**3. Materials and Methods**

#### *3.1. Plant Samples Collection and Preparation*

The aerial parts of Saudi *P. undulata* were collected from the Wadi Alsahbaa, Alkharj, Riyadh region (24◦16 34.1 N 47◦56 11.3 E), while the Egyptian sample was collected from Wadi Hagoul, the Eastern Desert, Egypt (30◦00 38.2 N 32◦05 35.5 E), during spring of 2019. The specimens were authenticated according to Tackholm [49] and Boulos [50]. Voucher specimens were prepared and deposited in the herbarium of the Department of Botany, Faculty of Science, Mansoura University with No. Mans.001162117 and Mans.001162118.

The samples were collected from two populations of *P. undulata* in separate plastic bags and immediately transferred to the lab. The samples were dried in a shaded place at room temperature (25 ± <sup>3</sup> ◦C) for 7 days, crushed into powder using a grinder (IKA® MF 10 Basic Microfine Grinder Drive, Breisgau, Germany) at a dimension of 3.0 mm, and packed in paper bags.

#### *3.2. EOs Extraction, GC–MS Analysis, and Chemical Compounds Identification*

About 150 g of the prepared samples of *P. undulata* were extracted with hydrodistillation via a Clevenger-type apparatus for 3 h. The oils were collected, water was removed using 0.5 g of anhydrous sodium sulfate, and stored in glass vials in the fridge (−4 ◦C) till further analysis [29]. Two samples of the plant were extracted by the same protocol afforded two samples of EOs. The two extracted EOs were analyzed via gas chromatography–mass spectrometry (GC-MS) at the National Research Center, Giza, Egypt, as described in our previously documented work [25,26,48,51]. Briefly, the apparatus has TRACE GC Ultra Gas Chromatographs (THERMO Scientific™ Corporate, Waltham, MA, USA), together with Thermo Scientific ISQ™ EC single quadrupole mass spectrometer. The GC–MS system is equipped with a TR-5 MS column (0.25 μm film thickness, 30 m × 0.32 mm internal diameter). Helium was used as a carrier gas at a flow rate of 1.0 mL min<sup>−</sup>1, with a divided ratio of 1:10. The temperature program was 60 ◦C for 1 min, rising by 4.0 ◦C min−<sup>1</sup> to 240 ◦C, and held for 1 min. An aliquot of 1 μL of the EO sample in hexane was injected at a ratio of 1:10 (*v*/*v*), and the detector and injector were adjusted at 210 ◦C. Mass spectra were recorded by electron ionization (EI) at 70 eV, using a spectral range of *m*/*z* 40–450. The chemical compounds identification was accomplished by Automated Mass spectral Deconvolution and Identification (AMDIS) software, as well as Wiley Spectral Library collection, NIST Library database (Gaithersburg, MD, USA; Wiley, Hoboken, NJ, USA), which were used for retention indices relative to n-alkanes (C8–C22), or appraisal of the mass spectrum with authentic standards.
