Influence of leaf temperature and blue light on the accumulation of rosmarinic acid and other phenolic compounds in Plectranthus scutellarioides (L.)

In this study, the influence of different lamp types on physiology and secondary metabolites of Plectranthus scutellarioides (Solenosternon scutellarioides, Coleus bluntel) was examined. The following four lamp systems were tested: a new microwave plasma lamp (MPL) emitting artificial sunlight, a commercial high-pressure sodium lamp (HPS), a ceramic metal halide lamp (CDM), and light-emitting diodes (LEDs). The lamps differed not only in the spectral properties of the emitted light but also in the emission of infrared radiation. The aim of this study was to investigate both the influence of the spectral quality and the influence of the leaf temperature from four different lamp systems on morphology and secondary metabolites in P. scutellarioides. Total phenolic compounds were quantified with colorimetric methods. For detailed description of the phenolic compounds, qualitative and quantitative analysis of selected phenolic compounds were performed by HPLC. Stem elongation was increased in plants grown with MPL and LED light, which was attributed to the higher amount of far-red light in the emission spectra. On the other hand, higher infrared radiation from MPL and HPS lamps led to increased leaf temperatures compared to plants grown with LED or CDM light, resulting in faster plant development indicated by greater leaf pair formation. In addition to rosmarinic acid (RA), which is typical for members of the family Lamiaceae, luteolin and apigenin glycosides were detected by mass spectrometry. The results demonstrated that P. scutellarioides leaves contained three acylated cyanidin diglycosides, one of them containing just a coumaroyl residue, the other two containing one and two additional malonyl residues, whereby malonylated compounds appeared in higher quantities. Leaves grown with LED and CDM light contained the highest amount of RA and flavone glycosides per dry weight, which was attributed to a lower leaf temperature compared to leaves developed under MPL or HPS light. Further, an increased blue light content in the emission spectra led to thicker leaves and consequently to a higher accumulation of secondary metabolites per leaf area.