**4. Conclusions**

The current study showed for the first time the chemical composition of the EO from the aerial parts of *P. lapathifolia*. The EO had 21 compounds, with 58.69% as nonterpenoids and 37.86% as terpenoids. The main compounds were *n*-dodecanal, α-humulene, 2,4 dimethylicosane, 2*E*-hexenoic acid, γ-nonalactone, and limonene. The EO of *P. lapathifolia* showed substantial herbicidal activity against the weed *E. colona*; 100 mg L−<sup>1</sup> of this EO inhibited the germination, seedling root, and shoot growth by 64.25, 82.48, and 95.25%, respectively. Also, the EO exhibited considerable antioxidant activity compared to ascorbic acid as a standard. The present results showed that the emergent leafy stems of aquatic plants such as *P. lapathifolia* have considerably low content of the EO, which exhibited substantial activities such as antioxidant and allelopathic activities. Further study is recommended to evaluate the effects of various environmental and climatic conditions on the production and composition of the EOs of *P. lapathifolia*.

**Author Contributions:** Conceptualization, A.M.A.-E. and A.I.E.; validation, A.M.A.-E., G.B., and A.I.E.; formal analysis, A.M.A.-E. and A.I.E.; investigation, A.M.A.-E., G.B., and A.I.E.; writing original draft preparation, A.M.A.-E., G.B., and A.I.E.; writing—review and editing, A.M.A.-E., G.B., S.A.A.-R., and A.I.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** The Researchers Supporting Project number (RSP-2021/200) King Saud University, Riyadh, Saudi Arabia.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors extend their appreciation to The Researchers Supporting Project number (RSP-2021/200) King Saud University, Riyadh, Saudi Arabia.

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

**Sample Availability:** Samples of the compounds are not available from the authors.
