**4. Discussion**

In the present work we tested the use of various eggplant grafting combinations and AM fungi, alone or in combination, to boost crop performance and fruit quality in "Birgah" eggplant cultivated in a protected environment. Our results showed that enhancements in terms of yield, yield related traits, NUE, and fruit nutritional and functional quality can be accomplished by harnessing the combined effects of specific grafting combinations and AM fungi. These results are in line with those reported by Sabatino et al. [12,13], who—exploring the effects of using eggplant hybrids and allied species as rootstocks on eggplant yield, plant vigor and overall fruit quality—found that *S. paniculatum* and the interspecific hybrid of *S. melongena* × *S. aethiopicum* gr. *Gilo* provided a higher yield performance and a better fruit quality compared to the ungrafted or self-grafted plants. Similarly, our results are in line with those observed by Oztekin et al. [29], who, studying the influence of AM fungi on salinity

tolerance in tomato grafted plants, stated that AM fungi significantly increased the total and marketable yields as well as the average fruit weight. Several authors [42,43] report that the phytostimulating effect of the AM fungi could be attributed to numerous mechanisms such as enhancing the uptake and translocation of major and trace elements, inducing a more developed root system, improving the water status and photosynthetic efficiency, bolstering the antioxidative defense system, balancing plant hormones, upregulating nutrient transporter action and promoting the production of enzymes such as phosphatases. Therefore, we might hypothesize that the higher yield and yield related traits of mycorrhized plants could have similar explanations. Our outcomes regarding the NUE are in agreement with those reported by Djidonou et al. [44], who—investigating the yield, water status and NUE in field-grown, grafted tomato—found that grafting the "Florida 47" tomato onto "Beaufort" or "Multifort" significantly increased N use. Our findings are also consistent with those of Colla et al. [45], who, studying the influence of selected rootstocks on the plant performance and nitrogen use efficiency of the "Proteo" melon, report that the use of melon grafted onto designated rootstocks would represent a prospective plan for increasing crop productivity and NUE. In the present study, AM fungi increased the NUE of eggplant plants. This accords with the results of Zhu et al. [46], who—evaluating the effects of AM fungi on growth, nitrogen uptake and NUE in wheat—found that AM symbiosis increased NUE. Our findings are also in line with those presented by Liu et al. [47], who, studying the influence of AM on NUE in soybean, concluded that AM fungi play an imperative function in increasing NUE. Likewise, Rouphael et al. [43], in a review, report that plant biostimulants, including microbial plant biostimulants, can enhance NUE. Our study showed that both the *S. torvum* and *S. macrocarpon* rootstocks increased SSC values. This is in congruence with the results of Sabatino et al. [12], who used the "Birgah" eggplant F1 hybrid as a scion. However, these findings are in contrast to the outcomes reported by Sabatino et al. [13], who tested the "Scarlatti" eggplant F1 hybrid as an eggplant scion. Furthermore, our results for SSC revealed that AM fungi increase the SSC level in eggplant. However, this is in contrast with previous reports [29] indicating that AM symbiosis does not affect SSC in tomato fruit. Our results on firmness are corroborated by the results of Sabatino et al. [12], who found that *S. torvum* rootstock increased fruit firmness in "Birgah" hybrid and by those reported by Miceli et al. [48], who, assessing the effect of AM fungi and grafting on yield and fruit quality in mini-watermelon, noted that AM symbiosis increases pulp firmness. In our study, at the end of the experiment, the mean mycorrhizal colonization rates were 71.8% and 2.5% in +AM and –AM plants, respectively. There are reports that mycorrhizal hyphae extending into the soil or substrate increase nutrient uptake [43,49] thanks to the mycorrhizal hyphae's capacity to penetrate into very small particles [50]. Our results on fruit mineral composition showed that the B/T, B/M and B/P scion/rootstock combinations improved the K, Ca and Fe fruit contents. Meanwhile, fruits from the B/M grafts revealed a decrease in terms of P content compared to the fruits from the control plots (ungrafted or self-grafted plants). Moreover, the present study showed that AM fungi treatment increased the K fruit content, without affecting the Ca and mg fruit content. In addition, our results highlighted that the B/T and B/P grafts benefit significantly from AM fungi in terms of P and Fe fruit concentrations. These outcomes are in accord with those of Kaya et al. [51] and Oztekin et al. [29], who supposed that a higher fruit mineral (macro and micro) concentration in grafted and mycorrhized plants could be attributed to a positive impact on water and nutrient uptake. Our trial showed that the *S. torvum* and *S. paniculatum* rootstocks increased fruit protein concentrations. This is in agreement with the results of Sabatino et al. [12] but in contrast to those of Sabatino et al. [13], who found no significant differences in terms of fruit protein content between ungrafted, self-grafted or *S. torvum* grafted plants. These results mark the important role played by the scion in terms of nutritional fruit quality in grafted plants. In addition, our results showed that AM fungi augmented the protein concentration in eggplant fruits. This seems to be in accord with the study of Baum et al. [52], who asserted that mycorrhizal inoculation induces a higher accumulation of proteins in onion. Contemporary outcomes exposed that AM symbioses are capable to adjust host plant primary and secondary metabolism, encouraging the synthesis of phytochemicals in the root system and shoots of mycorrhized plants [53]. Our data showed that the *S. torvum* rootstock

significantly increased ascorbic acid in eggplant fruit, whereas fruits from the B/M and B/P grafts did not show significant differences in terms of ascorbic acid compared to the fruits from the control plots, demonstrating that the rootstock plays an imperative role in eggplant fruit quality. This results are partially in accord with those reported by Oztekin et al. [29], who observed that the rootstock factor did not affect vitamin C in tomato fruits, and with those of Miceli et al. [48], who reported that grafting increases fruit ascorbic acid content in mini-watermelon. Conversely, our results on the positive effect of the AM fungi on fruit ascorbic acid content are fully in accord with those of Oztekin et al. [29] and Miceli et al. [48]. There is evidence that the increase in secondary metabolites in mycorrhized plants could be correlated to the higher contents of mineral nutrients [25]. In this regard, overall, our study indicated that AM fungi positively affected total anthocyanins and chlorogenic acid, with the sole exception of with the B/M scion/rootstock combination. Our findings seem to be in line with those reported by Walter et al. [54], who revealed that some graminaceous crops inoculated with AM fungi produce undergo important biochemical modulation leading to phenolic acid accumulation. Our results are in accord with those of other authors [55–57], who, investigating the influence of AM symbiosis on the production of phytochemicals in *Ocimum basilicum*, found higher concentrations of antioxidants when plants were treated with diverse AM fungi species. Baslam et al. [58] remarked that mycorrhized lettuce leaves display higher anthocyanin, carotenoid and phenolic contents than plants from control plots. Castellanos-Morales et al. [59] found that the strawberry mycorrhized plants produced fruits with a higher content of anthocyanidin cyanidin-3-glucoside. Ceccarelli et al. [60] state that artichoke, when treated with two AM fungi species, have increased leaf polyphenolic concentrations and antioxidant activity. Our results on glycoalkaloids support the findings reported by Sabatino et al. [12], who, although not showing a statistically significant effect of the rootstock on glycoalkaloids, highlighted a certain effect of rootstocks on glycoalkaloid content, remarking on the effectiveness of *S. torvum* and *S. paniculatum* in reducing glycoalkaloid concentrations. Furthermore, our results evidenced that +AM treatment had a positive effect on glycoalkaloid reduction which in turn could positively influence human health.

The effectiveness of PCA in interpreting species/cultivar differences across several agronomic and qualitative attributes in response to a wide range of pre-harvest factors such as the genetic material and agricultural practices (i.e., AM inoculation) has been reported previously by several researchers [61–63]. This was also evident in the present study, conducted under protected environment conditions, as the score plot of PCA integrated information on the yield, yield components, NUE, mineral profile and nutritive value of eggplant from five grafting combinations with or without inoculation with AM.

#### **5. Conclusions**

In the present work, eggplant wild/allied relatives' rootstocks and AMF significantly interacted, improving the crop performance and quality traits of "Birgah" eggplant. However, the B/T and B/P grafts combined with AMF stood out as producing the best results in terms of the yield traits, NUE, mineral profile, and nutritional and functional quality. Our findings could be useful information for vegetable crop nurseries interested in introducing new eggplant rootstocks that successfully respond when combined with AMF.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4395/10/5/693/s1, Figure S1: Main effects of arbuscular mycorrhizal inoculation on root colonization.

**Author Contributions:** Conceptualization, L.S., F.D. and G.I.; methodology, L.S. and Y.R.; validation, L.S. and G.I.; formal analysis, B.B.C. and L.S.; investigation, L.S.; resources, F.D.; data curation, B.B.C. and L.S.; writing—original draft preparation, L.S.; writing—review and editing, L.S., G.I. and Y.R.; visualization, L.S., G.I. and Y.R.; supervision, F.D., G.I. and Y.R.; project administration, F.D.; funding acquisition, F.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

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