*2.3. Sensory Evaluation*

Sensory measurements were made based on questionnaire results. Twelve trained tasting panellists evaluated 10 quality attributes (Table 1) according to ISO 13,299 standard. First, the sensory attributes and their corresponding reference values were determined, to reduce the variation in the resulting dataset. For the sensory test, small pieces of the fruit were boiled for 20 min, placed in numbered plates, and were immediately assessed. The questionnaire results were converted into a grade scale from 0 to 100. The combined results of the properties were plotted on profile diagrams, which were prepared by ProfiSens, a sensory analysis software. Tests were performed according to ISO 8589, and differences between data were evaluated with univariate ANOVA and Fisher least significant difference (LSD) significance level evaluation procedures.

**Table 1.** Definitions of sensory attributes used in the quantitative descriptive analysis.


#### *2.4. Statistical Procedures*

The experiment was arranged in a completely randomized design (CRD) with four replications and five plants in each replication. Data were statistically analysed using Statistix 8 software (Tallahassee, FL, USA). Data were subjected to the one-way analysis of variance (ANOVA) and means were separated using the least significant difference (LSD) test at *p* < 0.05.

#### **3. Results**

As shown in Table 1, grafting cv. 'Madonna' onto 'O and E rootstocks caused the lowest fruit length in comparison with the SR and SG rootstocks. Moreover, it has been observed that self-grafting cv. 'Madonna' significantly increased the width of fruit relative to the fruit harvested from control and other combinations (except ST). Statistical analysis showed that fruit shape index was not significantly (*p* < 0.05) influenced by rootstocks.

Our results showed that the total marketable fruit number significantly increased by grafting in comparison with SR and SG. Average fruit weight decreased by grafting onto O rootstocks in comparison with other treatment (except SG). Grafting cv. 'Madonna' on eggplant rootstocks (SH, ST, SI, and A) significantly increased total marketable fruit yield while tomato rootstocks (E and O) had no significant effect. Total marketable yield of *Solanum torvum* (ST) rootstock was two times higher than control, self-grafted plants (Table 2).


**Table 2.** Effect of different rootstock combinations on eggplant fruit shape, fruit number, and yield.

NS = not significant; \*\* = Significant at *p* ≤ 0.01, \*\*\* = Significant at *p* ≤ 0.001. Different letters indicate significant difference according to the least significant difference (LSD) test (*p* < 0.05). SR = self-root; SG = self-grafting; SH = *S. grandiflorum* × *S. melongena*; ST = *S. torvum*; SI = *S. melongena* × *S. integrifolium*; A = *S. integrifolium*; E = Emperador; O = Optifort.

According to ANOVA, L\* value and CIRG index (Color index of red) of fruit skin from all combinations was not significantly (*p*<0.05) different with the self-rooted (SR) plants. GraftingMadonna onto O, E, A, and SH significantly increased the a\* value of skin fruit relative to SR and SG (Table 3). Fruit harvested from O rootstocks had the highest b\* value compared with the SR. Our results showed that the highest Hue value was observed at E and O rootstock in comparison with SR and SG. Chroma value of skin fruit increased when O, E, and A was used as rootstock compared with either non-grafted or self-grafted (Table 3).


SH 25.36 a 3.62 abc −0.71 bc 3.70 abc 347.70 abc 6.53 a ST 25.58 a 3.32 bcd −0.75 bc 3.40 bcd 346.68 bcd 6.51 a SI 25.08 a 3.27 cd −0.79 c 3.72 cd 346.03 cd 6.65 a A 25.61 a 3.65 abc −0.69 ab 3.71 abc 348.69 ab 6.47 a **Tomato Rootstocks** E 25.53 a 3.89 ab −0.67 ab 3.95 ab 348.92 ab 6.46 a O 25.45 a 4.08 a −0.611 a 4.14 a 349.51 a 6.46 a CV 4.35 20.15 −22.19 28.66 1.26 5.03 *p* Value 0.47 0.001 0.006 0.001 0.002 0.084 Significance NS \*\*\* \*\* \*\*\* \*\* NS

**Table 3.** Effect of different rootstock combinations on chromatic characteristics of eggplant fruit skin.

NS = not significant; \*\* = Significant at *p* ≤ 0.01, \*\*\* = Significant at *p* ≤ 0.001. Different letters indicate significant difference according to the LSD test (*p* < 0.05). SR = self-root; SG = self-grafting; SH = *S. grandiflorum* × *S. melongena*; ST = *S. torvum*; SI = *S. melongena* × *S. integrifolium*; A = *S. integrifolium*; E = Emperador; O = Optifort.

The results of pulp colour measurements are presented in Table 4. No significant difference was observed in L0\*, a\*, CD and DW0 between grafted and SR plants. Moreover, our results showed that grafting onto tomato rootstocks (E and O) significantly decreased OP value of pulp fruit relative to SR and SG.


**Table 4.** Effect of different rootstock combinations on chromatic characteristics, colour difference, whiteness degree, and oxidation potential of eggplant pulp.

NS = not significant; \* Significant at *p* ≤ 0.05. Different letters indicate significant difference according to LSD's test (*p* < 0.05). SR = self-root; SG = self-grafting; SH = *S. grandiflorum* × *S. melongena*; ST = *S. torvum*; SI = *S. melongena* × *S. integrifolium*; A = *S. integrifolium*; E = Emperador; O = Optifort. CD = colour difference; DW = whiteness degree; OP = oxidation potential.

As shown in Table 5, Brix value of fruit flesh was negatively influenced by grafting onto SI, E, and O rootstocks, while SG resulted the highest Brix value in the pulp. Fruit firmness significantly decreased by grafting (except grafting onto SH rootstock). The pH value of the flesh was not influenced by different rootstocks.

**Table 5.** Effect of different rootstock combinations on pH, Brix, firmness, and seed number of eggplant fruits.


NS = not significant; \*\*\* = Significant at *p* ≤ 0.001. Different letters indicate significant difference according to the LSD test (*p* < 0.05). SR = self-root; SG = self-grafting; SH = *S. grandiflorum* × *S. melongena*; ST = *S. torvum*; SI = *S. melongena* × *S. integrifolium*; A = *S. integrifolium*; E = Emperador; O = Optifort.

In the current experiment, grafting eggplant onto SH and SI rootstocks sharply increased the seed number of fruits in comparison with SR (Table 4; Figure 2).

Sensory analysis data showed a significant difference in four parameters (flesh colour, firmness, sweet taste, and intensive odour) between selected rootstocks. Fruits harvested from SR, ST, and O had lighter flesh colour than other rootstock combinations. Respectively, SR and E had the lowest and highest flesh firmness between grafting combinations according to panellists' evaluation. Grafting onto ST showed significantly stronger sweet taste as compared to SR fruits. Fruits harvested from SI showed significantly intensive odour among all combinations (Figure 3). Bitter taste, pungent flavour, off flavour, aftertaste, flesh juiciness, and flavour intensity were not significantly influenced by grafting.

**Figure 2.** Effect of different rootstock combinations on fruit seed number. SR = self-root; SG = self-grafting; SH = *S. grandiflorum* × *S. melongena*; ST = *S. torvum*; SI = *S. melongena* × *S. integrifolium*; A = *S. integrifolium*; E = Emperador; O = Optifort.

**Figure 3.** Effect of different rootstock combinations on sensory evaluation of fruit by trained panellists.

#### **4. Discussion**

Vegetable quality is defined by external attributes, such as colour, shape, size and freshness, as well as internal features, such as texture, flavour, content of mineral, health-promoting compounds,

and sensory parameters. The quality of grafted vegetables can differ and conflicting from the grower and consumer perspectives. However, consumer demand determines economic value and it is an essential aspect in grafted vegetable productions. In the current experiment, we have tested the 'Madonna' eggplant cultivar grafted onto two groups of rootstocks (tomato and other *Solanum* spp.) to find the best combination in terms of yield and quality in both laboratory and consumer evaluation.

Our results showed that the lowest fruit length was observed at tomato rootstock (E and O) and highest fruit width was at SG rootstocks. Fruit shape is controlled genetically; however, Sabatino et al. [6,24] and Gisbert et al. [25] demonstrated that eggplant fruit shape (length and width) is affected by the rootstock. In line with our findings, Passam et al. [26] stated that grafting can increase eggplant fruit size. Moreover, Cassaniti et al. [27] harvested longer fruits from cv. 'Black Bell'—*S. torvum* combination than from self-grafted plants. In cucumber, shape index in grafted plants was higher than non-grafted ones [28], while in watermelon grafting it had no influence on fruit size and shape, according to some studies [29,30]. Therefore, these results are different due to genotype, vigorous rootstocks, and grafting combinations [3]. For instance, grafting tomato onto vigorous rootstocks (i.e., 'Maxifort'; 'Beaufort') increased the fruit size, while grafting onto less vigorous rootstocks (i.e., 'Brigeor'; 'Energy,'; 'Firefly', 'Linea9243,' and 'Nico') reduced tomato fruit size [31–33].

All eggplant rootstocks (SH, ST, A, and SI) improved total fruit yield. Yield, fruit number, and earliness of eggplant cv. 'Cristal F1' grafted onto eggplant rootstocks was higher than tomato rootstocks [25]. In line with our results, Sabatino et al. [19] reported that eggplant cv. 'Scarlatti' F1 grafted onto *S. melongena* × *S. aethiopicum* rootstock had higher total and marketable yield than plants grafted onto *S. aethiopicum*. Earlier, they found that higher marketable yield of grafted eggplant onto *S. torvum* and *S. macrocarpon* can be the result of higher fruit numbers, which associate with increasing water and nutrient absorption [6]. Gisbert et al. [25] confirmed that grafted plants with higher yield had earlier fruit harvesting. In the current experiment, the higher marketable yield is due to a higher harvested fruit number in grafted plants. Our results also correlate with Sabatino et al. [5] who confirmed a higher fruit number of grafted plants than on non-grafted. Higher yield of grafted plants may be related to higher water and nutrient absorption [29].

The darkest skin colour is connected to a high concentration of anthocyanin. Results of Moncada et al. [34] showed that grafting eggplant cv. 'Biragh' onto *S. torvum* caused a darker and less vivid fruit skin colour (lower value of L\* and chroma), while other cultivars ('Black Moon' and 'Black Bell') were not influenced by rootstock. Moreover, the highest Hue value was observed when eggplant cv. 'Scarlatti' was grafted onto *S. torvum* and *S. aethiopicum* rootstocks in an experiment by Sabatino et al. [24]. In our study, the most noticeable finding was that grafting cv. 'Madonna' onto O rootstock resulted in low-quality fruit skin, with the Hue value of 4.14, and chroma of 349.51. In tomato, variable results were found in fruit skin colour by grafted plants compared to non-grafted ones [31,35].

A CIRG index of 5 indicates red dark violet skin, and higher than 6 indicates blue–black skin. In the current experiment, 'Madonna' scion normally had dark black skin and grafting did not have any significant influence on CIRG. Kacjan Maršic et al. [13] reported that grafting eggplant cv. 'Blackbell' onto tomato rootstock cv. 'Beaufort' increased CIRG index, while in another cultivar (cv. 'Epic') decreased. It can be explained that scion variety has different responses to grafting according to Kacjan Maršic et al. [13]. Moreover, they concluded that higher vigour of grafted plants negatively influenced the anthocyanins, and the pruning of plants should be carried out during the cultivation to improve the fruit colour [13].

It seems that tomato rootstocks had lower browning degree and colour difference value than self-rooted and self-grafted plants, and fruit harvested from these combinations oxidized less than others (Table 3). High oxidation potential is expected due to the high phenolic compound content of the eggplant. Moreover, it may be influenced by many factors, e.g., scion genotype, rootstocks, and environmental conditions. Moncada et al. [34] and Sabatino et al. [6] reported that grafting eggplant onto *S. torvum* had little or no effect on pulp browning. However, other rootstocks had different reactions; for instance, *S. paniculatum* and *S. macrocarpon* showed lowest [6], while *S. aethiopicum* gave

the highest oxidation potential in eggplant [3]. Seed number and fruit size may lead to different colour and oxidization in fruit pulp after cutting. A negative correlation between the browning index and fruit size, and positive relation with seed number, has been reported by Radicetti et al. [36].

Several studies confirmed that fruit quality seldom varied significantly between grafted and non-grafted plants. For instance, previous results by Arvanitoyannis et al. [37] and Cassaniti et al. [27] confirmed a reduction in eggplant flesh firmness by grafting. Similarly, tomato fruits harvested from grafted plants were not firmer than control and self-grafted plants [31,38]. Lower water uptake in non-grafted plant caused lower water content in eggplant fruit and harder texture [37]. The current experiment showed that grafting significantly reduced TSS of eggplant cv. 'Madonna'. In line with our results, lower TSS was observed in eggplant fruits (cv. 'Faselis') obtained from grafted on *S. torvum*, while no significant difference was found in fruits (cv. 'Rima') obtained from tomato hybrid rootstocks compared to the rest of combinations [39]. Lee et al. [40] reported that grafting eggplant onto *S. torvum* had no influence on TSS. Previous studies showed that firmness and SSC of non-grafted and self-grafted melon was higher than grafted fruit in the Guan et al. [14] study, and they explained that it is due to the improved water status of grafted melon by enhancing leaf water potential, leaf stomatal conductance, transpiration rate, and amount of xylem sap, which then can reduce TSS of the fruit [14,41]. Fruits obtained from SH and SI × 'Madonna' combination had the highest seed number and it can be explained that higher nitrogen absorption in the grafted plant may cause a higher seed number in fruit [42].

Combining sensory and instrumental measurements provided a thorough evaluation of the effects of grafting and rootstock combination [14]. Grafting eggplant cv. 'Tsakoniki' onto *S. torvum* resulted in fewer empty spaces and harder fruits more than in case of other combinations [37]. Moreover, they reported that fruits grafted on *S. sisymbriifolium* had fewer seeds and less intensive tart flavour than the rest of the treatments. Another interesting finding was that all grafted plants resulted in less sweet fruits and lower acceptability ratings in the panellist test [37]. In our experiment, fruit harvested from ST had a sweeter taste and lighter colour, and fruit harvested from SI showed intensive odour, according to consumer evaluation. In tomato, according to the experiment of Di Gioia et al. [20], grafting onto cv. 'Beaufort' and 'Maxifort' rootstocks did not influence sweetness, sourness, and the tomato-like taste of the fruits. Barrett et al. [21] and Casals et al. [43] reported that grafting had negative effects on acceptability and the tomato flavour descriptors assessed by a consumer test.

In the *Cucurbitaceae* family, Guan et al. [14] reported that grafting Galia melon onto hybrid squash rootstocks caused lower sensory properties, while Honey Yellow cv. was not influenced by grafting. They explained that lower SSC can be one of the reasons of the lower sensory evaluation results. Similarly, in another experiment, Velkov and Pevicharova [19] confirmed that sensory evaluation of cucumber depended on scion-rootstock combinations and every scion showed a different reaction to the same rootstock.

#### **5. Conclusions**

In the present study, we examined the effect of grafting onto different rootstocks in case of an eggplant variety favoured by consumers. Either tomato or other *Solanum* species are recommended for the grafting of eggplant; thus, we also selected the rootstocks to be tested accordingly. Compared to previous studies, we also observed differences between each grafting combination, both in terms of yield and fruit quality.

In our study, *Solanum torvum* × Madonna, as well as *Solanum grandiflorum* × *Solanum melongena* × Madonna combinations, exhibited the highest marketable yield with a higher fruit number per plant and uniform skin colour. Our findings also revealed that tomato rootstocks (O and E) had the lowest pulp colour difference and oxidation potential.

Our results also confirm that it is very important to choose the right rootstock, primarily according to the goal of grafting to be achieved.

**Author Contributions:** All authors contributed to this research. The design of the experiment was done by N.K., M.M., and N.S.B.I.; M.M. and N.S.B.I. did the recording and processing of the data, as well as the result of the evaluation. The manuscript was written by M.M.; N.K. assisted in writing the paper. N.K. contributed to designing the research and revised the manuscript. The work presented in the paper was conceived within research projects led by N.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry for Innovation and Technology within the framework of the Higher Education Institutional Excellence Program (NKFIH-1159-6/2019) in the scope of plant breeding and plant protection research of Szent István University.

**Acknowledgments:** This publication was founded by EFOP-3.6.1-16-2016-00016. The specialize of the SZIE Campus of Szarvas research and training profile with intelligent specialization in the themes of water management, hydroculture, precision mechanical engineering, alternative crop production.

**Conflicts of Interest:** The authors declare no conflict of interest.
