Effects of polyethylene microplastic on the phytotoxicity of di-n-butyl phthalate in lettuce (Lactuca sativa L. var. ramosa Hort)
Introduction
Plastics are lightweight, durable, and convenient materials that are widely used around the world. The production and consumption of plastics has reached 300 million metric tons per year, and it is predicted that nearly 10% of plastic chips will eventually accumulate in the water environment after weathering (Wright et al., 2013; Law, 2016). Particles less than 5 mm in diameter are collectively referred to as microplastics by the National Oceanic and Atmospheric Administration (Wright et al., 2013). Microplastics are considered a new form of pollutant that has attracted increasing attention from researchers worldwide, since 1970 (Cole et al., 2011). In 2018, Tang et al. (2018) reported that the abundance of microplastics in surface seawater in the Xiamen coastal areas was 103–2017 particles·m−3. Microplastics can persist in water environments for long periods of time, during which they can drift independently and even interact with aquatic organisms. Sussarellu et al. (2016) reported that fluorescent polystyrene beads (0.023 mg L−1, 2 and 6 μm) caused damage to oyster feeding and reproductive systems and even affected the growth of the offspring. Recently, some studies have reported that plastic pollution on land may be more serious than pollution in the marine environment, and therefore highlight that more attention should be focused on issues related to terrestrial ecosystems (Lönnstedt and Eklöv, 2016; Horton et al., 2017). Domestic sewage, fertilizers, and vinyl coverings used in agriculture are the main sources of microplastics in soil and terrestrial environments (Lönnstedt and Eklöv, 2016; Farmer et al., 2017; Talvitie et al., 2017). Nizzetto et al. (2016) suggested that agricultural soils may contain more microplastic particles than marine basins. In the soil environment, numerous studies have shown that microplastics have adverse effects on Collembola, earthworms, isopods, and nematodes (Rodriguez-Seijo et al., 2017; Kokalj et al., 2018; Lei et al., 2018; Zhu et al., 2018). Microplastics characteristically have small particle sizes, large specific surface areas, and a high hydrophobicity. They can also be carriers of chemicals and contaminants in the environment, delivering harmful substances to living organisms and even to other environments (Zhang et al., 2015). Prata et al. (2018) investigated the toxicity of procainamide and doxycycline to the marine microalga Tetraselmis chuii and found that their toxicity increased after the addition of microplastics. The reduction ratios of chlorophyll content, photosynthetic rate, and growth rate of seawater microalgae were increased significantly with the addition of microplastics compared with treatments with the two pollutants alone (Prata et al., 2018).
Di-n-butyl phthalate (DBP) is a common industrial compound that can be released into the environment during its use or disposal (Bang et al., 2011; Gao et al., 2015). DBP usually accumulates in sediments, surface water, estuaries, and seawater, often seriously exceeding the safety limits set by national and international standards (Sha et al., 2007; Adeniyi et al., 2011; Yang et al., 2014; Li et al., 2016). In 2008, the DBP content in the North Canal of Beijing-Tianjin area reached 4.78 μg L−1 (Sui, 2008). DBP is regarded as a priority controlled hazardous substance by the United States and China (Liao et al., 2010). Sun et al. (2015) indicated that DBP can be absorbed by the lettuce roots and translocated to the leaves. Therefore, many researchers have focused on the biochemical and physiological mechanisms of DBP stress in plants. Excess DBP can lead to an increase in reactive oxygen species (ROS) in the chloroplasts, which can cause oxidative stress, cell membrane damage, and cell death (Lidon and Henriques, 1993; Gao et al., 2019).
In reality, it is common for multiple pollutants to coexist in polluted water, and such coexistence can lead to changes in the migration of pollutants and their toxicity to plants (Dong et al., 2018). For example, the combination of a high concentration of DBP and Cd can increase the toxicity of Cd to rapeseed (Yin et al., 2015). Plastic has become a common component of everyday objects used in all aspects of human life, and intensive research is needed to determine the overall toxicity of microplastics and other chemicals in the environment. Recent studies have shown that microplastics (polyvinyl chloride, polyethylene, and polystyrene) can adsorb DBP (Liu et al., 2019). However, it is unclear whether the toxicity of DBP to plants increases after microplastic polyethylene (MP) adsorption. Especially, the toxicity of a combination of pollutants on vegetable crops is still poorly understood. Therefore, with this study, we aimed to explore: 1) the influence of DBP on the physiological and biochemical parameters in lettuce in the presence of MP, and 2) the response mechanisms of lettuce to DBP in the presence of MP.
Section snippets
Materials and methods
Chemicals and reagents. The polyethylene we used in this study was pure and free of phthalate acid esters. It was purchased from Huachuang Chemical Co., Ltd. (Guangdong, China) with a particle size of ∼23 μm. Standard DBP reagent (99.8%) was purchased from Lark Technology Co., Ltd. (Beijing, China). Methanol (HPLC grade) and absolute ethanol (HPLC grade) were obtained from Thermo Fisher Scientific China Co. Ltd. (Shanghai, China). Assay kits for superoxide radicals (O2−), hydrogen peroxide (H2O2
Results
A methanol treatment was designed to determine the toxicity of methanol to lettuce. The results showed that there was no significant difference in the growth index, photosynthesis, and physiological and biochemical indexes between the methanol-treated and control groups (P > 0.05) (Table 1, Table 2, Table 3, Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5).
Growth parameters of lettuce. The growth parameters of the lettuce plants are shown in Table 1. Compared with the control, the fresh and dry weights
Discussion
Plant growth is the most intuitive characterization of toxicity and it can be inhibited by organic pollutants, including DBP (Gao et al., 2017). The growth of lettuce can be inferred from leaf and root dry weight and root length. In the present study, the MP-only, DBP-only, and all MP + DBP treatments significantly inhibited the growth of lettuce, as evidenced by the sharp decreases in these growth parameters. The findings of the present study are consistent with those of our previous study on
Conclusions
The findings of the present study indicated that the addition of exogenous MP increased the growth inhibition rate of DBP, the degree of photosynthesis inhibition, and the accumulated content of ROS in lettuce. Stress caused by the MP-only, DBP-only, and all MP + DBP treatments increased the activities of SOD, CAT, APX, GSH-Px, DHAR, MDHAR, and GR, and increased the contents of AsA and GSH. However, the results indicated that the antioxidant defense system could not completely remove the free
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgments
This study was funded by the National Key Research and Development Program of China (No. 2016YFD0800806), the STU Scientific Research Foundation for Talents (No. NTF19025), and the China Scholarship Council (No. 201708120026).
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