The role of reactive oxygen species, especially H202, in plant response to stresses has been the focus of much attention. Hydrogen peroxide has been postulated to play multiple functions in plant defence against pathogens. (1) H202 may possess direct microbicidal activity at the sites of pathogen invasion. (2) It is used for cell-wall reinforcing processes: lignification and oxidative cross-linking of hydroxyproline-rich proteins and other cell-wall polymers. (3) It was found to be necessary for phytoalexin synthesis. (4) H202 may trigger programmed plant cell death during the hypersensitive response that restricts the spread of infection. (5) H202 has been suggested to act as a signal in the induction of systemic acquired resistance and (6) it induces defence genes. Recently H202 has been proposed to be involved in the signal transduction pathways leading to acclimation and protection from abiotic stresses. The present review discusses new insights into the function of H202 in plant responses to biotic and abiotic stresses.The generation of reactive oxygen species (ROS) including singlet oxygen (102), superoxide radical anion (O2-), hydroxyl radical (OH) and hydrogen peroxide (H202) is an inevitable effect of the oxidative cellular metabolism. In plants ROS are synthesized as a by-product of photosynthesis, photorespiration, [3-oxidation of fatty acids and electron transport in mitochondria. Plants have evolved protective mechanisms to cope effectively with the potentially aggressive ROS. The ROS scavenging system includes enzymes such as superoxide dismutase, catalase and ascorbate peroxidase as well as non-enzymatic components: ascorbate, glutathione, (x-tocopherol and carotenoids. An excess production of ROS, resulting in oxidative stress, is promoted in plants exposed to biotic and abiotic stressful conditions. Such oxidative stress, potentially damaging to plant cells, was reported after herbivorous insect attack, viral, bacterial and fungal infection (Wojtaszek, 1997b) as well as in plants subjected to drought, flooding, high light intensity, chilling, heat, salinity, air pollutants and herbicides. An enhanced ROS generation has therefore been considered to be a general feature of a wide variety of stresses.

Hydrogen peroxide generation rates of uninfected and infected leaves of two tomato (Lycopersicon esculentum) cultivars showing differential susceptibility to Botrytis cinerea were determined. The superoxide anion, hydroxyl radical, ascorbate contents and changes in NADH peroxidase, superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) activities in the apoplast fraction were analysed. Infected leaves had an increased hydrogen peroxide level. It was greater and generally occurred earlier in plants of the less susceptible cv. Perkoz than in those of the more susceptible cv. Corindo. Induction of nitrotetrazolium blue reducing activity and SOD levels in apoplast were higher in cv. Perkoz 24 h after inoculation. In the controls, NADH peroxidase activity in apoplast was higher in the more susceptible cv. Corindo, but after infection it increased faster and to a higher level in the less susceptible cv. Perkoz. NADH oxidation was inhibited by only 15% by a specific inhibitor DPI (diphenylene‐iodonium) but was completely inhibited by KCN and NaN3. Similar increases in APX activity after 48 h and a small increase in catalase activities were observed in both cultivars soon after infection. These results indicate that resistance of tomato plants to infection by the necrotrophic fungus B. cinerea may result from early stimulation of hydrogen peroxide and superoxide radical generations by NADH peroxidase and SOD in apoplastic space, and they confirm the important role of their enhanced production in apoplastic spaces of plants.

We have studied the effects of precursor amino acids (phenylalanine and cystein)

o‐Hydroxyethylorutin, when applied exogenously to tomato plants was effective in enhancing resistance to Botrytis cinerea infection. Two tomato cultivars, high and low in susceptibility to B. cinerea, were analysed for superoxide anion, hydroxyl radical, hydrogen peroxide generation rates, for changes in lipid peroxidation and activities in superoxide dismutase, peroxidase and catalase. The reactions observed in uninfected tomato plants, in plants infected with B. cinerea, and in those treated with o‐hydroxyethylorutin and infected with the pathogen were compared. The most significant increase in superoxide anion and hydrogen peroxide generation was observed in plants pretreated with o‐hydroxyethylorutin and then infected with the pathogen. It was greater and generally occurred earlier in pretreated, less susceptible plants as compared to more susceptible ones. Some decrease in ascorbate peroxidase activity resulting from inoculation with B. cinerea plants pretreated with o‐hydroxyethylorutin was observed in studied plants. The results indicate that o‐hydroxyethylorutin‐induced tomato resistance to B. cinerea infection depends on its stimulation of reactive oxygen species generation and they confirm the important role of enhanced production of ROS in induced plant resistance reactions.

Changes in O 1 − and H 1 O 1 generation as well as super! oxide dismutase\ catalase\ ascorbate peroxidase and guai! acol peroxidase activities were examined in fusaric acid! treated tomato leaves[ Fusaric acid application resulted in a lasting activation of O 1 − and H 1 O 1 production[ Within the _rst day after toxin application the H 1 O 1 scavenging enzymes] catalase and ascorbate peroxidase showed a strong activity decrease followed by a gradual recovery to the control level after 1 and 2 days[ By con! trast fusaric acid markedly stimulated peroxidase activity measured with guaiacol as well as ferulic and coumaric acids\ and syringaldazine[ The maximum activity increase\ coinciding with the highest H 1 O 1 level\ was about two!fold and 4[4!fold for phenolic acids and syr! ingaldazine\ respectively[ The results obtained are dis! cussed in the light of the data on the role of activated oxygen species and enzymes of their metabolism in plant defence[ U[ S[ Copyright Clearance Center Code Statement] 9820Ð0674:88:3697Ð9274 , 03[99:9 dazin gemessen wurde[ Der maximale Aktivita Ãtsanstieg _el mit dem ho à chsten H 1 O 1 !Wert zusammen und lag bei den Phenolsa Ãuren bei ungefa Ãhr dem 1!fachen und bei Syringaldazin beim 4\4!fachen[ Die Ergebnisse werden im Hinblick auf die Daten zur Rolle von aktivierten Sau! ersto}spezies und die fu à r ihren Sto}wechsel verantwortli! chen Enzyme in der Abwehrreaktion von P~anzen diskutiert[