A novel procedure to assess the non-enzymatic hydrogen-peroxide antioxidant capacity of metabolites with high UV absorption

Assays assessing non-enzymatic hydrogen peroxide antioxidant capacities are often hampered by the high UV absorption of the sample itself. This is a typical problem in studies using plant extracts with high polyphenol content. Our assay is based on comparing the 405 nm absorption of the product of potassium iodine and hydrogen peroxide in the presence and absence of a putative hydrogen peroxide reactive antioxidant. This method is free of interference with either hydrogen peroxide or antioxidant self-absorption and it is also suitable for high-throughput plate reader applications

Comparative evaluation of total antioxidant capacities of plant polyphenols.

Thirty‐seven samples of naturally occurring phenolic compounds were evaluated using three common in vitro assays for total antioxidant activity (TAC) testing: the Trolox Equivalent Antioxidant Capacity (TEAC), the Ferric Reducing Antioxidant Potential (FRAP) and the 2,2‐ diphenyl‐1‐picrylhydrazyl (DPPH) radical scavenging assay, in addition to the Folin‐Ciocalteu reagent reactivity (FCR). We found that antioxidant hierarchies depended on the choice of assay and applied ANOVA analyses to explore underlying structure‐TAC dependencies. In addition tostatistically confirming the empirically established connection between flavonoid ring‐B catechol and high TEAC or FRAP, new correlations were also found. In flavonoids, (i) hydroxyl groups on ring‐B had a positive effect on all four TAC assays; (ii) the presence of a 3‐hydroxyl group on ring‐C increased TEAC and FRAP, but had no effect on DPPH or FCR; (iii) Phenolic acids lacking a 3‐hydroxyl group had significantly lower FRAP or DPPH than compounds having this structure, while TEAC or FCR were not affected. Results demonstrated that any TAC‐based ranking of phenolic rich samples would very much depend on the choice of assay, and argue for use of more than one technique. As an illustration, we compared results of the above four assays using either grapevine leaf extracts or synthetic mixtures of compounds prepared according to major polyphenols identified in the leaves

Post-harvest UV-B treatment of White Sultanina grapevine berries

The grapevine berries are an important source of phenolic compounds. In this study, White Sultanina berries were treated with UV radiation and followed quantitative change of these secondary metabolites. In the skin of cv. White Sultanina table grapes, a phenolic acid and several mono-glycosilated flavonols were identified by HPLC-DAD. The aim of this work was to alter this profile, to improve antioxidant properties. Resveratrol contents of berry skins have already been successfully increased using UV-C or UV-B radiation (Cantos et al. 2000) in another variety (cv. Napoleon). In this study, we report a UV-B-inducible increase in flavonoid-glycosides observed 2 h after 30 min exposure to 11.5 W/m2 radiation flux (physical dose) from a narrow band source (VL-215M centred at 312 nm, Vilbert Lourmat, France). This was accompanied by higher antioxidant capacities of berry skin extracts, in accordance with the observed strong antioxidant capacities of quercetin-glycosides in vitro (Csepregi et al. 2016, Csepregi and Hideg 2018). Differences between UV-B-treated berries and untreated controls were less pronounced when assayed after a longer storage period following irradiation at 20 ℃ under low fluxes (60 µmol m-2s-1) of photosynthetically active radiation. Berry skin photosynthesis, measured as photochemical yield using imaging PAM (Heinz Walz GmbH, Effeltrich, Germany) temporary increased after the UV-B treatment, then declined, suggesting a possible metabolic source of increased flavonol biosynthesis. These experiments suggest that UV-B irradiation of table grapes can be beneficial in terms of increasing the content of potentially health-promoting flavonol derivatives

Fight against cold: photosynthetic and antioxidant responses of different bell pepper cultivars (Capsicum annuum L.) to cold stress

The special metabolites of bell pepper (Capsicum annuum L.) leaves can protect the plant under possibly damaging circum- stances, such as high light, UV, unfavorable temperatures, or other environmental effects. In this study, we examined the cold stress tolerance of three different Hungarian pepper varieties (Darina, Édesalma, Rekord), focusing on the antioxidant and photosynthetic responses. The plants were developed in growth chambers under optimal temperature conditions (day/night 25 °C/20 °C) until the leaves on the fourth node became fully developed, then half of the plants received a cold treatment (day/ night 15 °C/10 °C). Via a detailed pigment analysis, the PS II chlorophyll fluorescence responses, gas exchange parameters and total antioxidant capacities, leaf acclimation to low temperatures has been characterized. Our results display some of the developing physiological and antioxidant properties, which are among the main factors in monitoring the damaging effects of cold temperatures. Nevertheless, despite their differences, the tested pepper varieties did not show different cold responses

Fotoreaktív nanorészecskék hatása vörösborszőlő-fajták levelének polifenol összetételére és tápelemtartalmára

A különleges tulajdonságokkal rendelkező nanoszerkezetű anyagok hasznosítása az iparban, a mezőgazdaságban és gyógyászatban egyaránt folyamatosan növekszik. A fotoreaktív titán-dioxid (TiO2) nanorészecskék sajátsága, hogy UV-fény hatására reaktív oxigén származékokat (ROS generálnak, melyek jelentős antimikrobiális hatással bírnak, így a növényre kijuttatva potenciális növényvédőszerként funkcionálhatnak. A növényekben keletkező ROS a növények élettani folyamatainak szabályozásában is részt vesznek, ezért a mesterségesen, TiO2 nanorészecskék segítségével keltett ROS a célzott antimikrobiális aktivitás mellett a levelek metabolizmusára is hatással lehet. Ez a hatás lehet negatív, mivel a ROS nagy mennyiségben sejtkárosító, de lehet pozitív is, a természetes úton keletkező ROS hatásához hasonlóan a stresszvédő utakat aktiváló. Habár a TiO2, mint alternatív növényvédőszer a gyakorlatban nagy érdeklődésre tarthat számot, ftotoxicitásáról még csak nagyon kevés információ áll rendelkezésünkre. Ebben a munkában azt tanulmányoztuk, hogy a levelek felszínére juttatott és ott napsugárzásnak kitett TiO 2 nanorészecskék miként befolyásolják a szőlőlevelek teljes fenolos tartalmát(antioxidáns kapacitását) és polifenol proflját. Ezek a paraméterek érzékenyen jelzik a növény oxidatív stresszre adott válaszreakcióit.Nyomon követtük továbbá a levelek makro- és mikroelem összetételének változását is. Szabadföldi kísérletben öt vörösborszőlő-fajtát (Cabernet sauvignon, Cabernet franc, Merlot, Kékfrankos és Kadarka) vizsgáltunk. Nagyhatékonyságú folyadékkromatográfás mérésekkel kimutattuk, hogy szőlőfajtától függően a levelek kaftársav és flavonol-glikozid tartalma a kezelést követően szignifkánsan emelkedett. A levelek teljes fenolos tartalmának növekedése szintén szignifkáns volt. Mindemellett ICP-AES méréseink azt mutatták, hogy TiO2 nanorészecskék a levelek főbb makro- és mikroelemeinek (Ca, Mg, K, B és Mn) koncentrációját ugyancsak növelték

Comparative Evaluation of Total Antioxidant Capacities of Plant Polyphenols

Thirty-seven samples of naturally occurring phenolic compounds were evaluated using three common in vitro assays for total antioxidant activity (TAC) testing: the Trolox Equivalent Antioxidant Capacity (TEAC), the Ferric Reducing Antioxidant Potential (FRAP) and the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay, in addition to the Folin-Ciocalteu reagent reactivity (FCR). We found that antioxidant hierarchies depended on the choice of assay and applied ANOVA analyses to explore underlying structure-TAC dependencies. In addition to statistically confirming the empirically established connection between flavonoid ring-B catechol and high TEAC or FRAP, new correlations were also found. In flavonoids, (i) hydroxyl groups on ring-B had a positive effect on all four TAC assays; (ii) the presence of a 3-hydroxyl group on ring-C increased TEAC and FRAP, but had no effect on DPPH or FCR; (iii) Phenolic acids lacking a 3-hydroxyl group had significantly lower FRAP or DPPH than compounds having this structure, while TEAC or FCR were not affected. Results demonstrated that any TAC-based ranking of phenolic rich samples would very much depend on the choice of assay, and argue for use of more than one technique. As an illustration, we compared results of the above four assays using either grapevine leaf extracts or synthetic mixtures of compounds prepared according to major polyphenols identified in the leaves

How do grapevine leaf phenolic contents respond to daily changes in environmental factors?

Grapevine leaves are rich in phenolic compounds, many of which are – among fulfilling other roles – efficient antioxidants (Csepregi et al. 2016, Csepregi and Hideg 2017). The long-term aim of a recently launched project is to explore the functional plasticity of phenolic compounds in grapevine leaves. Here we present results of a pilot study, registering and analysing hour-by-hour changes in photosynthesis, phenolic profiles and antioxidant capacities of South-facing Pinot noir leaves between 7 am and 7 pm during a clear summer day mid-July at Pécs (N46.071, E18.156). During this 12h period, photosynthetically active radiation (PAR) was 80-2130 µmol/m2/s and UV radiation (UV-A+B) varied between 5-46.1 kJ/m2 as physical dose measured on site. Local data were separated into UV-B (280-315 nm) and UV-A (315-400 nm) regions based on a model calculation (NCAR). Total adaxial flavonoid content (measured as Dualex flavonoid index) varied between 88% and 112% of the daily average and showed strong positive correlations with PAR, UV-A, UV-B, leaf temperature and net CO2 assimilation. Stomatal conductance (gs) was positively correlated with PAR, UV-A and UV-B. However, substomatal CO2 concentrations (Ci) were only correlated with PAR (positively) and not with UV. Hourly changes in leaf antioxidant capacities and phenolic profiles measured with HPLC-DAD complete the analysis

Postharvest UV-A and UV-B treatments may cause a transient decrease in grape berry skin flavonol-glycoside contents and total antioxidant capacities

Controlled postharvest abiotic stresses have already been shown to affect the secondary metabolism of freshproduce.Weappliedbroadbandultraviolet(either312nmcentredUV-Bor355nmcentredUV-A)radiationonharvested berry clusters of the red table grapeEmperor, and measured berry skin antioxidant capacities andphenolicprofiles2,24and48hafterthesetreatments.Therewasnosignificantchangeintheamountofcaftaricacid,thedominantphenolicacidcomponentfeaturinghigherUV-BthanUV-Aabsorbingcapacity.Ontheotherhand, but both types of UV treatment decreased the amounts of the two major quercetin-glycosides 2h afterirradiation. This effect was temporal, and concentration recovered during berry storage at 20°C under lowintensitylight.BerryskinUV-AandUV-Babsorbingcapacitiesshowedthesame,transitionaldecreaseregardlessof the wavelength of the irradiation. In the whole data set, antioxidant capacities and quercetin-glycosidecontent data showed strong and positive correlations. Berry skin peroxidase activity, as visualized with diami-nobenzidine-staining showed a transitional increase after 2h in response to both UV treatments. These datasuggestthattheobservedUV-inducedpostharvestchangesinphenolicmetabolitesarerelatedtotheoxidationofflavonols as peroxidase substrates rather than to direct UV photo-effects

Changes in grapevine leaf phenolic profiles during the day are temperature rather than irradiance driven

Photosynthesis parameters, adaxial flavonoid index, phenolic profiles and antioxidant capacities of south-facing sun exposed grapevine leaves (Vitis vinifera, Pinot Noir cultivar) were measured hourly between 7 a.m. and 7 p.m. on a clear summer day. Changes in these parameters were statistically compared to changes in environmental conditions, including solar irradiance (photosynthetically active and UV radiations), leaf and air temperature, and relative air humidity. Epidermal UV absorbance, characterised by the flavonoid index, and total extractable phenolic contents were correlated to distinct environmental parameters. The former was positively correlated to irradiance and leaf temperature, while the latter was positively correlated to air temperature. HPLC phenolic profiling identified a positive correlation between air temperature and amounts of the dominant flavonol component, quercetin-3-O-glucuronide. The only phenolic component statistically connected to the flavonoid index was quercetin-3-O-glucoside. This correlation was positive and both parameters decreased during the day, although changes in the amount of this flavonol component showed no correlation to environmental factors. Total antioxidant capacities of leaf extracts were positively correlated to solar UV, and leaf and air temperature, but not to photosynthetically active radiation. Positive correlations of quercetin-3-O-glucoside contents with the flavonoid index, with photosynthesis and with sub-stomatal CO2 concentration suggest a special protective role of this flavonol. A short-term negative effect of solar UV-A and UV-B on photosynthetic CO2 uptake was also identified, which was unrelated to changes in stomatal conductance. A hypothesis is presented assuming UV- and photorespiration-derived hydrogen peroxide as the driver of daily changes in leaf antioxidant capacities