Interactive Effect of Microplastics and Fungal Pathogen Rhizoctonia solani on Antioxidative Mechanism and Fluorescence Activity of Invasive Species Solidago canadensis

Microplastics and invasive species, driven by anthropogenic activities, significantly disrupt ecosystems and microbial communities. This study investigated the interactive effects of biodegradable microplastics (polylactic acid, or PLA, and polyhydroxyalkanoates, or PHAs) and the fungal pathogen <i>Rhizoctonia solani</i> on the invasive plant <i>Solidago canadensis</i>. One plant of <i>Solidago canadensis</i>/pot was cultivated in forest soil amended with 1% (<i>w</i>/<i>w</i>) microplastics and/or <i>R. solani.</i> PLA exhibited greater toxicity than PHAs, reducing the plant height, root length, and biomass by 68%, 44%, and 70%, respectively. Microplastics impaired the maximum quantum yield of photosystem II more severely than <i>R. solani</i>. However, <i>S. canadensis</i> demonstrated adaptive antioxidative and extracellular enzymatic mechanisms under combined stresses. A heatmap analysis revealed a positive correlation between PHAs and plant growth traits, while a redundancy analysis explained the 15.96% and 4.19% variability for the first two components (r² = 0.95). A structural equation model indicated the negative effects of morphology and physiology on biomass (β = -1.694 and β = -0.932; <i>p</i> < 0.001), countered by positive antioxidant contributions (β = 1.296; <i>p</i> < 0.001). These findings highlight complex interactions among microplastics, pathogens, and invasive species, offering insights into ecological management strategies under dual environmental pressures. Future studies should assess the long-term field effects and microbial mediation of these interactions.