Blue Light Enhances Photosynthetic Efficiency and Antioxidant Capacity in Mullein (Verbascum phlomoides L.) Seedlings

The orange mullein is a biennial plant whose tall yellow flower spikes contain mucilage, saponins, and other medicinal compounds that have a beneficial effect on respiratory problems. As light quality is known to influence plant morphology and physiology, with effects often depending on the species, understanding these responses in mullein is of particular interest. Therefore, this study aimed to investigate the combined effects of different light-emitting diodes (white, red and blue) and their corresponding photon flux densities (PPFD) on the morphology, pigment composition, antioxidant activity, fluorescence parameters and OJIP transient curves in mullein (Verbascum phlomoides L.) seedlings. Seedlings grown under blue light, which had relatively higher PPFD, showed the greatest root length, leaf number, leaf and root fresh and dry biomass. Red light, with lower PPFD, resulted in the lowest values for these parameters. Compared to white light, pigment analysis showed that blue light increased chlorophyll a, total chlorophyll, carotenoid content, and the Chl a/b ratio. Also, blue light enhanced antioxidant activity, as well as the accumulation of phenolic compounds and flavonoids, indicating that it appeared to enhance the synthesis of secondary metabolites under this spectrum. In contrast, seedlings under red light exhibited the lowest ferric reducing antioxidant power values and tended to reduce levels of phenols and flavonoids, indicating a weaker antioxidative response. It was found that white light appeared to enhance the photochemical activity of photosystem II (PSII) and energy dissipation. Blue light improved linear electron transport, photosystem I (PSI) activity and overall photosynthetic performance. Red light preferentially increased electron flow towards the final acceptors of PSI, affecting the terminal part of the electron transport chain. Analysis of OJIP curves revealed spectrum and intensity-specific changes in the L, K, H, and G bands, demonstrating that light treatments with differing PPFDs selectively modulate PSII and PSI function.