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.

UV-B radiation can substantially impact plant growth. To study UV-B effects, broadband UV-B tubes are commonly used. Apart from UV-B, such tubes also emit UV-A wavelengths. This study aimed to distinguish effects of different UV-B intensities on Arabidopsis thaliana wildtype and UVR8 mutant rosette morphology, from those by accompanying UV-A. UV-A promotes leaf-blade expansion along the proximal–distal, but not the medio-lateral, axis. Consequent increases in blade length: width ratio are associated with increased light capture. However, petiole length is not affected by UV-A exposure. This scenario is distinct from the shade avoidance driven by low red to far-red ratios, whereby leaf blade elongation is impeded but petiole elongation is promoted. Thus, the UV-A mediated elongation response is phenotypically distinct from classical shade avoidance. UV-B exerts inhibitory effects on petiole length, blade length and leaf area, and these effects are mediated by UVR8. Thus, UV-B antagonises aspects of both UV-A mediated elongation and classical shade avoidance. Indeed, this study shows that accompanying UV-A wavelengths can mask effects of UV-B. This may lead to potential underestimates of the magnitude of the UV-B induced morphological response using broadband UV-B tubes. Graphical abstract

Grapevine leaf synthesizes a wide variety of bioactive secondary metabolites, including polyphenols, which are also key components in ensuring development and growth of the whole plant even under adverse environmental conditions. Our study evaluates the nonanthocyanin polyphenolic composition in grapevine leaves of three varieties of Gohér conculta (Vitis vinifera L.) native to Hungary. A high performance liquid chromatography (HPLC) system including a diode array detector (DAD) coupled to a time-of-flight mass spectrometer (q-TOFMS) was successfully applied to profile intact glycoconjugate forms in samples. In-source fragmentation was utilized in order to provide structural information on the compounds. Using this method, the presence of 16 polyphenolic metabolites were confirmed, and eight of them were subjected to further quantification in sun acclimated and half shaded leaves. Intracellular microimaging detected accumulation of flavonols in cell nuclei, cell wall and chloroplasts. Our findings demonstrated that Gohér conculta—a special grapevine taxon of our viticultural heritage with berry color variants—is a suitable model to study the interaction between genetic and environmental factors in determination of grapevine phenolic composition.

The special metabolites of bell pepper (Capsicum annuum L.) leaves can protect the plant under possibly damaging circumstances, 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.

Bell pepper (Capsicum annuum) is one of the most popular vegetables consumed worldwide. The leaves of pepper are rich in phenolics, including phenolic acids and flavonoids. These compounds are well known for their ultraviolet (UV) absorbing and antioxidant properties. While the change of the phenolic pattern is an intensive research subject, it is not yet well-known in pepper leaves, particularly in outdoor conditions. In this experiment, we examined the effect of UV radiation on the leaves of outdoor grown peppers, focusing on the UV-absorbing properties and antioxidant capacities. Three different total antioxidant capacity (TAC) measurements have been compared: (I) Folin-Ciocalteu Reactivity (FC), (II) Ferric Reducing Antioxidant Power (FRAP) and (III) Trolox Equivalent Antioxidant Capacity (TEAC). Moreover, non-enzymatic hydrogen peroxide scavenging antioxidant capacity was measured. Significant increase was detected only in FRAP, suggesting an elevation exclusively in the level of phenolic acids in case of UV exposed outdoor grown pepper leaves.

Key message: Systemic changes in isoperoxidase profile and phenolic composition mirror those in leaves exposed to ultraviolet radiation, underscoring the essential role of systemic responses in plant health and development. Plants respond to direct exposure to broadband ultraviolet (UV) light (280–400 nm) through localized protective and regulatory mechanisms. In our recent study using Nicotiana tabacum as a model plant, we showed that protective responses triggered by moderate, non-harmful UV doses could spread from directly treated leaves to UV-unexposed leaves situated above (systemic leaves). This study aimed to broaden the understanding of previous findings by investigating the similarities between direct and systemic plant responses, concentrating on two main aspects: class III peroxidase (POD) isozymes and the phenolic profiles of leaves. We compared the responses of directly UV-exposed and systemic leaves from the same plant with those of negative (completely UV-unexposed) and positive control (fully UV-exposed) plants. Native gel electrophoresis confirmed a general increase in POD activity in both direct and systemic UV responses, demonstrating similar rearrangements in POD isozyme patterns, which is a novel finding. Furthermore, high-performance liquid chromatography analyses of leaf extracts showed similar quantitative and qualitative alterations in phenolic profiles between UV-exposed leaves and systemic leaves. These changes included increased levels of potential POD substrates, such as chlorogenic acids and flavonoids, particularly quercetin-3-O-rutinoside, which serves as an excellent UV-absorbing compound and antioxidant. In general, our findings indicate that the extent and nature of the systemic UV response are similar to those triggered by direct exposure to UV radiation. These results highlight the potential of applying localized low-dose UV treatment to activate systemic antioxidant responses that affect plant health and their growth.