Effect of polylactic acid microplastics and lead on the growth and physiological characteristics of buckwheat

Microplastics (MPs) and heavy metals are common, often co-existing pollutants, that threaten crop growth and productivity worldwide. We analysed the adsorption of lead ions (Pb²⁺) to polylactic acid MPs (PLA-MPs) and their single factor and combined effects on tartary buckwheat (Fagopyrum tataricum L. Gaertn.) in hydroponics by measuring changes in the growth characteristics, antioxidant enzyme activities and Pb²⁺ uptake of buckwheat in response to PLA-MPs and Pb²⁺. PLA-MPs adsorbed Pb²⁺, and the better fitting second-order adsorption model implied that Pb²⁺ was adsorbed by chemisorption. However, the similar Pb²⁺ contents in the plants treated with Pb²⁺ only and those treated with the combined PLA-MPs-Pb²⁺ suggested that the adsorption played no role in the uptake of Pb²⁺. Low concentrations of PLA-MPs promoted shoot length. At high concentrations of both PLA-MPs and Pb²⁺, buckwheat growth was inhibited, and leaf peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT) activities and malondialdehyde (MDA) contents were higher than in the control. No significant differences were observed in seedling growth between exposure to Pb²⁺ only and combined exposure to PLA-MPs with Pb²⁺, implying that PLA-MPs did not increase the toxicity of Pb²⁺ at macroscopic level. POD activity was higher and chlorophyll content was lower with PLA-MPs in the low Pb²⁺ dose treatments, suggesting that PLA-MPs may increase the toxicity of naturally occurring Pb²⁺. However, the conclusions must be verified in controlled experiments in natural soil conditions over the whole cultivation period of buckwheat.