Decreasing R:FR ratio in a grow light spectrum increases inflorescence yield but decreases plant specialized metabolite concentrations in Cannabis sativa

Cultivation of Cannabis sativa for recreational and pharmaceutical purposes has been increasing significantly in recent years due to legalization in many countries. Cultivation takes place regularly indoors under varying artificial lighting sources. There is a lack of scientific knowledge on the effect of light spectrum on the C. sativa morphology, yield, and quality, especially the cannabinoid and terpene concentrations in the female inflorescences in indoor environments. Furthermore, only a handful of the spectra studies conducted so far study or discuss the effect of far-red radiation, while the effect of other wavelengths, such as UV or blue, has gained more attention. This study had two aims: 1) to examine plant morphology and inflorescence yield under varying red to far-red ratio (R:FR) treatments with equal photon flux densities (380–780 nm), and 2) to examine the possible relationship of the cannabinoid and terpene concentrations with the spectrum’s R:FR ratio, Plant material was collected as cuttings from C. sativa ‘Finola’ mother plants and grown under 18 h photoperiod before transferring them under the light treatments for 49 days (550 μmol m −2 s −1 , 12 h/12 h dark/light). Light treatments were created with two types of LED fixtures, white spectrum (380-780 nm) and far-red (730 nm), which were used to create four R:FR ratio treatments; R:FR 3, 5, 9, and 12. Plant morphology was affected by the R:FR ratio; under the lowest R:FR (3) treatment plants were tallest, and the apical inflorescence dry weight decreased linearly with increasing R:FR ratio. The concentrations of many terpenes and cannabinoids including cannabidiolic acid (CBDA), tetrahydrocannabinolic acid (THCA), and cannabigerolic acid (CBGA), increased with increasing R:FR ratio. In conclusion, spectra with different R:FR ratios can be used as a tool at different growth phases to modify the plant morphology, inflorescence yield, and cannabinoid and terpene concentrations. • Inflorescence yield decreased linearly with increasing red to far-red (R:FR) ratio. • The total cannabinoid concentration increased with increasing R:FR ratio. • Plant height, inflorescence yield and quality can be manipulated with R:FR ratio.