**8. Structure-Activity Relationship Summary**

Based on the data presented in Tables 1–3 and correlation analysis between phytotoxic EOs derived from different plants and their major chemical constituents (Figures 1 and 2), we concluded that the phytotoxic activities of EOs increase with terpenoid content, particularly oxygenated terpenoid content. Almost all previous studies found that increasing the oxygenation of terpenoids led to an increase in allopathic activities via inhibition of germination and growth of noxious weeds [6,16,23].

As can be seen in Table 1, oxygenated monoterpenoids are the main components of phytotoxic EOs, and their phytotoxicity was observed to increase with the degree of oxygenation. For example, the mono-oxygenated monoterpenoid 1,8-cineole (eucalyptol, C10H18O), was to be the main compound in several allopathic EOs derived from plants from different botanical families such as *Euphorbia heterophylla* [6], *Callistemon viminalis* [19], *Eucalyptus cladocalyx* [20], *Nepeta flavida* [28], *Majorana hortensis* [40], *Peumus boldus* [43], *Eucalyptus lehmanii* [45], *Tanacetum aucheranum*, *Tanacetum chiliophyllum* [46], *Eucalyptus salubris*, *Eucalyptus brockwayii*, and *Eucalyptus dundasii* [14].

In addition, linalool and borneol were found to be the major compounds in numerous phytotoxic oils, such as *Salvia sclarea* [16], *Artemisia absinthium* [20], *Origanum acutidens* [22], *Eriocephalus africanus* [23], *Nepeta flavida* [28], *Zataria multiflora* [38], *Agastache rugosa* [44], *Salvia officinalis*, and *Ocimum basilicum* [40], and *Tanacetum chiliophyllum* [46]. It is therefore clear that the oxygenated monoterpenoids 1,8-cineole, linalool, and borneol play significant and effective roles as allopathic agents and more research into their phytotoxic activity and phytotoxic mechanism(s) is recommended.

Similarly, careful analysis of sesquiterpene-rich phytotoxic EOs revealed that an increase in oxygenated sesquiterpene levels can enhance the phytotoxic activity of an EO. The data supplied in Table 2 and PCA analysis suggest the major oxygenated sesquiterpenes caryophyllene and its derivatives, as well as hexahydrofarnesyl acetone, can be potent phytotoxic agents. The phytotoxic EOs derived from *Baccharis patens* [29,30], *Heliotropium curassavicum* [7], *Cullen plicata* [50], *Scutellaria strigillosa* [51], *Acroptilon repens* [52], *Lantana camara* [23], *Teucrium arduini*, *Teucrium maghrebinum*, *Teucrium polium*, *Teucrium montbretii* [31], and *Ailanthus altissima* [53] were reported to have all or one of *β*-caryophyllene, (-)-caryophyllene, and caryophyllene oxide as primary compounds. These reports indicate a strong correlation between the phytotoxic activities of EOs and the presence of these compounds as main components. Hexahydrofarnesyl acetone has been described as an essential compound in the phytotoxic EOs of *Launaea mucronata*, *Launaea nudicaulis* [6], *Heliotropium curassavicum* [7], and *Ailanthus altissima* [53]. The authors of these studies also concluded that compounds with hexahydrofarnesyl acetone as a main constituent can play a major role as phytotoxic mediators.

EOs derived from aromatic plants typically consist of low-molecular-weight terpenoids, including mono, sesqui-, and diterpenoids as well as non-terpenoid components [14]. Two plants belonging to the *Nepeta* genus were reported that containing iridoid-rich EOs such as *Nepeta meyeri* [57,58] and *Nepeta cataria* [59]. The two iridoids 4a-*α*,7-*α*,7a-*β*-nepetalactone and 4a-*α*,7-*β*,7a-*α*-nepetalactone were reported to be the main phytotoxic mediators in the EOs of these two species. The two compounds should, therefore, be subjected to further study to evaluate their allopathic abilities against several noxious weeds.

**Author Contributions:** A.I.E. and A.M.A.-E. contributed to the conceptualization, data collection, analysis of data, visualization, and writing of the first draft of the manuscript. A.I.E., A.M.A.-E., A.E.- N.G.E.G., A.M.A., S.L.A.-R., A.S.A., T.A.M., M.I.N., and Y.H.D. contributed to writing—review and editing of the manuscript. All authors discussed the results, commented on the paper, and approved the final manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Deputyship for Research & Innovation, "Ministry of Education" in Saudi Arabia for funding this research work through the project number IFKSURP-113 and the APC was funded also by the same project.

**Acknowledgments:** The authors extend their appreciation to the Deputyship for Research & Innovation, "Ministry of Education" in Saudi Arabia for funding this research work through the project number IFKSURP-113. The authors thank the Deanship of Scientific Research and RSSU at King Saud University for their technical support.

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