Synergistic Effect of a Plant-Derived Protein Hydrolysate and Arbuscular Mycorrhizal Fungi on Eggplant Grown in Open Fields: A Two-Year Study

Plant biostimulants, such as plant protein hydrolysates (PHs) and arbuscular mycorrhizal fungi (AM), are natural products capable of increasing the yield and quality of crops and decreasing the ecological impact of plant growing cycles. However, there is little research on the mutual application of different categories of biostimulants (microbial and non-microbial). The current study was conducted to examine the effects of “Trainer” PH application (0 or 3 mL L−1) and AM (R. irregularis) inoculation on the growth, yield, quality and nitrogen indices of ”Birgah” F1 eggplant cultivated for two years (2020 and 2021). Results revealed that the combined application of PH and AM significantly enhanced total and marketable yields, average marketable fruit weight and number of marketable fruits by 23.7%, 36.4%, 19.0% and 11.1% compared to non-treated plants (control), respectively. Moreover, biostimulants increased the soluble solids content (SSC), chlorogenic acid, total anthocyanins, K and Mg in the fruits by 16%, 4.6%, 6.4%, 8.6% and 23.9% compared to control plants, respectively. Interestingly, the mutual application of PH and AM improved fruit quality by reducing the glycoalkaloid concentration (−19.8%) and fruit browning potential (−38%). Furthermore, both biostimulants exerted a synergistic action, enhancing nitrogen use efficiency and nitrogen uptake efficiency by 26.7% and 18.75%, respectively. On the other hand, productive and fruit-quality features were significantly influenced by the year due to remarkable differences in terms of maximum temperature between the first and second cultivation cycles. Overall, our research underlined that PH and AM can positively interact to improve the performance of eggplant cultivated in open fields.