## 4 Discussion  
Oleoresin yield is a highly heritable trait that is genetically controlled by quantitative trait loci (QTLs) [36; 37]. To date, little is known about the underlying genetic mechanism of oleoresin biosynthesis in slash pine. In the present study, 20 terpenids were identified in the oleoresin of slash pine using GC-MS, and the major terpenoid compositions are exclusively composed of monoterenes and diterpenenes. Among monoterenes, \(\alpha\)- and \(\beta\)-pinene account for the majority, while myrcene, dipentene, and camphene make up less than 0.1% of the total monoterenes. The diterpenes are mainly composed of isopimaric acid, abietic acid, levopimaric acid, dehydrodiabetic acid, and neoabietic acid. Intriguingly, there are no sesquiterpenes (more than 0.1%) identified in oleoresin of slash pine, whereas sesquiterpenes are one kind of major constituent of turpentine oil in oleoresin from other pine species, such as Masson pine and Aleppo pine [38; 39]. The proportions of \(\alpha\)-pinene, \(\beta\)-pinene, abietic acid, levopimaric acid, and dehydrodiabetic acid of oleoresin from slash pine exhibited significant variation at P4, P7, and P10. The proportion of \(\alpha\)-pinene reached the highest level in oleoresin produced in July, while \(\beta\)-pinene, abietic acid, levopimaric acid, and dehydrodiabetic acid occupied greater percentages in oleoresin harvested in October than in any other months. Moreover, the proportion of these five terpenids in oleoresin of high- and low-yielding slash pines showed remarkable differences. The KEGG enrichment results showed that a total of 34 DEGs of L vs. H were enriched in pathways related to terpenoid backbone biosynthesis, including diterpenoid biosynthesis, carotenoid biosynthesis, and ubiquinone and other terpenoid biosynthesis pathways. In the MEP pathway, the chloroplast-located _dxs_ and _ispF/G_ are rate-limiting enzymes play pivotal roles in synthesizing 1-hydroxy-2-methyl-2-butenyl 4-diphosphate [40; 41; 42; 43]. The homologues of _dxs_ and _ispF/G_ in slash pine were significantly highly induced in the high-yielding trees compared with the low-yielding trees. However, enzymes in the mevalonate pathway, including the cytoplasm-located mevalonate kinase, peroxisome-located phosphomevalonate kinase, and diphosphomevalonate decarboxylase [44; 45; 46], showed nuanced differences between high- and low-yielding slash pines. The intermediate monoprenyl diphosphate is a crucial substrate for the biosynthesis of isoprene under the catalyzation of _ispS_[47]. The upregulated expression of _ispS_ in low-yielding pine was consistent with the fact that oleoresin of low-yielding pine had more isoprene. As for TPS, the homologous proteins of _PT30_ and _QH6_ in slash pine catalyzed the synthesis of \(\alpha\)- and \(\beta\)-pinene, respectively. Moreover, homologous proteins of _SPS_, _ZFPS_, and _idsA_ are responsible for the synthesis of ubiquinone, sesquiterpenoid, and diterpenoid, respectively. According to the previous studies, the homology of _PT30_ in Loblolly pine produces both (+)-\(\alpha\)-pinene and (\(-\))-\(\alpha\)-pinene in a 97:3 ratio [48], and _QH6_ can convert GPP to (\(-\))-\(\beta\)-pinene and (\(-\))-\(\alpha\)-pinene in a 94:6 ratio in Abies grandis and Artemisia annua [49; 50]. Therefore, homologies of _PT30_ and _QH6_ in slash pine are also implicated in controlling not only thecontent of but also the ratio between \(\alpha\)- and \(\beta\)-pinene. The expression levels of _PT30_ and _QH6_ in high- and low-yielding pines coincided with the content of \(\alpha\)- and \(\beta\)-pinenes in the slash pine. Orthologs of plastid-located _SPS_ in Arabidopsis are involved in the biosynthesis of plastoquinone-9 (PQ-9) [51] and can improve the tolerance of these plants to excess light energy [52]. Multiple sesquiterpenes in Solanum Iyopersicum and Solanum habrochaites have been found to be associated with _ZFPS_[53; 54].