Carbon quantum dots modulate light-responsive signaling and metabolic control during germination and early growth of lemon balm (Melissa officinalis L.)

This study investigated the interactive effects of carbon quantum dots (CQDs) and LED light spectra on seed germination, early seedling growth, physiological status, enzymatic activity, and metabolite and phytohormone profiles in Melissa officinalis L. CQDs and light quality significantly influenced all assessed traits. An intermediate CQDs dose (100 mg L⁻¹) consistently enhanced germination percentage, germination speed, germination energy, and reduced mean germination time, with the greatest improvements occurring under combined blue/red (1B/3 R and 1B/1 R) spectra. Water uptake and early seedling vigor were similarly stimulated, particularly under R and dichromatic lights, while shoot and root elongation increased markedly with 100 mg L⁻¹ CQDs. Oxidative stress indices (H₂O₂, MDA) declined at 100 mg L⁻¹ but rose under 200 mg L⁻¹ CQDs, whereas membrane stability peaked at the optimal dose, especially under 1B/3 R irradiation. Antioxidant and metabolic enzyme responses varied with light quality: combined spectra strongly enhanced SOD, GR, NiR, α-amylase, and protease activity, while B-LED light suppressed most hydrolytic and redox enzymes but stimulated PAL activity. CQDs and light spectra also reshaped metabolite accumulation; B light increased starch, phenolics, and flavonoids, whereas combined spectra increased total soluble sugars and total amino acids under 100 mg L⁻¹ CQDs. Phytohormone profiles demonstrated increased GA₃, IAA, and cytokinin levels-particularly under dichromatic light at 100 mg L⁻¹ -and reduced ABA under R light. Overall, the synergistic combination of moderate CQDs exposure and specific dichromatic B/R spectra (especially 1B/3 R) optimized germination performance, promoted early seedling vigor, enhanced stress mitigation, and induced favorable metabolic and hormonal reprogramming, highlighting a promising strategy for improving lemon balm seedling establishment.