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D. Mark Hodges, Charles F. Forney, and Wendy V. Wismer

The objective of this study was to assess responses of certain antioxidants in harvested leaves of selected cultivars of spinach (Spinacia oleracea L.) differing in postharvest senescence rates in order to explore the significance of these antioxidants in postharvest senescence regulation and dynamics. Ten cultivars were grown in both field plots and laboratory growth chambers, harvested at maturity, and their leaves detached and stored at 10 °C in the dark. Following postharvest analysis, two cultivars were identified consistently as having relatively high (`Spokane F1') and low (`BJ 412 Sponsor') postharvest senescence rates. These two cultivars were then grown in a growth chamber for 45 days and their leaves detached and stored as above. At the point of harvest (day 0) and on days 4, 8, 12, 16, and 20, samples were analyzed for activities of ascorbate peroxidase (ASPX; EC 1.11.1.11), catalase (CAT; EC 1.11.1.6), and superoxide dismutase (SOD; EC 1.15.1.1), and (ii) concentrations of malondialdehyde (MDA, an indicator of lipid peroxidation), total ascorbate, reduced ascorbate (AsA), oxidized ascorbate (DAsA), total glutathione, reduced glutathione (GSH), and oxidized glutathione (GSSG). Although MDA accumulated in leaves of both cultivars concomitant with time after detachment, levels became significantly higher in `Spokane F1'. It is argued that declining activities of ASPX and levels of ascorbate and increasing activities of SOD manifested in accumulation of hydrogen peroxide in `Spokane F1', leading to a greater potential for lipid peroxidation in this cultivar than for `BJ 412 Sponsor'. SOD activities and glutathione levels may have increased as a result of elevated oxidative stress in `Spokane F1'. Increased hydrogen peroxide accumulation in `Spokane F1' relative to `BJ 412 Sponsor' may have contributed to an increased rate of senescence in the harvested leaves of this cultivar.

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Yeh-Jin Ahn and Na-Hyun Song

plants, oxidative stress results in the accumulation of reactive oxygen species (ROS; Desikan et al., 2001 ). Increased ROS such as H 2 O 2 causes an imbalance in cellular redox state, thereby damaging the proteins, lipids, and nucleic acids of living

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Jing Tian, Li-Ping Wang, Yan-Juan Yang, Jin Sun, and Shi-Rong Guo

degradation ( Wahid et al., 2007 ). In addition to tissue dehydration, heat stress may induce oxidative stress in plants, which causes lipid peroxidation and subsequent membrane injury, protein degradation, enzyme inactivation, pigment bleaching, and

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Brandon R. Smith and Lailiang Cheng

The objective of this study was to quantify how photoprotective mechanisms in the leaves of `Concord' grapevines (Vitis labruscana Bailey) respond to a range of iron (Fe) supply. Own-rooted, 1-year-old container-grown vines were fertigated twice weekly for 11 weeks with a complete nutrient solution containing 1, 10, 20, 50, or 100 μm Fe from ferric ethylenediamine di (o-hydroxyphenylacetic) acid (Fe-EDDHA). Leaf total Fe content did not increase in response to Fe supply; however, “active” Fe (extracted with 2,2′-dipyridyl) and chlorophyll (Chl) increased on a leaf area basis as applied Fe increased. At the lowest active Fe level, leaf absorptance and the efficiency of excitation transfer (Fv′/Fm′) was lower, and nonphotochemical quenching (NPQ) was significantly greater. Photosystem II (PSII) quantum efficiency decreased curvilinearly, and the proportion of PSII reaction centers in the open state (qP) decreased linearly as active Fe content decreased. On a Chl basis, the xanthophyll cycle pool size [violaxanthin (V) + antheraxanthin (A) + zeaxanthin (Z)], lutein, and β-carotene increased curvilinearly as active Fe decreased, and neoxanthin (Neo) increased at the lowest Fe level. On a leaf area basis, as active Fe decreased, V+A+Z and β-carotene decreased curvilinearly, and lutein and Neo decreased linearly. At noon, conversion of V to A and Z increased as active Fe decreased. On a Chl basis, activities of antioxidant enzymes superoxide dismutase (SOD), monodehydroascorbate reductase (MDAR), and dehydroascorbate reductase (DHAR) increased curvilinearly, and glutathione reductase (GR) activity increased linearly as active Fe levels declined. Ascorbate peroxidase (APX) and catalase (CAT), on a Chl basis, were relatively constant. On a leaf area basis, a decrease in active Fe increased SOD and MDAR activity, whereas APX, CAT, DHAR and GR activity decreased. Antioxidant metabolites ascorbate (AsA), dehydroascorbate (DAsA), reduced glutathione (GSH) and oxidized glutathione (GSSG) also increased in response to Fe limitation when expressed on a Chl basis, whereas on a leaf area basis AsA and DAsA decreased and GSH increased curvilinearly. The GSH:GSSG ratio increased as active Fe declined, whereas the AsA:DAsA ratio did not change. In conclusion, both photoprotective mechanisms, xanthophyll cycle-dependent thermal dissipation and the ascorbate-glutathione antioxidant system, are enhanced in response to Fe deficiency to cope with excess absorbed light. In a low soil pH tolerant species such as V. labruscana, the foliar antioxidant system was upregulated in response to excess absorbed light from Fe deficiency-induced chlorosis, and there was no evidence of an increase in oxidative stress from high rates of applied Fe-EDDHA.

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John D. Williamson, Dianne B. Jennings, Wei-Wen Guo, D. Mason Pharr, and Marilyn Ehrenshaft

The traditional use of polyols as osmotica in plant culture media is based on the assumption that polyols are not taken up or metabolized by cells. In reality, polyols are significant photosynthetic products and efficiently utilized metabolites in a large number of plants. In addition to these metabolic roles, initial interest in polyols focused primarily on their function as osmoprotectants. This was hypothesized to be due to their ability to act as compatible solutes. More recent research, however, indicates much broader roles for polyols in stress responses based on their significant antioxidant capacity. These include protection against salt and photooxidative stress as well as a potential role in plant pathogen interactions.

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Hongmei Du, Zhaolong Wang, and Bingru Huang

Beard, 1992 ; Fry and Huang, 2004 ). Inhibition of photosynthetic activity under heat stress induces oxidative stress by enhancing the production of reactive oxygen species [ROS (such as O 2− , 1 O 2 , H 2 O 2 , OH − )] because of the imbalance between

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D. Mark Hodges and Gene E. Lester

The consumption of netted muskmelons (Cucumis melo L. Reticulatus group) has raised health concerns due to pathogenic bacteria attaching to sites on the netted rind inaccessible to sanitation. The purpose of this study was to compare 1) the enzymic and nonenzymic antioxidant capacity between representative cultivars of netted muskmelon and both green- and orange-fleshed honey dew muskmelons during storage for 17 days and 2) levels of non-nutrient phytochemicals between these genotypes in consideration of ultimately substituting netted orange-fleshed with non-netted orange-fleshed muskmelon. Netted muskmelon (`Cruiser'), green-fleshed (`Honey Brew'), and orange-fleshed (`Orange Dew') muskmelons were harvested in Texas at the beginning (21 May) and at the end (11 June) of the production season in 2004. Fruit were analyzed immediately (day 0) or stored simulating retail conditions for 7 or 14 days at 7 °C and 95% ± 2% relative humidity plus 3 days at 21 °C. Both `Orange Dew' and `Honey Brew' non-netted cultivars evinced similar and less lipid peroxidation, and hence postharvest senescence, during the 17-day storage period than the netted muskmelon `Cruiser'. In comparison with `Cruiser', `Orange Dew' generally exhibited higher concentrations of ß-carotene and phenolics and, with few exceptions, higher activities of the antioxidant enzymes ascorbate peroxidase (AsPX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), catalase (CAT), guaiacol peroxidase (POX), and superoxide dismutase (SOD). Higher AsPX and SOD activities in both `Orange Dew' and `Honey Brew' appear to confer a greater resistance to lipid peroxidation in these muskmelon genotypes than to the netted `Cruiser'. `Orange Dew' also appears to be a healthier food choice not only due to its lack of a netted rind which could potentially harbour human illness-related pathogens, but also that it is superior to both `Cruiser' and `Honey Brew' in overall beta-carotene and phenolic levels.

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Sarunya Yimyong, Tatsiana U. Datsenka, Avtar K. Handa, and Kanogwan Seraypheap

Effects of hot water treatment (HWT) on metabolism of mango (Mangifera indica cv. Okrong) fruit during low-temperature storage (LTS) and subsequent room temperature fruit ripening (RTFR) were examined. Mature-green ‘Okrong’ mango fruit were treated by immersing in hot (50 ± 1 °C) or ambient (30 ± 1 °C) water for 10 min, stored either at 8 or 12 °C for 15 days, followed by transfer to room temperature (30 ± 2 °C) for 5 days. Rate of ethylene production was significantly reduced by HWT during LTS and RTFR in all treatments. HWT increased catalase activity, suppressed ascorbate peroxidase activity, and had no effect on glutathione reductase activity during the ripening phase but showed a slight stimulatory effect during LTS. HWT altered RNA transcripts of manganese–superoxide dismutase, pectate lyase, β-galactosidase, and β-1,3-glucanase, which exhibited increases during LTS. RTFR of LTS fruit caused reduction in transcript levels of these genes, except pectate lyase. Total protein patterns were altered by all treatments during LTS and RTFR, but HWT arrested loss of several proteins during RTFR. Taken together, results provide strong evidence that HWT increases the storage period of mango by extending fruit shelf life through the regulation of a myriad of metabolic parameters, including patterns of antioxidant and cell wall hydrolase genes and protein expression during storage at low and ambient temperatures.

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Priscila L. Gratão, Carolina C. Monteiro, Lázaro E.P. Peres, and Ricardo Antunes Azevedo

physiological mechanisms related to the control of reactive oxygen species (ROS) normally produced in cells if oxidative stress conditions are to be studied. Given the vast complexity of antioxidative systems ( Garcia et al., 2006 ; Wahid and Ghani, 2008 ), we

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Ting-Ting Li, Zhi-Rong Li, Kang-Di Hu, Lan-Ying Hu, Xiao-Yan Chen, Yan-Hong Li, Ying Yang, Feng Yang, and Hua Zhang

, SAM to 1-aminocyclopropane-1-carboxylic acid (ACC) catalyzed by ACC synthase (ACS), and ACC to ethylene catalyzed by ACC oxidase (ACO) ( Adams and Yang, 1979 ; Mworia et al., 2010 ). Besides ethylene, ROS and oxidative stress are also responsible for