Physiological and Phytochemical Responses of Calendula officinalis L. to End-of-Day Red/Far-Red and Green Light

Calendula officinalis L. is a widely used medicinal plant whose secondary metabolism and morphology are influenced by light. This study evaluated the effects of 2 and 4 h end-of-day (EOD) red/far-red (R:FR) and green (G) light on the growth, physiology, and phytochemical profile of hydroponically grown C. officinalis under a constant red/blue light background, compared with a red/blue control without EOD treatment. Morphological, physiological (gas exchange, chlorophyll fluorescence), biochemical (chlorophyll, anthocyanin), and chemical composition (attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and Gas Chromatography-Mass Spectrometry (GC-MS)) were evaluated. EOD G 2 h enhanced photosynthetic pigments, anthocyanins, and biomass, while control plants showed higher phenolic content. EOD R:FR induced stem elongation but reduced pigment and metabolite accumulation. GC-MS revealed organ-specific metabolic specialization, with flowers displaying greater chemical diversity than leaves. EOD G favored sesquiterpene diversity in flowers, while EOD R:FR increased nitrogen-containing compounds and unsaturated fatty acids. Vibrational data supported these shifts, with spectral signatures of esters, phenolics, and lipid-related structures. Bioactive compounds, including α-cadinol and carboxylic acids, were identified across treatments. These findings demonstrate that EOD light modulates physiological and metabolic traits in C. officinalis, highlighting EOD G as an enhancer of biomass and phytochemical richness for pharmaceutical applications under controlled conditions.