Biochar Improves Panax notoginseng Resistance and Saponin Accumulation Under Soil Combined Microplastics and Cadmium Contamination

Microplastics and heavy metals are widely present in the soil, and their combined contamination affects plant growth and physiological characteristics. Currently, research on the growth, physiology, and quality of Panax notoginseng under combined microplastic-heavy metal contamination with biochar amendment is limited. Pine needle biochar was used as the experimental material, and the adsorption mechanism of pine needle biochar for microplastics and cadmium (Cd) was explored through adsorption experiments. Pot experiments were conducted under the combined stress of 0.1% (w/w) microplastics and 6 mg·kg−1 Cd (MP–Cd), with exogenous application of 0.5%, 1%, and 3% biochar. The biomass, Cd content, photosynthetic characteristics, antioxidant system (superoxide anion, O2−; hydrogen peroxide, H2O2; malondialdehyde, MDA; superoxide dismutase, SOD; peroxidase, POD; and catalase, CAT), saponin synthase activities (dammarenediol-II synthase, DS; squalene epoxidase, SE; glycosyltransferase, GT; and squalene synthase, SQS), and saponin content (notoginsenoside R1, ginsenoside Rg1, and ginsenoside Rb1) of P. notoginseng were analyzed. The results showed that under the 1% biochar application, the Cd2+ adsorption capacity was 1.31 mg·g−1, with a removal rate of 99.91%. Under biochar application, Cd content in all parts of P. notoginseng was significantly decreased, and the magnitude of the decrease increased with increases in the biochar application rate. Under the 3% biochar application, the plant height and biomass were increased by 18.92% and 29.72%, and the net photosynthetic rate (Pn) and stomatal conductance (Gs) were increased by 1.18-fold and 1.5-fold, respectively. Under the 1% biochar application, chlorophyll a and b contents were highest at 0.077 mg·g−1 and 0.052 mg·g−1, respectively, the activity of SE in rhizomes was increased by 20.24%, and the content of notoginsenoside R1 was significantly increased by 1.01-fold. Under the 3% biochar application, the activity of SQS in rhizomes was significantly increased by 10.38%, and the activities of GT and DS in fibrous roots were increased by 20.29% and 14.82%, respectively. The content of ginsenoside Rb1 in rhizomes was significantly increased by 69.64%, while the contents of ginsenoside Rg1 and total saponin in the main root were increased by 1.41-fold and 0.72-fold, respectively. The activities of saponin synthase in various parts of P. notoginseng were GT (717.167 U·L−1) > DS (472.752 IU·L−1) > SE (378.949 U·L−1) > SQS (144.918 U·L−1). In summary, the growth of P. notoginseng and saponin content were increased under combined MP–Cd contamination, with the 3% pine needle biochar application, with Cd content decreased to ensure the safe production of P. notoginseng.