyde in seedlings grown from seeds primed with SNP was lower than in the corresponding controls. It was concluded that nitrogen oxide enhanced the cold-induced activation of antioxidant and osmoprotective systems of cereals.

Effect of cold hardening on state of antioxidant system (AOS) of triticale (× Triticosecale Wittmack) seedlings of varieties, differed in frost resistance, namely Buket and Raritet (winter, high-frost-resistant), and Alexandra and Pidzymok Kharkovskiy (facultative, less resistant) was studied. In absence of cold hardening, a clear connection between studied indicators of the AOS state and frost resistance of the varieties was not observed. After hardening of seedlings at 2–4 °C for 6 days, the activity of superoxide dismutase (SOD) and catalase increased in the high-frost-resistant varieties Buket and Raritet. In less resistant ones—Alexandra and Pidzymok Kharkovskiy—the activity of these enzymes changed less significantly. At the same time, the activity of guaiacol peroxidase increased more noticeably after hardening in these varieties. Hardened seedlings of the Buket, Raritet and Alexandra varieties had significantly higher sugar content than the Pidzymok Kharkovskiy. The proline content in response to cold hardening increased in all varieties, while the absolute values in the varieties Buket, Raritet and Pidzymok Kharkovskiy were significantly higher than in the Alexandra. Cold hardening caused a significant increase in the content of anthocyanins and flavonoids, absorbing in UV-B, in the Buket, Raritet and Alexandra, but not in the Pidzymok Kharkovskiy. The content of lipid peroxidation product malondialdehyde after a 6-h damaging action of − 7 °C increased in the varieties Alexandra and Pidzymok Kharkovskiy and slightly changed in the Buket and Raritet. In general, a positive relationship was found between AOS activity and frost resistance of hardened triticale seedlings. This relationship was most clearly manifested for indicators of SOD activity and flavonoid content.

The effect of sodium hydrosulfide (NaHS) as a donor of hydrogen sulfide on the resistance to subzero temperatures was investigated in the seedlings of winter wheat (Triticum aestivum L.) and rye (Secale cereale L.). Treatment of nonhardened seedlings with NaHS at concentrations of 0.1 and 0.5 mM improved their survival after freezing at –5°С. Exposure to NaHS at the same concentrations also improved the survival of wheat and rye seedlings hardened at 2–4°С after their freezing at –9°С. Under the effect of the hydrogen sulfide donor, the seedlings of both species at normal temperature (20–22°С) and upon cold hardening accumulated more sugars and proline. After sodium hydrosulfide treatment, the content of anthocyanins rose only in wheat seedlings. The donor of hydrogen sulfide also induced a rise in the activity of catalase and guaiacol peroxidase in the seedlings of both species at normal and hardening temperatures, while the activity of superoxide dismutase remained essentially the same. Under the effect of NaHS, both species accumulated less malonic dialdehyde caused by cryogenic stress. The contribution of different components of stress-protective systems to chilling resistance induced by hydrogen sulfide and hardening is discussed in relation to plant species.

A time and concentration dependence of the hydrogen sulfide donor sodium hydrosulfide (NaHS) influence on the state of stomata of Arabidopsis thaliana (Col-0) leaves, as well as the role of calcium and phospholipases in the implementation of its effects, was studied. Treatment of leaves with NaHS in the concentration range of 5–250 μM caused a decrease in the size of the stomatal aperture. The maximal effect of the stomatal closure was observed 90 min after the beginning of the treatment with H2S donor, and the stomatal aperture in NaHS variants was, on the contrary, much wider than in the control after 180 min of exposure. The effect of treatment with NaHS solutions on the stomata state was completely eliminated by hydroxylamine, the hydrogen sulfide scavenger, which indicates the specificity of NaHS effects as an H2S donor. The decrease in stomatal aperture and relative number of open stomata caused by the donor of hydrogen sulfide was almost completely leveled off by the pretreatment of leaves with the calcium channel blocker lanthanum chloride, the extracellular calcium chelator EGTA, the phospholipase C inhibitor neomycin, and the antagonist of the formation of cyclic adenosine-5'-diphosphate ribose—nicotinamide. In addition, the stomatal effect of theH2S donor was partially eliminated by the calmodulin antagonist chlorpromazine. The leveling of the hydrogen sulfide donor action on the state of stomatal apparatus of Arabidopsis leaves was also noted at the pretreatment of leaves with butanol-1, an inhibitor of phospholipase D-dependent formation of phosphatidic acid. A conclusion about the importance of calcium intake into the cytosol from various compartments, as well as lipid signaling mediators formed with the involvement of phospholipases C and D, in the implementation of hydrogen sulfide action on the state of stomata was made.

Effects of plant treatment with a donor of hydrogen sulfide—sodium hydrosulfide—on the state of antioxidative and osmoprotective systems of young plants of winter wheat (Triticum aestivum L.) cv. Doskonala under conditions of soil drought (progressive decrease in soil moisture down to 25–30% of the total moisture capacity) were studied. Spraying of plants with NaHS solutions (0.1–0.5 mM) before the drought significantly alleviated the growth suppression and favored the conservation of the chlorophyll pool under drought conditions. The beneficial effect of sodium hydrosulfide on plants was eliminated by their simultaneous treatment with hydroxylamine, which is a scavenger of hydrogen sulfide. The plant pretreatment with NaHS prevented the drought-induced accumulation of hydrogen peroxide and that of a lipid peroxidation product (malonic dialdehyde) in leaves. Such pretreatment also promoted an increase in the activity of superoxide dismutase (SOD) and prevented a stress-induced decrease in activities of catalase and guaiacol peroxidase in the leaves. Furthermore, the hydrogen sulfide donor elevated the concentration of proline and considerably increased levels of anthocyanins and UV-absorbing flavonoids under the drought. It is concluded that the plant drought protection afforded by the hydrogen sulfide donor depends to large extent on SOD and flavonoid compounds.

Separate and combined effects of nitrate (NaNO3) and L-arginine as potential sources of nitric oxide (NO) on the content of endogenous NO in roots of wheat (Triticum aestivum L.) seedlings and on their heat resistance were studied. Both agents increased the seedling resistance to the damaging heating; the effect was maximal at 20 mM NaNO3 or 5 mM L-arginine. The treatment with L-arginine elevated the NO content in the roots within the first 2 h of the treatment. Nitrate caused a stronger and longer rise in nitric oxide. Activity of nitrate reductase considerably (2–3 times) increased in the roots exposed to nitrate. The augmentation in the nitric oxide level caused by nitrate or L-arginine was prevented by the root pretreatment with an inhibitor of nitrate reductase (sodium tungstate) or an inhibitor of animal NO-synthase—NG-nitro-L-arginine methyl ester (L-NAME). Upon the combined treatment with NaNO3 and L-arginine, the nitrateinduced stimulation of the nitrate reductase activity, NO level in the roots, and seedling heat resistance were less pronounced than after separate application. In the presence of L-NAME, the negative influence of L-arginine on nitrate effects was markedly attenuated. The plant exposure to nitrate or L-arginine increased the activities of antioxidant enzymes (superoxide dismutase, catalase, and guaiacol peroxidase). A mixture of NaNO3, and L-arginine caused weaker effects. It was suggested that nitrate-dependent and arginine-dependent pathways of NO formation are antagonistic to each other in wheat roots.

The effect of methyl jasmonate, donors and antagonists of nitric oxide (NO) on the state of stomata of Arabidopsis thaliana plants (Col-0) was studied. Treatment of the epidermis of leaves with a 50–400 μM solution of methyl jasmonate (MJ) caused a decrease in the size of the stomatal aperture and the percentage of open stomata. These effects were completely eliminated after preliminary treatment of epidermis cells with NO scavenger PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) and partially after treatment with inhibitors of animal NO synthase (NG-nitro-L-arginine methyl ester, L-NAME) and nitrate reductase (sodium tungstate). NO donors (L-arginine and sodium nitrite) in concentrations of 0.5–2.0 mM also caused a decrease in the number of open stomata and the size of the stomatal aperture. The action of L‑arginine on the state of stomata was leveled by pretreatment of cells with L-NAME, and effects of sodium nitrite were eliminated by preaction of sodium tungstate. A conclusion about the possible participation of nitrogen oxide, forming both as the result of L-arginine oxidation and during nitrate/nitrite reduction, in the regulation of the stomatal apparatus and about its role in the realization of an MJ effect on state of stomata in Arabidopsis thaliana was made.

Presowing treatment of wheat (Triticum aestivum L.) seeds with 10 or 100 μM salicylic acid (SA) reduced the inhibition of 14-day-old plant growth under soil drought. The same effect was caused by the spraying of 7-day-old seedlings with 0.5 or 2 mM nitrogen oxide donor (sodium nitroprusside, SNP) before drought. The protective effect was enhanced by the combination of seed treatment with 10 μM SA and plant spraying with 0.5 mM SNP, while their combinations in higher concentrations caused weaker effects. SA treatment in both concentrations and 0.5 mM SNP under drought conditions increased the antioxidant enzyme activity (superoxide dismutase, catalase, and guaiacol peroxidase) in leaves. This effect was especially significant when 10 μM SA was combined with 0.5 mM SNP. Spraying with 2 mM SNP and its combination with seed presowing with 100 μM SA did not significantly change the antioxidant enzyme activity; however, the proline content in the leaves increased. It is concluded that the SA stress-protective action on plants can be modified with exogenous nitrogen oxide.

The effect of hydrogen peroxide treatment on the salt tolerance of wild-type Arabidopsis thaliana L. plants (Col-0) and plants transformed with the bacterial salicylate hydroxylase gene (NahG) was studied. The base tolerance to salt stress caused by 200 mM of NaCl in solution culture was higher in plants with the NahG genotype in comparison with the wild-type plants. Growth inhibition was observed for wild-type plants under the action of exogenous hydrogen peroxide, which was not observed for the NahG transformants; salt tolerance increased in the both types of plants after treatment, which was assessed based on the growth indicators and the ability to preserve the chlorophyll pool following NaCl treatment. The content of endogenous Н2О2 in the leaves of wild-type plants increased significantly following exogenous hydrogen peroxide treatment and salt stress, while it practically did not change in the leaves of the NahG genotype. The SOD activity increased in both genotypes after treatment with exogenous hydrogen peroxide, and remained at an elevated level after salt stress in comparison with the nontreated plants. Furthermore, the catalase activity increased in leaves of the salicylate-deficient genotype but not in the Col-0 genotype. The guaiacol peroxidase activity increased in plants of both genotypes under the action of hydrogen peroxide and salt stress, with the NahG plants demonstrating a higher degree of increase. The Н2О2 treatment facilitated the increase of the proline content in leaves of the plants of both genotypes under conditions of salt stress. It was concluded that there were hydrogen peroxide signal transduction pathways in Arabidopsis plants that were salicylic acid independent and that the antioxidant system functioned more effectively in salicylate-deficient Arabidopsis plants.

To elucidate the possible role of the transcription factor (TRF) JIN1/MYC2 in implementation of stress-protective effects of abscisic acid (ABA), the effect of exogenous ABA on the state of stomata and the activity of antioxidant enzymes and proline content under salt stress conditions in Arabidopsis thaliana plants of wild type (Col-0) and jin1 mutants with impaired jasmonate signaling was investigated. Treatment of leaves’ epidermis with ABA (10 or 100 μM) caused the closing of stomata in Col-0 plants but has hardly any influence on stomatal aperture in jin1 mutants. Salt stress (200 mM NaCl exposure for 24 h) caused a reduction of the water content in the plant leaves of both genotypes. Addition of 10 μM ABA into the growing medium contributed to a maintaining of normal hydration in wild-type but not in jin1 plants under salt stress. ABA treatment caused an almost twofold increase in proline content in the leaves of plants of both genotypes under normal conditions. Pretreatment with phytohormone contributed to enhancing the proline content in wild-type plants at salt stress and had a less significant effect on its amount in jin1 plants. Treatment with ABA under physiologically normal conditions increased the catalase activity in wild-type plants. Both genotypes under ABA influence showed increased activity of superoxide dismutase (SOD). Under salt stress conditions, higher activity of SOD, catalase, and guaiacol peroxidase was observed in ABA-treated wild-type plants but not in jin1 mutants. A conclusion about the participation of TRF JIN1/MYC2 in the formation of certain ABA-induced physiological responses of Arabidopsis plants was made.

The effect of hydrogen sulfide (H2S) donor sodium hydrosulfide (NaHS) on the heat resistance of wheat (Triticum aestivum L.) coleoptile cells, the formation of reactive oxygen species (ROS), and the activity of the antioxidant enzymes in them was investigated. The treatment of coleoptiles with 100 µM NaHS caused transient enhancement of the generation of the superoxide anion radical (O2 •) and an increased hydrogen peroxide content. The activities of antioxidant enzymes—superoxide dismutase, catalase, and guaiacol peroxidase— and coleoptile resistance to damaging heat was later found to have increased. The biochemical and physiological effects of the hydrogen sulfide donor described above were inhibited by the treatment of wheat coleoptiles with the hydrogen peroxide scavenger dimethylthiourea, the NADPH oxidase inhibitor imidazole, the extracellular calcium chelator EGTA, and the phosphatidylinositol-specific phospholipase C inhibitor neomycin. A conclusion was made on the role of ROS generation, which is dependent on the activity of NADPH oxidase and calcium homeostasis, in the transduction of the H2S signal, which induces antioxidant enzymes and the development of plant cell heat resistance.

The participation of enzymatic systems carrying out generation and conversion of reactive oxygen species (ROS), in realization of the stress-protective effect of hydrogen sulfide (H2S) on wheat coleoptile cells was investigated. It has been shown that the treatment of isolated coleoptiles with a 100 μM hydrogen sulfide donor sodium hydrosulfide (NaHS) caused a transient enhancement of the generation of superoxide anion radical (O2•–), an increase of hydrogen peroxide content and superoxide dismutase (SOD) activity in them. The increase in ROS generation was eliminated by the inhibitor of NADPH oxidase imidazole, but not by the peroxidase inhibitor sodium azide. Treatment of coleoptiles with SOD inhibitor sodium diethyldithiocarbamate (DDC) enhanced the generation of O2•– and neutralized the effect of increasing H2O2 content induced by NaHS. One day after treatment with the H2S donor, the generation of ROS decreased to a control level, while the activity of antioxidant enzymes increased markedly and the resistance of coleoptiles to damaging heating­ was increased. These effects of the hydrogen sulfide donor were eliminated by coleoptiles’ treatment with inhibitors of NADPH oxidase (imidazole) and SOD (DDC). It was concluded that both NADPH oxidase, genera­ting O2•– , and SOD, which turns it into H2O2 performing signaling functions, are involved in the formation of a signal that induces protective systems and causes an increase in heat resistance of plant cells.

The effect of malonate and sedaxane, a compound with the fungicidal effect which act as succinate dehydrogenase inhibitors, on the resistance of etiolated wheat seedlings (Triticum aestivum L.) to osmotic stress caused by 12% PEG 6000 solution, was studied. The presowing treatment of seeds with 0.3 mM sedaxane solution significantly reduced the inhibitory effect of osmotic stress on seedling growth. The protective effect of 10 mM malonate was significant when it was added to the incubation medium of the roots; the effect of preseeding treatment with malonate was less significant. Unlike malonate, malate had no positive effect on seedling growth under osmotic stress. The activity of succinate dehydrogenase and the hydrogen peroxide content decreased in seedlings after the treatment of roots with malonate and sedaxane. Pretreatment with sedaxane and the addition of malonate to the incubation medium of roots prevented the accumulation of a lipid peroxidation product, malondialdehyde, which is caused by osmotic stress, and increased peroxidase activity. It was concluded that the stress-protective effect of sedaxane and malonate on wheat seedlings might be due to the inhibition of succinate dehydrogenase-dependent formation of reactive oxygen species and the prevention of oxidative cell damage.

The effect of NO donor sodium nitroprusside (SNP) on salt resistance of 4-week-old Arabidopsis thaliana L. wild-type Columbia-0 (Col-0) plants and jin1 mutants defective in the jasmonate signaling have been investigated. As affected by 0.5 mM, SNP salt resistance of wild-type plants rose, which was exhibited in a smaller growth inhibition and preserving the pool of photosynthetic pigments after salt stress (200 mM NaCl). The positive effect of SNP leveled by treatment of plant with NO scavenger: 0.5 mM PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). However, SNP treatment had no significant effect on the salt tolerance of jin1 genotype plants. In the case of wild-type plants but not jin1 mutants, treatment by NO donor increased activity of antioxidant enzymes—superoxide dismutase, guaiacol peroxidase, and catalase—in the leaves, which was especially noticeable in salt stress conditions. In wild-type plants treated by NO donor, proline content in the leaves after salt stress was lower and sugar content was higher than that in the untreated ones. In jin1 mutants, NO donor treatment resulted in a significant increase in proline content in the leaves under salt stress, without changing sugar content. A conclusion was made on the participation of transcript factor JIN1/MYC2 in NO-dependent induction of some plant defense responses to salt stress.

Functional interactions of calcium ions, hydrogen peroxide, and nitric oxide as signal mediators in root cells of wheat (Triticum aestivum L.) seedlings upon induction of their heat resistance was studied with use of inhibitor-based analysis. Treatment of the seedlings with hydrogen peroxide or a combination of calcium chloride with ionophore A23187 significantly increased their content of nitric oxide, which peaked 0.5–1 h after the start of the treatment. CaCl2 or exogenous NO donor (sodium nitroprusside, SNP) transitorily increased the hydrogen peroxide level in the roots. Seedlings pretreatments with calcium chelator (EGTA), blocker of Ca2+ channels (LaCl3), inhibitor of phospholipase C (neomycin), or antagonist of cyclic adenosine-5'-diphosphatribose formation (nicotinamide) more or less prevented the rise in the nitric oxide content in roots caused by exogenous H2O2; the SNP-induced rise in hydrogen peroxide was also damped down. However, the seedlings pretreatment with antioxidants ionol or dimethylthiourea did not hinder the increase in the NO level, which was caused by exogenous Ca2+. The inhibitors of NO synthase (NG-nitro-L-arginine methyl ester, L-NAME) or nitrate reductase (sodium tungstate) did not interfere in the accumulation of H2O2 in root tissues stimulated by exogenous calcium. Calcium antagonists diminished the seedlings heat resistance increased by hydrogen peroxide or SNP. Antioxidants and inhibitors of NO synthase or nitrate reductase weakened the calcium-stimulated enhancement in the seedlings heat resistance. It was concluded that calcium may activate NO- and H2O2-generating enzymatic systems as well as participate in the transduction of signals of these mediators into genetic apparatus and in the formation of physiological reactions underlying the enhanced heat resistance.

The influence of cold hardening of rye (Secale cereale L.) and wheat (Triticum aestivum L.) seedlings on their resistance to the oxidative stress (OS) agents, namely, 50 mM hydrogen peroxide or 5 mM iron (II) sulfate was studied. Unhardened rye seedlings were more resistant to hydrogen peroxide than those of wheat, since their growth was less inhibited, and they accumulated lesser amounts of lipid peroxidation products after a treatment with H2O2. The interspecific differences in responses to FeSO4 were less significant. The unhardened seedlings of rye, in comparison with those of wheat, possessed more active guaiacol peroxidase (GPO) and more levels of anthocyanins and proline. In response to the OS agents, the unhardened rye seedlings enhanced activities of superoxide dismutase and catalase, whereas the wheat seedlings enhanced GPO activity and proline content. The cold hardening (6 days at 2°C) increased activities of antioxidant (AO) enzymes, contents of proline, sugars, and anthocyanins in seedlings of both species, and made the seedlings more resistant to the OS agents. After the cold hardening, rye seedlings were more resistant to OS than wheat seedlings. The hardened seedlings of both species activated the AO enzymes in response to H2O2 or FeSO4 greater than the unhardened ones. However, the hardened wheat seedlings, in contrast to the unhardened ones, did not augment the proline content in contact with the OS agents. The conclusion was drawn on different contributions of AO enzymes and low-molecular weight compounds to the basal and induced by the cold—hardening resistances of rye and wheat seedlings to OS.