Fruits and vegetables play an important role in human nutrition and health, serving as sources of vitamins, fats, minerals, oils, and dietary fibers. The shelf-life of fruits and vegetables is a serious concern of economic importance to horticulturists (Islam et al., 2017; Etana et al., 2019). During storage and transportation, fruits and vegetables act as a suitable substrate for fungal pathogens, which cause rot, making fruits unfit for marketing use, thereby leading to huge post-harvest monetary losses (de Jesús Salas-Méndez et al., 2019; Ahmad et al., 2020). More than 20–25% of the harvested fruits and vegetables face decay by pathogens during post-harvest handling in the industrialized countries of the world (Kitinoja et al., 2019). This untoward situation is more severe in the developing countries, where post-harvest losses are estimated to be over 35% because of absence of proper storage facilities and transportation (Abano and Sam-Amoah, 2011; Kitinoja et al., 2019).

Attack of fungal pathogens elicits a broad spectrum of innate immunity in plants, which attempt to restrict or prevent pathogen invasion. Upon sensing the invading pathogen, plants mount a set of general defense reactions, such as cell wall reinforcement, accumulation of antimicrobial proteins (pathogenesis-related proteins), and production of phytoalexins (Dangl and Jones, 2001; Ho and White, 2005; de Freitas et al., 2012; Pandey et al., 2016; Jain and Khurana, 2018; Kumar et al., 2018). Plants also produce secondary metabolites including phenolics for curbing the microbial pathogens (Konuk and Ergüden, 2020). Generation of reactive oxygen species (ROS) is an intrinsic trait of any living cell, such that in normal conditions, ROS appear as inevitable by-products, formed as a result of reduction in molecular oxygen in chloroplasts and mitochondria. Moreover, the crucial role of ROS in the plant fungal interactions has been widely demonstrated (Segal and Wilson, 2018; Wang et al., 2019). Improved activities of catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) enzymes upon treatment with Bacillus subtilis JK-14 strain proved to play a key role in the control of post-harvest peach diseases caused by Alternaria tenuis and Botrytis cinerea (Zhang et al., 2019).

Tomato (Solanum lycopersicum L.) is an important horticultural crop with high nutritional value worldwide (Ghadage et al., 2019; Salehi et al., 2019; Liang et al., 2021). High moisture content and water-soluble nutrients in tomato fruits make them perishable and susceptible to a number of fungal pathogens causing post-harvest decays (Singh et al., 2017). Rots of tomato fruits are incited by fungal pathogens, such as Alternaria alternata, Phoma spp., Didymella lycopersici, Geotrichum candidum, Botrytis cinerea, Fusarium acuminatum, and Rhizopus stolonifer (Petrasch et al., 2019; Chudinova et al., 2020). Among filamentous fungi implicated in tomato fruit rots, A. alternata (Fr.) Keissler is a major storage decay biotic factor (Rizwana et al., 2021; Ventura-Aguilar et al., 2021). Furthermore, A. alternata becomes aggressive when the fruit develops injury or becomes debilitated during prolonged storage. Environmental factors have been shown to play crucial role in the development of fungal pathogens in fruits (Davari et al., 2020; Safari et al., 2021). Among these fungal pathogens, tomato fruit rot induced by Alternaria spp. appears as dark brown to black, smooth, slightly sunken lesions that are of firm texture and can become several centimeters in diameter (Yang et al., 2020).

To manage and minimize the post-harvest losses caused by various fungal pathogens in tomatoes for long-term storage, application of fungicides is a common preventive strategy. However, fungicidal applications to tomato crop have raised environmental issues and concerns for human safety, development of races resistant to pests, and residual toxicities (Tongnuanchan and Benjakul, 2014; Dagli et al., 2015; Devi et al., 2021). Application of biocontrol agents such as Trichoderma harzianum and B. subtilis JK-14 strain has been used to control fungal pathogens such as A. tenuis and Alternaria spp. (Zhang et al., 2019; El-Katatny and Emam, 2021). Leaf and fruit extracts of several medicinal plants have shown promising results in the effective management of tomato fruit rots. For instance, Annona muricata fruit extracts could control Alternaria spots on tomato fruit during post-harvest management (Rizwana et al., 2021). Plant extracts of Aloe barbadensis, Vitex trifolia, Allium sativum, Azadirachta indica, Acorus calamus, and Lantana camara have been effectively used to control the growth and sporulation of Alternaria solani (Devi et al., 2017). Similarly, leaf extracts of Moringa oleifera application showed significant reduction in A. solani-caused rot in tomato fruits (Mvumi et al., 2018). Plant-derived essential oils (EOs) also offer an effective management control of fungal pathogens in horticultural crops (Brochot et al., 2017; Wan et al., 2019; Kumar et al., 2020; Zhang et al., 2021). Most EOs are conferred with “generally recognized as safe” (GRAS) status by the Food and Drug Administration, USA (https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/cfrsearch.cfm?fr=182.20; Tongnuanchan and Benjakul, 2014; Dagli et al., 2015; Devi et al., 2021). Antimicrobial activities of Zanthoxylum armatum DC (Dhami et al., 2018; Li et al., 2021; Verma et al., 2021) and Z. alatum (Guleria et al., 2013) EOs have been widely documented. For instance, antifungal activity of Z. armatum EO as an antifungal agent against Aspergillus flavus has been demonstrated (Li et al., 2021). However, application of Z. armatum EO in the management of A. alternata-induced tomato fruit rot is least explored. Green consumerism awareness and desire among common people having fewer synthetic food additives and products and minimum impact on the environment further advocate the use of EOs for the management of fungal pathogens in edible crops (Dangl and Jones, 2001; Anupama et al., 2019; Singh et al., 2021; Tao et al., 2021). Objectives of the present investigation were set forth to evaluate the efficacy of Z. armatum DC fruit EO in controlling the A. alternata-mediated rot in tomato fruits, vis-a-vis its role in modulating antioxidant potential of tomato fruits.

The present investigation evaluated the efficacy of Z. armatum against A. alternata, the causal organism of black rot of tomato. Prior to evaluation of the antifungal activity, the EO of Z. armatum was characterized by GC-MS, revealing the presence of different compounds, with eucalyptol and sabinene being the major components (Figure 1 and Table 1). Interestingly, the composition of Zanthoxylum oil reported in the present investigation varied from an earlier report by Prakash et al. (2012); their report showed 20 compounds with linalool and methyl cinnamate as key constituents of the EO of Z. armatum seeds procured from the herbal market of Varanasi, India. Significantly, such chemotypic variations considerably affect biological activity of the oil. While evaluating antimicrobial potential, Zanthoxylum EO completely checked the growth of A. alternata from a concentration of 0.5–4.5 μl ml⁻¹. Our findings are in concordance with the EO of Z. armatum used to inhibit mycelial growth of Bipolaris sorokiniana (Manandhar and Tiwari, 2005). From the present study, eucalyptol emerged as a major component of the EO of Z. armatum; it may have played role in retarding the growth of the pathogen, A. alternata-induced tomato fruit rot. Moreover, besides eucalyptol, sabinenes present in higher concentration in the EO of Z. armatum may have also contributed toward the management of A. alternata-induced tomato fruit. For instance, the EO of Foeniculum vulgare has demonstrated strong inhibition of A. alternata (Mahmoudi, 2017). Similarly, the EO of Ocimum basilicum showed effective management of A. alternata-induced rot in tomato fruits (Perveen et al., 2020). Their findings showed methyl chavicol and linalool as the primary constituents of the EO; moreover, in vitro experiments demonstrated 88% reduction in fungus growth at the highest concentration of EO applied (Perveen et al., 2020). These observations are also followed in the present study, where the EO of Z. armatum at 75% concentration was able to reduce percent rot development by 84% (Figure 2).

Besides EO, the antioxidant system of plant tissues confers resistance to biotic and abiotic stressors (Guleria et al., 2013; Dhami et al., 2018; Zhang et al., 2021). The present study revealed a significant increase in the activity of antioxidant enzymes (SOD, CAT, GPOX, APX, and GR) of tomato fruits inoculated with A. alternata and treated with Z. armatum EO (Figure 3). These antioxidant enzymes may have worked synergistically to promote scavenging of ROS produced during abiotic and/or biotic stress (Staerck et al., 2017; Zhang and Feng, 2018; Dvorak et al., 2020). Although a plethora of information is available on the use of plant extracts for fungal stress amelioration (Prakash et al., 2012; Zhang and Feng, 2018), there is a paucity of information about the effects of plant extracts on mitigation of fungal stress in plants via modulating an antioxidant (enzymatic) system. Likewise, various non-enzymatic antioxidants (ASA, GSH, and TPC) evaluated in tomato fruits inoculated with A. alternata and treatment with Z. armatum fruit extract also showed a rising trend (Figure 4). Presumably, their accumulation helped in decimating the negative effects of oxidative stress. Among non-enzymatic antioxidants, rapid production of phenols at the infection site depicts initial activation of the defense mechanism in plants, thereby restricting or slowing down pathogen growth (Taheri and Kakooee, 2017). In addition, phenols also restrict the growth of invading pathogen by binding with hydrolytic enzymes released by fungal pathogens during cell division (Taheri and Kakooee, 2017; Zhang and Feng, 2018). Similarly, GSH is a rapidly accumulated antioxidant after fungal attack that may act as a systemic signal, carrying information concerning the attack to non-infected tissues (Staerck et al., 2017; Taheri and Kakooee, 2017; Zhang and Feng, 2018; Dvorak et al., 2020). Furthermore, GSH also functions as a reducing agent for other antioxidants such as ASA (Taheri and Kakooee, 2017). While a correlation between antioxidant capacities and abiotic stress has been demonstrated in several plant systems (Staerck et al., 2017; Taheri and Kakooee, 2017; Zhang and Feng, 2018; Dvorak et al., 2020), similar investigations on the correlation between antioxidant potential and biotic stress have been lacking. A high level of MDA was observed in tomato fruits inoculated with A. alternata. Further, MDA content was decreased by application of Z. armatum EO, suggesting its protective role on membrane damage by fungal infection (Figure 5). Similarly, an increase in PR was observed in EO-treated tomato fruits compared to control. This decrease in MDA upon EO application may be due to the free radical scavenging activity of Z. armatum that scavenge the ROS held responsible for lipid peroxidation (Guleria et al., 2013). An increase in PL content was observed during the present investigation (Figure 5). It might be due to its increased synthesis and reduced degradation of PL during stress. Moreover, it also acts as a molecular chaperon stabilizing the structure of proteins (Verbruggen and Hermans, 2008). Furthermore, there was a reduction in the free radical scavenging activity in the A. alternata-inoculated tomato fruits (Figure 6), which improved in EO-treated tomato fruits due to the improved antioxidant profile.

Application of Z. armatum fruit EO in the management of A. alternata rot at the physiological level showed that EO could decrease incidence of Alternaria rot of tomato considerably. The approach of using botanicals to manage various post-harvest losses without any adverse effects on the consumer's health can be extended to other post-harvest rots of fruits and vegetables.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

YPS and SP conceived and designed the experiments. SS, HRH, and MU performed experimentation. YPS, SP, and NSY analyzed the data and wrote the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.