**1. Introduction**

Humans have been cultivating plants for nearly 10,000 years ago. Today, any plant growing where it is not wanted is defined as a weed. Weeds represent an important constraint to agricultural production [1]. Weeds represent approximately 0.1% of the world's flora and they evolve with agricultural practices. Weeds can cause declines in crop yields via competition for resources such as light, water, space, and nutrients, and by producing chemical weapons known as allelopathic compounds [2]. Weed management is achieved using several techniques to limit infestation and minimize competition. These techniques evolved to mitigate crop yield losses, but weed control is typically used only after a problem has been identified.

**Citation:** Abd-ElGawad, A.M.; El Gendy, A.E.-N.G.; Assaeed, A.M.; Al-Rowaily, S.L.; Alharthi, A.S.; Mohamed, T.A.; Nassar, M.I.; Dewir, Y.H.; Elshamy, A.I. Phytotoxic Effects of Plant Essential Oils: A Systematic Review and Structure-Activity Relationship Based on Chemometric Analyses. *Plants* **2021**, *10*, 36. https://dx.doi.org/ 10.3390/plants10010036

Received: 16 November 2020 Accepted: 23 December 2020 Published: 25 December 2020

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Scientists and researchers address weed control through physical, chemical, and biological methods. Controlling weeds in an environmentally friendly way is often considered a challenge. Natural resources offer new approaches to producing eco-friendly, and safe bioherbicides that are effective against nuisance weeds. Plants produce the essential oils (EOs) in their various organs as a complex mixture of secondary metabolites such as mono-, sesquit-, and di-terpenoids in addition to hydrocarbons [3,4]. In plants, EOs were biosynthesized via different isoprenoid pathways such as methylerythritol phosphate (MEP) pathway and mevalonic acid (MVA) pathway [5]. The EOs have been described as potent biological agents such as phytotoxic [6–9], antimicrobial [10], anti-inflammatory, antipyretic [11], antiulcer [12], and hepatoprotective [13]. The bioactivities potential of EOs are directly correlated with the quality and quantity of their chemical constituents [6]. Many studies have been performed using the extracted EOs from various plants as phytotoxic chemicals (allelochemicals), where the phytotoxicity is usually correlated to the whole EO profile that contained a mixture of compounds. However, the activity of the EO could be ascribed to a specific compound(s) in the EO. Therefore, in the present review, we try to elucidate a framework of the most frequent and major allelochemicals that were identified in the EOs with a substantial phytotoxic activity using chemometric tools. Additionally, the activities of the authentic identified major compounds are discussed.

#### **2. Materials and Methods**

This review focuses on reports of EOs from plants that exhibit phytotoxic activity published between 1972 and early 2020, using Google, Sci-finder, Google Scholar, PubMed, Elsevier, and Springer databases. Based on the major compounds (those constituting > 5% of the total mass of the EO), the plants were categorized into three groups; mono-, sesqui-, and non-terpenoid–rich compounds. Firstly, the database of EOs rich in monoterpenes derived from plants comprised 46 species belonging to 12 botanical families, including Lamiaceae (18 species), Myrtaceae (nine species), Asteraceae (eight species), Anacardiaceae (three species), and Cannabaceae, Euphorbiaceae, Monimiaceae, Pinaceae, Poaceae, Verbenaceae, Winteraceae, and Apiaceae (a single species each). Additionally, the EOs of these plants were tested against 49 plant species.

Secondly, the plant EOs rich in sesquiterpenes from 25 plant species belonging to eight botanical families were studied. The most represented botanical families were Lamiaceae and Asteraceae (nine plant species each), while Anacardiaceae, Boraginaceae, Fabaceae, Myrtaceae, Simaroubaceae, Verbenaceae, and Chenopodiaceae were represented by a single species. All the EOs of these plants were investigated against 13 plant species. Thirdly, six plant EOs rich in non-terpenoid compounds were identified belonging to Lamiaceae (three species), Apiaceae (two), and Cucurbitaceae (one), were tested against 17 plants.

To assess the correlation of EOs phytotoxic activity and structural compounds, a data matrix of each group was performed as a spreadsheet in MS-EXCEL. A matrix of 42 major monoterpene compounds from 45 plant species was assembled, while a matrix of 26 sesquiterpene compounds, identified in the EOs of 22 plant species was prepared. These matrices were subjected to PCA using XLSTAT software version 14 (Addinsoft, New York, NY, USA).

#### **3. Phytotoxic EOs Derived from Plants Rich in Monoterpenes**

The EOs from different plant species with monoterpenes as the main compounds that exhibited significant phytotoxic activity against various target plant species are presented in Table 1. Zhang, et al. [14] concluded that monoterpene-rich EOs derived from *Eucalyptus salubris*, *E. dundasii*, *E. spathulata*, and *E. brockwayii* strongly inhibited germination and seedling growth in *Solanum elaeagnifolium* relative to commercial Eucalyptus oil and 1,8-cineole. Moreover, the EO of *E. salubris* was found to be the most powerful inhibitor of germination and roots and shoot growth, while *E. spathulata* exhibited the lowest effect [14].


**Table 1.** Monoterpene-rich EOs derived from various reported plants with significant allelopathic activity.

