Abstract

Microplastics are increasingly recognized as persistent contaminants in terrestrial ecosystems, yet their impacts on plant performance remain poorly understood under realistic environmental conditions. In natural and agricultural systems, plants rarely encounter contaminants in isolation, instead experiencing multiple interacting stressors. The work presented in this thesis investigated how soil microplastics influence plant growth and physiology both independently and in combination with freshwater flooding, a climate-relevant stressor associated with increased storm activity and shifts in precipitation patterns. Using white clover (Trifolium repens) as a model agronomic legume across multiple cultivars, the study examined how polymer identity, plant genotype, and environmental context interact to shape plant responses.The first experiment established polymer-specific effects by exposing three cultivars — Aber Ace, Aber Swan, and Aber Herald — to conventional Low-Density polyethylene (LDPE) or a biodegradable Polylactic Acid and Polybutylene Adipate Terephthalate blend (PLA/PBAT) microplastics applied individually. Microplastic exposure primarily affected belowground traits, altering root architecture, nodulation, and biomass allocation, while aboveground growth remained largely unaffected. Responses differed between polymer types and cultivars, with PE generally disrupting root development and nodulation while PLA/PBAT produced more variable responses ranging from neutral to mildly stimulatory, demonstrating structured, genotype-mediated variation in sensitivity.Building on these findings, a second experiment evaluated whether microplastic effects were modified under simulated freshwater flooding. Plants were exposed to either PE or PLA/PBAT under flooded and non-flooded conditions to assess multi-stressor interactions. Responses were non-additive and strongly dependent on polymer type and cultivar. Under flooding, PLA/PBAT was associated with increased nodule density, greater total biomass, increased stolon production, and maintained Fv/Fm in Aber Swan, whereas PE was associated with reduced Fv/Fm under the same conditions. These results demonstrate that microplastic impacts cannot be interpreted in isolation, as their ecological significance shifts substantially — and in some cases fundamentally reorganizes — under additional environmental stress.Across both experiments, microplastic effects were concentrated within the soil–root interface, suggesting indirect mediation through changes in belowground processes and biomass allocation strategies. Taken together, the work presented in this thesis demonstrates that microplastic impacts on plants are highly context-dependent, structured by the intersection of polymer chemistry, plant genotype, and interacting environmental stressors. These findings support a conceptual reframing of microplastics not as uniform contaminants but as context-dependent soil modifiers, and reinforce that multi-stressor, multi-genotype frameworks are essential for predicting plant responses to emerging soil contaminants under changing climatic conditions.

Awarding Institution(s)

University of Plymouth

Supervisor

Mick Hanley, Anne Plessis, Winnie Courtene-Jones

Document Type

Thesis

Publication Date

2026

Embargo Period

2026-07-29

Deposit Date

July 2026

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Additional Files

2026Kent1096706ResMAppendix.pdf (307 kB)

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