Correction: Ghani et al. Multi-Dimensional Antioxidant Screening of Selected Australian Native Plants and Putative Annotation of Active Compounds. Molecules 2023, 28, 3106

In the original publication [...].

Multi-Dimensional Antioxidant Screening of Selected Australian Native Plants and Putative Annotation of Active Compounds

Acacia implexa, Eucalyptus rossii and Exocarpos cupressiformis are native plants of Australia, which were used by the First Peoples for medicinal purposes. In this study, 70% aqueous ethanol crude extracts were prepared from A. implexa bark and leaves, E. rossii leaves and E. cupressiformis leaves, and partitioned via sequential extraction with n-hexane, dichloromethane (DCM), ethyl acetate and ethanol. The crude extracts and fractions were screened for antioxidant activity using a novel, high-throughput lipid-based antioxidant assay, as well as the aqueous ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) assay and the Folin-Ciocalteu test for total phenols. In the lipid-based assay, non-polar n-hexane and DCM fractions showed higher antioxidant activity against the formation of peroxides and thiobarbituric acid reactive substances (TBARS) than the other fractions, whereas the non-polar fractions were not effective in aqueous assays. This illustrates that the high potential of the lipid-soluble n-hexane and DCM fractions as antioxidants would have been missed if only aqueous-based assays were used. In addition, the potent antioxidant compounds were putatively annotated using liquid chromatography quadrupole time-of-flight mass spectrometry (LC-qTOF-MS). Gallic acid, (+)-catechin, (-)-epicatechin and tannins were found in most crude extracts.

A whole canopy gas exchange system for the targeted manipulation of grapevine source-sink relations using sub-ambient CO2

BACKGROUND

Elucidating the effect of source-sink relations on berry composition is of interest for wine grape production as it represents a mechanistic link between yield, photosynthetic capacity and wine quality. However, the specific effects of carbohydrate supply on berry composition are difficult to study in isolation as leaf area or crop adjustments can also change fruit exposure, or lead to compensatory growth or photosynthetic responses. A new experimental system was therefore devised to slow berry sugar accumulation without changing canopy structure or yield. This consisted of six transparent 1.2 m3 chambers to enclose large pot-grown grapevines, and large soda-lime filled scrubbers that reduced carbon dioxide (CO2) concentration of day-time supply air by approximately 200 ppm below ambient.

RESULTS

In the first full scale test of the system, the chambers were installed on mature Shiraz grapevines for 14 days from the onset of berry sugar accumulation. Three chambers were run at sub-ambient CO2 for 10 days before returning to ambient. Canopy gas exchange, and juice hexose concentrations were determined. Net CO2 exchange was reduced from 65.2 to 30 g vine- 1 day- 1, or 54%, by the sub-ambient treatment. At the end of the 10 day period, total sugar concentration was reduced from 95 to 77 g L- 1 from an average starting value of 23 g L- 1, representing a 25% reduction. Scaling to a per vine basis, it was estimated that 223 g of berry sugars accumulated under ambient supply compared to 166 g under sub-ambient, an amount equivalent to 50 and 72% of total C assimilated.

CONCLUSIONS

Through supply of sub-ambient CO2 using whole canopy gas exchange chambers system, an effective method was developed for reducing photosynthesis and slowing the rate of berry sugar accumulation without modifying yield or leaf area. While in this case developed for further investigations of grape and wine composition, the system has broader applications for the manipulation and of study of grapevine source-sink relations.

Grape Berry Flavonoid Responses to High Bunch Temperatures Post Véraison: Effect of Intensity and Duration of Exposure

Climate models predict an increase in the frequency and duration of heatwaves with an increase in intensity already strongly evident worldwide. The aim of this work was to evaluate the effect of two heatwave-related parameters (intensity and duration) during berry ripening and identify a threshold for berry survival and flavonoid accumulation. A Doehlert experimental design was used to test three temperature intensities (maxima of 35, 46, and 54 °C) and five durations (3 to 39 h), with treatments applied at the bunch level shortly after véraison. Berry skin and seeds were analysed by liquid chromatography-triple quadrupole-mass spectrometry (LC-QqQ-MS) for flavonoids (flavonols, anthocyanins, free flavan-3-ols, and tannins). Berries exposed to 46 °C showed little difference compared to 35 °C. However, berries reaching temperatures around 54 °C were completely desiccated, and all flavonoids were significantly decreased except for skin flavonols on a per berry basis and seed tannins in most cases. Some compounds, such as dihydroxylated flavonoids and galloylated flavan-3-ols (free and polymerised), were in higher proportion in damaged berries suggesting they were less degraded or more synthesised upon heating. Overall, irreversible berry damages and substantial compositional changes were observed and the berry survival threshold was estimated at around 50-53 °C for mid-ripe Shiraz berries, regardless of the duration of exposure.

Expression Patterns of Genes Encoding Sugar and Potassium Transport Proteins Are Simultaneously Upregulated or Downregulated When Carbon and Potassium Availability Is Modified in Shiraz (Vitis vinifera L.) Berries

A link between the accumulation of sugar and potassium has previously been described for ripening grape berries. The functional basis of this link has, as of yet, not been elucidated but could potentially be associated with the integral role that potassium has in phloem transport. An experiment was conducted on Shiraz grapevines in a controlled environment. The accumulation of berry sugar was curtailed by reducing the leaf photoassimilation rate, and the availability of potassium was increased through soil fertilization. The study characterizes the relationship between the accumulation of sugar and potassium into the grape berry and describes how their accumulation patterns are related to the expression patterns of their transporter proteins. A strong connection was observed between the accumulation of sugar and potassium in the grape berry pericarp, irrespective of the treatment. The relative expression of proteins associated with sugar and potassium transport across the tonoplast and plasma membrane was closely correlated, suggesting transcriptional coregulation leading to the simultaneous translocation and storage of potassium and sugar in the grape berry cell.

Development of a Method Suitable for High-Throughput Screening to Measure Antioxidant Activity in a Linoleic Acid Emulsion

An improved system for measuring antioxidant activity via thiobarbituric acid reactive substances and ferric thiocyanate assays is reported, on the basis of oxidation of a linoleic acid (LA) emulsion. Oxidation times were reduced from 20 h to 5 h by increasing the reaction temperature from 37 °C to 50 °C and with an acceptable precision of <10% coefficient of variation (CV). Antioxidants varying in polarity and chemical class—250 µM Trolox, quercetin, ascorbic acid and gallic acid—were used for method optimisation. Further reductions in reaction time were investigated through the addition of catalysts, oxygen initiators or increasing temperature to 60 °C; however, antioxidant activity varied from that established at 37 °C and 20 h reaction time—the method validation conditions. Further validation of the method was achieved with catechin, epicatechin, caffeic acid and α-tocopherol, with results at 50 °C and 5 h comparable to those at 37 °C and 20 h. The improved assay has the potential to rapidly screen antioxidants of various polarities, thus making it useful in studies where large numbers of plant extracts require testing. Furthermore, as this assay involves protection of a lipid, the assay is likely to provide complementary information to well-established tests, such as the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay.

Grape berry flavonoids: a review of their biochemical responses to high and extreme high temperatures

Climate change scenarios predict an increase in average temperatures and in the frequency, intensity, and length of extreme temperature events in many wine regions around the world. In already warm and hot regions, such changes may compromise grape growing and the production of high quality wine as high temperature has been found to affect berry composition critically. Most recent studies focusing on the sole effect of temperature, separated from light and water, on grape berry composition found that high temperature affects a wide range of metabolites, and in particular flavonoids-key compounds for berry and wine quality. A decrease in total anthocyanins is reported in most cases, and appears to be directly associated with high temperature. Changes in anthocyanin composition, and flavonol and proanthocyanidin responses are however less consistent, and reflect the complexity of the underlying biosynthetic pathways and diversity of experimental treatments that have been used in these studies. This review examines the impact of high temperature on the biosynthesis, accumulation, and degradation of flavonoids, and attempts to reconcile the diversity of responses in relation to the latest understanding of flavonoid chemistry and molecular regulation.

Vitis vinifera berry metabolic composition during maturation: Implications of defoliation

Leaves are an important contributor toward berry sugar and nitrogen (N) accumulation, and leaf area, therefore, affects fruit composition during grapevine (Vitis vinifera) berry ripening. The aim of this study was to investigate the impact of leaf presence on key berry quality attributes in conjunction with the accumulation of primary berry metabolites. Shortly after the start of véraison (berry ripening), potted grapevines were defoliated (total defoliation and 25% of the control), and the accumulation of berry soluble solids, N and anthocyanins were compared to that of a full leaf area control. An untargeted approach was undertaken to measure the content in primary metabolites by gas chromatography/mass spectrometry. Partial and full defoliation resulted in reduced berry sugar and anthocyanin accumulation, while total berry N content was unaffected. The juice yeast assimilable N (YAN), however, increased upon partial and full defoliation. Remobilized carbohydrate reserves allowed accumulation of the major berry sugars during the absence of leaf photoassimilation. Berry anthocyanin biosynthesis was strongly inhibited by defoliation, which could relate to the carbon (C) source limitation and/or increased bunch exposure. Arginine accumulation, likely resulting from reserve translocation, contributed to increased YAN upon defoliation. Furthermore, assessing the implications on various products of the shikimate pathway suggests the C flux through this pathway to be largely affected by leaf source limitation during fruit maturation. This study provides a novel investigation of impacts of leaf C and N source presence during berry maturation, on the development of key berry quality parameters as underlined by alterations in primary metabolism.

Vitis vinifera root and leaf metabolic composition during fruit maturation: implications of defoliation

Grapevine (Vitis vinifera) roots and leaves represent major carbohydrate and nitrogen (N) sources, either as recent assimilates, or mobilized from labile or storage pools. This study examined the response of root and leaf primary metabolism following defoliation treatments applied to fruiting vines during ripening. The objective was to link alterations in root and leaf metabolism to carbohydrate and N source functioning under conditions of increased fruit sink demand. Potted grapevine leaf area was adjusted near the start of véraison to 25 primary leaves per vine compared to 100 leaves for the control. An additional group of vines were completely defoliated. Fruit sugar and N content development was assessed, and root and leaf starch and N concentrations determined. An untargeted GC/MS approach was undertaken to evaluate root and leaf primary metabolite concentrations. Partial and full defoliation increased root carbohydrate source contribution towards berry sugar accumulation, evident through starch remobilization. Furthermore, root myo-inositol metabolism played a distinct role during carbohydrate remobilization. Full defoliation induced shikimate pathway derived aromatic amino acid accumulation in roots, while arginine accumulated after full and partial defoliation. Likewise, various leaf amino acids accumulated after partial defoliation. These results suggest elevated root and leaf amino N source activity when leaf N availability is restricted during fruit ripening. Overall, this study provides novel information regarding the impact of leaf source restriction, on metabolic compositions of major carbohydrate and N sources during berry maturation. These results enhance the understanding of source organ carbon and N metabolism during fruit maturation.

Impact of Fluorescent Lighting on Oxidation of Model Wine Solutions Containing Organic Acids and Iron

Previous studies have provided evidence that light exposure can increase oxygen consumption in wine and that the photodegradation of iron(III) tartrate could contribute to this process. In the present study, model wine solutions containing iron(III) and various organic acids, either alone or combined, were stored in sealed clear glass wine bottles and exposed to light from fluorescent lamps. Dissolved oxygen was monitored, and afterward the organic acid degradation products were determined and the capacity of the solutions to bind sulfur dioxide, the main wine preservative, was assessed. In the dark controls, little or no dissolved oxygen was consumed and the organic acids were stable. In the irradiated solutions, dissolved oxygen was consumed at a rate that was dependent on the specific organic acid present, and the latter were oxidized to various carbonyl compounds. For the solutions containing tartaric acid, malic acid, and/or citric acid, irradiation increased their sulfur dioxide-binding capacity.

Light-induced changes in bottled white wine and underlying photochemical mechanisms

Bottled white wine may be exposed to UV-visible light for considerable periods of time before it is consumed. Light exposure may induce an off-flavor known as "sunlight" flavor, bleach the color of the wine, and/or increase browning and deplete sulfur dioxide. The changes that occur in bottled white wine exposed to light depend on the wine composition, the irradiation conditions, and the light exposure time. The light-induced changes in the aroma, volatile composition, color, and concentrations of oxygen and sulfur dioxide in bottled white wine are reviewed. In addition, the photochemical reactions thought to have a role in these changes are described. These include the riboflavin-sensitized oxidation of methionine, resulting in the formation of methanethiol and dimethyl disulfide, and the photodegradation of iron(III) tartrate, which gives rise to glyoxylic acid, an aldehyde known to react with flavan-3-ols to form yellow xanthylium cation pigments.

A novel glutathione-hydroxycinnamic acid product generated in oxidative wine conditions

This study characterizes a novel glutathione-substituted dihydroxyphenyl compound formed during the oxidation of white wine and model wine solutions, which may contribute to the synergistic role of glutathione and hydroxycinnamic acids in delaying oxidative coloration. The critical components for the formation of the compound were found to be hydroxycinnamic acids and glutathione, while ascorbic acid enabled the product to accumulate to higher concentrations. The presence of the wine components important in other wine oxidation mechanisms, (+)-catechin, ethanol and/or tartaric acid, was not essential for the formation of this new compound. Via LC-MS/MS, HR-MS and (1)H NMR (1D and 2D NMR) analyses, the major isomer of the compound formed from glutathione and caffeic acid was found to be 4-[(E)-2'-(S)-glutathionyl ethenyl]-catechol (GEC). Equivalent products were also confirmed via LC-MS/MS for other hydroxycinnamic acids (i.e., ferulic and coumaric acids). Only trace amounts of GEC were formed with the quinic ester of caffeic acid (i.e., chlorogenic acid), and no equivalent product was found for cinnamic acid. GEC was detected in a variety of white wines supplemented with glutathione and caffeic acid. A radical mechanism for the formation of the styrene-glutathione derivatives is proposed.

Ascorbic acid: a review of its chemistry and reactivity in relation to a wine environment

Extensive reviews of research are available on the use of ascorbic acid, and its consequent degradation pathways, in physiological conditions or food matrices. However, very little information can be found for wine-related systems. This review highlights the relevant chemistry and reactivity of ascorbic acid with a focus on its behavior and potential behavior in a wine environment. The review describes the use of ascorbic acid as a complementary antioxidant preservative to sulfur dioxide along with the metal-catalyzed and radical-dependent manner by which it achieves this role. The relevant degradation products of ascorbic acid in aerobic and anaerobic conditions are presented as well as the interaction of these degradation products with sulfur dioxide and other wine-relevant sulfur compounds. Limitations in existing knowledge, especially regarding the crossover between the antioxidant and pro-oxidant roles of ascorbic acid, are identified.