## 2 Results and Discussion  
The studied kernels were characterized in terms of their VOCs, polar metabolites, and lipids. Information about the compounds found in fruit kernels is scarce or missing. Thus, this is considered a first comprehensive report about the composition of apricot, plum, and plum-apricot kernels. A total of forty-three compounds were identified from the samples (Table 1). Between them, amino acids, organic acids, sugar acids and alcohols, mono- and disaccharides, phenolic acids, and hydrocarbons were identified as existing groups. \begin{table}
\begin{tabular}{c c c c c} \hline
**RI** & **Class/Name** & **Modesto** & **Stanley** & **Stendesto** \\ \hline \multicolumn{5}{c}{**Amino acids**} \\  
\(\gamma\)-Aminobutyric acid & 0.13 \(\pm\) 0.04 ab & 0.16 \(\pm\) 0.05 a & 0.06 \(\pm\) 0.02 b \\
1727 & 2-Aminoadipic acid & 1.85 \(\pm\) 0.59 a & 1.36 \(\pm\) 0.44 a & 1.07 \(\pm\) 0.34 a \\
1816 & Isocitric acid & 0.39 \(\pm\) 0.12 ab & 0.24 \(\pm\) 0.08 b & 0.74 \(\pm\) 0.24 a \\ \hline \multicolumn{5}{c}{**Sugar acids and alcohols**} \\
1264 & Glycerol & 1.75 \(\pm\) 0.56 a & 0.36 \(\pm\) 0.11 b & 0.47 \(\pm\) 0.15 b \\
1541 & Ertrreonic acid & 1.19 \(\pm\) 0.38 a & 0.93 \(\pm\) 0.30 a & 0.52 \(\pm\) 0.17 a \\
1611 & Glutamic acid & 0.43 \(\pm\) 0.14 a & 0.14 \(\pm\) 0.04 b & 0.15 \(\pm\) 0.05 b \\
1695 & Xylitol & 2.30 \(\pm\) 0.74 a & 1.65 \(\pm\) 0.53 a & 0.87 \(\pm\) 0.28 a \\
1718 & Arabitol & 0.68 \(\pm\) 0.22 a & 0.50 \(\pm\) 0.16 a & 0.27 \(\pm\) 0.09 a \\
1801 & Glyceric acid-3-phosphate & 0.21 \(\pm\) 0.07 a & 0.19 \(\pm\) 0.06 a & 0.43 \(\pm\) 0.14 a \\ \hline \end{tabular}
\end{table} 
Table 1: Metabolites (mg/g dry weight) identified in studied kernels analyzed by HS-SPME-GC–MS. Amino acids were found in small quantities, yet the hybrid kernel had managed to keep all the amino acids present in both apricot and plum. The same trend applied to the other identified classes, where the hybrid had inherited all the metabolites present in its parents. A study about apricot fruit and seeds showed a resemblance in the metabolites found in the apricot kernel, although only 36 metabolites were identified [26]. Arginine was the most abundant in both hybrid and plum kernels, while serine had the highest value in the apricot kernel. Arginine is important to human nutrition since research has shown that it can increase lipolytic enzymes' activity and decrease insulin resistance [27]. Serine is also reported as exceptionally important, especially being a substrate for glucose and protein synthesis and building of phospholipids [28]. Mallic and 2-aminoadipic acids were the organic acids with the highest values in the studied kernels. The plum-apricot hybrid had the most malic acid, while the apricot had the most 2-aminoadipic acid. Aminoadipic acid is an object of scientific research due to its recent identification as a biomarker of insulin resistance and obesity [29]. Mallic acid is commonly found in fruits, and it is reported to be an antimicrobial agent combined with citric acid [30]. The current results show that organic acids also accumulate in kernels. Sorbitol had the highest values from the group of sugar acids and alcohols. The sorbitol pathway is a two-step one, where in the first step glucose is converted into sorbitol, and then in the second step sorbitol is converted into fructose [31]. The plum-apricot hybrid's kernel had lower quantities of sorbitol compared to its parents but had accumulated more glucose and fructose as isomers. Sorbitol is an alternative sweetener that is widespread in some _Prunus_ spp. [32]. Sucrose and glucose is formed were predominant in the plum-apricot hybrid kernels. The amount of sucrose in the apricot kernel was 50% higher compared to the hybrid. Kernels from Turkish apricots were also high in fructose, glucose, sucrose, and maltose [33]. Protocatechic acid was the dominant one in the plum-apricot hybrid kernels, while _trans-p_-coumaric acid had its highest values in both its parents.