Red light induced seed germination and seedling growth by modulating antioxidant defense system, Rubisco, and NADPH oxidase activities in Capsicum frutescens

In this study, the impact of light-emitting diodes (LEDs) in different spectrums was investigated on the seed germination and post-germinative performance of Capsicum frutescens seedlings. The seeds were exposed to different LED lights (full spectrum, white, red, blue, and red-blue) for 0, 1, 2, and 4 h (h). The seeds were placed for a week in darkness to investigate germination, and then the growth mechanisms were studied in four-week-old seedlings. Results indicated that germination percentage was promoted markedly under 2 h red and full lights and also in 1 h blue, which was accompanied by the regulation of H₂O₂ level and NADPH oxidase (NOX) activity. Sprout growth and height were more heightened under 2 h red light, but their contents decreased considerably under blue light with a rising incubation time. Red light induced more biomass yield, chlorophyll (Chl) pigments, Chl a/b ratio and florescence in four-week-old seedlings. Blue light also increased Chl pigments, but decreased biomass yield by enhancing malondialdehyde (MDA) level. Increased growth in seedlings treated to red light was associated with upregulating Rubisco gene expressions (rbcL and rbcS) and its activity. Red and red-blue lights promoted the activity of superoxide dismutase, glutathione reductase, and ascorbate peroxidase enzymes to increase ascorbic acid (ASA) production in the ascorbate-glutathione cycle. Total phenolic (0.22 mg DAG g^{- 1} DW), ASA (89.58 mg 100 g^{- 1} FW) and capsaicinoids (2.73 mg g^{- 1} DW) contents were heightened under red light, while carotenoid (11.78 µg g^{- 1} FW) content was more accumulated under blue light. The findings of this study suggest red light modulates NOX activity and H₂O₂ level for inducing seed germination and seedling quality in C. frutescens, which can create important implications for the production of antioxidant metabolites and increase the cultivation area of this plant.