## 4 Discussion
### Involvement of Key Metabolites in Response to Light Spectra  
Due to its unique and pleasant aroma, volatile organic compounds (VOCs) of basis grown under different light spectra qualities were identified using GC-MS/MS analysis. A total of 1129 metabolites belonging to 16 classes were identified in all samples (Table S1), among which the terpenoids contained a maximum of 251 species and accounted for about 22%. To the best of our knowledge, previous studies have not identified so many VOCs in basil (_O.__susilicum_ Linn. var. _pilosum_ (_Willd._) Benth.) [33; 56], and this study will therefore facilitate our understanding of the chemical components of basil. Furthermore, DAMs were screened to uncover the effects of light spectra on metabolites. All of the DAMs in the three comparison groups were mainly enriched in many of the same KEGG pathways, such as biosynthesis of secondary metabolites, monoterenoid biosynthesis, phenylalanine metabolism, plant hormone signal transduction, limonene and pinene degradation, etc. (Figure 5), suggesting that both red and blue light have significant effects on these pathways. The k-means cluster analysis showed that all of the DAMs could be classified into seven subclasses with different numbers (Figure S3, Table S4). In subclasses 1, 2, and 7, the contents of 373 metabolites were highest under white light. In subclasses 4, 5, and 6, the contents of 378 metabolites were highest under red light. Interestingly, the highest accumulation of 35 metabolites in subclass 3 was observed under blue light. Taken together, it can be seen that metabolites respond to different light qualities in specific ways.  
The top 10 most significant DAMs in each comparison group are listed in Figure 10. Among them, a total of seven significantly upregulated common metabolites were found in both the W vs. R and W vs. B groups, including debrisoquine, butanoic acid, 4-hexenyl ester, propanoic acid, 2-methyl-,phenylmethyl ester, (Z)-, (1 aR,7R,7 aR,7 bS)-(+)-1a,2,3,5,6,7,7a,7b-Octahydro-1,1,7,7a-tetramethyl-1H-cyclopropa [a] naphthalene -3-one, ethyl tridecanoate, and 1H-pyrrolo [2,3-b]pyridine, 2-(1-methylethyl)-, 7-(2-hydroxypropan-2-yl)-1,4a-dimethyldecahydronaphthalen-1-ol, which indicated that both red and blue light significantly promoted the accumulation of these metabolites. Similarly, 2-octanone, 3-octanone, and 5-hexenal,4-methylene- were three common metabolites among the top ten most significantly downregulated in both the W vs. R and W vs. B groups, suggesting that the accumulation of these metabolites was seriously inhibited under both red and blue light. In terms of the R vs. B group, the top 10 most upregulated metabolites included cyclohexacyclic acid, methyl ester, oleic acid, 2,6-piperdiedione, 3-ethyl-,2-hexenal, (E)-, and another 6 metabolites; it is worth noting that cyclohexanecarboxylic acid, methyl ester, oleic acid, and 3-octanol were among the top 10 most downregulated metabolites in the W vs. R group, indicating that these three key metabolites accumulated significantly under white and blue light but not under red light. The top 10 most downregulated metabolites in the R vs. B group consisted of iso-3-thuijl acetate, ketone, methyl 2,4,5-trimethylpyrrol-3-yl, fenchyl acetate, furfuryl pentanoate, pyrazine, 2,3-dimethyl-5-(1-methylpropyl)-, and another 5 metabolites; among them, 10-undecenal, (2E,4Z)-2,4-decadienal, and 3-cyclohexene-1-ethanol, beta.,4-dimethyl- were also included in the top 10 most upregulated metabolites in the W vs. R group, revealing that the accumulation of these three key metabolites was significantly strengthened by red light. Overall, these key metabolites could be used as the characteristic metabolites to distinguish the response of basil to blue, red, and white light.