Three-year-old Panax japonicus was exposed to elevated CO2 concentrations using open-top chambers: ambient CO2 (aCO2), moderately elevated (e1CO2, 550 μmol/mol), and highly elevated (e2CO2, 750 μmol/mol). Gas exchange parameters, photosynthetic pigments, sugar accumulation, and total saponin content were measured to assess the effects of CO2 enrichment on photosynthesis, sugar metabolism, and saponin biosynthesis. The e1CO2 treatment significantly increased net photosynthetic rate (by 17.22% at 36 days and 69.62% at 92 days), chlorophyll a content, and soluble sugar, sucrose, and starch accumulation. Key sugar metabolism enzymes, including sucrose phosphate synthase (SPS), also showed enhanced activity. Consequently, underground rhizome total saponins rose significantly by 15.16%. In contrast, e2CO2 initially (36 days) stimulated photosynthesis but lost this effect over prolonged exposure (92 days), with no significant impact on photosynthetic parameters, pigments, or sugar levels. Correlation analysis indicated that rhizome saponin content was positively associated with leaf sucrose levels and sucrose synthase (synthetic direction) activity. These findings suggest that moderate CO2 elevation (e1CO2) enhances P. japonicus photosynthesis and sugar metabolism, driving greater saponin accumulation. However, high CO2 (e2CO2) has transient benefits, with stimulatory effects diminishing over time.
Wang et al. (Thu,) studied this question.