Effects of Microplastic Particles and Microplastic Leachate on the Germination and Growth of Lolium multiflorum

Microplastics (MPs) are widespread environmental pollutants that have been detected in virtually all environmental compartments. Despite this, research has mainly focused on the impacts of microplastic on shorelines and at sea. The effects of MPs on terrestrial ecosystems has been sparsely investigated, and there are only a few studies on direct effects on terrestrial plants. Although plastic polymers are considered inert and non-hazardous, toxic additives are often added to the polymers during manufacture which may leach out into the environment, displaying ecotoxic effects. In this work, the effects of microplastic particles and microplastic leachate on the germination and growth of Lolium multiflorum (Italian ryegrass) was investigated. High density polyethylene (HDPE), which is one of the plastic polymers with the largest annual production, was chosen as the plastic material for investigation. New MPs, artificially aged MPs, MPs from the Lahti region, and MPs from Port Elizabet, South Africa was used in parallel to compare the effects of ageing and regional environmental factors on the ecotoxicity of MPs. The total germination percentage, mean germination rate, synchronization index, germination index, and time to 50% germination was investigated, as well as the root lengths, shoot lengths, root/shoot ratio, and fresh weights of the seedlings. The results showed that exposure to new and Lahti MPs and leachates severely inhibited the extent and speed of the germination of L. multiflorum, whereas all categories of MPs and leachates inhibited the growth to some extent. Most severe inhibition in germination and growth was seen for the new MP and new leachate, followed by Lahti MP and Lahti leachate. The root growth, shoot growth, and plant biomass were also severely reduced for these exposure media. For the aged and Port Elizabeth material, there were slight but significant (p < 0.05) inhibition in root lengths and fresh weights, but no significant inhibition in the germination parameters. These findings indicate that ageing severely reduces the ecotoxic effects of MPs, and that regional environmental factors affect the ecotoxicity of MPs. Microplastics from Port Elizabeth were significantly less toxic to L. multiflorum than MPs from the Lahti region, possibly due to the warmer climate in South Africa. Another explanation could be that the plastic material collected in Port Elizabeth wast older than the one from Lahti. There was little to no difference in germination and growth between seeds exposed to MPs or leachates of the same origin, indicating that it is the substances leaching out of the MPs that are responsible for their ecotoxicity.