Effects of Elevated CO2 on Saponin Accumulation of Panax japonicus are Associated with Changes in the Carbon and Nitrogen Balance

This study investigated the effects of elevated CO 2 concentrations (e1CO 2 : 550 mol mol -1 , e1CO 2 : 750 mol mol -1 ) at different durations (27 vs. 84 days) on the carbon (C) and nitrogen (N) balance and on saponin accumulation in Panax japonicus.After 27 days, no significant differences were observed in the leaf C or N values or in the C/N ratio or total saponin content across the treatments.However, after 84 days, e1CO 2 did not alter the N content in stems or rhizomes (medicinal organ), whereas it increased the C/N ratios in the leaves and stems.In contrast, e2CO 2 significantly reduced the N content in all organs and increased the C/N ratios in the rhizomes by 25.07%.The total saponin content in the leaves, stems, and rhizomes was significantly higher under both e1CO 2 and e2CO 2 compared to the control treatment, with the most pronounced increase in rhizomes under 750 ppm CO 2 after 84 days.The saponin content was positively correlated with the C/N ratio across all organs.Interactions between the CO 2 magnitude and duration significantly influenced the C/N ratio, stem/rhizome N content, and total saponin accumulation.These results demonstrate that elevated CO 2 (especially 750 ppm for 84 days) modulates the C/N balance to promote saponin biosynthesis, highlighting a CO 2 -mediated mechanism for secondary metabolite regulation in P. japonicus.