Abstract:

Microplastics (MPs) have emerged as novel pollutants in agricultural soils where they interact with heavy metals and influence plant growth and development. Cadmium (Cd) is a highly toxic heavy metal that significantly affects crop productivity and food safety. Understanding the combined effects of MPs and Cd on rice (Oryza sativa L.) seedlings is crucial for evaluating the ecological risks and formulating mitigation strategies. This study investigated the effects of polyethylene (PE) and polypropylene (PP) MPs on growth, oxidative stress response, antioxidant enzyme activity, and Cd accumulation in rice seedlings under Cd stress. By analyzing morphological parameters, reactive oxygen species (ROS) levels, antioxidant defense mechanisms, and Cd translocation, we aimed to elucidate the different roles of PE and PP MPs in regulating Cd toxicity in plants. A hydroponic experiment was conducted using four treatment groups: control (CT), Cd stress alone (Cd), Cd combined with PE MPs (PE-MPs+Cd), and Cd combined with PP MPs (PP-MPs+Cd). The results indicated that Cd exposure significantly inhibited rice seedling growth, and reduced the leaf length, plant height, and biomass. The addition of PE-MPs and PP-MPs caused slight variations in these inhibitory effects; however, the differences were not statistically significant. In leaf tissue, Cd exposure significantly reduced shoot length (42.7% vs. CT), whereas the PE-MPs+Cd (29.1% vs. CT) and PP-MPs+Cd (38.5% vs. CT) treatments also exhibited a significant decrease. Similarly, the leaf fresh weight was significantly reduced by 52.7% (Cd vs. CT), 44.1% (PE-MPs+Cd vs. CT), and 50.9% (PP-MPs+Cd vs. CT). For the root systems, there were no clear trends in biomass or length across the treatment groups. Overall, all experimental treatments reduced rice seedling biomass, but there was no significant difference in biomass reduction between Cd exposure alone and in combination with MPs in terms. Across all treatments, Cd primarily accumulated in the roots rather than translocated to the aerial parts. Compared with the control group, the Cd content in both the root and shoot tissues increased significantly in all treatments. Compared to Cd exposure alone, PE-MPs+Cd and PP-MPs+Cd increased root Cd accumulation by 9.2% (vs. Cd), and 26.9% (vs. Cd), respectively, while reducing the leaf Cd content by 27.9% (vs. Cd), and 25.1% (vs. Cd), respectively. Comparing the PE-MPs+Cd and PP-MPs+Cd treatments, the root Cd enrichment coefficient increased by 9.2% (PE-MPs+Cd vs. CT) and 26.9% (PP-MPs+Cd vs. CT), respectively. The leaf Cd enrichment coefficient decreased by 27.9% (PE-MPs+Cd vs. CT) and 25.1% (PP-MPs+Cd vs. CT), whereas the root-to-leaf Cd translocation factor decreased by 34.0% (PE-MPs+Cd vs. CT) and 40.9% (PP-MPs+Cd vs. CT), and the Cd translocation capacity in the PE-MPs treatment was 11.8% higher than that in the PP-MPs treatment. Overall, both PP-MPs and PE-MPs enhanced Cd accumulation in roots while reducing their translocation to aerial parts, although to different extents. This difference may be related to the varying Cd adsorption capacities of PE-MPs and PP-MPs. The higher Cd adsorption capacity of PP-MPs led to the highest Cd concentration in the roots under this treatment, while the higher Cd translocation factor in the PE-MPs+Cd group resulted in a slightly greater total Cd accumulation in the shoots compared to the PP-MPs+Cd treatment. Compared with the control group, all treatments reduced root activity. Relative to Cd exposure alone, the combination of MPs and Cd further suppressed root activity, with the inhibitory effect of PP-MPs+Cd treatment being stronger than that of PE-MPs+Cd treatment. However, there was no significant difference in photosynthetic performance between the two groups. Considering the Cd content in different plant parts, both PE-MPs and PP-MPs reduced Cd translocation compared with Cd alone. As a result, Cd exposure alone exerted stronger inhibitory effects on root activity and photosynthesis than combined exposure to MPs. The enhanced Cd adsorption capacity of PP-MPs resulted in greater Cd retention in the roots, leading to a more pronounced decline in root activity in the PP-MPs+Cd group than that in the PE-MPs+Cd group. In contrast, although Cd translocation was higher in the PE-MPs+Cd treatment, the lower Cd enrichment capacity of PE-MPs in the roots resulted in no significant difference in leaf Cd content between the two MP treatments. Consequently, their effects on photosynthesis inhibition were similar. Regarding oxidative stress and antioxidant responses, ROS and H₂O₂ levels were significantly elevated in the Cd-only treatment (vs. CT). Moreover, the levels of oxidative stress markers (ROS and H₂O₂) in the leaves were higher in the PE-MPs+Cd treatment than in the PP-MPs+Cd treatment. Antioxidant enzyme activity analysis showed that SOD and CAT activities in the roots significantly increased in the PP-MPs+Cd treatment, contributing to a reduction in ROS accumulation and TBARS formation. This indicates that PP-MPs play a positive role in regulating the antioxidant defence system of plants, thereby mitigating Cd toxicity. In contrast, although antioxidant enzyme activity was enhanced in the PE-MPs+Cd treatment group, the effect was weaker than that observed in the PP-MPs+Cd group. This may be related to the previously mentioned tendency of PE-MPs to slightly increase Cd translocation to leaves compared with PP-MPs, leading to more pronounced oxidative stress and antioxidant system impairment in leaves. Specifically, key antioxidant enzymes, such as SOD and CAT, exhibited reduced activity, ultimately weakening the ability of the PE-MPs+Cd treatment to scavenge ROS, particularly in leaf tissues. This study provides new insights into the combined effects of MPs and Cd in rice seedlings. The results demonstrated that Cd stress significantly inhibited rice seedling growth and increased Cd accumulation in aerial tissues. Additionally, Cd exposure suppresses root activity and photosynthesis while inducing significant oxidative stress. However, PE-MPs and PP-MPs promoted Cd accumulation in the roots while reducing Cd translocation to the aerial parts, thereby alleviating Cd toxicity in the shoots to some extent. PP-MPs mitigated Cd toxicity by enhancing antioxidant enzyme activity and reducing Cd translocation to leaves, thereby partially alleviating Cd-induced oxidative damage. In contrast, PE-MPs tended to enhance Cd accumulation in the leaves, exacerbating oxidative stress and causing greater damage to leaf function. The regulatory effects of different types of MPs on Cd toxicity differed significantly, likely because of the differences in their surface adsorption characteristics and interactions with Cd. These findings highlight the need for regulatory measures to limit MP contamination of agricultural soils. Future research should focus on the long-term exposure effects, potential interactions with other pollutants, and their implications for food safety. This study contributes to a better understanding of MP-heavy metal interactions and provides a scientific basis for developing effective pollution management strategies in agriculture.

Key words: polyethylene, polypropylene, microplastics, cadmium, rice seedlings, oxidative stress, toxicity effect

摘要：

为探讨聚乙烯（Polyethylene，PE）和聚丙烯（Polypropylene，PP）微塑料（Microplastics，MPs）在镉（Cadmium，Cd）胁迫下对水稻（Oryza sativa L.）幼苗生长及其抗氧化机制的影响，通过水培试验设置Cd单独暴露与Cd联合不同类型微塑料的处理，系统分析MPs对水稻幼苗的生长发育、生物量累积、Cd的吸收与分布、光合生理以及抗氧化系统的影响。结果表明，Cd胁迫显著抑制水稻幼苗生长，尤其对叶片生长和生物量产生明显抑制。PE-MPs与PP-MPs的添加均增强了Cd在根部的富集，降低了Cd向地上部的转运效率，但其作用强度存在差异。具体而言，PP-MPs在促进Cd滞留于根部方面效果更显著，进而对根系活力产生更显著影响；而PE-MPs的促进作用相对较弱，导致部分Cd仍转运至地上部，使叶片损伤程度更明显。抗氧化系统响应方面，PP-MPs+Cd处理组中超氧化物歧化酶（SOD）和过氧化氢酶（CAT）活性显著上升，有效降低了活性氧（ROS）与硫代巴比妥酸反应物（TBARS）的积累，表明PP-MPs可能通过增强抗氧化防御能力缓解Cd的毒害效应；相比之下，PE-MPs+Cd处理组的抗氧化酶活性提升较弱，ROS和MDA水平较高，反映出PE-MPs可能加剧了Cd诱导的氧化损伤。综上，不同类型MPs对Cd毒性的调控效应存在显著差异，PP-MPs通过激发植物抗氧化系统可在一定程度上减轻Cd毒性，而PE-MPs则更易促进Cd在地上部的积累，从而增强其氧化胁迫作用。该研究为揭示微塑料与重金属复合污染对农业生态系统的潜在危害提供了科学依据，并为农业环境污染的防控策略提供了理论支持。

关键词: 聚乙烯, 聚丙烯, 微塑料, 镉, 水稻幼苗, 生长抑制, 毒性效应