Arbuscular mycorrhizal fungi (AMF) participate in the process of plant secondary metabolism and thus affect the production of secondary metabolites. However, it is not clear whether and how AMF affect the growth and secondary metabolites of Polygonum cuspidatum, a medicinal plant rich in resveratrol and polygonin, under different phosphorus (P) levels. This study was performed to analyze the effects of Glomus mosseae on the growth, leaf gas exchange, P concentration, active ingredient concentrations, and expressions of associated genes of P. cuspidatum under P-deficient (0 mol/L P) and P-sufficient (0.2 mol/L P) conditions. The root mycorrhizal colonization rate of inoculated plants was 62.53–73.18%. G. mosseae improved shoot and root biomass as well as leaf P levels to some extent, but the improvement was more prominent under P-sufficient than P-deficient conditions. The fungal colonization also significantly increased leaf photosynthetic rate, stomatal conductance, transpiration rate, and intercellular CO₂ concentration, which was more prominent under P-deficient rather than P-sufficient conditions. P addition promoted the concentration of active medicinal components in leaves, especially in uninoculated plants. G. mosseae distinctly raised leaf chrysophanol, emodin, polydatin, and resveratrol concentrations, which was more prominent under P-deficient conditions. However, physcion was raised by G. mosseae only under P-sufficient conditions. AMF and P addition up-regulated expressions of PcCRS1, along with the up-regulation of PcRS11 by P addition and PcRS11 and PcSTS by AMF under P-sufficient conditions. It is concluded that an adequate P fertilizer and AMF facilitate the production of active medicinal components in P. cuspidatum, associated with expressions of associated genes such as PcCRS1.

Polygonum cuspidatum, an important medicinal plant, often experiences shading from surrounding vegetation during its growth phase, raising questions about the impact of such conditions on the functionality of arbuscular mycorrhizal fungi. This study investigated the effects of an arbuscular mycorrhizal fungus (Funneliformis mosseae) on the growth, leaf gas exchange, and concentrations of active ingredient concentrations in leaves and roots of P. cuspidatum under shading (with a 72% shading rate) conditions. A nine-week shading intervention significantly suppressed root colonization by F. mosseae and the formation of soil mycorrhizal mycelium. Shading significantly inhibited the above-ground growth performance, biomass production, leaf photosynthetic rate, transpiration rate, stomatal conductance, and intercellular CO₂ concentration, while F. mosseae significantly increased these variables in the absence of shading. Plant height, leaf biomass, stem biomass, leaf photosynthetic rate, transpiration rate, and stomatal conductance were all decreased by F. mosseae when the plants were shaded. The shading treatment also significantly diminished the concentrations of active components measured in both leaves and roots. Under no-shading conditions, F. mosseae significantly boosted the concentrations of polydatin, resveratrol, aloe-emodin, emodin, chrysophanol, and physcion in roots, as well as the concentrations of polydatin and chrysophanol in leaves. Conversely, in the presence of shading, F. mosseae distinctly reduced these active ingredient levels in roots, followed by an increase in leaf polydatin and chrysophanol concentrations. In summary, shading substantially impaired the mycorrhizal benefits on plant growth, leaf gas exchange, and root active ingredients in P. cuspidatum, highlighting the importance of sufficient light to maximize mycorrhizal contributions.