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Leslie A. Weston and Verónica M. Gonzalez

Sorgoleone, the oxidized quinone form of a hydrophobic p-benzoquinone was first isolated from Sorghum root exudates. Sorgoleone is a potent inhibitor of growth in several annual weed species and causes tissue bleaching at concentrations of <25 μ M. These investigations were designed to determine if soreoleone's allelopathic activity was related to an inhibition of photosynthetic electron transport. The effect of sorgoleone versus DCMU (diuron) on inhibition of O2 evolution by broken wheat thylakoids, and in oxygenevolving PSII membranes containing QA and QB primary and secondary electron acceptors in PSII was determined. Sorgoleone was a potent inhibitor of O2 evolution in this system with ∼ 0.04 and 0.78 μ M concentrations required for 50 and 100% inhibition as compared to -0.11 and 2.0 μ M DCMU, respectively. Sorgoleone caused no significant inhibition of PSI mediated photooxidation of ascorbate/dichlorophenolindophenol, establishing that the locus of inhibition by sorgoleone was within the PSII complex. The effect of trypsin treatment of chloroplasts and PSII membranes on sensitivity to inhibition by DCMU and sorgoleone was examined. The comparison of DCMU and sorgoleone upon the formation and decay of flash-induced chlorophyll a variable fluorescence indicates that sorgoleone specifically inhibited the oxidation of QA by QB.

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Sorkel Kadir and Robert Wample

Evaluation of thermostability of photosynthetic apparatus of intact leaves and isolated thylakoids of five cultivars of wine grapes (Vitis vinifera) was conducted. Four- week- old plants of Semillon, Chenin Blnac, Pinot Noir, Chardonnay, and White Riesling, were placed into a control environment chamber held at 20/15° 30/25°, and 40/35 °C day/night temperature for 14 days. Induced (F0), variable (Fv), and maximum fluorescence (Fm) and the quantum yield of net photosynthesis (Fv/Fm) were measured after 1-14 days exposure. All fluorescence parameters were not affected by 20/15° and 30/25°C. However, high temperature (40/35°C) increased F0 and decreased Fm, Fv, and Fv/Fm. These changes were severe in Semillon and Chenin Blanc, moderate in Chardonnay and White Riesling and scarce in Pinot Noir. Average high temperature data that are experienced in Yakima Valley area will be presented. Isolated thylakoid membranes from the cultivars were heated at 20-40°C. and uncoupled electron transport was determined. Thylakoid stability to heating varied similarly to whole-plant response to high temperature.

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Joo Hyun Lee, Yong-Beom Lee, and Kyu Sook Lee

Wasabi japonica plantlets were acclimatized in a hydroponic system to determine effective procedures. The plantlets were cultured on solid Murashige-Skoog medium with 3% sucrose. Shoots that formed roots were transplanted into hydroponic systems: 1) acclimatization in ebb-and-flow (EBB) for subirrigation (medium: granulated rockwool and coir); and 2) acclimatization in deep flow technique (DFT). The plantlets were acclimatized for 5 weeks under two irradiance treatments, 50 and 300 mmol·m-2·s-1. Photosynthetic capacity in high PPF was higher than that in low PPF during acclimatization. Electron transport rate from PS II (ETR) and biomass production increased significantly with increased light availability. The fresh weight, dry weight, and leaf area of plantlets in high PPF were higher than those in low PPF. In particular, the dry weight and ETR of the plantlets grown in high PPF increased more than twice as much as those in low PPF. At 50 mmol·m-2·s-1 PPF, growth indexes, such as number of leaves, leaf length, leaf width, leaf area, fresh weight, and dry weight, were higher in EBB (granulated rockwool) > EBB (coir culture) > DFT. At 300 mmol·m-2·s-1 PPF, those indexes were higher in DFT > EBB (granulated rockwool) > EBB (coir). The Wasabi japonica plantlets acclimatized in a hydroponic system also had a superior performance when they were transferred to the field.

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S. Kalantari, G. Samson, J. Makhlouf, and J. Arul

The application of ultraviolet light on fruit and vegetables is a promising new method to control storage diseases and to delay the onset of senescence. In this investigation, we studied the effects of hormic dose (1,4 Merg•cm-2) of UV-radiation on the ripening of tomato pericarp discs by measuring different characteristics of ripening and senescence during storage. We observed that UV-treatment induced significant delays of the red color development, chlorophyll degradation, and lycopene production compared to control discs. UV-treatment also retarded the decline of the chlorophyll-a fluorescence ratios Fv: Fm and *F : Fm′, two characteristics related, respectively, to the maximum and operational quantum yield of photosystem II electron transport. Furthermore, the climacteric ethylene peak was delayed in the treated discs. However, UV-treatment did not alter textural changes, and the respiratory climacteric peaks were observed concomitantly for both treated and untreated tomato discs. However, the respiratory rate was consistently higher in treated discs. These results indicate that UV irradiation of tomato pericarp discs delays some processes of ripening associated with chloroplast to chromoplast transition whereas other ripening processes seem unaffected.

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Zi Wei, Peter Jeranyama, Fan Zhang, Carolyn DeMoranville, and Harvey J.M. Hou

Yellow vine symptoms are often observed in cranberry bogs. To explore the mechanisms of the formation of yellow vine syndrome in cranberry leaves, the shade effect on the chlorophyll (Chl) content and photosynthetic activities in cranberry bogs were investigated by spectrometric, high-performance liquid chromatography (HPLC), and in vivo Chl fluorescence kinetics. Spectrometric and HPLC analyses revealed that the yellow vine leaves were associated with a 11% ± 5% and 14% ± 5% increase in Chl a/Chl b ratio after shading, respectively. The Chl a/Chl b ratio was the same in both types of leaves, suggesting the photosystem (PS) II organization remains invariant. The rise in chlorophyll content suggested that the number of reaction sites on PS II is increased in the shaded yellow vine leaves. The results of in vivo chlorophyll fluorescence analysis also indicated that the electron transport chain in the PS II is enhanced and that the size of the quinone pool is increased. In addition, the overall photosynthesis index is drastically improved by shading. These three lines of evidence imply that the shading of cranberry plants appeared to reduce the syndrome by improving the photosynthetic activity and increasing the chlorophyll content. The techniques presented here may be valuable for characterizing variations of plants by stress or disease.

Open access

S.Y. Wang, G.L. Steffens, and M. Faust

Abstract

A cyanide-resistant alternative pathway was found to exist in root tissue of apple (Malus domestica Borkh.). In the absence of potassium cyanide (KCN), an inhibitor of cytochrome electron transport, the alternative pathway did not contribute to overall root respiration. However, in the presence of KCN or carbonyl cyanide m-chlorophenyl hydrazone (CCCP), an uncoupler, active participation of the alternative pathway was detected. Inhibition of O2 uptake by salicylhydroxamic acid (SHAM) was observed in the presence of antimycin A (AA) or sodium azide (NaN3), but to a lesser degree than when KCN was present. The degree of inhibition by SHAM was greatest in the presence of KCN, followed by AA and then NaN3. The antioxidant n-Propyl gallate (PG) was found to be an effective inhibitor of the alternative pathway. The site of inhibition in apple root tissue by PG is very similar to that of SHAM. Sodium benzoate, another antioxidant and free radical scavenger, and tetraethylthiuram disulfide (disulfiram), a copper chelator, did not inhibit the alternative pathway in apple root tissue.

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Lailiang Cheng, Leslie H. Fuchigami, and Patrick J. Breen

Bench-grafted Fuji/M26 apple (Malus domestica Borkh) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 days. CO2 assimilation and actual photosystem II (PSII) efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O2 (2%) and saturated CO2 (1300 ppm) conditions. A single curvilinear relationship was found between true quantum yield for CO2 assimilation and actual PSII efficiency for leaves with a wide range of leaf N content. The relationship was linear up to a quantum yield of approximately 0.05 mol CO2/mol quanta, then became curvilinear with a further rise in quantum yield in response to decreasing PFD. This relationship was subsequently used as a calibration curve to assess the rate of linear electron transport associated with rubisco and partitioning of electron flow between CO2 assimilation and photorespiration in different N leaves in response to intercellular CO2 concentration (Ci) under normal O2 conditions. Both the rate of linear electron flow, and the rate to CO2 or O2 increased with increasing leaf N at any given Ci, but the percentage of linear electron flow to CO2 assimilation remained the same regardless of leaf N content. As Ci increased, the percentage of linear electron flow to CO2 assimilation increased. In conclusion, the relationship between actual PSII efficiency and quantum yield for CO2 assimilation and the partitioning of electron flow between CO2 assimilation and photorespiration are not affected by N content in apple leaves.

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A.C. Purvis, J.W. Gegogeine, and R.L. Shewfelt

Reactive O2 species produced when electron transport is disrupted have been implicated in several environmental stress-induced disorders. Superoxide ( \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}_{2}^{-}\) \end{document} ) is produced at two or more sites in mitochondria isolated from bell pepper fruit supplied with succinate and NADH. SOD and KCN completely inhibited \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}_{2}^{-}\) \end{document} production with both substrates. Antimycin A inhibited \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}_{2}^{-}\) \end{document} production with succinate, but not with NADH. Insensitivity of O2 uptake to KCN increased in mitochondria isolated from bell peppers stored at 2C and their \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}_{2}^{-}\) \end{document} production increased with NADH as substrate, but decreased with succinate. Disrupting the mitochondrial membranes enhanced \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}_{2}^{-}\) \end{document} production with NADH and reduced production with succinate. Greater \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}_{2}^{-}\) \end{document} production with NADH may result from the inability to transfer electrons from NADH through the alternative path. The KCN-insensitive alternative path in some plant tissues appears to reduce the potential production of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}_{2}^{-}\) \end{document} .

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A.C. Purvis

It has long been observed that chilling injury of warm-season fruit and vegetables during postharvest storage as well as during early seedling growth can be mitigated by maintaining high relative humidities during the exposure to low temperatures. A strong correlation between transpiration rates and chilling injury was observed among the fruit of several PI lines of greenhouse-type and field-type Cucumis sativus L. differing in their susceptibility to chilling injury. Transpiration rates and chilling injury of the F1s from crosses between resistant and susceptible lines were intermediate. Immature fruit lost moisture at faster rates and chill injured more severely than mature fruit of the same genotype. Coatings, applied as postharvest treatments to the fruit either reduced or increased chilling injury depending on the concentration applied and whether or not they retarded or enhanced moisture loss during low temperature storage. Fruit coated with surfactant-based waxes lost more moisture and developed more chilling injury than uncoated fruit or fruit coated with carnauba wax or polyethylene emulsions. The causal relationship between transpiration at low temperatures and chilling injury is not known, primarily because the precise mechanism of chilling injury has not been unequivocally delineated. The manifestation of chilling injury, however, occurs concomitantly with an increase in respiratory rate. We have postulated that chilling injury is caused by active oxygen species generated when the mitochondrial electron transport chain is impaired. In studies with germinating seed, desiccation injury was associated with free radicals generated by mitochondria. Thus, desiccation at low temperatures may intensify respiratory activity resulting in the generation of oxygen free radicals and extensive peroxidative damage to cellular membranes and enzymes.

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Krishna S. Nemali and Marc W. van Iersel

Optimal substrate volumetric water content (θ) and drought tolerance of impatiens, petunia, salvia, and vinca were investigated by growing plants under four constant levels of θ (0.09, 0.15, 0.22, and 0.32 m3·m-3). Gas exchange, quantum efficiency (ΦPSII), electron transport rate (ETR), non-photochemical quenching (NPQ), and leaf water potential (ϒ) were measured for all species, and response of photosynthesis (Pn) to internal CO2 concentration (Ci) was studied in petunia and salvia. Leaf photosynthesis (Pmax) was highest at a θ of 0.22 m3·m-3 for all species and did not differ between a θ of 0.15 and 0.22 m3·m-3 for vinca and petunia. The Pn-Ci response curves for petunia were almost identical at a θ of 0.22 and 0.15 m3·m-3. Regardless of species, ETR and ΦPSII were highest and NPQ was lowest at a θ of 0.22 m3·m-3. Based on these results, a θ of 0.22 m3·m-3 for salvia and impatiens and a slightly lower θ of 0.15 m3·m-3 for vinca and petunia, is optimal. Mean osmotic potential in all treatments was lower in vinca and salvia and resulted in higher turgor potential in these species than other species. Analysis of Pn-Ci response curves indicated that Pn at a θ of 0.09 m3·m-3 was limited by both gas phase (stomatal and boundary layer) and non-gas phase (mesophyll) resistance to CO2 transfer in salvia. At the lowest θ level, Pn in petunia was only limited by gas phase resistance, indicating that absence of mesophyll resistance during drought may play a role in the drought tolerance of petunia.