## 3 Materials and Methods  
### Materials  
Three kernels from the following fruit varieties were objects of analysis: \"Modesto\" (apricot), \"Stanley\" (plum), and \"Stendesto\" (plum-apricot hybrid). The kernels were collected from several trees of the same variety. All trees were part of the same plantation. A total of sixty kernels per variety (\"Modesto\", \"Stendesto\", and \"Stanley\") were gathered and thoroughly cleaned from the pulp. All kernels were extracted from ripe fruits. The kernels were left to air-dry, and then were broken into pieces with a Bosch MCM3 PM386 robot (Robert Bosch GmbH, Germany) and after that ground with the aid of WMF 0417070011 grinder (Tefal OBH Nordica Group AB, Sweeden). Each kernel variety was placed in a sterile container and kept in a cool, dark, and dry place prior to analysis. Headspace Solid-Phase Microextraction (HS-SPME) and Gas Chromatography-Mass Spectrometry Analysis (GC-MS)  
A 2 cm SPME fiber assembly Divinylbenzene/Carboxen/Polydimethylsiloxane (DVB/CAR/PDMS, Supelco, Bellefonte, PA, USA) was utilized for headspace sampling. The HS-SPME extraction technique of the studied kernels followed the description of [PERSON] et al. [48]. An online integrated sampling procedure was automatically performed with a G1888 Network Headspace Sampler. An Agilent 7890A GC unit coupled to an Agilent 5975C MSD and a DB-5 ms (30 m \(\times\) 0.25 mm \(\times\) 0.25 \(\upmu\)m) column were used to  
Figure 6: Pearson’s correlation heatmaps of the differential compounds in studied kernels: (A) primary metabolites; (B) VOCs. The values were normalized by log\({}_{10}\) transformation. analyze the volatile compounds in all samples. The oven temperature program was the following: from 40 \({}^{\circ}\)C (hold 1 min) to 250 \({}^{\circ}\)C (hold 5 min) at 2 \({}^{\circ}\)C/min; carrier gas: helium with flow rate: 1.0 mL/min; transfer line temperature: 270 \({}^{\circ}\)C; ion source temperature: 200 \({}^{\circ}\)C, EI energy: 70 eV, mass range: 50 to 550 m/z at 1.0 s/decade. The extraction procedure of the polar and lipid fractions was completed as described by [PERSON] et al. [49]: 0.05 g freeze-dried material was mixed with 1.0 mL methanol/water (75:25, \(v/v\)) solution and 50.0 \(\mu\)L of each internal standard (nonadecanoic acid methyl ester, ribitol, each in concentration 1.0 mg/mL for the quantification of metabolites of fractions A, and B, respectively), followed by heating at 70 \({}^{\circ}\)C for 1 h in a laboratory thermomixer (Analytik Jena AG). The solution, cooling to room temperature, was subjected to the following procedure: 500.0 mL chloroform and 200.0 mL water were added, and then the mixture was centrifuged (5 min/22 \({}^{\circ}\)C/13,000 rpm). The lower phase was designed for the analysis of non-polar substances (fraction A), whereas the upper phase for the polar constituents (fraction B). The two phases obtained were vacuum-dried in a centrifugal vacuum concentrator (Labconco Centrivap) at 40 \({}^{\circ}\)C. To the dried residue of fraction \"A\", 1.0 mL 2% H\({}_{2}\)SO\({}_{4}\) in methanol were added and the mixture was heated on Thermo-Shaker TS-100 (1 h/96 \({}^{\circ}\)C/300 rpm). After cooling, the solution was extracted with n-hexane (3 \(\times\) 500.0 mL). Combined organic layers were vacuum-dried in a centrifugal vacuum concentrator (Labconco Centrivap) at 40 \({}^{\circ}\)C.  
Prior to the gas chromatography-mass spectrometry (GC-MS) analysis, fractions \"A\" and \"B\" were derivatized by the following procedures: 100.0 \(\mu\)L pyridine and 100.0 \(\mu\)L BSTFA were added to the dried residue (fraction \"A\"), then heated on Thermoshaker, Analytik Jena AG, Germany (45 min/70 \({}^{\circ}\)C/300 rpm). 1.0 \(\mu\)L from the solution was injected into the GC-MS; 300.0 \(\mu\)L solution of methoxyamine hydrochloride (20.0 mg/mL in pyridine) was added to dried residue (fraction \"B\"), and the mixture was heated on Thermo-Shaker TS-100 (1 h/70 \({}^{\circ}\)C/300 rpm).