## 4 Discussion
### Cannabinoids and Monoterpenes Modulated by R:FR Ratio  
The influence of light on monoterpenes and cannabinoids is always accompanied by up- or down-regulation of the MEP pathway, in which chloroplast isoprenoids are given a core task [47]. As PIFs are direct regulators of the light-modulated expression of the MEP pathway genes [47], the shade avoidance induced by a low R:FR ratio, leading to higher PIF activity based on a reduction in PHYB [35], could therefore be a crucial factor in the modulation of secondary metabolism. The light spectra had the following R:FR ratios: AP67: 4.04, CHD: 2.83, SOL: 13.49. Based on our theory, a high R:FR ratio should result in lower PIF activity. In addition, Pfr, induced by red light, should further reduce their expression and promote the MEP pathway [47]. Therefore, the expected terpene yields in the main top buds were in the order SOL > AP67 > CHD. However, the data indicate that different terpen respond differently to light quality. Higher \(\beta\)-myrcene, ocimene and linalool concentrations were found under both LED spectra compared to CHD. In contrast, the concentration of \(\alpha\)-pinene was higher under CHD in A4 and E19. Considering all of the findings, we cannot confirm our hypothesis that a high R:FR ratio promotes monoterpene synthesis. However, our data indicated that this may be the case for CBDA.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Trait** & **Light Spectra** & **PFFD** & \
\begin{tabular}{c} **PFD\_Red** \\ **(umol m\({}^{-2}\) s\({}^{-1}\))** \\ \end{tabular} & **FFD\_Far-Red** &
\begin{tabular}{c} **Photosynthetic Rate** \\ **(umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\))** \\ \end{tabular} \\ \hline \multirow{3}{*}{A light spectra} & AP67 & 400 & 250 & 70 & 10.23 \(\pm\) 0.33 \({}^{*}\) \\  & CHD & 400 & 179 & 82 & 10.44 \(\pm\) 0.33 \({}^{*}\) \\  & SOL & 400 & 181 & 20 & 7.37 \(\pm\) 0.33 \({}^{*}\) \\ \hline \multirow{3}{*}{A RFR} & R & 400 & 400 & 0 & 7.44 \(\pm\) 0.09 \({}^{*}\) \\  & RFR 2.8 & 400 & 400 & 140 & 8.22 \(\pm\) 0.09 \({}^{*}\) \\ \cline{1-1}  & RFR 4 & 400 & 400 & 100 & 8.37 \(\pm\) 0.09 \({}^{*}\) \\ \cline{1-1}  & RFR 13.5 & 400 & 400 & 30 & 7.64 \(\pm\) 0.09 \({}^{*}\) \\ \hline \(p\)-values, light spectra & & & & & \\ \hline \multicolumn{3}{c}{\(p\)-values, RFR} & & & & 0.0049 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Photosynthetic rate (umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\)) of the strain KAN under the different light spectra (SOL, AP67 and CHD) as well as different R:FR ratios. Results are presented as mean values \(\pm\) standard errors (means \(\pm\) SEs). Means followed by at least one identical lower-case letter did not differ significantly at \(\alpha\) = 0.05, within a strain or flower, as indicated by a Tukey test.  
The CBDA concentrations measured in the main top buds were highest under the SOL treatment (Figure 3). No difference could be found between A4 and KAN under the LED lights, but A4 showed significantly less CBDA under CHD. As two synthesis pathways are involved in cannabinoid synthesis, MEP and PKS, it is difficult to draw spectra-related conclusions about PKS in particular. Therefore, the hypothesis that a high R:FR ratio enhances cannabinoid or terpene concentrations remains unconfirmed.