Growth conditions and pathogen inoculation.

An experiment was carried out from Sept. 2018 to Jan. 2019 under greenhouse conditions at the Faculty of Agricultural Sciences of the Universidad Nacional de Colombia, Bogotá (lat. 4°35′56″N, long. 74°04′51″W, altitude 2557 m above sea level). The general environmental conditions during the experiment were as follows: natural photoperiod of 12 h (photosynthetically active radiation of 1500 μmol·m⁻²·s⁻¹ at noon), ≈65% relative humidity, 28.4/12.5 °C day/night temperature, and 20.5 °C average temperature. Two-month-old ecotype ‘Colombia’ (P. peruviana L.) seedlings purchased from a local nursery were used. To rule out F. oxysporum infection, the plants were previously indexed using the methodology described by Leslie and Summerell (2006). Then, seedlings were subjected to an acclimation period for 15 d. When plants developed between three and four true leaves, they were transplanted into 2-L plastic pots, containing a soil-based substrate (clay loam) and rice husk (3:1 v/v) with and without Foph inoculum. Before substrate inoculation, the methodology described by Park (1961) was used to confirm Foph absence in the soil used for the substrate mixture.

Substrate inoculation was performed by adding 100 mL of a liquid Foph suspension at a concentration of 1 × 10⁶ microconidia/mL per 0.9 kg of substrate (soil + rice husk), guaranteeing a final concentration of 1 × 10⁴ microconidia/g of substrate (Osorio-Guarín et al., 2016). Finally, two inoculation conditions were obtained (with and without Foph presence). The Foph-Map5 strain supplied by the Biological Control Laboratory of the Corporación Colombiana de Investigación Agropecuaria (Agrosavia, Mosquera, Colombia) was used as a source of pathogen inoculum in the present experiment. Young mycelium segments of the supplied strain were cultured in 50 mL potato dextrose broth (Difco, Becton Dickinson, Sparks, MD) in 250-mL Erlenmeyer flasks and incubated for 7 d with constant agitation in an orbital shaker (Laboratory-Line, Melrose Park, IL) at 125 rpm and 28 °C under dark conditions (Moreno-Velandia et al., 2019).

Seedlings were irrigated daily with 50 mL of a nutrient solution prepared from a complete liquid fertilizer (Nutriponic; Walco SA, Bogotá, Colombia) at a dose of 5 mL·L⁻¹ H₂O from transplanting to the beginning of the waterlogging periods, and later from the end of the natural drainage to the end of the experiment. The concentration of the nutrient solution was as follows: 2.08 mm Ca (NO₃)₂·4 H₂O, 1.99 mm MgSO₄·7 H₂O, 2.00 mm NH₄H₂PO₄, 10.09 mm KNO₃, 46.26 nM H₃BO₃, 0.45 nM Na₂MoO₄·2 H₂O, 0.32 nM CuSO₄·5 H₂O, 9.19 nM MnCl₂·4 H₂O, 0.76 nM ZnSO₄·7 H₂O, and 19.75 nM FeSO₄·H₂O. The water volume used was obtained by daily quantification of the evapotranspiration needs of the plants by means of the gravimetric technique described by Hainaut et al. (2016).

Waterlogging and synthetic elicitor treatments.

At the time of transplantation, two groups of 40 plants each were obtained: the first group consisted of seedlings planted in substrate with Foph (inoculated), and the second corresponded to seedlings planted in substrate without Foph (noninoculated). Then, the two groups were subjected to a waterlogging period of 6 consecutive d. The stress condition due to waterlogging was imposed by placing the seedlings inside 120-L plastic boxes, to which 60 L tap water was added to guarantee a 5-cm level on the base of the stem 5 d after inoculation (DAI). At the end of the waterlogging period (11 DAI), the seedlings were removed from this condition and natural drainage was allowed until reaching the field capacity in the substrate (18 DAI). Afterward, the seedlings were maintained under normal irrigation conditions until the end of the experiment (51 DAI).

A group of 10 plants was selected to be sprayed with SA, BR, or BE (a commercial product based on E. purpurea, P. erecta, and A. vera extracts). The doses of each synthetic elicitor were 100 ppm, 1 ppm, and 2.5 mL of the commercial product per liter of H₂O, respectively. Eight different groups of treatments were obtained depending on Foph inoculation, waterlogging condition, and synthetic elicitor spray, which are summarized as follows: 1) waterlogged plants without Foph; 2) waterlogged plants with Foph; 3) waterlogged, noninoculated (Foph⁻) plants sprayed with BR, SA, or BE; and 4) waterlogged, inoculated (Foph⁺) plants sprayed with BR, SA, or BE. Likewise, two additional groups with and without Foph inoculation, without waterlogging, and without synthetic elicitors sprays were established as absolute control (Foph⁻) and pathogen control (Foph⁺). The general characteristics of the synthetic elicitors are presented as follows: SA (2-Hydroxybenzoic acid; Panreac Applichem, Barcelona, Spain); Biomex DI-31 [(25 R) 3β,5α-dihydroxy-spirostan-6-one; Minerales exclusivos SA, Bogotá, Colombia]; Loker (E. purpurea, P. erecta, and A. vera extracts, K and Mg salts; Biolchim S. p. A., Medicina, Bologna, Italy).

Foliar synthetic elicitor applications were performed at three different times during the experiment: 1) foliar application 8 d before inoculation, 2) foliar application at the time of inoculation, and 3) foliar application 8 DAI. In general, each synthetic elicitor spray was carried out between 0700 and 0900 hr, using a 1.8-L manual spray pump (Royal Condor Garden, Soacha, Colombia) with an application volume of 20 mL H₂O per plant, wetting the upper and lower leaf surfaces. A coadjuvant [Tween 20 (Merck, Darmstadt, Germany)] was used at a rate of 0.02% (v/v) in all foliar applications. Control plants were sprayed with distilled water and the adjuvant at the same concentration. The length of the waterlogging period, number of foliar applications, and doses used for each elicitor were determined from previous studies (Chávez-Arias et al., 2019). A total of 10 treatment groups were obtained and arranged in a completely randomized design in which each treatment consisted of 10 plants (repetitions). Finally, the experiment lasted 74 d.

Analysis of vascular wilt severity.

Vascular wilt severity was determined from the beginning of the waterlogging period (5 DAI) to the end of the experiment (51 DAI). Severity was estimated through visual assessments of the characteristic symptoms of the disease every 3 d. To perform the evaluation, the scale described by Moreno-Velandia (2017) was followed: 0) asymptomatic plants; 1) slight epinastic response and mild chlorosis of the lower third of the plant; 2) epinastic response in 30% to 50% of the leaves and moderate chlorosis in mature leaves; 3) epinastic response in 60% to 80% of the leaves and moderate chlorosis in the middle third; 4) epinastic response in all the leaves of the plant, severe chlorosis and defoliation; and 5) wilting, severe defoliation, and/or dead plant. The disease severity index can be used to indicate plant performance to compare treatment effectiveness (in this case, synthetic elicitor application) to reduce disease. The disease severity index was calculated using Eq. [1], described by Chiang et al. (2017).$$\text{Disease\hspace{0.17em}severity\hspace{0.17em}index}=\left({{\displaystyle \sum }}^{\text{}}\left(nv\right)/V\right)\hspace{0.17em}\text{,}$$[1]

where n is the level of infection according to the scale, v is the number of plants present in each level, and V is the total number of evaluated plants.

Afterward, disease intensity was analyzed by calculating the area under the disease progress curve (AUDPC), which is a useful quantitative summary of disease intensity over time, for comparison across management tactics (in this case, foliar application of synthetic elicitors). The AUDPC was estimated in each treatment by the trapezoidal integration method shown in Eq. [2] (Alves et al., 2017; Campbell and Madden, 1990):$$\text{AUDPC\hspace{0.17em}}=\left\{{\displaystyle \sum }_{\text{i}=1}^{n-1}\left[\left(y_i+y_{i+1}\right)/2\right]*(t_{i+1}-t_i)\right\}\text{,}$$[2]

where n is the number of evaluations, y_i and y_i+1 are the values of the severity scale at evaluation time, and $(t_{i+1}-t_i)$ is the time interval between evaluations.

Finally, pathogen presence or absence in Foph-inoculated and noninoculated plants was confirmed by isolates in potato dextrose agar (PDA) medium (Oxoid, Basingstoke, UK) from explants taken from the base of the stem at each evaluation time (11 and 51 DAI) (Leslie and Summerell, 2006).

g_S and Ψ_wf.

g_S and Ψ_wf were estimated on the fourth fully expanded leaf from the upper portion of the canopy. A steady-state porometer (SC-1; Decagon Devices Inc., Pullman, WA) was used to estimate g_S. Then, Ψ_wf was measured with a Scholander pressure chamber (Model 615; PMS, Corvallis, OR) on the same leaf used to estimate g_S on completely sunny days at 11 and 51 DAI between 0900 and 1200 hr.

Chlorophyll fluorescence parameters.

The same leaves used to estimate g_S were used for the measurement of maximum quantum yield of PSII (F_v/F_m), qP, NPQ, and ETR using a modulated fluorometer (MINI-PAM; Walz, Effeltrich, Germany). Before taking the measurements, leaves were adapted to darkness using the fluorometer’s clips for 10 min. Then, leaves received a pulse of actinic light of up to 2600 μmol·m⁻²·s⁻¹ on their surface to obtain the fluorescence parameters. These measurements were also taken at 11 at 51 DAI.

Growth parameters.

Leaves, stems, and roots of each plant per treatment were collected separately at 11 and 51 DAI and dried in a compressed dry air oven (Model 27; Thelco, Chicago, IL) at 80 °C for 48 h to estimate their dry weight. The stem diameter for each treatment was estimated using a caliper and LA was obtained from digital images in TIFF (Tagged Image File Format) format using a digital camera (D3300; Nikon, Bangkok, Thailand). Images were analyzed using a Java image processing program (Image J; National Institute of Mental Health, Bethesda, MD). Based on the total dry weight (TDW) of the plants from all the treatments, the relative tolerance index (RTI) was estimated according to Eq. [3] (Dutta Gupta et al., 1995; Roussos et al., 2010).$$\text{RTI\hspace{0.17em}}=\left(\frac{\text{Total\hspace{0.17em}biomass\hspace{0.17em}under\hspace{0.17em}the\hspace{0.17em}interaction\hspace{0.17em}Foph\hspace{0.17em}×\hspace{0.17em}waterlogging}}{\text{\hspace{0.17em}Total\hspace{0.17em}biomass\hspace{0.17em}of\hspace{0.17em}plants\hspace{0.17em}without\hspace{0.17em}stress}}\right)\times \hspace{0.17em}100$$[3]

Leaf photosynthetic pigments.

The equations described by Wellburn (1994) were used to estimate total chlorophyll (TChl) and carotenoid (Cx+c) contents. Leaf samples of 30 mg from the middle third of the canopy were collected and then homogenized in 3 mL of 80% acetone. Then, the samples were centrifuged (centrifuge model 420101; Becton Dickinson Primary Care Diagnostics, Sparks, MD) at 5000 rpm for 10 min to remove particles. The supernatant was diluted to a final volume of 6 mL by adding acetone (Sims and Gamon, 2002). Chlorophyll content was determined at 663 and 646 nm, and carotenoids were measured at 470 nm using a spectrophotometer (Spectronic BioMate 3 ultraviolet-vis; Thermo, Madison, WI).

Proline and MDA content.

The method described by Bates et al. (1973) was used to estimate proline content in each evaluated treatment. Samples of 300 mg from leaves of the middle third of the canopy were homogenized in liquid nitrogen and stored for further analysis. Then, 10 mL of a 3% aqueous sulfosalicylic acid solution was added to the stored samples, which were filtered through Whatman paper (No. 2); 2 mL of this filtrate was reacted with 2 mL of ninhydrin acid and 2 mL of glacial acetic acid. The mixture was placed in a water bath at 90 °C for 1 h, and the reaction was stopped by incubation in ice. The resulting solution was dissolved in 4 mL toluene, and test tubes were shaken using a vortex shaker (V-1; BOECO, Hamburg, Germany). Finally, the absorbance readings were determined at 520 nm with the spectrophotometer. Proline content was calculated using the fresh weight of the sample with a standard calibration curve, as in Eq. [4].[4]$$\frac{\text{μmol\hspace{0.17em}Proline}}{\text{\hspace{0.17em}fresh\hspace{0.17em}vegetal\hspace{0.17em}material}}=\frac{\left[\frac{\left(\frac{\text{μg\hspace{0.17em}Proline}}{\text{mL}}\times \text{\hspace{0.17em}mL\hspace{0.17em}Toluene}\right)}{\frac{115.5\text{\hspace{0.17em}μg}}{\text{μmol}}}\right]}{\left[\frac{\text{g\hspace{0.17em}sample}}{5}\right]}\hspace{0.17em}$$

The thiobarbituric acid method described by Hodges et al. (1999) was used to estimate membrane lipid peroxidation (MDA). Leaf samples of 300 mg from the middle third of the canopy of plants from the different treatments were macerated and stored in liquid nitrogen. Samples were centrifuged at 5000 rpm for 10 min and their absorbances were estimated at 440, 532, and 600 nm with the spectrophotometer. Finally, an extinction coefficient (157 M·mL⁻¹) was used to obtain the MDA concentration. Proline and MDA were measured at 51 DAI.

Experimental design and data analysis.

Data were analyzed by a factorial arrangement with inoculation with the pathogen (with and without Foph) as the first factor, and synthetic elicitors (SA, BR, or BE) as the second factor. Each treatment group consisted of 10 plants. A principal components analysis was performed using the InfoStat 2016 program (analytical software; Universidad Nacional de Cordoba, Córdoba, Argentina) to select the best synthetic elicitors under the interaction between waterlogging and F. oxysporum f. sp. physali (Foph) inoculation. Then, the selected treatments were compared with the absolute control (Foph⁻) and pathogen control (Foph⁺) through a completely randomized analysis. An analysis of variance was performed in all cases, and when significant differences (P ≤ 0.05) were found, a Tukey post hoc test was used for means comparison. The percentage values were transformed using the arcsine function. Data were analyzed using the Statistix v 9.0 software (Analytical Software, Tallahassee, FL). The figures and cluster analysis were carried out using the software SigmaPlot (version 12.0; Systat Software, San Jose, CA).

Vascular wilt severity expressed as AUDPC and disease index.

The evaluation of the AUDPC and vascular wilt index showed differences at P ≤ 0.001 between plants of all Foph-inoculated treatments at 51 DAI. Foph presence was confirmed by isolates in PDA from affected material (Fig. 1). The pathogen was not isolated from plants without inoculation, without any synthetic elicitor sprays, without any stress, or from plants with only waterlogging (Fig. 1A).

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The highest AUDPC values were obtained in inoculated plants with and without waterlogging, and with no synthetic elicitor sprays (80.13 and 76.36, respectively) at the end of the experiment (51 DAI), which may show a higher vascular wilt progress (Table 1; Fig. 1B and C). An intermediate development of the disease was observed in plants with Foph, under waterlogging, and sprayed with BE (68.15), whereas plants with exogenous SA (52.93) and BR (45.73) applications showed the lowest levels of the disease (Table 1; Fig. 1D–F). Trends similar to those obtained in the AUDPC were observed on the vascular wilt index (Table 1).

Table 1.Area under the disease progress curve (AUDPC) and severity index of vascular wilt caused by Fusarium oxysporum f. sp. physali (Foph) in cape gooseberry plants under conditions of hypoxia in the soil and sprayed with synthetic elicitors [brassinosteroids (BR), salicylic acid (SA), or botanical extracts (BE) of Echinacea purpurea, Potentilla erecta, and Aloe vera] at 51 d after inoculation.

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g_S and Ψ_wf.

Differences were also found on g_S in the interaction between Foph inoculation and synthetic elicitors at 11 (P = 0.0002) and 51 DAI (P = 0.0000) in plants under waterlogging conditions. In general, Foph presence caused a greater g_S reduction in both sampling points. At 11 DAI, foliar sprays, mainly with BR or SA, obtained the highest g_S values (208.9 and 194.3 mmol·m⁻²·s⁻¹, respectively) under waterlogging conditions in nondiseased plants. On the other hand, it was observed that SA sprays favored a higher g_S (116.8 mmol·m⁻²·s⁻¹) only in Foph-inoculated cape gooseberry plants and subjected to hypoxia in the soil (Fig. 2A). At 51 DAI, the plants without pathogen presence continued recording the highest g_S values. In these plants, foliar BR sprays increased this variable under oxygen deficit in the soil (241.7 mmol·m⁻²·s⁻¹). A positive effect on g_S was evidenced when the inoculated plants were treated with BR, SA, or BE (≈132.9 mmol·m⁻²·s⁻¹) compared with diseased plants, with no synthetic elicitor sprays and under waterlogging conditions (79.2 mmol·m⁻²·s⁻¹) (Fig. 2B). Differences between the evaluated factors (presence of the pathogen and synthetic elicitors) were also recorded on Ψ_wf at 11 and 51 DAI (P = 0.0328 and 0.0000, respectively) in waterlogged cape gooseberry plants. Foph presence in waterlogged cape gooseberry plants generated more negative Ψ_wf values at both sampling moments. At 11 DAI, SA or BR sprays favored the water status of noninoculated plants through a higher Ψ_wf under hypoxia conditions (≈−0.53 Mpa) (Fig. 2C).

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On the other hand, foliar BR, SA, or BE sprays had a positive effect on Ψ_wf (≈−0.74 Mpa) in Foph-inoculated cape gooseberry plants and subjected to hypoxia conditions in the soil (Fig. 2C). At 51 DAI, noninoculated plants and with BR sprays showed a slight increase in Ψ_wf values (−0.22 Mpa) under waterlogging conditions. Likewise, foliar BR, SA, or BE sprays under oxygen deficiency in the soil caused an increase of Ψ_wf in Foph-inoculated plants (≈−0.48 Mpa) compared with diseased and untreated plants (−0.71 Mpa) (Fig. 2D).

Plant growth variables.

Growth parameters (LA, TDW, and stem diameter) of cape gooseberry plants subjected to hypoxia showed differences (P = 0.0002, 0.0000, and 0.0164, respectively) in the interaction between Foph presence and synthetic elicitors at 51 DAI. In general, the group of plants without Foph showed the highest growth parameters compared with inoculated plants under waterlogging conditions (Fig. 3). Foliar BR, SA, or BE sprays favored LA in Foph-inoculated plants (BR 260.1 cm², SA 238.8 cm², and BE 204.4 cm²) under waterlogging conditions. In addition, foliar BR and SA sprays enhanced the same variable in noninoculated (BR 413.2 cm² and SA 373.3 cm²) plants under soil oxygen deficiency (Fig. 3A). The TDW of cape gooseberry plants subjected to hypoxia in the soil was mainly favored in the treatment with BR sprays in both situations of pathogen inoculation (with and without Foph) (4.93 and 6.52 g, respectively). However, SA and BE favored this variable in inoculated cape gooseberry plants under hypoxia (Fig. 3B). Finally, foliar BR, SA, or BE sprays also produced an increase in stem diameter values in plants with and without Foph presence under the condition of oxygen deficit in the soil (Fig. 3C).

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Chlorophyll a fluorescence parameters.

Significant differences were observed between Foph inoculation and synthetic elicitor sprays on the chlorophyll a fluorescence parameters (Fv/Fm, ETR, qP, and NPQ) of cape gooseberry plants under waterlogging conditions at 51 DAI. Foph inoculation caused lower values of the Fv/Fm ratio, ETR, and qP of cape gooseberry plants under waterlogging conditions; however, an opposite trend was obtained with NPQ for these same plants (Fig. 4). BR, SA, or BE sprays showed two effects on the fluorescence parameters in cape gooseberry plants with waterlogging and without Foph. In the first case, the use of synthetic elicitors did not cause any differences on Fv/Fm and qP (Fig. 4A and C); in the second case, the application of these elicitors generated higher ETR (≈10.9) and lower NPQ (≈1.38) values (Fig. 4B and D). On the other hand, Foph-inoculated plants under waterlogging showed an increase in the parameters Fv/Fm (≈0.62), ETR (≈7.71), and qP (≈0.47) after the foliar treatment with BR, SA, or BE compared with inoculated plants subjected to waterlogging and without any elicitor application (0.37, 5.87, and 0.27 for Fv/Fm, ETR, and qP, respectively) (Fig. 4A–C). In contrast, a decrease in NPQ values was registered with the use of synthetic elicitors (≈1.58) compared with diseased, waterlogged, and untreated plants (2.03) (Fig. 4D).

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Leaf photosynthetic pigments.

Figure 5 shows the content of leaf photosynthetic pigments (TChl and Cx+c). Differences (P = 0.0000) were found in the photosynthetic pigment content between the evaluated factors (Foph presence × synthetic elicitors) at the end of the experiment (51 DAI). In general, the content of TChl and Cx+c was lower in Foph-inoculated plants compared with noninoculated plants under waterlogging conditions. BR, SA, or EB sprays generated an increase in the photosynthetic pigment concentration (TChl and Cx+c) in cape gooseberry plants subjected to a waterlogging period for both inoculation conditions (with and without Foph) (Fig. 5A and B).

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MDA production and leaf proline content.

Significant differences were observed between Foph inoculation and synthetic elicitor sprays on MDA production (P = 0.0000) and leaf proline content (P = 0.0008) of cape gooseberry plants under waterlogging conditions at 51 DAI. Regarding these biochemical variables, opposite effects were observed in waterlogged cape gooseberry plants under both inoculation conditions (with and without Foph). A reduction of lipid peroxidation (MDA) was obtained with the application of elicitors (BR, SA, or BE), whereas a higher proline production was recorded under the foliar sprays of these inducers (Fig. 5C and D).

Biplot analysis of physiological and biochemical responses to Foph management with synthetic elicitors under waterlogging conditions.

The principal component analysis showed that the evaluated variables explained 96.6% of the physiological response of cape gooseberry plants infected with Foph, subjected to waterlogging, and treated with synthetic elicitors at 51 DAI. Figure 6A shows the evaluated variables represented by vectors and the plants of the different treatments identified with points. The vectors generated for the variables g_S, Ψ_wf, TDW, LA, stem diameter, F_v/F_m, ETR, qP, TChl, Cx+c, and proline present angles close to the origin, showing that there is a high correlation between these variables and the plant physiological behavior. It was observed that Foph-inoculated plants without the application of elicitors under waterlogging conditions (plants + Foph + W) form a single group (I). It can be inferred that this group presented the greatest effects due to the evaluated interaction of biotic and abiotic stresses. In contrast, noninoculated plants, subjected to a short waterlogging period (6 d) and with and without BR, SA, or BE sprays (group III), were located in the sector opposite to group I, observing a positive effect of the treatments on plant physiology. On the other hand, plants with Foph under waterlogging and sprayed with the synthetic elicitors (BR, SA, or BE) showed an intermediate behavior (group II), evidencing a positive effect of the treatments with elicitors on the mitigation of the interaction between the two stresses (Foph + waterlogging). This analysis is corroborated by the cluster analysis, with the dendrogram reflecting a lower Euclidean distance between waterlogged plants with and without Foph inoculation and synthetic elicitor sprays (BR, SA, or BE). This group of plants is notably farther from the plants inoculated with the pathogen and without synthetic elicitor sprays under waterlogging conditions (Fig. 6B).

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Comparative analysis of vascular wilt and waterlogging mitigation by synthetic elicitor sprays.

Table 2 shows the effect of foliar BR, SA, or BE sprays on Foph-inoculated cape gooseberry plants under waterlogging conditions (plants + W + Foph), Foph-inoculated plants without the waterlogging condition (plants + Foph), plants without Foph inoculation under waterlogging conditions (plants + W), and noninoculated plants without waterlogging and without elicitor sprays (plants without stress) at 51 DAI. These results demonstrated that three foliar BR, SA, or BE sprays helped plants to cope with the combined condition of vascular wilt and waterlogging, because a positive stimulus of these elicitors was observed on physiological and biochemical variables, such as g_S, Ψ_wf, TDW, LA, TChl, Cx+c, F_v/F_m, ETR, MDA, and proline. In addition, the RTI based on the TDW corroborated that the foliar applications with BR, SA, or BE help to tolerate the double-stress condition in cape gooseberry plants, observing an RTI between 61.3% and 71.4% compared with the RTI of 51.3% observed in plants subjected to the two stress conditions (Foph and waterlogging) (Fig. 7).

Table 2.Comparative analysis of vascular wilt and hypoxia in the soil mitigation in cape gooseberry plants by synthetic elicitor [brassinosteroids (BR), salicylic acid (SA), or botanical extracts (BE) of Echinacea purpurea, Potentilla erecta, and Aloe vera] sprays on stomatal conductance (g_S), leaf water potential (Ψ_wf), total dry weight (TDW), leaf area (LA), stem diameter (SD), total chlorophyll (TChl), carotenoids (Cx+c), maximum PSII efficiency (F_v/F_m), electron transport rate (ETR), MDA production, and leaf proline content at 51 d after inoculation (DAI).

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