As nanoplastics become an increasing environmental and human health concern, greater understanding of their absorption by crop plants is vital. Many plant species, including lettuce, have been reported to transport nanoplastics from roots to leaves, but most research has not been conducted with crops grown in commercial production systems to harvestable size. Across two greenhouse experiments, we compared ‘Winter Density’ romaine lettuce grown with and without the addition of 10 mg.liter-1 yellow-green fluorescent-labeled 0.2 µm and 0.05 µm polystyrene nanoplastics (PSNP) added to municipal irrigation water in commercial-style hydroponic Nutrient Film Technique (NFT) systems. No difference was observed between lettuce grown with or without PSNPs based on plant width; shoot and root fresh and dry weights; number of true leaves; or chlorophyll content. Stomatal resistance of plants in both treatments was also not different and indicated very high transpiration rates that would support PSNP translocation to shoots. Fluorescence of shoots and roots from each treatment was evaluated via confocal microscopy. No PSNPs were found in the shoot tissue. In the roots, we observed PSNPs aggregated on the surface and possibly penetrated a maximum of 10 to 25 microns deep within outer root tissue. In root samples processed with acetone washes, no PSNPs were observed on or in root tissue by either TEM or confocal microscopy. As such, we found no evidence that 0.2 µm or 0.05 µm PSNPs are absorbed into the vascular system of hydroponically-grown lettuce plants and thus are not translocated to shoot tissue. These findings emphasize problems with current literature and the need for further research to investigate PSNP uptake by plants in realistic production systems.
