Effects of microplastics and their combination with cadmium on Pinus massoniana are dependent on the type of microplastics

The widespread occurrence of microplastics (MPs) in various environments globally often coincides with the presence of toxic heavy metals, leading to co-contamination toxicity in plants. However, the differences between conventional and degradable MPs and their effects on tree responses to heavy metals remain inadequately understood. To address this issue, we conducted a three-factor pot experiment (MP type: polyethylene (PE) and polylactic acid (PLA); dose: 0%, 1% and 10% w/w; cadmium(Cd): (5 mg kg⁻¹)) to investigate the effects of different types of MPs, in combination with Cd on the growth, physiological and biochemical traits of Pinus massoniana seedlings. The results showed that 1% MPs (alone or with Cd) had no significant effect on growth, whereas both high-dose MPs (10%) alone and their co-exposure with Cd significantly reduced seedling biomass and photosynthetic rates. PLA+Cd caused stronger biomass inhibition (51.5% vs. PE+Cd: 34.3%), while PE+Cd suppressed photosynthesis more severely (68.4% vs. PLA+Cd: 52.8%). PE (alone or with Cd) and PLA+Cd triggered oxidative stress, elevating proline (Pro), peroxidase (POD), and superoxide dismutase (SOD) activities, whereas PLA-only increased Pro and POD. Both 10% MPs-only and their co-exposure with Cd significantly reduced non-structural carbohydrates (NSC) in needles. Overall, the effects of MPs and their combination with cadmium on P. massoniana were dependent on the type of MPs, providing significant implications for tree seedling cultivation and forest management in the context of increasing human activities.