**2. Taxonomy**

The genus *Ocimum* is known as one of the most prominent genera in the Lamiaceae family and currently comprises more than 150 species [1,25]. The distribution is mainly in the tropical and template regions and is likely to have originated (mainly the holy basil) in India [26]. Recently, they have been cultivated worldwide as culinary herbs and for essential oil extraction [27]. Taxonomical identification within the genus and between the varieties can be made by the morphological characteristics such as leaf shape and its colour, flower, and seed morphology [4,28]. Numerous polymorphisms resulting from extended cultivation and inter- and intra-specific cross hybridisation result in a vast range of subspecies, each with its own chemical makeup and biological activity [25,29,30].

The commonly available *Ocimum* plants were studied in the previous work for their distinct morphological characteristics [6]. The leaf is generally simple, petiolate and the

leaf blade is ovate with a rounded base, oblique, and the apex is acute. *O. gratissimum* has a large leaf size (~45 cm2), whereas *O. citriodorum* has a leaf size of around 3.5 cm<sup>2</sup> (Figure 1c,d). The *O. sanctum* of var. Rama and Shyama can be distinguished by having the aerials of red and white (Figure 1a). Similarly, different leaf and stem colours were noticed, ranging from red, purple-green, and green among the different varieties of the *O. basilicum* L. used in Iran [31]. Singh [32] used the number of leaf veins to show that *O. americanum* was described to have seven distinct veins, and the mid-vein reached the apex, while *O. tenuiflorum* has nine distinct veins, and the mid-vein does not reach the apex. The typical inflorescence of *Ocimum* spp. is a thyrse composed of opposite 1–3-flowered cymes (Figure 1b) [33]. The calyx is generally a short tube or funnel-shaped; it is straight or slightly curved. The corolla is formed forward (sometimes bent downwards), larger upper lip and a smaller lower one and declinate stamens [34]. The posterior lip of the corolla comprises four lobes. There are always four stamens, an anterior pair that attaches near the corolla mouth and a posterior pair that connects close to the corolla base. The size of basil seeds varies depending on the phenotype, cultivating location, and moisture content [35]. Its colour can occasionally be used to differentiate between varieties [36]. The complex polysaccharide structure gives the seed a unique mucilaginous characteristic after soaking in water, which is prominent in *O. citriodorum* and *O. basilicum* var. thyrsiflorum. Table 1 illustrates the taxonomical characteristics of different basil species.

**Figure 1.** Morphological characteristics of some *Ocimum* species; aerial part (**a**) inflorescence (**b**) upper (**c**) and lower (**d**) leaf surface of lemon basil (*O. citriodorum*), Thai basil (*O. basilicum* var. thyrsiflorum), red holy basil (*O. sanctum* var. Rama), white holy basil (*O. sanctum* var. Shyama) and tree basil (*O. gratissimum*).



n/d = no data.

### **3. Volatile Chemical Compositions of Basil Essential Oils**

A number of unique epidermal structures known as trichomes are developed on the surface of the aerial part, which may or may not be secretory [45]. These include the glandular trichomes where the essential oil is localised and the non-glandular trichomes for pest defence [46]. Essential oils are refined lipophilic mixes derived as liquids that possess aromatic properties due to the volatile aroma-active components (i.e., molecules that elicit a distinctive taste and smell) [47]. According to the French Agency for Normalization (AFNOR), the essential oil is defined as follows (NF T 75-006): "The essential oil is the product obtained from a vegetable raw material, either by steam distillation or mechanical processes, from the epicarp of citrus, or dry." [48]. The conventional essential oil extractions

are steam distillation [49,50] and hydro-distillation [51,52]. However, a few techniques have been used to enhance the efficiency of the extraction process, including microwave-assisted extraction [52,53] and ultrasonication [51]. The extraction techniques and processes used to influence the quality and quantity of the extract result in a range of bioactive levels, for example, biopesticide activity against stored-grain pests [51]. Basil plants contain up to 1% of the essential oil, depending on genotypes, cultivation, growing location, and post-harvest managemen<sup>t</sup> [54–57]. The essential oils are more concentrated in leaves and flowers and much less in the stems [58]. In the study of different basils used as culinary herbs, the essential oil yield of white holy basil (*O. sanctum* var. Rama) and Thai basil (*O. basilicum* var. thyrsiflorum) was ~0.4%, followed by lemon basil (*O. citriodorum*) and red holy basil (*O. sanctum* var. Shyama) ~0.3%, and tree basil *O. gratissimum*) was the least (<~0.2%) [6]. Variation of essential oil colours also depends on the genotypes, harvesting stages as well as different extraction techniques [58,59]. Under the visible light, the essential oil of *O. gratissimum*, *O. citriodorum*, *O. sanctum,* and *O. basilicum* var. thyrsiflorum colour are orange, yellow, and colourless, respectively. However, the colour difference is not noticed within the same species, such as those of white and red holy basils (*O. sanctum* var. Rama and Shyama) [6,58,60]. According to this, the volatile chemical compositions of essential oils may play a crucial role in the colour characteristic of the essential oils [61]. Other factors include thermal degradation, oxidation, isomerisation, dehydrogenation, and polymerisation [62–64].

Essential oils are a complex mixture of various classes of volatile organic components such as alcohols, aldehydes, esters, ketones, phenylpropanoids, and terpenoids [65]. Table 2 illustrates the different volatile classes in the essential oils with the representative descriptors of the *Ocimum* plants. The essential oil profiles are displayed by the heat map of mass spectrums of the different volatile components from Thai basil plants (Figure 2). It is apparent that there is the closest relationship between the volatile organic compounds of plants within the same species (white and red holy basils). The phenylpropanoids (estragole, eugenol, and methyl eugenol) are dominant with a proportion of up to 30–50% of analysed compounds, followed by the sesquiterpenes (i.e., trans-caryophyllene, transα-bergamotene, τ-cadinol, cis-α-bisabolene, β-elemene, and germacrene) and monoterpenes (i.e., trans-ocimene, linalool, 1,8-cineole, and camphor) [57,65,66]. The principal constituents of *O. citriodorum* essential oil are estragole, citral, and neral, which serve as crucial fingerprints representing its distinctive citrus scent [6]. Holy basil oil comprises a mixture of 17 volatile compounds with methyl eugenol, trans-caryophyllene, eugenol representing clove-like aroma being dominant [6,66,67]. In the essential oil of *O. basilicum*, estragole is the key volatile element. At the same time, others, such as those of alcohols (i.e., linalool), ketones (i.e., camphor), and esters, are variable among different varieties [68]. It also illustrates that *O. gratissimum* essential oil is enriched with eugenol, trans-ocimene, trans-α-bergamotene, and linalool as the significant components [6,66] projected away from the other basil species. In another study, thymol, eugenol, and geraniol were used as volatile markers to distinguish sub-varieties grown in the USA [69].


**Table 2.** Chemical classes of the volatile organic compounds in the essential oils of the *Ocimum* spp.




 =

1 The Good Scents Company Information System [70]; 2 Zeller and Rychlik [71]; 3 Pripdeevech et al. [72];45678

 =

Genovese et al. [73]; Miyazawa et al. [74]; Jiang and Kubota [75]; Tangpao et al. [6]; Tangpao et al. [66], LB = lemon basil (*O. citriodorum*); RB = red holy basil (*O. sanctum* var. Rama); TB = Thai basil (*O. basilicum* var. thyrsiflorum);TrBtreebasil(*O.gratissimum*);WBwhiteholybasil(*O.sanctum*var.Shyama),n/dnodata.

 =

**Figure 2.** Heat map on volatile organic components in the essential oil of different basil species. The volatile components of different basil species as from the previous studies [6,66]. The heat map was generated using XLSTAT version 2020 (Addinsoft Inc., New York, NY, USA).

#### **4. The Applications of the Basil Essential Oils in the Production of the Tropical Fruits**

Plants synthesise various volatile organic compounds in the essential oils to defend themselves from environmental stresses, both biotic and abiotic [76]. These compounds' beneficial or adverse effects on the living matter are known as "biological activity". Consequently, essential oils have been used in many industrial applications and mainly replace synthetic chemicals [77–79]. During the pre- and post-harvest productions of the horticultural crops, essential oils were tested for their antipathogenic and pest control properties, such as insecticidal agents [80], repellents, attractants [81], and microbial disease controlling agents. In the production of most tropical fruits, the Oriental fruit flies attack the soft-skin fruits by laying eggs and feeding the larvae inside the fruits [66]. At post-harvest, *Colletotrichum* spp. is a major fungus causing anthracnose disease, accelerating the fruit deterioration process [82].
