Enhancing biological activities and phenolic content of wild grapevine roots by severe drought stress

Abstract Wild grapevines are a rich source of polyphenolic compounds. Various environmental factors can influence these bioactive metabolites, contributing to stress mitigation and adaptation. The present study aimed to investigate the impact of prolonged drought stress on the accumulation of phytochemicals, antioxidants, and antimicrobial activity in the roots of the endangered Tunisian wild grapevine (Vitis vinifera subsp. sylvestris) "Tebaba" genotype. The results revealed that under prolonged drought stress, there was a significant increase in total phenolic compounds, total flavonoid content, and stilbenes, which showed a positive and significant correlation with the total antioxidant activity. Furthermore, the ethanolic extracts demonstrated an enhanced in vitro antibacterial effect against selected human pathogenic bacterial strains, including Staphylococcus aureus, Methicillin-resistant S. aureus, and Enterococcus faecalis. The minimum inhibitory concentration (MIC) and the minimal bactericidal concentration (MBC) values, relative to prolonged drought stress, ranged from 125 to 250 µg/mL. The values of the ratio between MBC and the MIC showed that the activity of the root extract was bactericidal in all cases (MBC/MIC ≤ 2). This study highlights the potential of wild grapevine roots enriched with drought-inducible phenolic compounds, as a promising source of antimicrobial agents relevant to human health applications. Graphical Abstract


Introduction
Wild grapevine species are interesting germplasms for bioactive compounds (Burin et al. 2014;Chen et al. 2018).They are valuable genetic resources of resilience to environmental stress such as the case of the Tunisian salt tolerant "Tebaba" genotype (Askri et al. 2018;Daldoul et al. 2022Daldoul et al. , 2023)).It has been reported that wild grapevines have a higher content of bioactive compounds than cultivated ones (Shi et al. 2014;Ahn et al. 2015).Their root tissues were found to be particularly rich in oligostilbenes (Huang and Lin 1999;Huang et al. 2000) and resveratrol (Huang et al. 2001(Huang et al. , 2005) ) known for their antimicrobial activity (Mattio et al. 2020).The roots of Taiwanese wild grapes contain several antibacterial compounds, such as viniferin, ampelosin C, miyabenol A, vitisin A, vitisin C, vitisinol A-D (Huang et al. 2005) and heyneanol A (Peng et al. 2008).Such richness of secondary metabolites contributed to their clinical relevance for human health (Salehi et al. 2019).Plant based antibiotic agents for therapeutic use became of high interest (Guadie et al. 2020;Archana and Bose 2022).Infectious diseases caused by bacteria are a major public health problem (eggleston et al. 2010).Antibiotic treatment is a preferred choice to treat bacterial infections.However, the emergence of antimicrobial resistance and toxicity issues subside the use of antibacterial agents (Tacconelli 2017).Among human pathogenic bacteria, the methicillin-resistant Staphylococcus aureus (MRSA) strains could induce multiple human infections (Boucher and Corey 2008).The rise of multidrug resistance (MDR) in MRSA represents a significant menace, leading to the ineffectiveness of treatment and control measures.MRSA developed new effective mechanisms to resist modern antibiotics, which is considered a life-threatening microorganism for humans and animals (Tang et al. 2014;Gajdács 2019).The World Health Organization (Tacconelli 2017) has classified MRSA as a high-priority pathogen for the development of novel antibiotics (Tacconelli 2017).S. aureus is another major human pathogen that causes a wide range of clinical infections (Tong et al. 2015).Approximately 30% of the human population is colonized by S. aureus (Wertheim et al. 2005).The bacterium Enterococcus faecalis (E.faecalis) plays a significant role in the gastrointestinal tracts of both humans and farm animals (Vu and Carvalho 2011).Infections caused by E. faecalis are hard to treat with common antimicrobials (Jabbari Shiadeh et al. 2019).E. faecalis clinical strains show resistance towards commonly used antimicrobials, particularly penicillins, and tetracyclines (Ahmed and Baptiste 2018).Due to the limitations of antibiotics, a special focus on biological research regards the antimicrobial role of plants as a natural source of bioagents against infectious diseases (Alviano and Alviano 2009).The pharmacological activities of some wild grapevines were proven to be efficient (Burin et al. 2014;Chen et al. 2018).For example, the root extracts of the Taiwanese wild grape (Vitis thunbergia) showed antimicrobial activities against several Gram-positive pathogen strains (Peng et al. 2008).Furthermore, Vitis amurensis roots exhibited an anti-melanogenic (Jin et al. 2014a(Jin et al. , 2014b) ) and an antitumor activity (Lee et al. 2006).In traditional medicine, roots from Vitis amurensis have been widely used to treat pains due to stomachache injury, neuralgic, and abdominal complaints in China (Huang and Lin 1999;Jang et al. 2007).The roots of Vitis thunbergii are traditionally used for the treatment of diarrhea, fracture and injury, jaundice, and hepatitis in Taiwan (Chiu and Chang 1986).Nevertheless, the antimicrobial potential of the wild grapevine [Vitis vinferra subsp.sylvestris (i.e.hereafter referred to as Vitis sylvestris)] was limited to only one report on leaf tissue (Amara and Melouk 2016).However, root tissues still remain quite unexplored.Stress-induced phenolic metabolites, as an endogenous protective defense against abiotic stress, have been proposed as an alternative to improve the biosynthesis of phenolic metabolites for antimicrobial applications (Christopher et al. 2021).elicitation of grapevine plants by drought induced an accumulation of diverse polyphenolic substances including phenolic compounds (Villangó et al. 2016;Pinasseau et al. 2017) and stilbenes (Valletta et al. 2021).The protective defense mediated by phenolics enables plants to serve as free radical scavengers, mitigating oxidative and dehydration stress (Nakabayashi et al. 2014).Furthermore, secondary metabolites could inhibit further loss of water through stomatal closure (yadav et al. 2021).Higher antimicrobial activity was reported in leaf extracts from water-stressed tree species (Netshiluvhi and eloff 2016).The antimicrobial potential of these stress-inducible phenolics could be mediated through signal transduction processes or modulation of cellular targets (Su et al. 2016).They function as inhibitors in microbial biosynthetic pathways, as chelating agents that reduce the availability of micronutrients for bacterial growth, and as hydrophobic molecules that permeabilize or destabilize the bacterial cell membrane, all of which can help inhibit the growth of pathogenic bacteria (Daglia 2012;Radulovic et al. 2013;Bouarab Chibane et al. 2019).Regarding the growing need to discover novel plant-based antimicrobial agents, it is of great interest to develop new tools that can promote their biosynthesis.In this context, we assessed the impact of severe drought on phenolic accumulation in the roots of a wild grapevine genotype "Tebaba" and their biological activities (antioxidant and antimicrobial properties).

Wild grapevine culture and drought stress application
Woody cuttings from wild grapevine [Vitis sylvestris (Arnold et al. 2010)] "Tebaba" genotype were cultivated in sand for two months.When the grapevine shoots had reached 12-14 nodes, they were transferred into 7 l pots of inert sandy soil and grown for two additional months under a controlled greenhouse (16/8 h light period, PAR of 300 µmol m −2 s −1 , 30/22 °C ± 3 °C day/night temperatures, and an average humidity of 70%).Six months old plants were subjected to drought stress treatment.Stressed plants were not irrigated at all for 15 and 20 days.Control plants were continuously watered with 5X diluted commercial nutrition solution Villmorin universal.Roots were sampled for phytochemical analysis.Three independent biological experiments were performed consisting of two treatments: irrigated condition, referred to as control (C) and drought-stressed (DS) with three plants per treatment.

Plant growth and water status parameters
The length of the main shoot of grapevine plants was measured between the beginning (L 1 ) and the end (L 2 ) of each drought stress treatment period, t = 15 days and t = 20 days, respectively, to determine the shoot elongation rate by SeR (cm d −1 ) = (L 2− L 1 )/t.Leaf water potential (Ψ leaf ) was determined according to Scholander et al. (1964) using a pressure chamber (PMS Instrument Albany, uSA).Measurements were taken between 11:00 am and 1:00 pm using healthy, fully expanded leaves at the same developmental stage.

Total phenolic, flavonoid and antioxidant activity
The phenolic extraction was performed according to Gouvinhas et al. (2018).For both control and drought-treated plants, 40 mg of freeze-dried root tissue powders were extracted with 1.5 mL of methanol/distilled water (70:30, v/v) by agitation for 30 min at room temperature.The mixture was then centrifuged for 15 min at 10,000 rpm and 4 °C and the supernatants were used for further spectrophotometric analysis.Total phenolic (TPC) and flavonoid (TFC) contents were determined according to Machado and Domínguez-Perles (2017).The total phenolic content of root extracts was determined based on the Folin-Ciocalteu method using gallic acid as standard.Briefly, 20 μL of each diluted sample was mixed with Folin-Ciocalteu aqueous (100 μL, 10% v/v) reagent and Na 2 CO 3 (80 μL, 7.5%).The mixture solution was first kept at 40 °C, for 30 min in the dark and then the optical density was read at 760 nm.TFC in grape root extracts was determined by mixing 24 µL of diluted sample with 28 μL NaNO 2 (50 g/L) for 5 min.Then, 120 μL of NaOH (1 M) was added and the absorbance was immediately recorded at 510 nm.The flavonoid content was determined using catechin as standard.Total antioxidant activity was measured using the DPPH (1,1-diphenyl-2-picrylhydrazyl) assay, as described by yen and Chen (1995).15 µL of each diluted extract was combined with 1 mL of a freshly prepared methanolic solution of DPPH (25 mg/L).The absorbance was then measured at 517 nm after a 10 min incubation period.A Trolox calibration curve was used.

Stilbene extraction and HPLC-DAD analysis
The stilbene extraction was performed according to Rayne et al. (2008) with some modifications.A three-stage extraction was conducted on 100 mg of freeze-dried grapevine root powder.The extraction involved continuous stirring at room temperature in the absence of light, using 800 µL of ethanol/ distilled water (80:20, v/v) with the addition of 20 µL of t-OH-stilbene (100 µg/mL), as internal standard, during the first extraction.After each extraction, the mixture was centrifuged at 13,000 rpm for 15 min at room temperature and obtained supernatants were combined before proceeding to the evaporation step.The recovery of the obtained residues was done using 200 µL of methanol/distilled water (50:50, v/v) and extracts were analyzed using HPLC-DAD system.The analysis of stilbenes was performed as previously reported by Vincenzi et al. (2013) with slight modifications.Stilbenes were separated on a C18 Lichrospher column (4 mm × 250 mm, 5 μm, Agilent Technologies, Milano, Italy) at 40 °C, using an HPLC system Nexera XR (ShimadzuItalia, Milano) equipped with PDA detector SPD-M30A (ShimadzuItalia, Milano).The gradient program used was reported by De Bona et al. (2020) The concentration of individual stilbenes was determined based on peak areas using calibration curves of commercially available standards of trans-resveratrol, trans-ɛ-viniferin, trans-piceatannol and trans-piceid.All stilbene standards were obtained from extrasynthese (Genay Cedex, France).Data were analyzed by the Shimadzu Lab Solution software.

Bacterial strains
The antibacterial potency of plant extracts was evaluated using three Gram-positive bacteria: S. aureus 6538, methicillin-resistant S. aureus (MRSA) and E. faecalis.These bacterial strains were provided by the Laboratory of Bioactive Compounds (LSBA), Center of Biotechnology of Borj Cedria (CBBC).

Determination of minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC)
The MIC was carried out using a 96-well plate microbroth dilution method (Grieco et al. 2013).Serial two-fold dilutions of extracts ranging from 15.6 to 2000 µg/mL were performed in Luria Bertani broth.Aliquots (50 µL) of bacterial suspension were added to the wells at 10 6 CFu/mL and the plates were incubated at 30 °C for 18 h.The MIC was determined as the lowest concentration of the extract that inhibited the bacterial growth.All tests were carried out in triplicate.MBC was determined from the same experiments as previously described with minor modifications (Tabbene et al. 2010).10 µl were taken from wells with MIC, 2 × MIC and 4 × MIC and were spread on LB plates.The colony count was conducted following a 24 h incubation period.The lowest concentration of the extract that kills 99.9% of bacterial strains was considered as the MBC (Mogana et al. 2020).The experiments were repeated three times.The MBC/MIC ratio has been calculated to determine whether extracts have bactericidal (MBC/MIC ≤ 4) or bacteriostatic (MBC/MIC > 4) effects (Jung et al. 2022).

Statistical analysis
Statistical analysis was conducted using IBM SPSS Statistics software for Windows (Version 20.0,IBM Corp, Armonk, Ny).Student's t-test and one-way ANOVA with Student-Newman-Keuls post-hoc tests were employed to determine statistically significant differences between mean values at the 5% significance level.Pearson correlation was performed at the 5 and 1% significance levels.All experimental data were presented as means of three independent biological replicates expressed with standard deviation (means ± SD).GraphPad Prism software (Version 8.0.0,GraphPad Software Inc., San Diego, California, uSA; available at www.graphpad.com)was utilized to visually present the results in graphical form.

Drought induced physiological changes
The rate of shoot elongation exhibited a notable decline of 58% on day 15 and a more significant decrease of 79.2% on day 20 (Figure 1a, b).The measurement of leaf water potential (Ψ leaf ) was conducted on day 15 and day 20 (Figure 1c).The results indicated a significant fluctuation in Ψ leaf during the period of water stress.On day 15, there was a reduction in leaf water potential, reaching values as low as −2.1 MPa.This decrease further intensified to reach −2.8 MPa after 20 days of stress, in contrast to the control condition where the Ψ leaf values were around −0.7 MPa (Figure 1c).

Drought induced phytochemical accumulation and enhanced antioxidant activity
The exposure to water stress resulted in a significant increase in the total polyphenol content (TPC), total flavonoid content (TFC), and total antioxidant activity (TAA) as measured by the DPPH parameter.The findings from this study demonstrated that the TPC, TFC, and TAA were progressively affected by drought stress (Figure 2).Specifically, after 15 days of drought stress treatment, there was a significant increase of 21.7% in TPC, 15% in TFC, and 17.82% in TAA compared to their respective controls.With a longer period of water deprivation of 20 days, these percentages further increased to 34% in TPC, 22.2% in TFC, and 23.5% in TAA compared to the controls.The investigation also focused on non-flavonoid stilbene compounds (Figure 3).The results revealed a decrease in stilbenic compounds after 15 days of drought stress treatment.However, with an extended stress period of 20 days, their contents significantly increased.Specifically, there was a percentage increase of 78.8% in t-resveratrol, 79.8% in t-piceatannol, 139.2% in t-ɛ-viniferin, and 26% in t-piceid compared to their respective controls.Analysis of the fold change ratios of stress versus control contents revealed that the long-term stress exposure of 20 days enhanced the synthesis of secondary metabolites compared to 15 days of stress.The ratios for total stilbenes, TPC, and TFC at 20 days versus 15 days were 1.9 versus 0.8, 1.3 versus 1.2, and 1.2 versus 1.1, respectively.We further examined the changes in the individual stilbenic compound profiles (Fig. S1, supplementary data).Quantitative analysis demonstrated an increase in t-ɛ-viniferin (fold change: 2.3 versus 1.1), t-resveratrol (1.7 versus 1.0), t-piceatannol (1.7 versus 0.5), and t-piceid (1.2 versus 0.5) at 20 days compared to 15 days (Figure 3).

Correlation between total antioxidant activity and TPC, TFC and TSC
To assess the relationship between total antioxidant activity (TAA) and phenolic compounds in root extracts under drought stress treatments for 15 and 20 days, Pearson correlation analysis was conducted.The analysis revealed significant correlations (p < 0.01) between the total polyphenol content (TPC) of the extracts and TAA.The correlation coefficients (r values) were found to be 0.953** and 0.972** at 15 and 20 days of drought stress treatments, respectively (Table 1).The total flavonoid content (TFC) showed a significant correlation (p < 0.01) with TAA only at 15 days of drought stress treatment, with an r-value of 0.930**.However, no significant correlation was observed between TFC and TAA at the prolonged stress duration of 20 days.The non-flavonoid stilbenic compounds exhibited a significant (p < 0.01) correlation with TAA.Specifically, the non-flavonoid stilbene (TSC) showed a significant and positive correlation (p < 0.05) with TAA at 20 days, with an r-value of 0.953**.Similarly, the individual stilbenic compounds demonstrated significant correlations with TAA, as indicated by their respective r values of 0.971** for t-resveratrol, 0.933** for t-piceatannol, 0.959** for t-ɛ-viniferin, and 0.911* for t-piceid.These findings suggest that phenolic compounds, including specific stilbenic compounds, positively contribute to the antioxidant capacity of the extracts by reducing levels of free radicals.

Drought stress-enhanced antibacterial activity
The antibacterial activities of the ethanolic extracts of irrigated and drought-stressed Vitis sylvestris "Tebaba" roots were evaluated, and the results are presented in Table 2. Results indicate the effectiveness of the extracts against the tested bacteria.Both control and drought-stressed aqueous extracts from "Tebaba" roots demonstrated interesting antibacterial activity against all tested bacteria.At 15 days of stress exposure, the MIC value was 250 μg/mL for both well-watered and water stress treatments against the bacterial strains.
However, after a prolonged stress period of 20 days, the root extract from drought stress treatment exhibited a higher antimicrobial activity than that obtained from the well-watered treatment.The extract from drought stress treatment showed MIC values between 125 and 250 µg/mL against tested bacterial strains.However, the extract from well-watered treatment showed a MIC value of 500 µg/mL against all tested bacteria.The MBC/MIC ratio was also determined for tested extracts.Interestingly, root extracts from 15 and 20 days of stress exposure and well-watered treatments exhibited bactericidal effects as MBC/MIC ratio ranged between 1 and 2 (Table 2).

Drought stress enhances antioxidant defense by inducing phenolic compounds accumulation
environmental stresses led to disturbance in the cellular redox homoeostasis, which in turn, induces secondary oxidative stress and the production of reactive oxygen species (ROS) (Huang H et al. 2019).The intensity of damage caused by ROS mainly depends on its balance between production and elimination by the antioxidant scavenging system.Secondary metabolic products, which are intensively synthesized under drought, acts as antioxidants (Nascimento and Fett-Neto 2010).A significant increase of TPC, TFC, and TAA after 15 days was observed to achieve the highest values at 20 days of drought stress treatment (Figure 2).These findings corroborate those previously reported by Weidner et al. (2009) who observed an increase in TPC of grapevine roots under drought stress.Previous studies have also indicated that drought tolerance of "Tebaba" genotype involves an efficient antioxidant defense to protect cell membranes and photosystems from oxidative damage (Azri et al. 2020).The application of drought stress treatment at different phenological stages results in higher concentrations of phenolic compounds (Matthews and Anderson 1988;Deluc et al. 2009;Savoi et al. 2016).In this study, we investigated the content of polyphenols, flavonoids and non-flavonoid stilbenes during the drought stress treatment since they act as antioxidants, protecting the cells from oxidative damage (Waffo Teguo et al. 1998;Privat et al. 2002;Biais et al. 2017).Our results indicated that long-term water deprivation significantly increased the stilbene content from 0.8 (FC) to 1.9 (FC) at 15 compared to 20 days of stress, respectively (Figure 3) which is on line with the results obtained in cultivated grapevines (Deluc et al. 2011;Herrera et al. 2017).The antioxidant activity of the root extract from 15 days of drought stress treatment was significantly correlated with TPC and TFC.However, after 20 days of prolonged drought stress treatment, stilbenic compounds and TPC were significantly correlated with the TAA (Table 1).Our results are also in accordance with Pérez-Álvarez et al. ( 2021) indicating a close relationship  between the antioxidant activity with total phenolic and flavonoid content in drought-stressed Monastrell Grapes as well as for the stilbene compounds (Gris et al. 2011).The in vivo elicitation through inducing the natural plant defense is a useful strategy to improve the plant's health properties (Cantos et al. 2003) and could be used for antimicrobial application (Christopher et al. 2021).Stilbenes possess several antipathogenic properties including antibacterial (Peng et al. 2008), antifungal (Schnee et al. 2008;Viret et al. 2018), and insecticidal (Gabaston et al. 2018).Studies have demonstrated that monomeric stilbenes like piceatannol (Piotrowska et al. 2012) and oligomeric stilbenes such as viniferins (Nivelle  Furthermore, natural and synthetic stilbenic compounds have antimicrobial activity against some pathogenic bacteria such as S. aureus and MRSA strains (Mattio et al. 2020).

Stress-induced phenolic metabolites enhance the antimicrobial potential of wild grapevine root extracts
Plants have developed a natural adaptive response to mitigate environmental stresses.A key aspect of this response is the biosynthesis of stress-inducible phenolics, along with other secondary metabolites (Qaderi et al. 2023).These compounds have demonstrated antimicrobial properties (Christopher et al. 2021).Wild grapevines are a promising source of antibacterial agents.Their genetic diversity and bioactive compounds make them an interesting target for discovering natural antimicrobial substances (Chen et al. 2018).We discovered that root extracts of wild grapevine contained high levels of polyphenolic compounds with increased concentrations over 20 days of severe drought stress.Stress-inducible phenolics conferred potent antioxidant properties upon the extracts.It was also observed that prolonged drought stress treatment for 20 days significantly increased the antimicrobial activity of the root extracts.The MIC values were as low as 125 µg/mL, particularly against MRSA strains.These results showed a similar pattern as observed for phenolic contents (Figures 2a, 3a), indicating that total phenolic contents might be involved in antibacterial activity.These findings align with the study by Smirnova et al. (2010) showing a positive correlation between high levels of polyphenols from leaves of the Siberian medicinal plants (Chamerion angustifolium, Filipendula vulgaris, and Pyrola rotundifolia) and the antimicrobial activity against Escherichia coli strain.In a recent study by Krasteva et al. (2023), it was found that grape seed extracts obtained from red grape varieties exhibited greater antimicrobial activity compared to those from white grapes.This difference was attributed to the higher concentration of total phenols and phenolic compounds present in red grape varieties.The activity of polyphenols on microorganisms may be attributed to the hydroxyl groups of polyphenols that interact with bacterial cell membranes, disrupting their composition and causing the loss of cellular components (Papuc et al. 2017).Moreover, these hydroxyl groups can affect the active sites of enzymes, thereby impairing the metabolic processes of microorganisms (Górniak et al. 2019).The position of the hydroxyl group in the aromatic ring of polyphenols and the length of the saturated side chain have also been implicated in enhancing antimicrobial activity (Cueva et al. 2010;Gyawali and Ibrahim 2014).Wild grapevine roots enriched with flavonoids and stilbenes exhibited a potent bactericidal activity (Peng et al. 2008) This study provides evidence that validates the historical usage of Vitis roots in traditional medicine for treating human diseases (Chiu and Chang 1992;Huang and Lin 1999;Jang et al. 2007).Overall study emphasizes the positive impact of prolonged water scarcity on enhancing antimicrobial properties in the wild grapevine (Vitis sylvestris).This species is, currently, endangered across its entire distribution range (Di Vecchi-Staraz et al. 2009).Conservation efforts are crucial to preserve the genetic integrity and survival of the remaining populations (Nick 2012).The maintenance and conservation of these wild populations are of primary importance as they provide valuable genetic resources for future breeding processes, aiming to develop enhanced health-promoting compounds.

Conclusion
Wild grapevines are a valuable genetic reservoir of bioactive compounds that can be utilized for discovering and developing antimicrobial agents.This potential can be further enhanced by subjecting these plants to stress conditions.The current study contributes to ongoing research focused on exploring alternative strategies that utilize the antimicrobial properties of natural plant agents.When exposed to severe drought stress treatment, the wild grapevine root extracts showed an increase in phenolic content and antioxidant capacities.Importantly, these extracts exhibited bactericidal activity against human bacterial pathogens.Therefore, inducing stress-responsive phenolic metabolites offers an effective approach for enhancing the production of bioactive compounds that can be exploited in pharmaceutical industries.all experiments were designed in triplicate and there was no standard deviation as the results obtained confirmed each other.

Figure 1 .
Figure 1.Phenotypic and physiological evaluation of wild grapevine "tebaba" genotype grown under control (c) and drought stress (Ds) treatment for 15 and 20 days (a); changes in shoot elongation rate (b); leaf water potential Ψ leaf (c) after 15 and 20 days of water stress.Bars represent means ± sD of three replicates.asterix represents significant differences between control (c) and drought stress (Ds) treatments according to the t-student test at p < 0.05.

Figure 2 .
Figure 2. changes in total phenolic content (a), total flavonoid content (b), and total antioxidant activity (c) in the roots of "tebaba" grown under control (c) and drought stress (Ds) treatments for 15 and 20 days.Bars represent means ± sD of three replicates.asterix represents significant differences between well-watered and water stress according to the t-student test at p < 0.05.

Figure 3 .
Figure3.stilbenic compounds contents in the roots of "tebaba" cultivated under control (c) and drought stress (Ds) treatments for 15 and 20 days.total stilbene content and the repartition of t-resveratrol, t-piceatannol, t-ɛ-viniferin, t-pterostilbene and t-piceid in roots (a).changes in individual stilbene content of t-resveratrol (b), t-piceatannol (c), t-piceid (d) and t-ɛ-viniferin (e).Bars represent means ± sD of three replicates.asterix represents significant differences between control (c) and drought stress (Ds) treatments according to the t-student test at p < 0.05.

Table 1 .
Pearson's correlation coefficients between taa and phenolic compounds at 15 days and 20 days of drought stress treatments.
Jung et al. (2022)al.2018)rootextracts from 15 and 20 days of stress exposure and well-watered treatments exhibited bactericidal effects as MBC/MIC ratio ranged between 1 and 2. Bactericidal are preferred over bacteriostatic agents in clinical settings, as they are more effective in controlling infections by killing bacteria(Rivas-Cáceres et al. 2018).Similar results regarding the antimicrobial activity of wild Vitis sylvestris root extracts against MRSA strains were reported byPeng et al. (2008)in roots of several wild Vitis species and byJung et al. (2022)using other medicinal plants.

Table 2 .
minimum inhibitory concentration (mic) and minimum bactericidal concentration (mBc) of ethanol extracts and fractions from control (c) and drought stress (Ds) treatments for 15 and 20 days in the root extracts of "tebaba" against Staphylococcus aureus 6538 (S. aureus 6538), methicillin-resistant S. aureus (mrsa) and Enterococcus faecalis (E.faecalis) strains.