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Article

Polypropylene Microplastics and Cadmium: Unveiling the Key Impacts of Co-Pollution on Wheat–Soil Systems from Multiple Perspectives

by
Zhiqin Zhang
1,2,*,
Haoran He
3,
Nan Chang
4 and
Chengjiao Duan
5
1
School of Materials Engineering, Shanxi College of Technology, Shuozhou 036000, China
2
College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China
3
Key Laboratory of Green Utilization of Critical Non-Metallic Mineral Resources, Ministry of Education, Wuhan University of Technology, Wuhan 430070, China
4
State Key Laboratory of Herbage Improvement and Grassland Agro-Ecosystems, College of Ecology, Lanzhou University, Lanzhou 730000, China
5
College of Resources and Environment, Shanxi Agricultural University, Taigu, Jinzhong 030801, China
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(8), 2013; https://doi.org/10.3390/agronomy15082013
Submission received: 21 July 2025 / Revised: 13 August 2025 / Accepted: 20 August 2025 / Published: 21 August 2025
(This article belongs to the Section Agroecology Innovation: Achieving System Resilience)
Browse Figures
Versions Notes

Abstract

The interaction between microplastics (MPs) and heavy metals and their ecological risks to the soil–plant system has attracted widespread attention. This study explored the effects of polypropylene (PP) alone or combined with cadmium (Cd) pollution on wheat seed germination, plant growth, and the soil environment from multiple perspectives through seed germination experiments and pot experiments. The results of the seed germination experiment showed that the addition of 50 mg L−1 PP could promote the growth of seeds. However, medium and high concentrations of PP had significant inhibitory effects on seeds. For PP + Cd co-pollution, the addition of 50 mg L−1 PP could partially alleviate the stress of Cd alone. However, with the increase in PP concentration, the co-pollution showed stronger toxicity to seeds. Moreover, the synergistic effect of PP and Cd was greater than the antagonistic effect; both of them aggravated the stress on wheat. The results of the pot experiment showed that the soil microenvironment was significantly affected by PP alone or combined with Cd pollution. It was manifested as reducing soil moisture and pH, affecting soil nutrient cycling, and inhibiting the activities of soil enzymes (except for catalase). In addition, the MPs and Cd significantly affected the physiological characteristics of plants. Specifically, the addition of 50 mg L−1 PP alone promoted or had no significant effect on wheat growth. However, with the increase in PP concentration, the biomass and chlorophyll content of plants decreased significantly, while carotenoids, oxidative damage, and antioxidant enzyme activities increased significantly. Moreover, PP + Cd co-pollution led to stronger phytotoxicity. Moreover, PP exposure caused an increase in plant shoot and root Cd concentrations, promoting Cd transport from roots to shoots. Correlation heat maps and RDA analysis revealed that plant Cd concentration was significantly correlated with soil environmental factors and plant physiological indicators. Finally, the results of the linear model (%) of relative importance indicated that pH and MDA content were important soil and plant variables affecting the increase in Cd concentration in plant tissues. This study is of great significance for evaluating the ecological risks of MPs-Cd composite pollution.

1. Introduction

We live in a “plastic age” [1]. Plastics bring great convenience to human production and life but also cause serious pollution. Over the past few decades, with the significant increase in the production of different plastic polymers, a large amount of plastic waste has entered the soil environment [2]. These plastics are gradually decomposed into microplastics (MPs, particle size < 5 mm) [3], resulting in the soil being the largest reservoir of MPs [4]. Therefore, MPs pollution in the soil environment, as a serious global problem, has been receiving extensive attention [5].
It is noteworthy that any changes in soil properties caused by MPs may affect plant growth [6]. Studies have demonstrated that MP exposure can affect plant growth, alter many physiological and biochemical processes, and even produce toxic effects [7,8]. Furthermore, micro- and submicron-sized MPs may be absorbed by crops and translocated to edible tissues [8], ultimately affecting human health [9]. However, due to the diversity of MP types, sizes, and exposure concentrations, the biological effects of MPs exposure on terrestrial higher plants, especially crops, remain controversial [10]. For example, a study showed that 10% polylactic acid can significantly reduce maize biomass and chlorophyll content, while 0.1% and 1% exposure had no effect [11]. Another study showed that common polystyrene (PS) and sulfonated polystyrene (SPS) with different groups had different effects on wheat at the same exposure concentration. Specifically, the growth-promoting effect of PS on wheat showed a significant concentration-dependent relationship. SPS had a growth-promoting effect at low concentrations but a significant inhibitory effect at higher concentrations [12]. Actually, regarding wheat, as one of the most widely cultivated crops globally, some studies have been explored the impact of MPs on its physiological characteristics. For instance, different types of MPs (PP, HDPE, and PLA) had varying effects on the growth of wheat [13], and PE reduced the plant height of wheat [10]. There was no significant difference in the germination rate of wheat seeds between different exposure concentrations of PLA and whether it was aged, etc. [14]. In summary, there is no unified conclusion on the effects of MPs on wheat under different types, concentrations, and systems. Therefore, further study of the effect and mechanisms of MPs on plants, for risk assessment and management in the agricultural ecosystem pollution, is crucial.
In addition, due to human activities, such as mining, smelting, metal plating, sewage sludge treatment, and fertilizer use, there is often heavy metal pollution in the natural environment, of which cadmium (Cd) pollution is the most common [15,16]. As we all know, Cd, as a highly mobile and potentially bioavailable element, is easily absorbed and accumulated by plants [17,18]. MPs are natural carriers of many organic and inorganic pollutants. Therefore, MPs and Cd are likely to coexist in environmental media, and their co-exposure may cause greater toxicity to plants than single exposure [19,20]. For example, Wang et al. [21] showed that HDPE alone had no significant inhibitory effect on plant biomass, while combined pollution of HDPE and Cd could significantly reduce dry biomass. Another study showed that 1% PE increased soil Cd bioavailability and exacerbated Cd uptake by lettuce [19]. However, it has also been suggested that MP-heavy metal interactions were not always harmful to plants. For example, HDPE and PES microfibers did not affect Cd accumulation in maize [11] and lettuce [18], respectively. MPs can even partially mitigate the toxic effects of metals on plants and reduce their bioavailability [21,22]. Therefore, the co-exposure of MPs and heavy metals had no generally accepted conclusion [5], and there may be synergistic, antagonistic, or enhancing effects between them [20]. The interaction of MPs and heavy metals was affected by environmental exposure (such as physical and chemical weathering, microbial degradation, etc.), as well as the changes in surface charge, polarity, porosity, and roughness caused by the MP aging process, resulting in different adsorption behaviors [23]. In conclusion, the coupling effects of Cd and MPs in the soil–plant system remain largely unknown, especially regarding their impacts on plant toxicity and soil properties. Meanwhile, the interaction between MPs and Cd and the resulting differences in plant growth characteristics and other environmental effects under different hydroponic and soil culture systems are also lacking. Therefore, it is necessary to further understand the key ecological effects of MPs combined with heavy metals on plants and their related mechanisms.
As polypropylene (PP) is a typical MP [24], and Cd is one of the most common and toxic metal environmental pollutants. Wheat is the highest total yield and most widely cultivated food crop in the world [10] and has long demonstrated the ability to abnormally accumulate metal ions [25]. Therefore, based on this, this study conducted seed germination experiments and pot experiments to reveal the effects of different doses of polypropylene (PP) alone or combined with cadmium (Cd) on soil–plant systems from multiple perspectives. We hypothesized that (1) with the increase in MPs concentration, the inhibition of wheat germination or growth gradually increased; (2) either MPs alone or in combination with Cd pollution can significantly affect soil properties or the microenvironment; and (3) the presence of MPs will promote the uptake of Cd by wheat. The results of this study provide theoretical support for evaluating the ecological risks of MPs-heavy metal interactions to agricultural systems.

2. Materials and Methods

2.1. MPs and Their Characterization

PP is a typical and common MP [24]. The PP used in the seed germination experiment and pot experiment of this study was purchased from Shenzhen Guangyuan Plasticizing Co., Ltd. in Shenzhen City, China. The particle sizes of the plastics were 13 μm and 150 μm, respectively. In this study, the surface morphology and crystal structure of plastics were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. For detailed descriptions of the entire process, please refer to the Supplementary File Text S1.

2.2. Plant Used in This Study and Their Pretreatment

The wheat seeds were purchased from Yayi Agricultural Planting Co., Ltd. in Suzhou City, China, and the variety was Jinchun No. 6. The seed germination rate of this wheat is more than 85%. It has strong disease resistance and tolerance and is sensitive to water and temperature. Seeds with uniform and full particles were selected, disinfected with 3% H2O2 for 30 min, and then rinsed clean with distilled water.

2.3. Seed Germination Experiment

2.3.1. Experimental Design and Procedure

Refer to previous studies [26]; three different concentrations of PP (50, 200, and 500 mg L−1) were set up in this experiment. For the PP alone treatment, MPs concentrations were labeled as PA, PB, and PC from low to high, and a control treatment was set up at the same time (CK, adding an equal volume of deionized water). For the PP + Cd combined treatment, the MPs concentrations were labeled as HA, HB, and HC from low to high in sequence; the Cd concentration was 5 mg L−1, while a single Cd treatment (Cd) was set simultaneously. And each treatment was replicated five times.
Specifically, place a filter paper in a 9 cm glass culture dish. For single exposure of PP, add 10 mL of MPs suspension to each dish. For PP + Cd combined exposure, add 5 mL of MPs suspension and 5 mL of Cd solution per dish. After extruding the bubbles under the filter paper, place the soaked seeds neatly on the filter paper, 30 seeds per dish. The culture dish was placed in 25 °C biochemical incubator for 7 days (11–17 March 2024). Seed germination was recorded at 17:00 every day (germination was defined as the length of the radicle reaching 1/2 of the seed length). Add an appropriate amount of deionized water daily to compensate for evaporating water and keep the concentration of the culture medium constant.

2.3.2. Method of Index Determination

The germination rate (GR), germination potential (GV), germination index (GI), vitality index (VI), average germination time (MGT), and average germination speed (MGS) of the seeds were determined. Details of germination index calculations (GI, VI, and ΔI) were provided in Supplementary Text S2. After the completion of seed germination experiment, 10 seeds were randomly selected for each treatment to determine bud length and root length. In addition, the interaction between MPs and Cd on seed was evaluated by calculating the net growth change (ΔI) [27]. Details were described in Supplementary File Text S2.

2.4. Pot Experiment

2.4.1. Soil Used in This Study

The test soil was collected from the farmland topsoil (0–20 cm) in Shuozhou City, Shanxi Province (39°19′ N, 112°25′ E). The soil was air-dried after removing impurities and then passed through 2 mm sieve. The basic properties of soil were as follows: pH, 8.60; soil organic carbon (SOC), 11.05 g kg−1; total nitrogen (TN) 0.54 g kg−1; total phosphorus (TP) 1.88 g kg−1; Clay content (20%); Silt content (40%); Sand content (40%); and Cd 0.25 mg kg−1.

2.4.2. Experimental Design and Procedure

The pot experiment was conducted with 8 treatments: CK (no MPs and Cd added, control treatment), PA (0.1% PP), PB (1.0% PP), and PC (5.0% PP); Cd-only treatment (Cd), HA (0.1% PP + Cd), HB (1.0% PP + Cd), and HC (5.0% PP + Cd); the Cd concentration was 5 mg L−1, and each treatment was replicated three times. The concentration of MPs was determined according to previous research [28,29].
Specifically, 500 g of soil was added to each pot along with the corresponding proportion of PP or Cd. All materials were thoroughly mixed and allowed to stabilize at room temperature for 30 days to ensure soil equilibrium. Planted 20 uniform-sized, plump, and surface-sterilized wheat seeds. The experiment was conducted in a greenhouse maintained at temperatures between 25 and 35 °C, with relative humidity ranging from 50% to 65%, under natural daylight conditions (from 8:00 to 20:00 daily). Throughout the experimental period, the gravimetric method was employed to maintain soil moisture at approximately 60% of field capacity. After 35 days of growth (5 April–10 May 2024), plants and soil were collected, respectively. Shoots and roots were carefully rinsed thoroughly with deionized water and stored for further analysis.

2.4.3. Determination and Analysis of Plant and Soil Indexes

(1)
Plant indexes
Firstly, the height of the plants and the length of the roots were measured with a ruler, and then fresh biomass was determined. Then, the shoots and roots were dried at 60 °C for 48 h to a constant weight and weighed respectively to determine the dry biomass.
The leaves were extracted in the dark with anhydrous ethanol until they turned completely grayish-white. Then, the absorbance values were measured at specific wavelengths using a UV-spectrophotometer, and the contents of chlorophyll and carotenoids in the plants were calculated.
The contents of hydrogen peroxide (H2O2), superoxide anion (O2·−), malondialdehyde (MDA), and the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were determined using the kit (Suzhou Keming Biochemical Reagent Co., Ltd., Suzhou City, China)
(2)
Soil indexes
Dried the soil to a constant weight and measured the moisture content. The pH value was determined using air-dried soil at a soil–water ratio of 1:25 (w/v). SOC was determined by dichromate oxidation method [30]. TN was determined by Kjeldahl method. TP was determined by molybdenum blue method [31]. In addition, soil urease, alkaline phosphatase, sucrase, and catalase activities were determined [32], as detailed in Supplementary Materials Text S3.
(3)
Cd analysis
After dissolving the plant samples (shoots and roots) and soil samples, the total Cd concentration was determined using an atomic absorption spectrophotometer (Hitachi, FAAS Z-2000, Tokyo City, Japan). In addition, the available cadmium content in the soil was extracted with DTPA solution and determined. Simultaneously calculated the uptake and transport coefficients (TFs) of heavy metals in plants. The detailed determination and calculation steps are shown in Supplementary Materials Text S4.

2.5. Statistical Analysis

Statistical analysis was performed using IBM SPSS Statistics 27, including one-way ANOVA (comparing the differences among various treatments) and two-way ANOVA (MPs × Cd, examining the main effects and interaction effects of MPs and Cd on soil physicochemical properties and plant physiological indicators). Figures were generated using Origin 2025. To further explore the key drivers affecting plant growth and Cd enrichment, correlation heatmaps and the linear model (%) of relative importance analysis were performed using R software (Version 4.2.2). Redundancy Analysis (RDA) was conducted using Canoco 5. Both RDA and the linear model (%) of relative importance analysis require data normalization as a preliminary step.

3. Results

3.1. Microplastics Characterization

PP was characterized using SEM and XRD (Figure 1). SEM images showed that 13 μm PP had a regular, long cylindrical shape with a rough surface (Figure 1A,B), and 150 μm PP had a blocky shape with a relatively flat surface (Figure 1C,D).
The XRD results showed that the 13 μm PP had a relatively low crystallinity, a high proportion of amorphous regions, and a small crystal size (Figure 1E). The 150 μm PP had a relatively high crystallinity, a low proportion of amorphous regions, and a large crystal size (Figure 1F). The position and shape of the characteristic peaks (crystallinity) of them are significantly different, and this difference may be caused by the processing technology.

3.2. Seed Germination Rate

The results showed that most of the seeds could still germinate under MPs or Cd treatment, and the final germination rates of seeds were 90.00–97.33% and 83.33–90.67%, respectively (Figure 2A,B). Compared with CK, the treatment with 50 mg L−1 PP alone relatively promoted the germination of seeds, while the exposure of medium and high concentrations of PP significantly reduced the germination rate of seeds. In addition, the combined treatment further inhibited the germination rate of seeds compared with the single treatment of PP. However, compared with single Cd treatment, low-concentration PP relatively promoted the germination rate of seeds. In addition, seed germination rates varied with time at different exposure concentrations (Figure 2C,D). Specifically, the germination rates of the treatments increased gradually over time, increasing rapidly on day 2, increasing slowly on day 4, and finally leveling off. Overall, the germination rate of seeds decreased significantly with the increase in MPs concentration regardless of Cd addition.

3.3. Seed Growth Characteristics

Table 1 shows the effects of different treatments on seed growth characteristics. The results demonstrated that, compared to the CK treatment, under MPs single exposure, the addition of low-concentration PP (50 mg L−1) had a promoting effect on the GV, GI, and VI of seeds. However, further increasing the PP concentration resulted in insignificant inhibition of seed growth characteristics. In addition, PP + Cd treatment suggested that low concentration PP addition relatively alleviated the adverse effects of single Cd treatment on seeds; however, with the increase in MPs concentration, the GV, GI, and VI of seeds were significantly reduced. The toxicity of PP + Cd combined pollution was stronger than that of PP single pollution. Furthermore, Table 1 showed that MGT and MGS of seeds were independent of Cd addition and increased overall with increasing PP exposure concentrations.
The effects of PP single or combined with Cd treatment on the bud length and root length of seeds are presented in Figure S1. The results indicated that exposure to different concentrations of PP alone, or combined exposure with Cd, had different effects on seed bud and root length. Specifically, for the PP treatment alone, compared with CK, the addition of low-concentration PP slightly promoted seed growth, while medium- and high-concentration PP significantly inhibited (p < 0.05). For the PP + Cd treatment, the addition of low-concentration PP alleviated the stress of Cd alone on seed root growth (15.48%). However, with the increase in PP concentration, the toxicity of seeds was significantly enhanced by combined exposure, which was significantly higher than that of single exposure. For instance, compared with CK treatment, the bud and root length of seeds under PC treatment decreased by 18.45% and 17.28% respectively, while under HC treatment, they decreased by 31.63% and 34.37%, respectively.

3.4. Interaction Between PP and Cd

This study investigated the overall effects of PP alone or combined with Cd on the germination and growth characteristics of wheat seeds (Figure 3). Our study indicated that 50 mg L−1 PP could promote seed growth, while medium- and high-concentration PP produced a significant inhibitory effect (Figure 3A). Similarly, under combined pollution, 50 mg L−1 PP can alleviate single Cd stress. However, with increasing PP concentration, combined pollution resulted in stronger phytotoxicity than single pollution (Figure 3B). There was an obvious “concentration-dose effect” between PP concentration and many indexes of seed growth characteristics. Furthermore, Table S1 evaluated the combined effects of MPs and Cd on seed germination by calculating net growth changes (ΔI). Specifically, MPs and Cd showed an antagonistic effect at low concentration but a synergistic effect at high concentration. Overall, the synergistic effect of MPs and Cd on many indexes of seeds was greater than the antagonistic effect, which indicated that the combination of MPs and Cd enhanced the toxicity of plants.

3.5. Soil Physicochemical Properties and Enzyme Activities

Table S2 presents the basic physicochemical properties of soil under different treatments. The results showed that compared with CK, soil water content and pH decreased significantly with the increase in PP concentration, regardless of whether Cd was added; the combined pollution decreased more obviously than the single pollution. Soil SOC, TN, and TP contents basically increased gradually with the increase in PP concentration. Overall, there is a significant “concentration-dose effect” between SOC and PP concentration. However, there was no difference in TN content between PB and PC.
The changes in soil enzyme activities under different treatments are presented in Figure S2. The results showed that soil enzyme activities were significantly affected by MPs alone or in combination with Cd exposure (p < 0.05). Specifically, the activity of soil urease decreased significantly with the increase in PP concentration, showing a distinct “concentration-dose effect”, and the co-pollution further inhibited the activity of urease (Figure S2A). In addition, the activities of alkaline phosphatase and sucrase were also significantly inhibited (Figure S2B,D). However, the addition of low-concentration PP showed no significant difference compared with CK treatment, while medium- and high-concentration exposure led to a significant decrease in enzyme activities. Soil catalase activity, especially at medium and high concentrations, increased significantly after PP addition. The PP + Cd treatment resulted in higher catalase activity than the PP alone treatment (Figure S2C).
Two-way ANOVA (Table S4) revealed that PP exerted a significant main effect on all soil indicators, except for SOC and sucrase. Cd, PP × Cd also had significant main effects and interaction effects, respectively (p < 0.5).

3.6. Plant Height, Root Length, Plant Biomass, and Chlorophyll Content

The biomass, plant height, and root length of wheat under different treatments are shown in Figure S3. The results indicated that under single exposure, compared with CK treatment, low-concentration PP treatment relatively promoted the growth of wheat, such as root length, fresh biomass, and branch dry biomass. However, the addition of medium and high concentrations of PP significantly inhibited multiple growth indicators of wheat. For the PP + Cd treatment, the combined pollution had a stronger inhibitory effect on wheat growth indicators than PP or Cd alone exposure and had an obvious “Concentration-dose effect”.
The effects of PP alone or PP + Cd co-exposure on the chlorophyll content of wheat are shown in Table S3. The results showed that compared with CK, there was no difference in the chlorophyll content of wheat under low concentration PP treatment. However, exposure to medium and high concentrations of PP significantly reduced the contents of Chla, Chlb, and Chla+b in wheat while significantly increasing the contents of Chla/b and Car. For the PP + Cd treatment, with the increase in PP concentration, the chlorophyll content of the plants was further reduced. Similarly, Car content increased significantly, whereas Chla+b content was lowest only in the HC treatment, with no difference between the other treatments.
Similarly, two-way ANOVA (Table S4) revealed that PP exerted a significant main effect on many growth indexes of wheat. In addition to Chlb and Chla/b, Cd also has a significant main effect on other indicators. Furthermore, PP and Cd had significant interaction effects on indicators such as plant fresh biomass, root length, and Chla.

3.7. Plant Oxidative Damage and Antioxidant Enzyme Activity

The contents of oxidative damage (MDA, H2O2, and O2·−) and the activities of antioxidant enzymes (SOD, POD, and CAT) in plant tissues are displayed in Figure 4. The results showed that the contents of MDA, H2O2, and O2·− in wheat tissues were significantly increased by PP alone or combined with Cd, and there was a significant “Concentration-dose effect”. In addition, co-pollution leads to higher oxidative damage (Figure 4A–C). Correspondingly, compared with CK treatment, the antioxidant enzyme activity of plants also increased significantly with the increase in PP concentration. However, there was no difference in POD and SOD activities under low-concentration PP exposure compared with CK treatment, while there was no difference in SOD and CAT activities under HB and HC treatment (Figure 4D–F). In general, the oxidative damage and antioxidant enzyme activities of plants reached the maximum under HC treatment (5.0% PP + Cd). Moreover, two-way ANOVA (Table S4) revealed that PP and Cd had significant main effects and interaction effects on antioxidant enzyme activities, while only significant main effects on oxidative damage indicators.

3.8. Cd Concentration in Plant and Soil

Table 2 demonstrates the Cd concentrations in soil and plant tissues under different treatments. The results indicated that roots had higher Cd concentrations than shoots. Furthermore, with the increase in MPs concentration, the Cd concentration in wheat shoots and roots increased significantly and reached the maximum value of 0.79 and 3.23 mg kg−1 under HC treatment, respectively. Cd uptake by plants was affected by plant dry biomass, which reached the maximum under Cd alone treatment. The overall Cd uptake of shoots and roots decreased with increasing PP concentration; however, there was little difference in Cd uptake between HA and HB treatments.
In addition, Table 2 suggests that soil Cd concentration increased significantly with increasing PP concentration, while DTPA-Cd concentration decreased under HB and HC treatments. Two-way ANOVA (Table S4) revealed that both PP and Cd had significant main effects and interaction effects on Cd-related indicators of plants and soils.

3.9. Factors Affecting Cd Concentration in Plants

Figure 5 displays the correlations between soil properties and plant indicators. The results show that soil moisture and pH were negatively correlated with soil nutrients (SOC, TN, and TP) and catalase activity and positively correlated with other enzyme activities, regardless of single treatment or combined exposure. Soil nutrient indicators were opposite to moisture and pH. In addition, under PP + Cd treatment, soil total Cd concentration was positively correlated with soil nutrients and catalase and negatively correlated with other soil environmental factors, while DTPA-Cd was opposite to soil total Cd (Figure 5B). The heatmap results of the correlation between soil and plant indicators show that under PP single pollution, soil environmental factors were highly correlated with almost all plant indicators (p < 0.01), except plant height and Chla/b (p = 0.01–0.5) (Figure 5C). Under PP + Cd treatment, TF was not correlated with soil physicochemical properties, and Chla/b, TF were not correlated with soil Cd, while other soil environmental factors were significantly correlated with plant physiological indexes (p < 0.05) (Figure 5D).
To further understand the relationship between heavy metal indicators and other indicators, we conducted an RDA analysis, as presented in Figure 6. The results demonstrated that soil environmental factors and plant physiological indicators could well explain heavy metal indicators, with an explanation rate of 90.43%. And the first and second axes explained 90.34% and 0.09%, respectively. Specifically, soil nutrient indicators, total Cd concentration, plant oxidative damage, and antioxidant enzyme activities were the main positive factors affecting Cd concentration and TF in plant tissues, while plant biomass and photosynthetic pigments were the main negative factors.
In addition, Figure S4 shows the correlation of Cd concentration of plant tissues with plant and soil indicators. After screening the key indicators, we further constructed a linear model of relative importance (%) to deeply explore the influence of environmental variables on Cd enrichment in plant tissues (Figure S5). The results suggested that the model could explain 98.91% of the soil factors and 94.02% of the plant factors on shoot Cd concentration (Figure S5A,C) and 85.57% of soil factors and 95.35% of plant factors on Cd concentration in roots (Figure S5B,D). Moreover, TP and pH were the most important soil variables affecting Cd concentration in plant tissues, while MDA and plant height were the most important plant variables.

4. Discussion

4.1. Effects on Seed Germination

Wheat is one of the most important food crops in the world [10]. Seed germination is the beginning of wheat life and the most sensitive critical period to the external environment [33,34]. Therefore, the exposure test of MPs on plant seed germination is expected to preliminarily evaluate the toxicological effects of pollutants on plants.
This study investigated the overall effects of PP alone or combined with Cd on the germination and growth characteristics of wheat seeds (Figure 3). The results showed that the addition of low-concentration PP had a corresponding promoting effect on seed growth. This was inconsistent with our hypothesis that “with the increase in MPs concentration, the inhibition of wheat germination or growth gradually increased”. This is possibly due to a “priming effect”. Lian et al. [34] showed that exposure to PSNPs significantly increased the water absorption of seeds during the early stage of seed germination, when the seeds were in a rapid absorption stage. At this stage, PSNPs rapidly initiated the recovery of basal metabolism caused by the gradual increase in hydration. However, the mechanism of MPs promoting plant growth or seed germination is still unclear and needs further study. In addition, the results displayed that medium- and high-concentration PP produced a significant inhibitory effect (Figure 3A). This may be due to the accumulation of MPs on the seed epidermis and root hairs, resulting in physical blockages that inhibit seed growth [35]. In addition, many harmful substances contained in MPs themselves may leach out, leading to oxidative damage in plants, especially exposure to high concentrations of MPs [34].
Similarly, for the combined pollution of PP and Cd, the results showed that the addition of low concentrations of PP could alleviate the toxicity of single Cd to plants. Meanwhile, by calculating the net growth change (ΔI) (Table S1), the results indicated that PP and Cd showed synergistic effects on seed growth under HA treatment, which was consistent with previous results [27]. However, it is noteworthy that the toxicity of the co-pollution gradually increased with increasing PP concentration. This may be attributed to the substantial accumulation of MPs in plant roots with increasing PP concentrations, which hinders plant water uptake and thus growth inhibition [36]. In addition, MPs can adsorb heavy metal ions in the environment, and when MPs increase, they can enhance the probability of heavy metals entering and accumulating in organisms through their own migration advantage, thus causing a more toxic inhibition effect on organisms [23]. In conclusion, the complex interaction between MPs and Cd needs to be further explored.

4.2. Effects on Soil Properties

The results of the pot experiment indicated that MPs alone or combined with Cd significantly affected soil properties and the microenvironment, which was consistent with our hypothesis. Specifically, the results of this study showed that PP addition significantly reduced soil moisture content, regardless of single exposure or combined pollution (Table 1). This may be due to the strong hydrophobicity of MPs, which will directly affect the water-holding capacity of soil. In addition, MP exposure affects the physical structure, aggregation, and porosity of soil [37]. For example, a previous study showed that MPs added to soil can increase the evaporation rate of soil moisture by establishing water flow channels [38]. Furthermore, PP also led to a significant decrease in soil pH. This may be due to changes in the soil microenvironment or the colonization of MPs’ surfaces by microorganisms [37]. Our study demonstrates that soil SOC content increases significantly (p < 0.05) with rising PP concentration. This enhancement was driven by MPs’ inherent organic carbon components and additive release, substantially elevating soil SOC levels [39]. Furthermore, MP addition stimulated the increase in TN and TP simultaneously. This may be due to the following reasons: Firstly, MPs have a positive impact on soil aggregation, resulting in a higher nutrient retention capacity [37]. Second, studies have proven that MPs contain multiple additives that may contain multiple elements such as P, N, and Cl and eventually release these elements into the soil after mineralization [40]. Finally, MPs can influence soil nutrients by mediating microbial communities and activities [41].
Soil enzyme activities are important indicators of soil quality and fertility. The results of this study showed that PP decreased the activities of sucrase, urease, and alkaline phosphatase but increased the activity of catalase, regardless of whether Cd was added (Figure S2). The results of this study were consistent with previous studies [42]. The decreased activity of soil sucrase was due to the toxic compounds leached from MPs that may inhibit the expression of functional genes related to organic carbon [43]. Studies have proven that MPs entering the soil can affect the porosity or aggregates of the soil [37], which may disturb the nitrogen cycle process of the soil [41] and thus inhibit urease activity. In addition, Li et al. [44] showed that co-exposure of MPs and Cd further inhibited urease activity. However, some studies suggested that MPs stimulate urease activity, potentially due to their influence on soil nitrogen storage and nitrogen cycling efficiency [45]. Furthermore, the increase in soil catalase might be due to the addition of MPs, altering the soil structure and affecting the quantity and activity of aerobic microorganisms. Previous studies have also shown that different concentrations of PVC, PS, and PE can stimulate the activity of soil catalase [46,47]. Overall, MPs induce direct and indirect impacts on soil nutrients and enzyme activities through multiple mechanisms, which vary depending on polymer type, MP shape, dose, and size [37,48].

4.3. Effects on Plant Growth

Our study demonstrated that PP or Cd had different and extensive effects on plant physiological characteristics. The results showed that compared with CK treatment, low concentration PP addition could promote the growth of wheat or had no significant difference with CK. This might be because low concentrations of PP provide a small amount of nutrients for plants and promote the release of soil nutrients [49]. However, the plant growth was obviously inhibited by medium and high concentrations of PP exposure. It was manifested as a reduction in biomass, plant height, and root length (Figure S3), inhibition of photosynthetic pigments (Table S3), oxidative damage (Figure 4), etc. Studies have suggested that MPs tend to attach to plant roots, hindering their access to nutrients and thus adversely affecting plant growth [7]. Furthermore, harmful substances released by PP, including plasticizers, flame retardants, and stabilizers, may leach into the soil and pose a hazard to wheat [50]. In addition, for PP + Cd combined pollution, the results showed that a low concentration of PP addition could partially alleviate the stress of Cd alone. However, with the increase in PP concentration, co-pollution led to stronger phytotoxicity than single pollution. This is due to the fact that MPs, in combination with Cd, may cause harmful effects on plants by increasing the concentration of accumulated Cd in plant tissues under the Trojan horse effect [51]. This is also confirmed in Table 2 of the results of this study.
It is well known that the content of chlorophyll will directly affect the intensity and rate of photosynthesis, thus affecting the growth and yield of plants [52]. The results of this study demonstrated that both single exposure to PP and combined pollution with Cd led to a decrease in chlorophyll content in wheat. Studies have proven that exogenous pollutants can cause intracellular ROS accumulation and damage chloroplasts, hindering chlorophyll synthesis [53]. The results of this study also confirmed that the content of ROS in wheat tissues increased significantly under PP or Cd stress (Figure 4). In addition, studies have shown that metal elements were involved in plant photosynthesis [54]. Therefore, the combined stress of heavy metals will further reduce chlorophyll, leading to weakened photosynthesis. For instance, the study by Jiang et al. [55] indicated that PP + Cd reduced the chlorophyll content of rice seedlings. However, notably, this study showed that carotenoid content increased under the stress of PP or Cd. The increase in carotenoid content may regulate carotenoid synthesis to remove accumulated ROS and protect plant leaves from oxidative damage [10]. Furthermore, the results of our study showed that the content of oxidative damage in wheat tissues increased with increasing PP concentration, and the combination with Cd resulted in the accumulation of higher ROS content (Figure 4). When plants are under stress, ROS levels increase dramatically. And oxidative stress occurs once the antioxidant capacity of the plant is exceeded [56]. In addition, it has been demonstrated that Cd can damage cell membrane structure and cause oxidative damage in plants, thereby increasing ROS content [57]. Therefore, wheat produced higher levels of antioxidant enzymes to resist external stress. This result was consistent with many previous studies [12,58]. However, when external stress induces excessive ROS production in plants, it may disrupt the antioxidant system balance, paradoxically leading to decreased enzyme activity [59].

4.4. Driving Factors on Plant Cd Enrichment

The results of this study indicated that the presence of PP promoted the Cd concentration in the shoots and roots of plants (Table 2), which was consistent with our hypothesis. The results were similar to previous studies [19,60]. However, the impact of MPs on plant enrichment of metal ions can be highly complex. There are also corresponding studies indicating that the presence of MPs can alleviate the toxicity of Cd to plants. For example, Zhang et al. [48] showed that PS addition reduced Cd uptake by Brassica chinensis L., and Yang et al. [61] suggested that MPs combined with Cd promoted tomato growth and significantly reduced oxidative damage in plants. The reduced Cd bioavailability caused by MPs may be related to their size and surface properties. Nevertheless, the prevailing view currently tends to suggest that MPs facilitate plants’ uptake of heavy metals [62] for the following reasons: Firstly, the addition of MPs altered the soil microenvironment. Studies have shown that MPs can accumulate heavy metals on surfaces due to their strong hydrophobicity and high specific surface area [63]. However, accumulated MPs can be easily released back into the soil solution, especially under the constantly changing soil physicochemical conditions [19]. The correlation heat map (Figure S4) and RDA analysis (Figure 6) of this study further confirmed that most soil physicochemical properties were highly correlated with the Cd concentration in the stems and roots of plants (Figure S4). In addition, Figure S5 reveals that soil pH was the most important soil factor affecting Cd concentration in wheat roots. Studies have suggested that MPs can change soil pH [45], thereby improving the availability of soil Cd and significantly increasing the bioaccumulation of plant Cd [19]. Secondly, the increased Cd enrichment in wheat tissues and enhanced bioavailability of soil Cd may be attributed to MPs altering the adsorption and desorption processes of Cd in the soil [64]. For example, Abbasi et al. [65] reported that heavy metals (Pb, Cd, and Zn) were initially adsorbed onto MPs and subsequently desorbed in the wheat rhizosphere. Although this study indicated that PP addition reduced the content of DTPA-Cd in HB and HC treatments. This might be caused by the complex influence of MPs and soil environmental conditions. Thirdly, we found that the transport coefficient (TF) of heavy metals within plants was also higher than that of Cd alone exposure. According to the research of Takahashi et al. [66], the plasma membrane of plant cells contains OsHMA2, which may promote the transfer of Cd from roots to branches. Finally, MPs have the potential to influence the behavior of soil organisms, such as microorganisms involved in Cd cycling [18], due to altered soil nutrient properties and enzyme activities [48]. Studies have shown that MPs can promote cadA/czcA gene expression in soil microbes and enhance Cd bioavailability and toxicity [18,19].
Furthermore, Figure S5 shows that MDA was the most important factor influencing the concentration of Cd in plant tissues. This is because MDA is the final product of lipid peroxidation and the main indicator of plant response to Cd stress [53]. Moreover, Figure S4 also suggests that there was a significant correlation between the Cd concentration of plants and the chlorophyll content. Figure S5 also confirms that chlorophyll contents were the important indicators affecting the Cd concentration of plants. This may be due to the presence of Cd significantly inhibited the accumulation of Mg in leaves and further affected the production of chlorophyll [55]. In conclusion, the complexity of plant toxicity resulting from combined MPs-Cd exposure is significantly modulated by multiple factors: MPs-related parameters (type, morphology, particle dimensions, and dosage), Cd concentration, and heterogeneity in soil milieu.

5. Conclusions

This study explored the effects of PP alone or combined with Cd pollution on wheat seed germination, plant growth, and soil environment from multiple perspectives through seed germination experiments and pot experiments. The conclusions of this study were as follows:
(1)
The results of the seed germination experiment showed that the addition of low concentration PP could promote the growth of seeds. However, medium and high concentrations of PP had significant inhibitory effects on the growth characteristics of seeds. For PP + Cd co-pollution, the addition of low-concentration PP could partially alleviate the single stress of Cd. However, with the increase in PP concentration, the co-pollution showed stronger toxicity to the growth of wheat seeds than the single pollution. Moreover, the synergistic effect of PP and Cd was greater than the antagonistic effect; both of them aggravated the stress on wheat.
(2)
The results of the pot experiment showed that both single PP and combined Cd pollution had a significant impact on the soil microenvironment. It was manifested as reducing soil moisture and pH, affecting soil nutrient cycling, and inhibiting the activities of soil urease, sucrase, and alkaline phosphatase, while increasing the activity of catalase. And the combined pollution had a stronger adverse effect on the soil.
(3)
In addition, the MPs-Cd system significantly affected the physiological characteristics of plants. Specifically, compared with the CK treatment, the addition of low concentration PP alone promoted or had no significant effect on plant growth. However, with the increase in PP concentration, the biomass and chlorophyll content of plants decreased significantly, while carotenoids, oxidative damage, and antioxidant enzyme activities increased significantly. Moreover, PP + Cd co-pollution led to stronger phytotoxicity. In addition, PP exposure caused an increase in plant shoot and root Cd concentrations, promoting Cd transport from roots to shoots. Correlation heat maps and RDA analysis revealed that plant Cd concentration was significantly correlated with soil environmental factors and plant physiological indicators. Moreover, the results of the linear model (%) of relative importance indicated that soil pH and plant MDA content were important soil and plant variables affecting the increase in Cd concentration in plant tissues.
In conclusion, the results of this study provide theoretical support for the compound ecological risk assessment of MPs and Cd. However, this work only used one type of MP polymer and was based on laboratory conditions. Future research needs to further explore the true exposure doses and types of MPs in the natural environment, as well as long-term and large-scale field experiments. In addition, it is necessary to combine the plant–soil–microorganism system to reveal the complex mechanism of MPs-heavy metal co-pollution on agroecosystems.

Supplementary Materials

Please add the section and the following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15082013/s1, Text S1 Characterization of PP by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Text S2 Calculation of seed germination indicators. Text S3 Determination of soil enzyme activity. Text S4 Cd analysis. Table S1 Interaction between MPs and Cd co-exposure on seed germination and growth. Table S2 Effects of PP alone or PP + Cd co-exposure on soil physicochemical properties. Table S3 Effects of PP alone or PP + Cd co-exposure on chlorophyll content in wheat. Table S4 Significance levels (F values) of PP, Cd, and the impact of their interactions on measured variables according to two-way ANOVA analysis. Figure S1 Effects of PP alone (A) or PP + Cd (B) co-exposure on seed bud length and root length. Figure S2 Effects of PP alone or PP + Cd co-exposure on the enzyme activities of soil. Figure S3 Effects of PP alone or PP + Cd co-exposure on plant height (A), root length (B), fresh biomass (C and D) and dry biomass (E and F) of wheat. Figure S4 The correlation of Cd concentration of plant tissues with soil (A) and plant (B) indicators. Figure S5 Linear models reveal the relative importance (%) of environmental drivers for Cd concentration in shoots (A) and roots (B).

Author Contributions

Z.Z.: Writing—original draft, Funding acquisition, and Resources. H.H.: Methodology and Review and editing. N.C.: Review and editing and Visualization. C.D.: Review and editing and investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Fundamental Research Program of Shanxi Province (202403021212116) and the Science and Technology Innovation Program for Higher Education Institutions of Shanxi Province (2024L438), as well as the Start-up Funds for high-level talent research of Shanxi College of Technology (013016).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author(s).

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 15 02013 g001
Figure 1. SEM and XRD images of PP. 13 μm PP: (A,B,E) and 150 μm PP: (C,D,F).
Figure 1. SEM and XRD images of PP. 13 μm PP: (A,B,E) and 150 μm PP: (C,D,F).
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Figure 2. Effects of PP alone (A,C) or PP + Cd (B,D) co-exposure on seed germination rate. Note: The lowercase letters indicate significant differences (p < 0.05) between treatments. PA: 50 mg L−1 PP; PB: 200 mg L−1 PP; PC: 500 mg L−1 PP; HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1. Each value represents mean ± SE.
Figure 2. Effects of PP alone (A,C) or PP + Cd (B,D) co-exposure on seed germination rate. Note: The lowercase letters indicate significant differences (p < 0.05) between treatments. PA: 50 mg L−1 PP; PB: 200 mg L−1 PP; PC: 500 mg L−1 PP; HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1. Each value represents mean ± SE.
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Figure 3. Effects of PP alone (A) or PP + Cd (B) co-exposure on seed growth characteristics. Note: GR: germination rate; GV: germination potential; GI: germination index; VI: vigor index; MGT: mean germination time. MGS: mean germination speed. PA: 50 mg L−1 PP; PB: 200 mg L−1 PP; PC: 500 mg L−1 PP; HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1.
Figure 3. Effects of PP alone (A) or PP + Cd (B) co-exposure on seed growth characteristics. Note: GR: germination rate; GV: germination potential; GI: germination index; VI: vigor index; MGT: mean germination time. MGS: mean germination speed. PA: 50 mg L−1 PP; PB: 200 mg L−1 PP; PC: 500 mg L−1 PP; HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1.
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Figure 4. Effects of PP alone or PP + Cd co-exposure on plant oxidative damage (A–C) and antioxidant enzyme activities (D–F). Note: The lowercase letters indicate significant differences (p < 0.05) between treatments. PA: 50 mg L−1 PP; PB: 200 mg L−1 PP; PC: 500 mg L−1 PP; HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1. Each value represents mean ± SE. MDA: malondialdehyde; H2O2: hydrogen peroxide; O2·−: superoxide anion. SOD: superoxide dismutase; POD: guaiacol peroxidase; CAT: catalase. * p < 0.05; ** p < 0.01; *** p < 0.001.
Figure 4. Effects of PP alone or PP + Cd co-exposure on plant oxidative damage (A–C) and antioxidant enzyme activities (D–F). Note: The lowercase letters indicate significant differences (p < 0.05) between treatments. PA: 50 mg L−1 PP; PB: 200 mg L−1 PP; PC: 500 mg L−1 PP; HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1. Each value represents mean ± SE. MDA: malondialdehyde; H2O2: hydrogen peroxide; O2·−: superoxide anion. SOD: superoxide dismutase; POD: guaiacol peroxidase; CAT: catalase. * p < 0.05; ** p < 0.01; *** p < 0.001.
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Figure 5. Heat maps of the correlation between soil environmental factors under the treatment of PE alone or in combination with Cd (A,B), as well as the heat map of the correlation between soil and plant physiological characteristics (C,D). Note: S: shoot; R: root; FB: fresh biomass; DB: dry biomass; Chla: Chlorophyll a; Chlb: Chlorophyll b; Chla+b: Chlorophyll a and Chlorophyll b; Chla/b: Chlorophyll a/Chlorophyll b; Car: carotenoids; SOD: superoxide dismutase; POD: guaiacol peroxidase; MDA: malondialdehyde; H2O2: hydrogen peroxide; O2·−: superoxide anion; CAT: catalase; SOC: soil organic carbon; TN: total nitrogen; TP: total phosphorus; AP: alkaline phosphatase; DTPA Cd: soil DTPA-extractable Cd concentration. * p < 0.05; ** p < 0.01; *** p < 0.001.
Figure 5. Heat maps of the correlation between soil environmental factors under the treatment of PE alone or in combination with Cd (A,B), as well as the heat map of the correlation between soil and plant physiological characteristics (C,D). Note: S: shoot; R: root; FB: fresh biomass; DB: dry biomass; Chla: Chlorophyll a; Chlb: Chlorophyll b; Chla+b: Chlorophyll a and Chlorophyll b; Chla/b: Chlorophyll a/Chlorophyll b; Car: carotenoids; SOD: superoxide dismutase; POD: guaiacol peroxidase; MDA: malondialdehyde; H2O2: hydrogen peroxide; O2·−: superoxide anion; CAT: catalase; SOC: soil organic carbon; TN: total nitrogen; TP: total phosphorus; AP: alkaline phosphatase; DTPA Cd: soil DTPA-extractable Cd concentration. * p < 0.05; ** p < 0.01; *** p < 0.001.
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Figure 6. RDA analysis of heavy metal indicators with plant physiological indicators and soil environmental factors. Note: S: shoot; R: root; FB: fresh biomass; DB: dry biomass; Chla: Chlorophyll a; Chlb: Chlorophyll b; Car: carotenoids; SOD: superoxide dismutase; POD: guaiacol peroxidase; MDA: malondialdehyde; H2O2: hydrogen peroxide; O2·−: superoxide anion; CAT: catalase; SOC: soil organic carbon; TN: total nitrogen; TP: total phosphorus; AP: alkaline phosphatase; DTPA Cd: soil DTPA-extractable Cd concentration. PA: 50 mg L−1 PP; PB: 200 mg L−1 PP; PC: 500 mg L−1 PP; HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1.
Figure 6. RDA analysis of heavy metal indicators with plant physiological indicators and soil environmental factors. Note: S: shoot; R: root; FB: fresh biomass; DB: dry biomass; Chla: Chlorophyll a; Chlb: Chlorophyll b; Car: carotenoids; SOD: superoxide dismutase; POD: guaiacol peroxidase; MDA: malondialdehyde; H2O2: hydrogen peroxide; O2·−: superoxide anion; CAT: catalase; SOC: soil organic carbon; TN: total nitrogen; TP: total phosphorus; AP: alkaline phosphatase; DTPA Cd: soil DTPA-extractable Cd concentration. PA: 50 mg L−1 PP; PB: 200 mg L−1 PP; PC: 500 mg L−1 PP; HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1.
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Table 1. Effects of PP alone or combined with Cd on seed growth characteristics.
Table 1. Effects of PP alone or combined with Cd on seed growth characteristics.
TypeTreatmentmg L−1GVGIVIMGTMGS
PPCK00.87 ± 0.02 ab47.26 ± 1.24 b609.81 ± 30.75 b2.14 ± 0.27 bc4.48 ± 0.02 cd
PA500.88 ± 0.02 a49.67 ± 0.94 a694.52 ± 38.67 a2.11 ± 0.11 c4.45 ± 0.02 d
PB2000.83 ± 0.03 bc44.20 ± 2.32 c505.05 ± 29.09 c2.23 ± 0.21 abc4.51 ± 0.06 bc
PC5000.78 ± 0.04 cd41.36 ± 1.78 de434.30 ± 57.66 d2.38 ± 0.11 ab4.56 ± 0.03 ab
PP + CdCd00.79 ± 0.02 cd43.02 ± 0.79 cd513.98 ± 49.17 c2.26 ± 0.16 abc4.50 ± 0.03 bc
HA500.81 ± 0.07 c43.58 ± 1.50 c524.07 ± 66.61 c2.25 ± 0.17 abc4.52 ± 0.03 bc
HB2000.74 ± 0.04 de40.11 ± 0.85 e409.59 ± 41.82 d2.29 ± 0.16 abc4.54 ± 0.05 b
HC5000.69 ± 0.04 e37.19 ± 0.80 f327.98 ± 20.07 e2.41 ± 0.06 a4.60 ± 0.03 a
Factor (Df)--13.3640.7133.931.948.08
p--*********ns***
Note: Different letters in the same column indicate significant differences (p < 0.05) between different treatments. GV: germination potential; GI: germination index; VI: vitality index; MGT: mean germination time; MGS: average germination speed. PA: 50 mg L−1 PP; PB: 200 mg L−1 PP; PC: 500 mg L−1 PP; HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1. Each value represents mean ± SE. * p < 0.05; ** p < 0.01; *** p < 0.001; ns, no significant difference.
Table 2. Cd concentrations and the total uptake of Cd in plant tissue and Cd concentration and bio-effectiveness in soil.
Table 2. Cd concentrations and the total uptake of Cd in plant tissue and Cd concentration and bio-effectiveness in soil.
TreatmentsCd Concentration (mg kg−1)Total Uptake (μg pot−1)TFSoil Total Cd ConcentrationDTPA-Cd Concentration
ShootRootShootRoot (mg kg−1)(mg kg−1)
Cd0.64 ± 0.01 c2.82 ± 0.05 c0.56 ± 0.02 a1.33 ± 0.09 a0.23 ± 0.01 c0.64 ± 0.01 d0.36 ± 0.01 a
HA0.67 ± 0.01 b2.86 ± 0.02 bc0.48 ± 0.03 ab1.1 ± 0.15 b0.23 ± 0.01 bc0.68 ± 0.01 c0.36 ± 0.01 a
HB0.78 ± 0.02 a2.91 ± 0.02 b0.49 ± 0.07 ab1.01 ± 0.12 bc0.27 ± 0.01 a0.70 ± 0.01 b0.34 ± 0.01 b
HC0.79 ± 0.01 a3.23 ± 0.04 a0.41 ± 0.03 b0.84 ± 0.08 c0.24 ± 0.01 b0.72 ± 0.01 a0.34 ± 0.01 b
Factor (Df)71.7182.926.239.8123.4733.3826.26
p******************
Note: Different letters in the same column indicate significant differences (p < 0.05) between different treatments. HA: 50 mg L−1 PP + Cd; HB: 200 mg L−1 PP + Cd; HC: 500 mg L−1 PP + Cd; Cd concentration was 5 mg L−1. Each value represents mean ± SE. * p < 0.05; ** p < 0.01; *** p < 0.001.
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Zhang, Z.; He, H.; Chang, N.; Duan, C. Polypropylene Microplastics and Cadmium: Unveiling the Key Impacts of Co-Pollution on Wheat–Soil Systems from Multiple Perspectives. Agronomy 2025, 15, 2013. https://doi.org/10.3390/agronomy15082013

AMA Style

Zhang Z, He H, Chang N, Duan C. Polypropylene Microplastics and Cadmium: Unveiling the Key Impacts of Co-Pollution on Wheat–Soil Systems from Multiple Perspectives. Agronomy. 2025; 15(8):2013. https://doi.org/10.3390/agronomy15082013

Chicago/Turabian Style

Zhang, Zhiqin, Haoran He, Nan Chang, and Chengjiao Duan. 2025. "Polypropylene Microplastics and Cadmium: Unveiling the Key Impacts of Co-Pollution on Wheat–Soil Systems from Multiple Perspectives" Agronomy 15, no. 8: 2013. https://doi.org/10.3390/agronomy15082013

APA Style

Zhang, Z., He, H., Chang, N., & Duan, C. (2025). Polypropylene Microplastics and Cadmium: Unveiling the Key Impacts of Co-Pollution on Wheat–Soil Systems from Multiple Perspectives. Agronomy, 15(8), 2013. https://doi.org/10.3390/agronomy15082013

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