**4. Conclusions**

The EO composition of the Saudi and Egyptian ecospecies of *P. undulata* showed substantial variation in both quantity and quality. The Saudi ecospecies had 61 compounds, with *β*-pinene, isoshyobunone, 6-epi-shyobunol, *α*-pinene, and *α*-terpinolene as major compounds, while the EO of the Egyptian ecospecies attained a lower number (34 compounds), with spathulenol, hexahydrofarnesyl acetone, *α*-bisabolol, and τ–cadinol as main compounds. This variation could be attributed to the difference in the environmental and climatic conditions. The EO of the Egyptian ecospecies showed more phytotoxic activity against *D. aegyptium* than *B. pilosa*, as well as more phytotoxic, compared with the Saudi ecospecies. This variation might be ascribed to the difference in their major constituents. Therefore, further study is recommended for the characterization of authentic materials of these compounds as allelochemicals against various weeds, either singular or in combination.

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

**Funding:** This study was funded by the Researchers Supporting Project, Number (RSP-2021/403), 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-2021/403), 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.
