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▪ Abstract  To cope with environmental fluctuations and to prevent invasion by pathogens, plant metabolism must be flexible and dynamic. Active oxygen species, whose formation is accelerated under stress conditions, must be rapidly processed if oxidative damage is to be averted. The lifetime of active oxygen species within the cellular environment is determined by the antioxidative system, which provides crucial protection against oxidative damage. The antioxidative system comprises numerous enzymes and compounds of low molecular weight. While research into the former has benefited greatly from advances in molecular technology, the pathways by which the latter are synthesized have received comparatively little attention. The present review emphasizes the roles of ascorbate and glutathione in plant metabolism and stress tolerance. We provide a detailed account of current knowledge of the biosynthesis, compartmentation, and transport of these two important antioxidants, with emphasis on the unique insights and advances gained by molecular exploration.

Cleft grafting methods were applied in order to evaluate the effect of grafting on tomato fruit yield and quality. The tomato cultivars used as scions were Yeni Talya, Swanson and Beri. The cultivars Beaufort and Arnold were used as rootstocks. The following characteristics of grafted and nongrafted plants were recorded: fruit index, number of fruits/truss, fruit weight, fruit yield, dry matter, pH, concentration of soluble solids, titratable acidity, concentrations of total sugar, lycopene and vitamin C. The results showed that fruit yield and fruit index, number of fruits/truss and fruit weights were improved by grafting. Fruit quality, measured in terms of concentrations of dry matter, soluble solids, total sugar, and vitamin C, was lower in grafted plants than in nongrafted ones. No significant difference in lycopene level and pH was found. Titratable acidity was improved by grafting. A positive effect of grafting was recorded when Beaufort was used as rootstock. These results showed that grafting could be an advantageous alternative in tomato production.

In this study, fruits, fruit flesh and seeds of Rosa damascena and Rosa canina were assayed for the composition of fatty acids, ascorbic acid, alpha-tocopherol, beta-carotene, and mineral elements. The concentration of linoleic acid in seed oil of R. damascena (54.18%) was higher than in R. canina (48.84%). Alpha-tocopherol concentrations were found to be 7.10 microg/g and 34.20 microg/g for R. damascena and R. canina fruits, respectively. Ascorbic acid conc. was determined as the highest in the fruit flesh (546 mg/100 g in R. damascena and 2,200 mg/100 g in R. canina), and as the lowest in the seeds of both species. R. damascena fruits were found to be richer in minerals such as Ca, Fe, K, Mn, Na, P, and Zn than R. canina fruits. The results of the present study showed that R. damascena fruits could be used as food and food additive equally as rose hip fruits.

Furan and its derivatives were identified in a small number of heat-treated foods back in the 60's and 70's. In May 2004, US Food and Drug Administration published a report on the occurrence of parent furan in a number of thermally treated foods. Since furan has been classified as possibly carcinogenic to human by IARC, a great concern has been addressed to the analysis of this substance naturally-occurring in food. This paper gives a short overview on the mechanistic pathways of the parent furan formation in food by degradation of amino acids and/or reducing sugars, and oxidation of ascorbic acid and polyunsaturated acids which can be induced by thermal or irradiation treatments; further, it deals with the metabolism and toxicology of furan as well as with the comparison of the methods of furan determination.

The presence of the enzymes of the ascorbate-glutathione cycle was investigated in mitochondria and peroxisomes purified from pea (Pisum sativum L.) leaves. All four enzymes, ascorbate peroxidase (APX; EC 1.11.1.11), monodehydroascorbate reductase (EC 1.6.5.4), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2), were present in mitochondria and peroxisomes, as well as in the antioxidants ascorbate and glutathione. The activity of the ascorbate-glutathione cycle enzymes was higher in mitochondria than in peroxisomes, except for APX, which was more active in peroxisomes than in mitochondria. Intact mitochondria and peroxisomes had no latent APX activity, and this remained in the membrane fraction after solubilization assays with 0.2 M KCl. Monodehydroascorbate reductase was highly latent in intact mitochondria and peroxisomes and was membrane-bound, suggesting that the electron acceptor and donor sites of this redox protein are not on the external side of the mitochondrial and peroxisomal membranes. Dehydroascorbate reductase was found mainly in the soluble peroxisomal and mitochondrial fractions. Glutathione reductase had a high latency in mitochondria and peroxisomes and was present in the soluble fractions of both organelles. In intact peroxisomes and mitochondria, the presence of reduced ascorbate and glutathione and the oxidized forms of ascorbate and glutathione were demonstrated by high-performance liquid chromatography analysis. The ascorbate-glutathione cycle of mitochondria and peroxisomes could represent an important antioxidant protection system against H2O2 generated in both plant organelles

We investigated how salicylic acid (SA) enhances H2O2 and the relative significance of SA-enhanced H2O2 in Arabidopsis thaliana. SA treatments enhanced H2O2 production, lipid peroxidation, and oxidative damage to proteins, and resulted in the formation of chlorophyll and carotene isomers. SA-enhanced H2O2 levels were related to increased activities of Cu,Zn-superoxide dismutase and were independent of changes in catalase and ascorbate peroxidase activities. Prolonging SA treatments inactivated catalase and ascorbate peroxidase and resulted in phytotoxic symptoms, suggesting that inactivation of H2O2-degrading enzymes serves as an indicator of hypersensitive cell death. Treatment of leaves with H2O2 alone failed to invoke SA-mediated events. Although leaves treated with H2O2 accumulated in vivo H2O2 by 2-fold compared with leaves treated with SA, the damage to membranes and proteins was significantly less, indicating that SA can cause greater damage than H2O2. However, pretreatment of leaves with dimethylthiourea, a trap for H2O2, reduced SA-induced lipid peroxidation, indicating that SA requires H2O2 to initiate oxidative damage. The relative significance of the interaction among SA, H2O2 and H2O2-metabolizing enzymes with oxidative damage and cell death is discussed

Recent studies have revealed that dietary flavonoids are potent radical scavengers, acting in a manner similar to ascorbate and alpha-tocopherol. However, it is still not clear whether flavonoids have a similar antioxidative function in plants. We examined the possibility that flavonoids could function as stress protectants in plant cells by scavenging H2O2. Two major flavonoids, quercetin and kaempferol glycosides, were isolated from leaves of the tropical tree Schefflera arboricola Hayata. Both glycosides and aglycones of isolated flavonols were oxidized by H2O2 in the presence of horseradish peroxidase and/or in a soluble fraction of S. arboricola leaf extract. The rates of oxidation were in the order quercetin kaempferol quercetin glycoside kaempferol glycoside. Judging from the effects of inhibitors such as KCN, p-chloromercuribenzoate, and 3-amino-1H-1,2,4-triazole, we conclude that guaiacol peroxidase in the soluble fraction catalyzes H2O2-dependent oxidation of flavonols. In the flavonol-guaiacol peroxidase reaction, ascorbate had the potential to regenerate flavonols by reducing the oxidized product. These results provide further evidence that the flavonoid-peroxidase reaction can function as a mechanism for H2O2 scavenging in plants

Earlier studies with Arabidopsis thaliana exposed to ultraviolet B (UV-B) and ozone (O3) have indicated the differential responses of superoxide dismutase and glutathione reductase. In this study, we have investigated whether A. thaliana genotype Landsberg erecta and its flavonoid-deficient mutant transparent testa (tt5) is capable of metabolizing UV-B- and O3-induced activated oxygen species by invoking similar antioxidant enzymes. UV-B exposure preferentially enhanced guaiacol-peroxidases, ascorbate peroxidase, and peroxidases specific to coniferyl alcohol and modified the substrate affinity of ascorbate peroxidase. O3 exposure enhanced superoxide dismutase, peroxidases, glutathione reductase, and ascorbate peroxidase to a similar degree and modified the substrate affinity of both glutathione reductase and ascorbate peroxidase. Both UV-B and O3 exposure enhanced similar Cu,Zn-superoxide dismutase isoforms. New isoforms of peroxidases and ascorbate peroxidase were synthesized in tt5 plants irradiated with UV-B. UV-B radiation, in contrast to O3, enhanced the activated oxygen species by increasing membrane-localized NADPH-oxidase activity and decreasing catalase activities. These results collectively suggest that (a) UV-B exposure preferentially induces peroxidase-related enzymes, whereas O3 exposure invokes the enzymes of superoxide dismutase/ascorbate-glutathione cycle, and (b) in contrast to O3, UV-B exposure generated activated oxygen species by increasing NADPH-oxidase activity

Measurements of the quantum efficiencies of photosynthetic electron transport through photosystem II (phi(PSII)) and CO2 assimilation (phi(CO2)) were made simultaneously on leaves of maize (Zea mays) crops in the United Kingdom during the early growing season, when chilling conditions were experienced. The activities of a range of enzymes involved with scavenging active O2 species and the levels of key antioxidants were also measured. When leaves were exposed to low temperatures during development, the ratio of phi(PSII)/phi(CO2) was elevated, indicating the operation of an alternative sink to CO2 for photosynthetic reducing equivalents. The activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and superoxide dismutase and the levels of ascorbate and alpha-tocopherol were also elevated during chilling periods. This supports the hypothesis that the relative flux of photosynthetic reducing equivalents to O2 via the Mehler reaction is higher when leaves develop under chilling conditions. Lipoxygenase activity and lipid peroxidation were also increased during low temperatures, suggesting that lipoxygenase-mediated peroxidation of membrane lipids contributes to the oxidative damage occurring in chill-stressed leaves

The basic chemical characteristics of 22 quince genotypes and cultivars were determined: dry matter content, soluble solid content, the contents of organic acids, pectins, and mineral elements (nitrogen, phosphorus, potassium, calcium, and sodium). Quince fruits were harvested in the course of October and analysed at the stage of consumption ripeness after storage at 2 deg C and relative air humidity of 85%. The levels of pectins in fruits were high - the cultivar Hruskovita contained 3.51+/-0.19 g/100 g FW. The concentrations of vitamin C were also high, the cultivar Muskatova containing as much as 79.31+/-2.01 g/100 g FW. The affinity of chemical properties of the individual cultivars was expressed by means of cluster analysis and it was found out that there were no marked differences between pear-shaped (Cydonia oblonga subsp. pyriformis) and apple-shaped (Cydonia oblonga subsp. maliformis) forms of fruit.