Abstract
Microplastics are ubiquitous in the atmosphere globally, resulting in deposition and accumulation on plant leaves. Little is known about the effects of foliar microplastic exposure on plants, particularly in particles larger than 1 µm, however microplastic exposure is known to induce oxidative stress in plants. This study aimed to explore the effects of foliar microplastic exposure in mung bean (Vigna radiata) in growth cabinet and glasshouse environments. The objectives were to determine if foliar exposure to 1–90 µm microplastics at various exposure doses would result in physiological or biochemical responses, and to investigate whether effects were systemic or limited to the site of exposure. The first experiment used a growth cabinet with one size of microplastics (10–45 µm) which were simply brushed onto the leaves. Analysis of oxidative stress markers found that oxidative stress occurred in the microplastic exposed plants, showing that foliar exposure to microplastics >10 µm can cause damage. This led into a larger scale glasshouse study with a range of particle sizes (1–4 µm, 10–45 µm, 45–75 µm, and 75–90 µm), as well as three doses for each size range, the low dose of which was based on environmental levels. The results from this experiment showed that after both one and two weeks of exposure, oxidative stress was induced in plants exposed to all doses of 1–4 µm microplastics, and the high dose of 10–45 µm microplastics, with no effects above 45 µm particle size. No growth or photosynthetic effects were observed. There were clear trends of increased oxidative stress with decreasing particle size and increasing dose. This aligns with scientific understanding of plastic leaching behaviour — toxic additives leach more quickly from smaller particles. Time and location relative to the site of exposure also influenced plant response. In the primary leaf that was not exposed to microplastic a smaller stress response was observed than the treated leaf, however in the subsequently developed trifoliate leaf a decrease in oxidative stress levels occurred. Similarly, between one and two weeks, oxidative stress levels decreased in the treated leaf, and in the untreated leaves, levels decreased below control, suggesting a hormetic effect. Future research into foliar microplastics should focus on more glasshouse and field trials, investigating reproduction and yield, as well as using a wider mixture of plastics that is more environmentally realistic. There needs to be a particular focus on the smaller size ranges which are poorly understood in the environment, as they cause far more harm to plants, as this study showed.