Uptake, Distribution, and Impact of Micro- and Nano-Plastics in Horticultural Systems Using Lettuce (Lactuca sativa) as a Model Crop

Plastics are integral to horticulture, with millions of tonnes used annually for crop protection and packaging. Despite enhancing productivity, plastic fragmentation, contamination, and accumulation pose underexplored environmental and crop cultivation concerns. This thesis enhances the knowledge of micro- and nanoplastics fate in soil, plants, and horticultural systems, emphasising environmentally representative approaches. Soils are sinks for horticultural plastics, necessitating reliable quantification to contextualise risks to plant health and production. Accordingly, an extraction, fluorescent staining, and FTIR characterisation procedure was optimised using organic content extremes and representative microplastic polymers, densities, dimensions, and degradation phases, yielding high matrix removals and plastic recoveries >96 %. Recognising further fragmentation, polystyrene nanoplastics (~30 nm, spherical, uniform, negatively charged, minimally aggregating) were synthesised and characterised, providing insights into their impact and uptake potential during hydroponic lettuce (Lactuca sativa) exposures. Across 0.01-1000 mg/L nanopolystyrene exposures, leaf and root development inhibition was significantly concentration dependent (9.3-96.4 %). Conversely, chlorophyll and elemental concentrations remained adequate for growth. Synthesis impurity controls attributed toxicity primarily to nanopolystyrene, potentially through root blockage, growth reallocation, or uptake-related damage. Carbon-14-radiolabelling enabled highly sensitive nanopolystyrene quantification and localisation in lettuce at an environmentally relevant concentration, 0.3 µg/L [14C]nPS, facilitated by liquid scintillation counting and autoradiography. Strong root association (6.68 ± 1.40 ng/g), limited leaf uptake (0.08 ± 0.005 ng/g; 4.36 % translocation), and greater accumulation in older leaves suggest entrapment or protective sequestration, demonstrating nanoplastic pathways into edible tissue. A Life Cycle Assessment contextualised laboratory-derived data, linking realistic plastic contamination scenarios (0.06-1.74 %) to reduced lettuce growth, demonstrating that increased land or inputs to maintain marketable yields could elevate global warming potential, terrestrial and freshwater ecotoxicity, and fossil resource consumption by up to 39 %. Collectively, these novel findings evidence nanoplastic impact, detection, and localisation in lettuce, highlighting significant, emerging horticultural concerns under environmentally relevant conditions.