Root transcriptomic provides insights on molecular mechanisms involved in the tolerance to water deficit in Pisum sativum inoculated with Pseudomonas sp.

MAIN CONCLUSION: Root transcriptomics and biochemical analyses in water-stressed Pisum sativum plants inoculated with Pseudomonas spp. suggested preservation of ABA-related pathway and ROS detoxification, resulting in an improved tolerance to stress. Drought already affects agriculture in large areas of the globe and, due to climate change, these areas are predicted to become increasingly unsuitable for agriculture. For several years, plant growth-promoting bacteria (PGPB) have been used to improve legume yields, but many aspects of this interaction are still unclear. To elucidate the mechanisms through which root-associated PGPB can promote plant growth in dry environments, we investigated the response of pea plants inoculated with a potentially beneficial Pseudomonas strain (PK6) and subjected to two different water regimes. Combined biometric, biochemical, and root RNA-seq analyses revealed that PK6 improved pea growth specifically under water deficit, as inoculated plants showed an increased biomass, larger leaves, and longer roots. Abscisic acid (ABA) and proline quantification, together with the transcriptome analysis, suggested that PK6-inoculated plant response to water deficit was more diversified compared to non-inoculated plants, involving alternative metabolic pathways for the detoxification of reactive oxygen species (ROS) and the preservation of the ABA stress signaling pathway. We suggest that the metabolic response of PK6-inoculated plants was more effective in their adaptation to water deprivation, leading to their improved biometric traits. Besides confirming the positive role that PGPB can have in the growth of a legume crop under adverse conditions, this study offers novel information on the mechanisms regulating plant-bacteria interaction under varying water availability. These mechanisms and the involved genes could be exploited in the future for the development of legume varieties, which can profitably grow in dry climates.

Photoprotective Role of Photosynthetic and Non-Photosynthetic Pigments in Phillyrea latifolia: Is Their "Antioxidant" Function Prominent in Leaves Exposed to Severe Summer Drought?

Carotenoids and phenylpropanoids play a dual role of limiting and countering photooxidative stress. We hypothesize that their "antioxidant" function is prominent in plants exposed to summer drought, when climatic conditions exacerbate the light stress. To test this, we conducted a field study on Phillyrea latifolia, a Mediterranean evergreen shrub, carrying out daily physiological and biochemical analyses in spring and summer. We also investigated the functional role of the major phenylpropanoids in different leaf tissues. Summer leaves underwent the most severe drought stress concomitantly with a reduction in radiation use efficiency upon being exposed to intense photooxidative stress, particularly during the central hours of the day. In parallel, a significant daily variation in both carotenoids and phenylpropanoids was observed. Our data suggest that the morning-to-midday increase in zeaxanthin derived from the hydroxylation of ß-carotene to sustain non-photochemical quenching and limit lipid peroxidation in thylakoid membranes. We observed substantial spring-to-summer and morning-to-midday increases in quercetin and luteolin derivatives, mostly in the leaf mesophyll. These findings highlight their importance as antioxidants, countering the drought-induced photooxidative stress. We concluded that seasonal and daily changes in photosynthetic and non-photosynthetic pigments may allow P. latifolia leaves to avoid irreversible photodamage and to cope successfully with the Mediterranean harsh climate.

Seasonal and Diurnal Variation in Leaf Phenolics of Three Medicinal Mediterranean Wild Species: What Is the Best Harvesting Moment to Obtain the Richest and the Most Antioxidant Extracts?

Mediterranean plants biosynthesize high amounts of polyphenols, which are important health-promoting compounds. Leaf polyphenolic composition changes according to environmental conditions. Therefore, it is crucial to know the temporal variation in their production. This study aimed to: i) evaluate the monthly and daily changes in polyphenols of Phyllirea latifolia, Cistus incanus, and Pistacia lentiscus to identify their best harvesting moment, ii) verify the possible correlations between phenolic production and temperature and irradiation, iii) evaluate their antioxidant capacity using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and hydroxyl radical (OH)scavenging assays. The extracts of leaves harvested at 8:00, 13:00 and 18:00, in May, July, and October for two years were analysed by HPLC-DAD. Both "month" and "time of the day" affected the polyphenolic content in all species. July at 13:00 was the best harvesting moment for all polyphenolic classes of P. latifolia and only for some classes of C. incanus and P. lentiscus. Environmental parameters positively correlated with the polyphenols of C. incanus and P. latifolia, while the antioxidant capacity only varied in this last species, reaching the highest value in July. Results of the study allow to determine the balsamic time for each species. Moreover, the relationship between polyphenols and environmental data can be useful for the cultivation of these plants under controlled conditions.

Dynamic changes in ABA content in water-stressed Populus nigra: effects on carbon fixation and soluble carbohydrates.

BACKGROUND AND AIMS: Hydraulic and chemical signals operate in tandem to regulate systemic plant responses to drought. Transport of abscisic acid (ABA) through the xylem and phloem from the root to shoot has been suggested to serve as the main signal of water deficit. There is evidence that ABA and its ABA-glycosyl-ester (ABA-GE) are also formed in leaves and stems through the chloroplastic 2-C-methylerythritol-5-phosphate (MEP) pathway. This study aimed to evaluate how hormonal and hydraulic signals contribute to optimize stomatal (gs), mesophyll (gm) and leaf hydraulic (Kleaf) conductance under well-watered and water-stressed conditions in Populus nigra (black poplar) plants. In addition, we assessed possible relationships between ABA and soluble carbohydrates within the leaf and stem. METHODS: Plants were subjected to three water treatments: well-watered (WW), moderate stress (WS1) and severe stress (WS2). This experimental set-up enabled a time-course analysis of the response to water deficit at the physiological [leaf gas exchange, plant water relations, (Kleaf)], biochemical (ABA and its metabolite/catabolite quantification in xylem sap, leaves, wood, bark and roots) and molecular (gene expression of ABA biosynthesis) levels. KEY RESULTS: Our results showed strong coordination between gs, gm and Kleaf under water stress, which reduced transpiration and increased intrinsic water use efficiency (WUEint). Analysis of gene expression of 9-cis-epoxycarotenoid dioxygenase (NCED) and ABA content in different tissues showed a general up-regulation of the biosynthesis of this hormone and its finely-tuned catabolism in response to water stress. Significant linear relationships were found between soluble carbohydrates and ABA contents in both leaves and stems, suggesting a putative function for this hormone in carbohydrate mobilization under severe water stress. CONCLUSIONS: This study demonstrates the tight regulation of the photosynthetic machinery by levels of ABA in different plants organs on a daily basis in both well-watered and water stress conditions to optimize WUEint and coordinate whole plant acclimation responses to drought.

Dissecting Adaptation Mechanisms to Contrasting Solar Irradiance in the Mediterranean Shrub Cistus incanus.

Molecular mechanisms that are the base of the strategies adopted by Mediterranean plants to cope with the challenges imposed by limited or excessive solar radiation during the summer season have received limited attention. In our study, conducted on C. incanus plants growing in the shade or in full sunlight, we performed measurements of relevant physiological traits, such as leaf water potential, gas exchange and PSII photochemistry, RNA-Seq with de-novo assembly, and the analysis of differentially expressed genes. We also identified and quantified photosynthetic pigments, abscisic acid, and flavonoids. Here, we show major mechanisms regulating light perception and signaling which, in turn, sustain the shade avoidance syndrome displayed by the 'sun loving' C. incanus. We offer clear evidence of the detrimental effects of excessive light on both the assembly and the stability of PSII, and the activation of a suite of both repair and effective antioxidant mechanisms in sun-adapted leaves. For instance, our study supports the view of major antioxidant functions of zeaxanthin in sunny plants concomitantly challenged by severe drought stress. Finally, our study confirms the multiple functions served by flavonoids, both flavonols and flavanols, in the adaptive mechanisms of plants to the environmental pressures associated to Mediterranean climate.

Phenotypic differences determine drought stress responses in ecotypes of Arundo donax adapted to different environments.

Arundo donax has been identified as an important biomass and biofuel crop. Yet, there has been little research on photosynthetic and metabolic traits, which sustain the high productivity of A. donax under drought conditions. This study determined phenotypic differences between two A. donax ecotypes coming from stands with contrasting adaptation to dry climate. We hypothesized that the Bulgarian (BG) ecotype, adapted to drier conditions, exhibits greater drought tolerance than the Italian (IT) ecotype, adapted to a more mesic environment. Under well-watered conditions the BG ecotype was characterized by higher photosynthesis, mesophyll conductance, intrinsic water use efficiency, PSII efficiency, isoprene emission rate and carotenoids, whereas the IT ecotype showed higher levels of hydroxycinnamates. Photosynthesis of water-stressed plants was mainly limited by diffusional resistance to CO2 in BG, and by biochemistry in IT. Recovery of photosynthesis was more rapid and complete in BG than in IT, which may indicate better stability of the photosynthetic apparatus associated to enhanced induction of volatile and non-volatile isoprenoids and phenylpropanoid biosynthesis. This study shows that a large phenotypic plasticity among A. donax ecotypes exists, and may be exploited to compensate for the low genetic variability of this species when selecting plant productivity in constrained environments.

Dissecting molecular and physiological response mechanisms to high solar radiation in cyanic and acyanic leaves: a case study on red and green basil.

Photosynthetic performance and the expression of genes involved in light signaling and the biosynthesis of isoprenoids and phenylpropanoids were analysed in green ('Tigullio', TIG) and red ('Red Rubin', RR) basil. The aim was to detect the physiological and molecular response mechanisms to high sunlight. The attenuation of blue-green light by epidermal anthocyanins was shown to evoke shade-avoidance responses with consequential effects on leaf morpho-anatomical traits and gas exchange performance. Red basil had a lower mesophyll conductance, partially compensated by the less effective control of stomatal movements, in comparison with TIG. Photosynthesis decreased more in TIG than in RR in high sunlight, because of larger stomatal limitations and the transient impairment of PSII photochemistry. The methylerythritol 4-phosphate pathway promoted above all the synthesis and de-epoxidation of violaxanthin-cycle pigments in TIG and of neoxanthin and lutein in RR. This enabled the green leaves to process the excess radiant energy effectively, and the red leaves to optimize light harvesting and photoprotection. The greater stomatal closure observed in TIG than in RR was due to enhanced abscisic acid (ABA) glucose ester deglucosylation and reduced ABA oxidation, rather than to superior de novo ABA synthesis. This study shows a strong competition between anthocyanin and flavonol biosynthesis, which occurs at the level of genes regulating the oxidation of the C2-C3 bond in the dihydro-flavonoid skeleton.

Physiological significance of isoprenoids and phenylpropanoids in drought response of Arundinoideae species with contrasting habitats and metabolism.

Physiological, biochemical and morpho-anatomical traits that determine the phenotypic plasticity of plants under drought were tested in two Arundinoideae with contrasting habitats, growth traits and metabolism: the fast-growing Arundo donax, which also is a strong isoprene emitter, and the slow-growing Hakonechloa macra that does not invest on isoprene biosynthesis. In control conditions, A. donax displayed not only higher photosynthesis but also higher concentration of carotenoids and lower phenylpropanoid content than H. macra. In drought-stressed plants, photosynthesis was similarly inhibited in both species, but substantially recovered only in A. donax after rewatering. Decline of photochemical and biochemical parameters, increased concentration of CO2 inside leaves, and impairment of chloroplast ultrastructure were only observed in H. macra indicating damage of photosynthetic machinery under drought. It is suggested that volatile and non-volatile isoprenoids produced by A. donax efficiently preserve the chloroplasts from transient drought damage, while H. macra invests on phenylpropanoids that are less efficient in preserving photosynthesis but likely offer better antioxidant protection under prolonged stress.

Characterisation and Antioxidant Activity of Crude Extract and Polyphenolic Rich Fractions from C. incanus Leaves.

Cistus incanus (Cistaceae) is a Mediterranean evergreen shrub. Cistus incanus herbal teas have been used as a general remedy in traditional medicine since ancient times. Recent studies on the antioxidant properties of its aqueous extracts have indicated polyphenols to be the most active compounds. However, a whole chemical characterisation of polyphenolic compounds in leaves of Cistus incanus (C. incanus) is still lacking. Moreover, limited data is available on the contribution of different polyphenolic compounds towards the total antioxidant capacity of its extracts. The purpose of this study was to characterise the major polyphenolic compounds present in a crude ethanolic leaf extract (CEE) of C. incanus and develop a method for their fractionation. Superoxide anion, hydroxyl and DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging assays were also performed to evaluate the antioxidant properties of the obtained fractions. Three different polyphenolic enriched extracts, namely EAC (Ethyl Acetate Fraction), AF1 and AF2 (Aqueos Fractions), were obtained from CEE. Our results indicated that the EAC, enriched in flavonols, exhibited a higher antiradical activity compared to the tannin enriched fractions (AF1 and AF2). These findings provide new perspectives for the use of the EAC as a source of antioxidant compounds with potential uses in pharmaceutical preparations.

Isoprenoids and phenylpropanoids are part of the antioxidant defense orchestrated daily by drought-stressed Platanus × acerifolia plants during Mediterranean summers.

The hypothesis was tested that isoprenoids and phenylpropanoids play a prominent role in countering photooxidative stress, following the depletion of antioxidant enzyme activity in plants exposed to severe drought stress under high solar irradiance and high temperatures. Platanus × acerifolia, a high isoprene-emitting species, was drought-stressed during summer (WS) and compared with unstressed controls (WW). Water relations and photosynthetic parameters were measured under mild, moderate, and severe drought stress conditions. Volatile and nonvolatile isoprenoids, antioxidant enzymes, and phenylpropanoids were measured with the same time course, but in four different periods of the day. Drought severely inhibited photosynthesis, whereas it did not markedly affect the photochemical machinery. Isoprene emission and zeaxanthin concentration were higher in WS than in WW leaves, particularly at mild and moderate stresses, and during the hottest hours of the day. The activities of catalase and ascorbate peroxidase steeply declined during the day, while the activity of guaiacol peroxidase and the concentration of quercetin increased during the day, peaking in the hottest hours in both WW and WS plants. Our experiment reveals a sequence of antioxidants that were used daily by plants to orchestrate defense against oxidative stress induced by drought and associated high light and high temperature. Secondary metabolites seem valuable complements of antioxidant enzymes to counter oxidative stress during the hottest daily hours.

Rapid determination of phenolic composition in chamomile (Matricaria recutita L.) using surface-enhanced Raman spectroscopy.

Flavonoids and hydroxycinnamic acids are the main responsible of the antioxidant activity of chamomile (Matricaria recutita L.). Traditional methods for the analysis of the phenolic content in vegetables often suffer from limitations such as being expensive, time-consuming, and complex. In this study, we propose, for the first time, the use of surface-enhanced Raman spectroscopy (SERS) for the rapid determination of the main components of the polyphenolic fraction in chamomile. Results demonstrate that SERS can serve as an alternative or complementary technique to main analytical strategies for qualitative and quantitative determination of polyphenol compounds in plant extracts. The method can be proposed for quasi real-time analysis of herbal teas and infusions, facilitating rapid screening of their main antioxidant components.

Unveiling resilience mechanisms of Quercus ilex seedlings to severe water stress: Changes in non-structural carbohydrates, xylem hydraulic functionality and wood anatomy.

Over the last few decades, extensive dieback and mortality episodes of Quercus ilex L. have been documented after severe drought events in many Mediterranean forests. However, the underlying physiological, anatomical, and biochemical mechanisms remain poorly understood. We investigated the physiological and biochemical processes linked to embolism formation and non-structural carbohydrates (NSCs) dynamics in Q. ilex seedlings exposed to severe water stress and rewatering. Measurements of leaf gas exchange, water relations, non-structural carbohydrates, drought-related gene expression, and anatomical changes in wood parenchyma were assessed. Under water stress, the midday stem water potential dropped below - 4.5 MPa corresponding to a ~ 50 % loss of hydraulic conductivity. A 70 % reduction in stomatal conductance led to a strong depletion of wood NSCs. Starch consumption, resulting from the upregulation of the ß-amylase gene BAM3, together with the downregulation of glucose (GPT1) and sucrose (SUC27) transport genes, suggests glucose utilization to sustain cellular metabolism in the wood parenchyma. After rewatering, the presence of residual xylem embolism led to an incomplete recovery of leaf gas exchanges. However, the partial restoration of photosynthesis allowed the accumulation of new starch reserves in the wood parenchyma and the production of new narrower vessels. In addition, changes in the cell wall composition of the wood parenchyma fibers were observed. Our findings indicate that thirty days of rewatering were sufficient to restore the NSCs reserves and growth rates of Q. ilex seedlings and that the carryover effects of water stress were primarily caused by hydraulic dysfunction.

Quercus ilex L. dieback is genetically determined: Evidence provided by dendrochronology, δ13C and SSR genotyping.

Quercus ilex L. dieback has been reported in several Mediterranean forests, revealing different degree of crown damages even in close sites, as observed in two Q. ilex forest stands in southern Tuscany (IT). In this work, we applied a novel approach combining dendrochronological, tree-ring δ13C and genetic analysis to test the hypothesis that different damage levels observed in a declining (D) and non-declining (ND) Q. ilex stands are connected to population features linked to distinct response to drought. Furthermore, we investigated the impact of two major drought events (2012 and 2017), that occurred in the last fifteen years in central Italy, on Q. ilex growth and intrinsic water use efficiency (WUEi). Overall, Q. ilex showed slightly different ring-width patterns between the two stands, suggesting a lower responsiveness to seasonal climatic variations for trees at D stand, while Q. ilex at ND stand showed changes in the relationship between climatic parameters and growth across time. The strong divergence in δ13C signals between the two stands suggested a more conservative use of water for Q. ilex at ND compared to D stand that may be genetically driven. Q. ilex at ND resulted more resilient to drought compared to trees at D, probably thanks to its safer water strategy. Genotyping analysis based on simple-sequence repeat (SSR) markers revealed the presence of different Q. ilex populations at D and ND stands. Our study shows intraspecific variations in drought response among trees grown in close. In addition, it highlights the potential of combining tree-ring δ13C data with SSR genotyping for the selection of seed-bearing genotypes aimed to preserve Mediterranean holm oak ecosystem and improve its forest management.

An Improvement of SPME-Based Sampling Technique to Collect Volatile Organic Compounds from Quercus ilex at the Environmental Level.

Biogenic Volatile Organic Compounds (BVOCs) include many chemical compounds emitted by plants into the atmosphere. These compounds have a great effect on biosphere-atmosphere interactions and may affect the concentration of atmospheric pollutants, with further consequences on human health and forest ecosystems. Novel methods to measure and determine BVOCs in the atmosphere are of compelling importance considering the ongoing climate changes. In this study, we developed a fast and easy-to-handle analytical methodology to sample these compounds in field experiments using solid-phase microextraction (SPME) fibers at the atmospheric level. An improvement of BVOCs adsorption from SPME fibers was obtained by coupling the fibers with fans to create a dynamic sampling system. This innovative technique was tested sampling Q. ilex BVOCs in field conditions in comparison with the conventional static SPME sampling technique. The results showed a great potential of this dynamic sampling system to collect BVOCs at the atmosphere level, improving the efficiency and sensitivity of SPME fibers. Indeed, our novel device was able to reduce the sampling time, increase the amount of BVOCs collected through the fibers and add information regarding the emissions of these compounds at the environmental level.

Phenolic Compounds from Leaves and Flowers of Hibiscus roseus: Potential Skin Cosmetic Applications of an Under-Investigated Species.

The use of plant extracts in skin-care cosmetics is a modern trend due to their richness in polyphenols that act as anti-aging molecules. Hibiscus roseus is a perennial species naturalized in Italy, with beautiful soft pink flowers; its phenolic composition and biological activities have not been studied yet. The aim of this study was to characterize and quantify the phenolics and to evaluate the antioxidant, sun protection factor (SPF), and anti-collagenase activities of the ethanolic extracts of H. roseus leaves (HL) and flowers (HF). p-Coumaric, chlorogenic, and trans-ferulic acids derivatives as well as quercetin and kaempferol flavonoids were the main phenolic compounds detected. Catechin, epicatechin, kaempferol-3-O-rutinoside, kaempferol-3-O-glucoside, kaempferol-7-O-glucoside, tiliroside, oenin, and peonidin-3-O-glucoside were detected only in HF, while phloridzin was exclusive from HL, which also showed greater amounts of hydroxycinnamic acid derivatives. HF was richer in flavonoids and total phenolics, also exhibiting greater antioxidant capacity. The SPF and anti-collagenase activity of both extracts were similar and comparable to those of synthetic standards. The overall results demonstrate that H. roseus extracts are promising sources of bioactive phenolic compounds that could be potentially applied as anti-aging agents in skin-care cosmetics.

Ethyl acetate extract from Cistus x incanus L. leaves enriched in myricetin and quercetin derivatives, inhibits inflammatory mediators and activates Nrf2/HO-1 pathway in LPS-stimulated RAW 264.7 macrophages.

Cistus x incanus L. is a Mediterranean evergreen shrub used in folk medicine for the treatment of inflammatory disorders but the underlying mechanisms are not fully understood. We therefore investigated the anti-inflammatory effects of an ethyl acetate fraction (EAF) from C. x incanus L. leaves on lipopolysaccharide (LPS) activated RAW 264.7 macrophages. HPLC analysis revealed myricetin and quercetin derivatives to be the major compounds in EAF; EAF up to 1 µM of total phenolic content, was not cytotoxic and inhibited the mRNA expression of interleukin-6 (IL-6) and cyclooxygenase-2 (COX-2) (p < 0.05) and the production of prostaglandins E2 (PGE2) (p < 0.05). Meanwhile, EAF triggered the mRNA expression of interleukin-10 (IL-10) and elicited the nuclear translocation of nuclear factor erythroid 2-related factor 2 (Nrf2), as well as the expression of its main target gene, heme oxygenase-1 (HO-1) (p < 0.05). These data indicate that EAF attenuates experimental inflammation via the inhibition of proinflammatory mediators and at least in part, by the activation of Nrf2/HO-1 pathway. These effects are likely due to myricetin and quercetin derivatives but the role of other, less abundant components cannot be excluded. Further studies to confirm the relevance of our findings in animal models and to highlight the relative contribution of each component to the anti-inflammatory activity of EAF should be conducted.

Optimization of a Green Ultrasound-Assisted Extraction of Different Polyphenols from Pistacia lentiscus L. Leaves Using a Response Surface Methodology.

Pistacia lentiscus leaves are used in several applications, thanks to their polyphenolic abundance. Thiswork aimed to characterize the polyphenols and to optimize the extraction conditions to shorten the time, decrease the consumption of solvent, and to maximize the yield of different classes of phenolics, which have diverse industrial applications. The variables were optimized by applying a Box-Behnken design. Galloyl and myricetin derivatives were the most abundant compounds, and two new tetragalloyl derivatives were identified by LC-MS/MS. According to the models, the maximum yields of polyphenols (51.3 ± 1.8 mg g-1 DW) and tannins (40.2 ± 1.4 mg g-1 DW) were obtained using 0.12 L g-1 of 40% ethanol at 50 °C. The highest content of flavonoids (10.2 ± 0.8 mg g-1 DW) was obtained using 0.13 L g-1 of 50% ethanol at 50 °C, while 0.1 L g-1 of 30% ethanol at 30 °C resulted in higher amounts of myricitrin (2.6 ± 0.19 mg g-1 DW). Our optimized extraction decreased the ethanolic fraction by 25% and halved the time compared to other methods. These conditions can be applied differently to obtain P. lentiscus extracts richer in tannins or flavonoids, which might be employed for various purposes.

Changes in abscisic acid content during and after drought are related to carbohydrate mobilization and hydraulic recovery in poplar stems.

Drought compromises plant's ability to replace transpired water vapor with water absorbed from the soil, leading to extensive xylem dysfunction and causing plant desiccation and death. Short-term plant responses to drought rely on stomatal closure, and on the plant's ability to recover hydraulic functioning after drought relief. We hypothesize a key role for abscisic acid (ABA) not only in the control of stomatal aperture, but also in hydraulic recovery. Young plants of Populus nigra L. were used to investigate possible relationships among ABA, non-structural carbohydrates (NSC) and xylem hydraulic function under drought and after re-watering. In Populus nigra L. plants subjected to drought, water transport efficiency and hydraulic recovery after re-watering were monitored by measuring the percentage loss of hydraulic conductivity (PLC) and stem specific hydraulic conductivity (Kstem). In the same plants ABA and NSC were quantified in wood and bark. Drought severely reduced stomatal conductance (gL) and markedly increased the PLC. Leaf and stem water potential, and stem hydraulic efficiency fully recovered within 24 h after re-watering, but gL values remained low. After re-watering, we found significant correlations between changes in ABA content and hexoses concentration both in wood and bark. Our findings suggest a role for ABA in the regulation of stem carbohydrate metabolism and starch mobilization upon drought relief, possibly promoting the restoration of xylem transport capacity.

Are Flavonoids Effective Antioxidants in Plants? Twenty Years of Our Investigation.

Whether flavonoids play significant antioxidant roles in plants challenged by photooxidative stress of different origin has been largely debated over the last few decades. A critical review of the pertinent literature and our experimentation as well, based on a free-of-scale approach, support an important antioxidant function served by flavonoids in plants exposed to a wide range of environmental stressors, the significance of which increases with the severity of stress. On the other side, some questions need conclusive answers when the putative antioxidant functions of plant flavonoids are examined at the level of both the whole-cell and cellular organelles. This partly depends upon a conclusive, robust, and unbiased definition of "a plant antioxidant", which is still missing, and the need of considering the subcellular re-organization that occurs in plant cells in response to severe stress conditions. This likely makes our deterministic-based approach unsuitable to unveil the relevance of flavonoids as antioxidants in extremely complex biological systems, such as a plant cell exposed to an ever-changing stressful environment. This still poses open questions about how to measure the occurred antioxidant action of flavonoids. Our reasoning also evidences the need of contemporarily evaluating the changes in key primary and secondary components of the antioxidant defense network imposed by stress events of increasing severity to properly estimate the relevance of the antioxidant functions of flavonoids in an in planta situation. In turn, this calls for an in-depth analysis of the sub-cellular distribution of primary and secondary antioxidants to solve this still intricate matter.

Short-Term Pre-Harvest UV-B Supplement Enhances the Polyphenol Content and Antioxidant Capacity of Ocimum basilicum Leaves during Storage.

Ocimum basilicum (basil) leaves are rich in polyphenols, conferring them a high antioxidant activity. The application of UV-B can be used to maintain the post-harvest nutraceutical quality of basil leaves. We aimed to investigate the effects of pre-harvest UV-B application on polyphenolic and pigment contents, antioxidant capacity, and the visual quality of basil stored leaves. We also evaluated the applicability of the non-invasive Dualex® for monitoring the accumulation of leaf epidermal phenolics (Flav Index). After exposing plants to white light (control) and to supplemental UV-B radiation for 4 d, the leaves were harvested and stored for 7d (TS7). The UV-B leaves showed both a higher phenolic content and antioxidant capacity than the controls at TS7. In addition, the correlations between the Flav Index and phenolic content demonstrated that Dualex® can reliably assess the content of epidermal phenolics, thus confirming its promising utilization as a non-destructive method for monitoring the phytochemical quality of O. basilicum leaves. In conclusion, a pre-harvesting UV-B application may be a tool for enhancing the content of polyphenols and the antioxidant potential of basil stored leaves without detrimental effects on their visual quality. These results are important considering the nutraceutical value of this plant and its wide commercial distribution.