Lolium multiflorum germination and growth affected by virgin, naturally, and artificially aged high-density polyethylene microplastic and leachates

Abstract

With the detection of microplastics in soil, coupled with the various sources continuously contributing to its delivery to and distribution in soils, understanding the effects of microplastics on plants are necessary. Plastics discarded in the environment continually degrade into micro- and nano-sizes, subsequently leaching hazardous chemicals with time. Not only time but also environmental factors related to the climate of the area where the plastic is degrading will contribute to the breakdown process. Thus, this study aimed to understand the phytotoxic effects of microplastic derived from a frequently discarded plastic item, i.e., high-density polyethylene soda bottle caps. The commonly occurring perennial Lolium multiflorum (Italian ryegrass) was exposed to microplastic derived from new and artificially aged bottle caps as well as bottle caps collected from the cities of Lahti, Finland and Gqeberha, South Africa. Additionally, leachates were prepared from these samples and used for exposure. Germination, root and shoot growth, and fresh weight were measured as indicators of adverse effects, and various growth parameters were calculated. Microplastic and leachates from new and Lahti collected bottle caps adversely affected the germination and growth of the plant, indicating that aging and environmental factors affect the phytotoxicity of plastics as environmental pollutants in soil.

Impacts of Pristine, Aged and Leachate of Conventional and Biodegradable Plastics on Plant Growth and Soil Organic Carbon

“…Similar to the results of the present study, Esterhuizen et al (2022) found the inhibition of root and shoot growth to be comparable, irrespective of exposure to particles or leachates of the same plastic. Esterhuizen et al (2022), however, also looked at artificially aged (high-density) PE fragments, finding the difference between new and aged microplastics and leachates to indicate that the aging of the plastic significantly decreases the toxicological effect on root and shoot growth. These results, as well as those conducted by others within their group (Pflugmacher et al 2020(Pflugmacher et al , 2021a(Pflugmacher et al , 2021b, indicate a correlation between aging and a decrease in phytotoxicity, conflicting the results of the present study, where the suppression of root and shoot biomass was similar, regardless of whether the exposure was to pristine or aged films of the same polymer.…”

“…Concentrations used to leach additives from plastics, simultaneously producing "aged" plastics, has varied remarkably among published studies (e.g. from 2 g L −1 (Lee et al 2022) to 100 g L −1 (Esterhuizen et al 2022); these values represent the mass of plastic per litre of water), but has been primarily based on measured concentrations of environmental plastic pollution in the field (Coffin et al 2018;Bridson et al 2021). The environmental aging of MP cannot be fully simulated by mechanical stress alone, as photodegradation also plays a significant role (Liu et al 2021).…”

“…The environmental aging of MP cannot be fully simulated by mechanical stress alone, as photodegradation also plays a significant role (Liu et al 2021). To mimic plastic weathering by aging the plastics in an accelerated laboratory setting, 2.5 g each plastic was shaken in 1 L deionised water (a concentration of 0.25% (w/v), corresponding to the lower values in literature) at 120 rpm at 50 °C under a UV light, for 2 weeks (Rummel et al 2019;Kim et al 2020;Esterhuizen et al 2022). From each leaching chamber, 0.5 g plastic in 200 mL water was extracted, with the plastic added to soil of Experiment 2, at a concentration of 0.1% (w/w), and the resulting leachate (the by-product from the aging process) used for Experiment 3, at a concentration of 0.25% (w/v).…”

“…The presence of phthalate esters in soil, for example, has been shown to reduce the contents of total phosphorus, total nitrogen, and available potassium (Cao et al 2023), inducing phytotoxic effects, decreasing germination rates and inhibiting root development of plants (Zou et al 2017;Gao et al 2021). In a study by Esterhuizen et al (2022), Lolium multiflorum (Italian ryegrass) planted in soil containing (3% w/w) pristine and naturally aged (high-density) PE fragments (4 mm), and their leachates (0.44% w/v), resulted in roots and shoots with a 77-83% lower fresh weight than the controls. Similar to the results of the present study, Esterhuizen et al (2022) found the inhibition of root and shoot growth to be comparable, irrespective of exposure to particles or leachates of the same plastic.…”

“…In a study by Esterhuizen et al (2022), Lolium multiflorum (Italian ryegrass) planted in soil containing (3% w/w) pristine and naturally aged (high-density) PE fragments (4 mm), and their leachates (0.44% w/v), resulted in roots and shoots with a 77-83% lower fresh weight than the controls. Similar to the results of the present study, Esterhuizen et al (2022) found the inhibition of root and shoot growth to be comparable, irrespective of exposure to particles or leachates of the same plastic. Esterhuizen et al (2022), however, also looked at artificially aged (high-density) PE fragments, finding the difference between new and aged microplastics and leachates to indicate that the aging of the plastic significantly decreases the toxicological effect on root and shoot growth.…”