The Effect of Microplastics with Different Types, Particle Sizes, and Concentrations on the Germination of Non-Heading Chinese Cabbage Seed

Abstract

Microplastics (MPs) are a new type of pollutant widely distributed in the environment. The ecological risks caused by MPs are becoming increasingly serious, especially in cultivated land where pollution is more likely to accumulate. In this paper, the effects of different types, particle sizes, and concentrations of MPs on the seed germination of non-heading Chinese cabbage were analyzed to reveal their potential mechanisms. Five types of MPs, polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polystyrene (PS), were used for correlation analysis. The results showed that the effect of PVC and PET on seed germination was greater than that of PP, PS, and PE. PVC and PP promoted the growth of germinated seeds, while PET and PS showed a certain degree of inhibition. The effect of MPs with a particle size of 6.5–150 μm on seed germination was obvious. Low-concentration MPs (<1 g/L) had a weak inhibitory effect on seed germination. When the concentration was 1 g/L, 75 μm-PP, 75 μm-PVC, and 150 μm-PS promoted the growth of germinated seeds, while 48 μm PET showed inhibition. At high concentration, PP and PS inhibited amylase activity. In general, MPs’ effects showed significant differences according to different types, particle sizes, and concentrations.

1. Introduction

Microplastics (MPs) are solid particles or fragments with a diameter of less than 5 mm, gradually formed by plastic waste through photodegradation, mechanical abrasion, and biodegradation [1]. As a new pollutant, MPs are becoming one of the most serious threats to the earth’s surface ecosystem [2]. MP pollution in terrestrial ecosystems is more serious, and its abundance is four to twenty-three times that in marine ecosystems [3]. The migration ability of MPs in soil is low, and their content will continue to accumulate [4]. With the rapid development of facility agriculture worldwide, the use of agricultural plastics has increased significantly, and waste agricultural film has become the main source of MPs in farmland [5]. For example, in 2018, the consumption of greenhouse film in China was 2.465 million tons, and that of mulching film was 1.404 million tons, but the recycling rate was less than 60% [6]. Similar situations are common in other countries and regions. In addition, fertilizer and water irrigation, dumping of plastic waste, and the use of soil amendments are also important sources of MPs in agricultural ecosystems [2].

Plants are an important part of the soil ecosystem, and their growth and metabolism are inevitably affected by MPs in soil [7]. For example, MPs in soil can directly affect plants by blocking seed pores, limiting root absorption of water and nutrients, and accumulating in roots, stems, and leaves [8]. MPs can also indirectly affect plants by changing the soil microbial community and soil physical and chemical properties [9,10].

The effect of MPs on plants is related to their type, particle size, concentration, and crop species. For example, PS, PE, and PP had significantly different effects on amaranth seed germination [11]. The effect of PE on tomato seed germination was greater than that of PP and PS [12]. Different types and concentrations of MPs inhibited the germination and root growth of Lepidium sativum L., and the inhibitory effect of PVC was significantly stronger than that of PP and PE [13]. PE with different particle sizes reduced the germination potential, bud length, and seed vigor of maize and cucumber seeds [14], and the inhibitory effect increased with the increase in particle size.

Seed germination is the key stage of plant growth and development, and its germination quality is closely related to the plant’s subsequent growth and stress resistance. Many studies found that MPs had an inhibitory effect on plant seed germination, and the inhibitory effect increased with the increase in concentration. PE significantly inhibited soybean seed vigor, and the inhibitory effect was positively correlated with concentration [15]. The germination potential and germination rate of Trifolium repens L., Orychophragmus violaceus L., and Impatiens balsamina L. decreased in an MPs exposure environment, and the decline increased with the increase in MP concentration and particle size [16]. However, there are also different conclusions. For example, a low concentration (<0.5 g/L) of PE inhibits wheat seed germination, while a high concentration (1 g/L) promotes it [17]. The effect of MPs on plant physiological and biochemical metabolism may be one of the reasons for these results. MPs can induce oxidative stress, cytotoxicity, and genotoxicity in plants [18]. When plants are exposed to high concentrations of MPs, they can often reduce their adverse effects by regulating certain metabolic processes, such as changes in soluble sugar content, starch content, amylase activity, and protective enzyme activity [16,19].

Non-heading Chinese cabbage is a popular year-round supply of fast-growing leafy vegetables, often produced in greenhouses. The cultivation environment is inevitably affected by MPs, which has attracted the attention of some researchers [20]. MPs in greenhouse soil can affect the growth of non-heading Chinese cabbage by altering soil physicochemical properties [21]. However, due to the diversified composition of MPs and the differences in particle size and concentration, the conclusions drawn from studies using a given type of MPs are uncertain. This paper tested the effects of five types of MPs with different particle sizes and concentrations on the germination of non-heading Chinese cabbage seeds. By observing the changes in germination parameters, morphological characteristics, and some physiological and biochemical indicators, this study attempts to reveal the mechanism of MPs and provide a reference for the ecological risk assessment of vegetable cultivation.

2. Materials and Methods

2.1. Materials

Non-heading cabbage seeds were purchased from Nongjia Seed Industry Co., Ltd. (Fuzhou, China). MPs included PVC, PE, PET (with particle sizes of 550 μm, 250 μm, 150 μm, 106 μm, 75 μm, 48 μm, 15 μm, and 6.5 μm), PS (with particle sizes of 550 μm, 250 μm, 150 μm, 106 μm, 75 μm, 48 μm, and 15 μm), and PP (with particle sizes of 250 μm, 150 μm, 106 μm, 75 μm, 48 μm, and 15 μm), all purchased from Zhongxin Plastics Co., Ltd. (Guangzhou, China).

2.2. Seed Germination Testing in Exposure Environments with Different Types and Particle Sizes of MPs

MPs of different types and particle sizes were prepared into a suspension with a concentration of 1 g/L, using double-distilled water (ddH₂O) as the solvent. Tween 60 (0.1%) was added to the solution to maintain uniform and stable dispersion of MPs. Before use, the suspension was subjected to ultrasonic oscillation for 0.5 h (25 °C, 40 kHz) to avoid the aggregation of MP particles. The abbreviation 550 μm PVC was used to mark the PVC treatment group with a particle size of 550 μm. Other treatment groups were also marked using the same method. According to particle size, the tested MPs were classified into large particle sizes (550 and 250 μm), medium particle sizes (150, 106, 75, and 48 μm), and small particle sizes (15 and 6.5 μm). The ddH₂O treatment group without MPs was used as the control.

The seeds were disinfected with a 3% H₂O₂ solution for 10 min and then rinsed with sterile ddH₂O. The seed germination test was conducted using the filter paper germination method in a petri dish. Two sterile filter papers were placed in each petri dish, and 3 mL of MP suspensions of different types and particle sizes were added separately. The seeds were cultured under dark conditions at 25 °C. During the germination test, an equal amount of sterile ddH₂O was added as needed to keep the filter paper moist. The germination of seeds was observed and recorded every 6 h during the period of 12–48 h, and then every 24 h. The absence of newly germinated seeds for three consecutive days was considered a marker of the end of the test. On the last day, the germination rate and germination index were counted. The length, dry weight, and fresh weight of the embryonic axis, epicotyl, and hypocotyl for germinated seeds were also measured simultaneously. Epicotyl refers to the part of the germinating seed between the cotyledons and the first true leaf. Hypocotyl refers to the part between the cotyledons and roots. Embryonic axis refers to the total value of epicotyl and hypocotyl. Each treatment combination was repeated 4 times, with 30 seeds per repetition. The length of the embryonic root that broke through the seed coat exceeding 2 mm was considered the standard for germination. Germination rate (%) = (number of germinated seeds at the end of the experiment/total number of tested seeds) × 100 Germination index (GI) = ∑ (Gt/Dt) (Gt: the number of germinated seeds on day t, Dt: the corresponding number of germinated days). Vitality index (VI) = average embryonic axis length of germinating seeds × GI. The measurement of the embryonic axis length was completed using a vernier caliper. Dry and fresh weight were measured using an FA2204B electronic balance.

2.3. Seed Germination Testing in Exposure Environments with Different Types and Concentrations of MPs

Through step 2.3, it was found that 75 μm-PP, 75 μm-PVC, and 150 μm-PS significantly promoted seed germination and growth, while 48 μm-PET showed significant inhibition. These four treatment combinations were used to test the effect of MP concentrations on seed germination. The concentrations were set to 0.25, 0.5, 1, 2, 4, 8, and 16 g/L, respectively annotated as high concentrations (8 g/L, 16 g/L), medium concentrations (1 g/L, 2 g/L, 4 g/L), and low concentrations (0.25 g/L and 0.5 g/L). The abbreviation-marking methods for different combinations were the same as before. The seed germination test method was the same as 2.2. The sterile ddH₂O treatment group without MPs was used as a control.

2.4. The Effect of MPs on Starch Content and Amylase Activity During Seed Germination

Through step 2.3, it was found that 75 μm-PVC with a concentration of 1 g/L (marked as 1 g/L–75 μm-PVC) significantly promoted seed germination, while 2 g/L–75 μm-PP and 1 g/L–150 μm-PS showed significant inhibition. In order to analyze the effect of MPs on starch metabolism during seed germination, the above three treatments were repeated, and samples (whole seeds were ground and used for sampling) were taken at 0, 12, 24, 36, and 48 h after treatment to determine starch content, soluble sugar content, and amylase activity. All indicators were measured using a 0.1 g sample of the entire seed, which had been frozen and ground into powder using liquid nitrogen. Starch content was determined using the anthrone colorimetric method [22]. The activities of α-amylase and β-amylase were determined using an improved 3,5-dinitrosalicylic acid method [23]. The enzyme activity that can catalyze the release of 1 mg of glucose from a substrate within 1 min was defined as 1 U. Soluble sugars were measured using the anthrone colorimetric method [22]. All indicators were measured using a reagent kit from Solaibao Technology Co., Ltd. (Beijing, China). The absorbance value was measured using a multifunctional enzyme-linked immunosorbent assay (ELISA) reader (Infinite M200 PRO, Tecan, Salzburg, Austria).

2.5. Data Analysis

Statistical and significance analyses were conducted using IBM SPSS Statistics 26.0 software, and one-way ANOVA and Least Significant Difference (LSD) analyses were performed. Data were represented as x ± SD, where x represents the sample mean, and SD represents the sample standard deviation. The figures and tables were created using Excel 2021 and GraphPad Prism 8.0.1. Principal Component Analysis (PCA) of morphological, physiological, and biochemical indicators was completed using IBM SPSS Statistics software 26.0 and Origin software 2023.

3. Results

3.1. Effects of MPs with Different Types and Particle Sizes on Seed Germination Parameters

Overall, PVC and PET showed varying degrees of inhibitory effects on the germination index (

Table 1. Effects of MPs with different types and particle sizes on the seed germination parameters of non-heading Chinese cabbage seeds.

MPs Type	Particle Size	Germination Rate/%	Germination Index	Vitality Index
PVC	Ck	96.00 ± 4.18 a	1.79 ± 0.09 ab	99.50 ± 3.41 c
	550 μm	100.00 ± 0.00 a	1.80 ± 0.13 ab	124.73 ± 3.80 b
	250 μm	96.00 ± 4.18 a	1.67 ± 0.10 cd	107.97 ± 7.59 c
	150 μm	96.00 ± 4.18 a	1.57 ± 0.03 d	90.56 ± 8.95 c
	106 μm	98.00 ± 2.74 a	1.73 ± 0.05 bc	148.67 ± 12.44 a
	75 μm	100 ± 0.00 a	1.85 ± 0.04 a	156.35 ± 19.93 a
	48 μm	100 ± 0.00 a	1.75 ± 0.06 abc	104.62 ± 2.70 cc
	15 μm	98.00 ± 2.74 a	1.70 ± 0.05 bc	155.87 ± 18.02 a
	6.5 μm	100 ± 0.00 a	1.79 ± 0.05 ab	158.46 ± 10.52 a
PP	Ck	100 ± 0.00 a	1.50 ± 0.06 a	69.24 ± 5.21 bc
	250 μm	98.00 ± 2.74 a	1.51 ± 0.06 a	79.94 ± 20.78 bc
	150 μm	99.00 ± 2.24 a	1.59 ± 0.10 a	84.59 ± 14.01 bc
	106 μm	98.00 ± 2.74 a	1.59 ± 0.06 a	67.83 ± 10.89 c
	75 μm	98.00 ± 2.74 a	1.57 ± 0.11 a	102.73 ± 12.02 a
	48 μm	98.00 ± 4.47 a	1.49 ± 0.09 a	80.48 ± 8.96 bc
	15 μm	99.00 ± 2.24 a	1.53 ± 0.10 a	85.78 ± 6.84 b
	6.5 μm	100.00 ± 0.00 a	1.58 ± 0.05 a	66.80 ± 12.77 c
PE	Ck	96.00 ± 4.18 a	1.52 ± 0.07 b	89.34 ± 8.40 ab
	550 μm	97.00 ± 4.47 a	1.60 ± 0.09 ab	101.94 ± 12.41 ab
	250 μm	99.00 ± 2.24 a	1.57 ± 0.07 ab	80.80 ± 23.93 b
	150 μm	100.00 ± 0.00 a	1.60 ± 0.11 ab	107.54 ± 23.88 a
	16 μm	96.00 ± 2.24 a	1.50 ± 0.03 b	80.30 ± 12.18 b
	75 μm	98.00 ± 4.47 a	1.54 ± 0.06 ab	87.68 ± 9.73 ab
	48 μm	97.00 ± 4.47 a	1.65 ± 0.08 a	87.47 ± 10.06 ab
	15 μm	97.00 ± 4.47 a	1.66 ± 0.10 a	82.71 ± 14.22 b
	6.5 μm	97.00 ± 2.74 a	1.48 ± 0.13 b	82.47 ± 11.38 b
PET	Ck	87.00 ± 13.51 ab	1.19 ± 0.20 ab	71.65 ± 10.09 a
	550 μm	89.00 ± 9.62 ab	1.03 ± 0.17 bcd	55.14 ± 12.27 abc
	250 μm	88.00 ± 7.58 ab	0.97 ± 0.08 cd	42.99 ± 3.59 d
	150 μm	95.00 ± 7.07 a	1.10 ± 0.02 abc	64.04 ± 13.19 ab
	106 μm	99.00 ± 2.24 a	1.11 ± 0.08 abc	68.09 ± 10.12 ab
	75 μm	88.00 ± 10.37 ab	0.96 ± 0.10 cd	47.27 ± 13.28 de
	48 μm	82.00 ± 9.08 b	0.87 ± 0.07 d	39.39 ± 10.57 d
	15 μm	97.00 ± 6.71 a	1.18 ± 0.12 ab	71.12 ± 11.27 a
	6.5 μm	96.00 ± 2.24 a	1.24 ± 0.10 a	59.11 ± 5.04 abc
S	Ck	99.00 ± 2.24 a	2.03 ± 0.08 ab	76.11 ± 16.72 b
	550 μm	99.00 ± 2.24 a	2.07 ± 0.07 a	79.52 ± 11.54 b
	250 μm	98.00 ± 2.74 a	1.98 ± 0.15 ab	108.54 ± 19.56 a
	150 μm	98.00 ± 2.74 a	2.07 ± 0.12 a	114.50 ± 19.21 a
	106 μm	99.00 ± 2.24 a	2.11 ± 0.09 a	88.59 ± 5.08 b
	75 μm	96.00 ± 2.24 a	2.01 ± 0.14 ab	85.73 ± 12.48 b
	48 μm	96.00 ± 4.18 a	1.91 ± 0.10 b	80.85 ± 18.14 b
	15 μm	99.00 ± 2.24 a	1.89 ± 0.10 b	67.61 ± 7.06 b

Note: Significance analysis was conducted within the same type of MPs treatment group. The significance analysis results of the same column data are marked with lowercase letters (p < 0.05).

). PVC with a particle size of 150 μm significantly inhibited the germination index, with an inhibition rate of 11.86%. Except for the small particle size, PET inhibited the germination index. The inhibitory effects were significant at 250 μm, 75 μm, and 48 μm, with the 48 μm particle size showing the strongest inhibition, an inhibition amplitude of 27.11%. Some small and medium-sized PE enhanced the germination index, such as 15 μm and 48 μm size treatments, which increased the germination index by 9.21% and 8.55%, respectively, compared to the control.

PVC, PP, and PS enhanced the seed vitality index, while PET showed inhibition (Table 1). The 75 μm-PP group increased the vitality index by 1.48 times compared to the control. The activity index of PS treatment with particle sizes of 150 μm and 250 μm increased by 42.12% and 50.45%, respectively, compared to the control. Different particle sizes of PVC increased the vitality index by 1.25 to 1.59 times. PET with large or medium particle sizes significantly reduced the vitality index, with the strongest inhibitory effect observed at 48 μm, resulting in a decrease of 45.03%.

3.2. Effects of MPs with Different Types and Particle Sizes on the Morphology and Biomass of Germinated Seeds

Overall, PVC, PP, and PS with medium particle sizes promoted the elongation of the embryonic axis and biomass accumulation of germinating seeds to some extent, while PET showed inhibition, and the effect of PE was not significant (Figure 1 and Figure 2). PVC of different particle sizes can increase the length of the embryonic axis and the biomass of germinating seeds. The total length of the embryonic axis and hypocotyls in the medium particle size treatment group increased significantly, approximately 1.63 and 1.51 times that of control group, respectively (Figure 2A,B). The small particle size group showed significant effects on the epicotyl, hypocotyl, and total length (Figure 1 and Figure 2A,B). The effect of different particle size treatments on biomass was not significant. Only 75 μm-PVC increased the epicotyl fresh weight by 11.79%, while the 550 μm increased the hypocotyl fresh weight by 28.45% (Figure 2C,D).

PP with a medium particle size increased the embryonic axis length, and the biomass showed a trend of first increasing and then decreasing with the increase in particle size (Figure 1). The 75 μm group had a significant impact, with the length of the hypocotyl and the embryonic axis increasing by 78.46% and 54.84%, respectively, and the fresh weight increasing by 28.98% and 46.07%, respectively (Figure 2A,B,D,E). In the PP group, the dry weight index showed a trend of first increasing and then decreasing with the decrease in particle size, reaching its peak in the 48 μm group (Figure 2F–H). The 6.5 μm group increased the embryonic axis fresh weight by 17.30%, and the 150 μm group increased the hypocotyl dry weight by 4.10% (Figure 2E,G). The effect of PE with other particle sizes was not significant.

PET overall inhibited embryonic axis elongation and biomass increase, with the 48 μm group showing the most significant inhibition, showing a decrease of 29.92% and 25.81%, respectively (Figure 2A,B). The epicotyl fresh weight also showed a decrease of 10.33%–26.06% (Figure 2C). The treatment of small and medium-sized particles significantly reduced the dry weight index (Figure 2F), but there was a certain degree of increase in the hypocotyl fresh weight (Figure 2D). PS with a particle size of 150–250 μm promoted the elongation of the embryonic axis and increased biomass. The lengths of the epicotyl and hypocotyl were approximately 1.3 and 1.5 times those of the control (Figure 1 and Figure 2A,B), respectively. The hypocotyl dry weight also significantly increased, reaching 1.37–1.50 times that of the control (Figure 2G).

3.3. Effects of MPs with Different Types and Concentrations on Seed Germination Parameters

The effects of different types and concentrations of MPs on seed germination parameters are shown in

Table 2. Effects of MPs with different types and concentrations on the seed germination parameters of non-heading Chinese cabbage seeds.

MPs Type	Concentration (g/L)	Germination Rate/%	Germination Index	Vitality Index
PVC	CK	80.00 ± 10.95 b	1.68 ± 0.27 ab	138.78 ± 18.39 c
	0.25	96.67 ± 8.16 a	1.94 ± 0.22 ab	199.24 ± 18.14 ab
	0.5	86.67 ± 13.66 ab	1.68 ± 0.19 ab	174.39 ± 28.92 b
	1	85.00 ± 10.49 ab	1.66 ± 0.20 b	188.79 ± 10.40 ab
	2	83.33 ± 12.11 ab	1.85 ± 0.33 ab	193.86 ± 8.94 ab
	4	93.33 ± 10.33 a	1.93 ± 0.21 a	220.83 ± 9.65 a
	8	93.33 ± 8.16 a	1.94 ± 0.25 ab	221.91 ± 32.13 ab
	16	96.67 ± 5.16 a	1.80 ± 0.17 ab	182.11 ± 22.20 ab
PET	CK	80.00 ± 5.00 b	1.60 ± 0.19 c	72.33 ± 16.50 abc
	0.25	89.00 ± 8.22 ab	1.89 ± 0.17 ab	91.65 ± 12.16 a
	0.5	95.00 ± 6.12 a	2.03 ± 0.10 a	78.64 ± 15.08 ab
	1	88.00 ± 14.40 ab	1.61 ± 0.29 c	54.50 ± 17.70 c
	2	95.00 ± 5.00 a	1.89 ± 0.10 ab	81.66 ± 10.64 ab
	4	87.00 ± 4.47 ab	1.82 ± 0.05 abc	59.89 ± 9.34 bc
	8	90.00 ± 7.91 ab	1.74 ± 0.23 bc	73.24 ± 20.12 abc
	16	89.00 ± 4.18 ab	1.85 ± 0.06 abc	73.78 ± 11.03 bc
PS	CK	100.00 ± 0.00 a	2.01 ± 0.11 a	67.30 ± 8.07 b
	0.25	99.00 ± 2.24 a	1.95 ± 0.05 a	64.33 ± 7.30 b
	0.5	100.00 ± 0.00 a	2.03 ± 0.11 a	91.40 ± 15.52 a
	1	99.00 ± 2.24 a	2.00 ± 0.25 a	73.91 ± 18.75 ab
	2	100.00 ± 0.00 a	2.00 ± 0.19 a	64.22 ± 17.73 b
	4	100.00 ± 0.00 a	2.08 ± 0.12 a	65.69 ± 12.52 b
	8	100.00 ± 0.00 a	2.15 ± 0.12 a	75.45 ± 17.81 ab
	16	100.00 ± 0.00 a	2.02 ± 0.19 a	71.61 ± 10.34 ab
PP	CK	90 ± 8.67 ab	1.74 ± 0.16 ab	75.56 ± 17.62 abc
	0.25	83.00 ± 7.58 b	1.54 ± 0.06 c	74.26 ± 15.62 bc
	0.5	89.00 ± 4.18 ab	1.72 ± 0.12 ab	90.47 ± 18.55 ab
	1	91.00 ± 6.52 ab	1.67 ± 0.11 abc	90.54 ± 13.04 ab
	2	91.00 ± 6.52 ab	1.63 ± 0.08 abc	61.30 ± 4.98 c
	4	89.00 ± 4.18 ab	1.60 ± 0.13 abc	71.66 ± 14.21 bc
	8	89.00 ± 8.94 ab	1.65 ± 0.21 abc	101.27 ± 23.70 a
	16	96.00 ± 4.18 a	1.89 ± 0.08 a	84.52 ± 28.04 abc

Note: Significance analysis was conducted within the same type of MPs treatment group. The significance analysis results of the same column data are marked with lowercase letters (p < 0.05).

. The different concentrations of the PVC group all increased the germination rate and vitality index to a certain extent, with the germination rate being 1.17–1.21 times that of the control, and vitality index being 1.26–1.59 times that of the control. PET increased the germination rate and germination index, reaching their peak in the 0.5 g/L group, which were 1.39 and 1.26 times higher than the control, respectively. PS increased the vitality index, but the germination index and germination rate were not significantly affected. The vitality index of the 0.5 g/L group was the highest, 1.36 times that of the control. PP overall inhibited the germination index but had no significant regularity in its effects on germination rate and seed vitality.

3.4. Effects of MPs with Different Types and Concentrations on the Morphology and Biomass of Germinated Seeds

Overall, PVC improved embryonic axis elongation and biomass accumulation, PET showed inhibition, PP and PS increased embryonic axis length, and partial concentration treatment inhibited biomass accumulation (Figure 3 and Figure 4). Except for the 16 g/L concentration, the length and dry weight of epicotyl and hypocotyl in the PVC group increased, and all reached their peak at the 1 g/L concentration (Figure 4A,F). The epicotyl fresh weight and the total fresh weight also reached their peak at this concentration, increasing by 30.76% and 29.30%, respectively, compared to the control, while there was no significant change in the fresh weight of the hypocotyl (Figure 4C–E). All treatment groups increased hypocotyls dry weight and total fresh weight, reaching a peak at 1 g/L (Figure 4G,H).

PP increased the length and fresh weight of the embryonic axis, and overall, inhibited dry weight accumulation. The hypocotyl and embryonic axis lengths of the 8 g/L concentration increased by 51.00% and 41.00%, respectively, compared to the control (Figure 4A,B), and the hypocotyl fresh weight increased by 43.00% (Figure 4D). Except for the 0.25 g/L concentration, PP significantly inhibited the accumulation of epicotyl dry weight, with a decrease of 13.72–19.72% (Figure 4F–H). The effects of PET on most morphological indicators did not show significant regularity. The medium concentration group inhibited the embryonic axis length, with a decrease of 25.61% and 27.09% at the 1 g/L and 4 g/L concentrations, respectively (Figure 4B). Both medium and high concentrations increased the hypocotyl’s fresh weight, which was about 1.5 times that of the control (Figure 4D).

PS overall inhibited the biomass accumulation of germinated seed. For example, compared with the control group, the decrease in hypocotyl’s fresh weight in the 0.25 g/L and 1 g/L concentrations was 24.68% and 24.67%, respectively. The hypocotyl’s fresh weight reached its lowest value at the 16 g/L concentration, with a decrease of 33.63% (Figure 4D). The decrease in total fresh weight at the 1 g/L concentration was 27.40% (Figure 4F). The decrease in epicotyl dry weight and total dry weight was the greatest at the 1 g/L concentration, with reductions of 29.26% and 23.20%, respectively (Figure 4F,H). However, the hypocotyl’s dry weight in some concentration groups showed a certain degree of increase (Figure 4G). The trend of changes in embryonic axis length was “low concentration promotion and high concentration inhibition”. For example, at the 0.5 g/L concentration, the hypocotyl length and total length were 1.51 and 1.34 times that of the control group, respectively (Figure 4A,B), but epicotyl length at the 1 g/L concentration decreased by 33.00% (Figure 3).

3.5. Effects of PVC, PS, and PP on Starch Metabolism During Seed Germination

The effects of PVC, PS, and PP on the soluble sugar content of germinating seeds are shown in Figure 5. In the early stage of germination, the soluble sugar content in the PVC group decreased significantly compared to the control, but in the middle and later stages, it was mostly close to or slightly higher than the control (Figure 5A). The soluble sugar content in the PP group only decreased by 34.67% compared to the control at 24 h, while other sampling points were close to or slightly higher than the control. The soluble sugar content of the PS group decreased significantly and was significantly lower than the control throughout germination. For example, at 36 h and 48 h, its content decreased by 68.60% and 62.54%, respectively, compared to the control. The starch content of all groups reached its peak at 12 h and continued to decrease (Figure 5B), with the control group showing a more significant and rapid decline. At 24 h and 36 h, the starch content in the PVC and PS groups increased by 20–35% compared to the control, while the increase in the PP group was not significant or close to the control.

PP and PS significantly inhibited the activity of α-amylase (Figure 5C). Compared with the control, the average decrease in α-amylase activity in the PS group at different time points was about 42%. At 36 h and 48 h, the α-amylase activity in the PP group was the lowest among all groups, decreasing by 46.68% and 59.91% compared to the control, respectively. The α-amylase activity in the PVC group was basically the same as that in the control group, both increasing at 12 h and gradually decreasing over time. PP significantly inhibited β-amylase activity, with the lowest values among all groups at each sampling point (Figure 5D). Except for 36 h, the effects of PVC and PP on β-amylase activity were not significant at other sampling points and were basically close to the control.

3.6. PCA Analysis

The PCA analysis of germination characteristic parameters and morphological indicators showed that vitality index, epicotyl length, and hypocotyl length were the main indicators that responded to MPs (Figure 6). The weight values of the three were 13.24%, 12.47%, and 13.40%, respectively, with a cumulative total proportion of 39.11%. There were complex correlations between different indicators (Figure 6A). There was a positive correlation between embryonic axis length, dry weight, and fresh weight. The length of the hypocotyl and its dry weight and fresh weight were positively correlated with germination rate, germination index, and vitality index. The dry weight and fresh weight of the epicotyl were negatively correlated with germination rate, germination index, and vitality index.

The correlation heatmap results (Figure 6C) further refine the above conclusions. The germination rate and vitality index were positively correlated with embryonic axis length and negatively correlated with germination index and embryonic axis fresh weight. The germination index and vitality index were positively correlated with hypocotyl fresh weight and negatively correlated with germination rate and embryonic axis dry weight. The vitality index was positively correlated with germination rate, germination index, embryonic axis length, and hypocotyl fresh weight and negatively correlated with epicotyl fresh weight. The fresh weight of the embryonic axis was positively correlated with germination rate. The embryonic axis dry weight was positively correlated with epicotyl length, and negatively correlated with germination index. The measured values of relevant indicators in the PVC treatment group were relatively dispersed, with a greater degree of dispersion compared to other treatment groups, indicating that their impact on seeds was greater than that of other types of MPs (Figure 6B).

PCA analysis of starch metabolism-related indicators showed that soluble sugar content and β-amylase activity were the main indicators responsive to MPs (Figure 7A). The weight values of the two were 25.10% and 25.56%, respectively, with a cumulative proportion of over 50.66%. The correlation between different indicators is shown in Figure 7B. There was a positive correlation between soluble sugar content and α-amylase activity. There was a negative correlation between soluble sugar content and β-amylase activity. The correlation between α-amylase activity, β-amylase activity, and soluble sugar content was weak. The activity of β-amylase was negatively correlated with soluble sugar content, starch content, and α-amylase activity. There was a positive correlation between soluble sugar content, starch content, and α-amylase activity.

4. Discussion

Seed germination is the initial critical stage of the plant life cycle and is extremely sensitive to the external environment. The effects of MPs on seed germination are related to plastic type, particle size, concentration, interactions with other environmental factors, and plant species. The conclusions drawn from different research methods and materials are often inconsistent, even contradictory. For example, relevant papers showed that 12 different types and particle sizes of MPs have no significant effect on the germination rate of carrot seeds [24]. In most reports, MP usually shows adverse effects on seed germination. For example, PE can significantly inhibit the germination rate and vigor of corn and cucumber seeds [14].

In our study, the effects of PVC and PET on the germination of non-heading Chinese cabbage seeds were greater than those of PP, PS, and PE, and the results were related to the particle size and concentration of MPs. PVC promoted the elongation of embryonic axis and biomass accumulation of germinating seeds. PP, PET, and PS treatments exhibited diverse effects due to differences in concentration and particle size, while the effect of PE was much weaker than the other four types of MPs. It is evident that there are significant differences in the effects of different types of MPs on plant seed germination. Even for the same type of MP, its impact will vary depending on the particle size. The inhibitory effects of PS, PE, and PP with larger particle sizes on amaranth seeds are greater than those with smaller particle sizes [11]. Our research also showed that PET with a particle size of 48–550 μm had a stronger inhibitory effect on germination than PET with a particle size of 6.5–15 μm. The concentration of MP is another key factor. MPs with concentrations exceeding a certain threshold may reduce the biological activity of internal substances in seeds, thereby inhibiting seed germination. Our study showed that exposure to different concentrations of MPs with the same particle size had different effects on seed germination. Our research showed that different concentrations of MP with the same particle size exhibited different effects on seed germination. Except for PS, PVC, PET, and PS had no significant adverse effects on the growth of germinated seeds under low concentration conditions, but high concentrations often manifested as inhibition. Even the same treatment concentration may show differences in its effects on different parts of the plant. For example, 50 μm-PS promotes aboveground elongation of amaranth, but inhibits root growth [11].

In summary, the effects of MPs on seed germination are multifaceted, and the mechanisms behind these varied outcomes are intricate. MPs can accumulate within the pores of the seed epidermis, creating blockages that hinder the seed’s water absorption, ultimately delaying germination [25]. The inhibition of root nutrient and water absorption, coupled with a reduction in stomatal conductance, will inevitably lead to the suppression of subsequent biomass accumulation [26]. Meanwhile, plants can cope with or reduce the adverse effects of MPs through various pathways [16]. For instance, seeds exposed to MPs rapidly initiate metabolism, increase water absorption, and thereby mitigate the toxicity of MPs on seeds [27]. This rapidly enhanced metabolism may facilitate the excessive absorption of nutrients and water by plants within a short time [11]. This could explain the increased elongation of the embryonic axis and biomass accumulation observed in certain treatment combinations in our study. When the environmental impact of MPs surpasses the regulatory capabilities of plants, this temporary “pseudo-promotion” may swiftly transition into inhibition, manifesting as a “low-promotion, high-inhibition” pattern [28]. For instance, the dry matter mass of wheat seedlings diminishes as the concentration of MPs increases [17]. In our tests, the changes in embryonic axis length after PS treatment showed a similar pattern.

Starch is the main energy source during seed germination, and amylase is the main enzyme in starch hydrolysis. Starch is hydrolyzed into soluble sugars by amylase. Soluble sugars are important substances for maintaining plant metabolism and growth and are highly sensitive to environmental stress [11]. In response to the adverse effects of MPs, plants may mobilize more starch to participate in related metabolic processes during seed germination. The combination treatment of some PVC or PS can increase the starch content during seed germination, which obviously provides more abundant energy and nutrients. The functions of soluble sugars are multifaceted, serving as energy substances, osmoregulatory substances, and signaling molecules. Soluble sugar content of seeds at different germination stages decreased in the 1 g/L−150 μm-PS group, which may be the main reason for their growth inhibition. The effect of PS on some herbaceous ornamental plant seedlings also showed similar characteristics [16]. The changes in amylase activity may be the main cause of changes in soluble sugar content, such as the inhibition of α-amylase activity during rapeseed seed germination by MPs [19]. In this study, both the 2 g/L−75 μm-PP group and the 1 g/L−150 μm-PS group inhibited the activity of α-amylase and β-amylase. The effect of MPs on amylase activity during plant seed germination may be a promising research direction. However, it is worth noting that changes in amylase activity are not always accompanied by corresponding changes in soluble sugar content. In this study, some combinations of PVC or PP treatments had no significant effect on soluble sugar content.

The impact of MPs on plants is extremely complex, and different types and particle sizes of MPs may produce completely opposite results. Similar results have also been obtained in studies related to onions and pakchoi [29,30]. MPs may affect the apparent biomass of plants by influencing physiological and biochemical processes such as photosynthesis, protective enzyme activity, and osmoregulatory substance content [30,31,32]. Research on Chinese cabbage has shown that MPs can indirectly affect plant growth by affecting the activity of major soil enzymes [33]. A noteworthy problem is that MPs can accumulate and transport within plants, ultimately affecting the quality of agricultural products, which has been confirmed in cucumbers [34]. Modern molecular biology techniques can help clarify the interaction between MPs and plants, but there are currently few related reports. Research has shown that PS can accumulate in lettuce, leading to upregulation of stress-related genes and changes in the composition of root exudates [35]. These related studies will help us understand the multifaceted effects of MPs on plants and provide different approaches for addressing this new type of pollutant.

5. Conclusions

Among the five MPs tested in this paper, PVC and PET had a more significant impact on the germination of non-heading Chinese cabbage seeds compared to PE, PS, and PP. Most PVC treatment combinations led to an increase in embryonic axis length and biomass accumulation to some extent. The effects of PP, PET, and PS treatments varied with concentration and particle size, showing no clear pattern. Most PE treatment combinations had no significant impact. Treatments with 75 μm-PP and 75 μm-PVC, as well as 150 μm-PS, significantly increased the hypocotyl length and biomass accumulation of germinating seeds, whereas 48 μm-PET significantly inhibited these processes. Low concentrations of MPs had either no significant effect or slightly promoted seed germination, whereas medium to high concentrations typically exhibited inhibitory effects. Certain PVC or PS treatment combinations increased starch content during the seed germination process. PS and PP treatments somewhat inhibited amylase activity. PS treatment also led to a decrease in soluble sugar content.