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Lailiang Cheng and Fengwang Ma

Xanthophyll cycle conversion and the antioxidant system in the peel of apple fruit (Malus ×domestica Borkh. `Liberty') were monitored in the field over a diurnal course at about 3 months after full bloom. Compared with leaves, sun-exposed peel of apple fruit had much lower photosystem II operating efficiency at any given photon flux density (PFD) and a larger xanthophyll cycle pool size on a chlorophyll basis. Zeaxanthin (Z) level increased with rising PFD in the morning, reached the highest level during midday, and then decreased with falling PFD for the rest of the day. At noon, Z accounted for >90% of the xanthophyll cycle pool in the fruit peel compared with only 53% in leaves. Efficiency of excitation transfer to PSII reaction centers (F v′/F m′) was negatively related to the level of Z in fruit peel and leaves throughout the day. In fruit peel, the antioxidant enzymes in the ascorbate-glutathione cycle, ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) showed a diurnal pattern similar to that of incident PFD. In contrast, the activities of APX and GR in leaves did not change significantly during the day although activities of both MDAR and DHAR were higher in the afternoon than in the morning. In both fruit peel and leaves, superoxide dismutase activity did not change significantly during the day; catalase activity showed a diurnal pattern opposite to that of PFD with much lower activity in fruit peel than in leaves. The total ascorbate pool was much smaller in fruit peel than in leaves on an area basis, but the ratio of reduced ascorbate to oxidized ascorbate reached a maximum in the early afternoon in both fruit peel and leaves. The total glutathione pool, reduced glutathione and the ratio of reduced glutathione to oxidized glutathione in both fruit peel and leaves also peaked in the early afternoon. We conclude that the antioxidant system as well as the xanthophyll cycle responds to changing PFD over the course of a day to protect fruit peel from photooxidative damage.

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Mingjun Li, Xuesen Chen, Pingping Wang, and Fengwang Ma

-dependent monodehydroascorbate reductase [MDHAR (EC] and dehydroascorbate reductase [DHAR (EC], respectively ( Noctor and Foyer, 1998 ). The importance of MDHAR and DHAR in controlling AsA levels has been demonstrated in transgenic plants by expressing enzymes

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Shiow Y. Wang, Kim S. Lewers, Linda Bowman, and Min Ding

main antioxidant enzymes in strawberry fruit are superoxide dismutase (SOD), guaiacol peroxidase (G-POD), glutathione peroxidase (GSH-POD), ascorbate peroxidase (AsA-POD), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR), and

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Lixin Xu, Liebao Han, and Bingru Huang

enzymes such as superoxide dismutase, catalase, peroxidases, monodehydroascorbate reductase (MR), glutathione reductase (GR), dehydroascorbate reductase (DR), and other nonenzymatic constituents such as α-tocopherol, ascorbate, and reduced glutathione; the

Open access

Zhou Li, Yan Peng, and Bingru Huang

) monodehydroascorbate reductase ( MDHAR ), ( E ) dehydroascorbate reductase ( DHAR ), and ( F ) glutathione reductase ( GR ) in creeping bentgrass under well-watered and drought condition for 20 d. Vertical bars indicate ± se (n = 4). Different letters above columns

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Chien Wang, Korakot Chanjirakul, Shiow Wang, and Jingtair Siriphanich

The effect of naturally occurring volatile compounds on decay and antioxidant activities in fresh-cut papayas (Carica papaya L.) was studied. Exposure to methyl jasmonate (MJ), methyl salicylate (MS) or allyl isothiocyanate (AITC) substantially delayed the onset and reduced the severity of decay during and after storage at 5 °C. Treatment with tea tree oil (TTO) or ethanol (ETOH) was also effective in retarding decay, but to a lesser extent. No beneficial effect was obtained with the use of vinegar vapor. MJ and MS increased oxygen radical absorbance capacity and elevated the activities of several antioxidant enzymes, including glutathione reductase, glutathione peroxidase, guaiacol peroxidase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and superoxide dismutase. The nonenzyme components in the ascorbate-glutathione cycle were also increased by MJ and MS treatments, including ascorbate and glutathione. It is possible that MJ and MS treatments enhanced the antioxidant system and increased the resistance of tissue to decay. However, while AITC also suppressed the development of decay in papaya slices, it had little effect on antioxidant levels and antioxidant enzyme activities. Apparently, AITC exerted its effect through different mechanisms. Studies are in progress to determine if AITC inhibits decay directly via its antimicrobial properties.

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Brandon Smith* and Lailiang Cheng

One-year-old `Concord' grapevines (Vitis labrusca L.) were fertigated twice weekly for 11 weeks with a complete nutrient solution containing 1, 10, 20, 50 or 100 μmol iron (Fe) from ferric ethylenediamine di (o-hydroxyphenylacetic) acid (Fe-EDDHA). Leaf total Fe content did not increase in response to Fe supply, however both “active” Fe (extracted with 2, 2'-dipyridyl) and chlorophyll (Chl) content increased as applied Fe increased. At the lowest active Fe level, leaf absorptance and maximum PSII efficiency (Fv/Fm) were slightly decreased, and non-photochemical quenching was significantly greater. PSII quantum efficiency decreased curvilinearly as active Fe content decreased. On a Chl basis, the xanthophyll cycle pool size, lutein, and beta-carotene increased curvilinearly as active Fe decreased, and neoxanthin increased at the lowest Fe level. Activities of antioxidant enzymes superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase followed a similar trend and increased under Fe deficiency, when expressed on a Chl basis. Antioxidant metabolites also increased in response to Fe limitation. On a Chl basis, ascorbate (AsA), dehydroascorbate (DAsA), reduced glutathione (GSH) and oxidized glutathione (GSSG) content was greater at the lowest active Fe levels. We did not find a difference in the ratio of AsA to DAsA or GSH to GSSG. In conclusion, both photoprotective mechanisms, xanthophyll cyle-dependent thermal dissipation and the ascorbate-glutatione antioxidant system, are enhanced in response to iron deficiency to cope with excess absorbed light.

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Rui Zhou*, Lailiang Cheng, and Abhaya Dandekar

Sorbitol is the primary photosynthetic end product in the leaves of many tree fruit species in the Rosaceae family, but its physiological role remains unclear. In this study, we determined the effect of decreased sorbitol synthesis on the antioxidant system that scavenges reactive oxygen species (ROS) in apple leaves. Sorbitol synthesis was decreased in apple leaves by antisense inhibition of aldose-6-phosphate reductase activity. Dehydroascorbate reductase (DHAR), glutathione reductase, and catalase (CAT) activities increased in the leaves of the transgenic plants with decreased sorbitol synthesis, whereas superoxide dismutase, ascorbate peroxidase, NADH dependent and NADPH dependent monodehydroascorbate reductase activity did not show significant changes. Ascorbate and glutathione concentrations were higher in leaves of the transgenic plants compared with the control. The effect of decreased sorbitol synthesis on the antioxidant enzyme activity was dependent on leaf developmental stages. Larger changes in the enzyme activities of CAT, DHAR, and GR were observed in the old leaves than in the young leaves. These results suggest that sorbitol may play a role in ROS scavenging in apple leaves.

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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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Feng wang Ma and Lailiang Cheng*

About 80 days after full bloom, well-exposed fruit on the south part of the canopy of mature Liberty/M.9 apple trees were randomly assigned to one of the following two treatments. Some fruit were turned about 180 degrees to expose the original shaded side to full sun whereas the rest served as untreated controls. On day 0, 1, 2, 4, 7, and 10 after treatment, fruit peel samples were taken from the original shaded side of the treated fruit and both the sun-exposed side and the shaded side of the control fruit at midday to determine photosynthetic pigments and enzymatic and non-enzymatic antioxidants. Maximum photosystem II efficiency of the original shaded side decreased sharply after 1 day exposure to full sun, and then gradually recovered to a similar value of the sun-exposed side of the control fruit by day 10. The shaded side of the control fruit had much lower xanthophyll cycle pool size and conversion and antioxidant enzymes and soluble antioxidants of the ascorbate-glutathione cycle than the sun-exposed side. In response to full sun exposure, xanthophyll cycle pool size of the original shaded side increased, reaching a similar value of the sun-exposed side by day 10. Ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase and total pool size and reduction state of both ascorbate and glutathione of the original shaded side all increased to the corresponding values found in the sun-exposed side of the control fruit over a 10-day period. It is concluded that both xanthophyll cycle and the ascorbate-glutathione cycle in the original shaded side are up-regulated in response to fullsun exposure to minimize photo-oxidative damage and contributes to its re-acclimation to full sun.