Impacts of Micro/Nanoplastics on Crop Physiology and Soil Ecosystems: A Review

Abstract

Long-term exposure of plastics to the environment causes them to disintegrate, resulting in the formation of micro/nanoplastics as well as the release of additives and chemicals into the soil. The micro/nanoplastics are able to readily migrate into the soil, destabilize the soil microbiota, and finally enter crop plants. Endocytosis, apoplastic transport, root adsorption, transpiration pull, stomatal entry, and crack-entry mode are well-known pathways by which microplastics enter into plants. Roots of vegetable crops were able to transfer 0.2 µm–1.0 µm of microplastics through root adsorption and by transpiration pull to the xylem and then further transported them to the plant tissues through apoplastic pathways. Beads of 1000 nm size were also engulfed by BY-2 protoplast cells through endocytosis. Micro and nanoplastics that enter crops affected the physiological and biochemical activities of the plants. Aquaporins were needed to aid the symplastic pathway which made the symplastic pathway difficult for MPs/NPs transport. Microplastics block seed capsules and roots of seedlings, thereby negatively affecting the uptake and efficient use of nutrients supplied. Photosynthesis of plants was affected due to the reduction in chlorophyll contents. Exposing soils to MPs/NPs drastically affected the pH, EC, and bulk density of the soil. This review focused on bridging the knowledge gap with understanding how microplastics prevent nutrient uptake and nutrient use efficiency in plants. This understanding is essential for assessing the broader ecological impacts of plastic contamination and for developing effective mitigation strategies. Further research is needed on microorganisms capable of degrading plastics, as well as on developing analytical methods for detecting plastics in soil and plant tissues. Also, further research on how to replace plastic mulching and still provide the same benefits as plastic mulch is needed.

1. Introduction

Plastic is a product that is produced through the addition and condensation of monomers and additives [1]. Generally, plastics have physical, mechanical, electrical, optical/colorability, and thermal properties [2]. Examples of these properties are modulus of elasticity, tensile strength, compressive strength, melt index, expansion, water absorption, thermal conductivity, volume resistivity, and transparency (optical property) [2]. PE, which is one of the commonly used plastics, is extensively used in agriculture as mulch, covering rows, films in greenhouses, plastic nursery pots, and bags for silage [3]. Commonly used plastics include polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyethylene terephthalate, and polytetrafluoroethylene [4,5]. The accumulation and pollution of plastics has emerged as an environmental concern since the increase in the production of plastics in the 1940s [6]. Over the last 30 years, annual plastic production surpassed a threshold of 360 million tons in 2018 [7]. This increase in production has brought a negative impact on both aquatic life [8] and the terrestrial biota [9]. Exposing plastics to the environment over a long time, the various additives and chemicals used in producing them begin to slowly desorb/release and, through photochemical deterioration, leach into the soil environment and adversely affect soil biota and their function [10,11]. Examples of these additives and chemicals that can cause harm to soil environment and other ecosystem biota are stabilizers and plasticizers such as di-2-ethlyhexyl phthalate, di-n-propyl phthalate, and polyethylene terephtalate for PVCs. Conventional plastics exposed to the environment are broken into fragments [12]. There are three main known processes of fragmentation, which are physical abrasion, chemical degradation, and biological processes [13].

The grouping of plastics and their typical examples are shown in Figure 1. As depicted in Figure 1, plastic comes in different fragments; thus, plastics in general, plastic particles, microplastics, nanoplastics, oligomers and monomers, and chemical fragments are seen as a result of being exposed to physical abrasion and UV light for a long time [14,15,16]. A diagrammatic representation by [17] distinguished plastic litter and microplastic by definite sizes. Thus, plastic litter was 1 m, microplastic was 5000 µm to 150 µm, and nanoplastics, which were the smallest, ranged between 0.1 µm and 0.05 µm. Pinto-Poblete et al. and Moshood et al. [3,18] reported that microplastics could be termed as fragments of industrial plastic (plastic mulch and other agricultural plastic materials) and household plastic garbage of particle size smaller than 5 mm. These fragments can be identified with Fourier transform infrared spectroscopy [19]. Other methods of identifying MPs/NPs are scanning electron microscopy (SEM) and fluorescence labelling methods. SEM detects the qualitative presence of MPs/NPs in tissues of plants, whereas fluorescence labelling traces the absorption pathways of the particles [20].

EC50 (median effect concentration) was also used to detect the phytotoxicity of MPs/NPs on crops and soil microfauna [21,22]. Plastic mulching, which began in the 1950s, significantly changed the direction of crop production in terms of yield [23]. The emergence of plastic mulch became a norm in agricultural production because of the numerous benefits derived from using the plastic mulching technology. Some of the benefits of plastic mulching are modifying radiation budget, increasing water use efficiency, controlling soil temperature and moisture content, increasing water holding capacity of the soil, maintaining good plant growth, increasing yield and quality, and lastly, regulating weed and insect pest infestation without using chemical products to control them [24,25].

However, the use of plastic mulching contributes to microplastic contamination in agricultural soils. Behind all the significant benefits derived from the use of plastic mulch lies the devastating long-term effects of microplastics on our crops, soil, climate, animals, and, lastly, humans. The presence of micro/nanoplastics found in fruits or vegetables has been examined to pose a great risk to human and animal health [26,27]. Microplastic contamination can cause soil structural loss and a reduction in the activities of some soil microorganisms, and some other negative effects of using plastic mulches may need further investigation. It has also been reported that plastic mulch, compost from municipal solid waste and other plastic materials always end up in landfills or agricultural lands, which are capable of destroying the chemical and physical properties of soil [3]. Plastic mulch is reported to affect soil negatively by impeding the disintegration of organic matter and increasing water run-off/erosion as a result of reducing the ability of the soil to hold water. Also, the habitat of soil organisms is reported to be destroyed by plastic mulch [28]. In recent times, most of the reviews on microplastic toxicity in plants mostly focused on the damage to the ecosystem, ignoring the specific effects on the plants in detail; thus, their nutrient uptake and the efficient usage of the nutrient uptake [29,30]. These negative effects of microplastics bring out the focus of this review paper.

The negative impacts of plastics used on farm soils bring out the purpose of this review paper, which precisely talks about the impact of these microplastics on nutrient uptake, mechanisms for entry and uptake, and effects on the efficient usage of the nutrient supplied. It compares varieties of crops (monocots and dicots) and how these crop varieties are affected by several parameters such as germination, photosynthesis, growth, yield, and several other specific parameters entailed in this paper.

Therefore, the purpose of this review paper is to comprehend how micro or nanoplastic fragments enter plants, how they impact these plants (directly/indirectly), and how these fragments affect the uptake of nutrients supplied and the efficient use of the nutrients supplied. Then, lastly, this paper will provide some recommendations for further thorough investigations to help mitigate the devastating effects of micro/nanoplastics.

2. The Criterion for Paper Selections

In respect to this review, the search strategy which conforms to PRISMA methodology was used by the combination of these search terms ‘AND’ and ‘OR’ with keywords such as “microplastics”, “effects”, “plants”, “growth parameters”, “impacts”, “entry pathways”, “plastic mulching”, “soil”, “crops adsorption”, “higher plants”, “decomposition”, “plastic decomposition”, “monomer”, “additives”, “plastic particles”, “plastic bottles”, “cosmetic plastics”, “oligomers and monomers”, “chemical fragments”, “polymer”, “polyethylene”, “polyester”, “polystyrene”, “polycyclic aromatic hydrocarbons”, “polyfluoroalkyl substances”, “high density polythene”, “phthalates”, “low density polythene”, “microfibers”, “microbeads”, “nano-meter sized plastics”, and “nurdles” used. These keywords were combined to search for related research and review articles for this paper. The published literature was searched in the “Taylor and Francis Journal”, “Web Science”, “Science Direct”, “Google Scholar”, “Frontiers in Plant Science”, “Springer”, the “MDPI Journal”, etc. Mendeley’s referencing tool was used to organize the papers. For citation and selecting the bibliography, a folder was created first, and the downloaded PDFs which matched our topic were imported into the folder by dragging the files into the folder. These downloaded PDFs were used to create our Mendeley library. Mendeley Cite plugin for word was installed and where a citation was needed, it was inserted directly from our library. For reference list, Mendeley automatically creates a reference list based on what is cited in the main text. The referencing style which suited the MDPI journal was selected from Mendeley cite menu and used. For accuracy, all in-text citations were manually verified if they appeared in the bibliography. And also, manual verification was performed for duplicate/incomplete entries in the reference list. Approximately 200 matching articles were downloaded, which included primary research articles and book chapters from journals, such as reviews, bulletins, and several others, of which a total of 155 papers and articles were reviewed in this paper. In total, 42 articles from the 200 matching articles were deleted due to several reasons, such as studies not related to plants. The abstracts, introductions, and conclusions of these papers were critically studied, and the results of the reports were selected, primarily focusing on the research published from 1994 to 2024. The information gathered to draw the tables came from 30 articles labelled as reports of included studies. Three duplicate articles were deleted, and two articles that were also not in the English language were deleted. The search for articles for this review paper focused on (A) fragmentation of plastics, (B) entry modes and transportation of MPs/NPs within plants and soil environment, and (C) effects of microplastics on (a) crop physiology and (b) soil environment. The extracted information included (C, a) effects on the type of crops, (C, ai) monocot species, (C, aii) dicot species, and the type of experiment (soil medium/hydroponics). For effects on soil environment, the focus was on the following: (C, bi) MPs/NPs effects on the physical properties of soil, (C, bii) MPs/NPs effects on the chemical properties of soil, and (C, biii) MPs/NPs effects on the biological properties of soil. Following the flow diagram, articles for this manuscript were added manually in order to provide relevant information. To ensure the eligibility of the selection criteria, we considered only peer-reviewed journal articles which were published in English. Most of these studies concentrated on types and sizes of microplastics and their effects on plants and soil. Studies which were unrelated to microplastic effects on crops and soil properties were excluded. For example, studies which focused on microplastic effects on human beings were excluded. Figure 2 illustrates the flow of the literature search for this article.

3. Microplastics in the Soil

3.1. Sources of Microplastics in Soil

The long-term usage of plastic mulch is the main source of MPs in soils [31]. In a Chinese nationwide survey, soils of different years (5, 15, and 24 years) of continuous plastic mulching contained 80.3 ± 49.3, 308 ± 138.1, and 1075.6 ± 346.8 microplastic particles per kg soil, respectively [32]. In total, 13 mg per kg of soil contained microplastics after cultivating with plastic mulch for long years compared to other soils in Germany [33]. Materials made of plastics used (on farmlands or in households) have a decomposing effect, which does not always lead to 100% decomposition [34]. With regards to the notion that plastics do not decompose, it has now been discovered that plastics can decompose at 30 °C to generate a monomer [35]. MP/NP is then generated from the crushed plastic/polymer. Plastics constitute Polyhydroxy butyrate (PHB), Polybutylene succinate (PBS), Polycaprolactone (PCL), Polyhydroxy alkanoate (PHA), Polybutylene adipate-co-terephthalate (PBAT), Polylactic acid (PLA), polyglycolide, starch-based plants, recycled food wastes, straws, vegetable fats, and oils [36,37]. As plastics degrade, a considerable number of MPs/NPs are released into the agricultural soils [34]. These additives are often low molecular and exist in a dispersed polymer matrix, rather than chemically bonded. They leach out of the plastic over time, and unlike polymers, they dissolve in water and can be metabolized by microorganisms. On the other hand, Alteromonas plasticoclasticus MED 1 has been reported to be able to degrade and assimilate PHBV (poly 3-hydroxybutyrate-co-3-hydroxyvalerate), a biofilm [38]. It has been established that this bacterium is able to isolate MED 1, degrade PHBV, and use it as energy and a carbon source [39]. MED 1 means the bacterium was isolated under aerobic conditions from the Mediterranean seawater [39]. Also, phthalate esters, which are a component of plastics, were reported to leach out easily into the agricultural soils [40,41]. It has also been reported that irrigation and rainwater penetrate into the soil along with MPs and NPs, ending up in groundwater [42]. These can reside in the soil for decades, tempering with soil and plant health [43,44].

The soil plastisphere refers to the soil environment and the microbial environment that is mostly affected by MPs directly or indirectly [45]. It has been found out that the soil plastisphere can indirectly affect crop development and growth [7]. The soil plastisphere comprises four parts, which are the detritusphere, drilosphere, rhizosphere, and aggregatusphere. It has been reported that microplastics attack the rhizosphere, which contains numerous kinds of soil microbes, as well as affecting nutrient cycling and the growth of crops [46]. Chia et al. [47] and Gündoğdu et al. [48] gave some standardized methods for analyzing soil microplastics which stipulated that MPs/NPs could be found in soils. The most continuous contact of microplastics in the environment and food chain can therefore be found in soils through the aquatic ecological environments.

3.2. Bioaccumulation and Toxicity of Microplastics

The bioaccumulation and toxicity results of microplastics have been discovered to be seen in both soil fauna and flora, thus, reducing their reproduction and growth, altering root structure and nutrient uptake, and affecting nutrient cycling by attacking microbial communities as well. Recently, studies in toxicology have found that both conventional MPs and BPs have the same toxic potency on soil biota [49]. Mei et al. [50] reported that MPs have the propensity to absorb hydrophobic organic compounds (HOCs), for example, PBDE, PAHs, PCBs, and NPE, because of their minute particle size and mobility coupled with strong hydrophobicity [51,52]. Supplementary Table S1 shows how PS and PMMA on barley increase the adsorption and accumulation of plastics, thereby increasing ROS activities. Some studies have also discovered that the process by which some plastics degrade is very fast [53], which also acts as a vector for pollutant chemicals such as phenanthrene into the soil ecosystem [54]. Microplastics generally become embedded in soils by the prolonged usage and decomposition of plastic mulches on agricultural soils. Also, the use of other products on our farms that contain microplastics results in their release into soils after usage, which ends up increasing the toxicity levels in the soil. Contamination of PTEs and MPs can result in ecological risks and adversely impact the soil biota. Another astonishing discovery is the ability of MPs to uptake triazole fungicides as a result of electrostatic forces and hydrophobicity [55]. Wang et al. [56] also concluded that MPs were able to absorb five pesticides as a result of the hydrophobic interaction from plastic mulches used on agricultural soil. The use of pesticides [56,57,58] and polymer-based slow-release fertilizer [59] coupled with the weathering of plastic mulch on agricultural soils has been stipulated to cause microplastic pollution [60,61]. In addition, microplastics enhanced heavy metal uptake by plants. In Supplementary Table S2, bioaccumulation, uptake, and toxicity of heavy metal (Cd) in lettuce increased in the presence of PE and PS. Adsorption of heavy metal (Cd) was enhanced in sweet potato in the presence of PE in Supplementary Table S2. It has also been reported that the activities of microorganisms like earthworms incorporate micro/nanoplastics from the topsoil through their movements and biological processes [62].

3.3. Microplastic Effects on Soil Nutrients

In the soil, microplastics also have the ability to affect the availability of nutrients supplied. They mostly affect nutrient availability by preventing plant roots from absorbing nutrients and disrupting catalysts for enhancing nutrient uptake. Soil nutrients are attained from the breaking down of minerals and the decomposition of organic matter [63], whereby important soil enzymes, such as dehydrogenase, urease, phosphatase, or β-glucosidase, are deliberately involved in regulating nutrient cycles, becoming indicators of soil fertility [64,65]. However, PS nanoplastics have been reported to reduce the activities of N-(leucine aminopeptidase), which is a primary nitrogen cycling enzyme [66,67]. Research articles have also stipulated that mulching with plastic films has the ability to affect the nitrogen cycle and then reduce the content of inorganic nitrogen [68]. In nutrient cycling, water-stable aggregates need to be maintained for easy nutrient distribution and adsorption by the plants, but PS fibres have been reported to decrease water-stable aggregates, thereby limiting the availability of nutrients supplied to plants [69]. Supplementary Table S1 shows that mineral (Mg, Fe, and Ca) content was decreased as a result of PS accumulation in cucumber plant. The efficiency of cycling nutrients in the soil is prohibited by MPs/NPs by changing the physical structure of the soil, such as affecting water retention and porosity of the soil [44,70]. The nitrogen use efficiency (NUE) of soils polluted by MPs/NPs has been reported to be reduced due to the interference of processes such as nitrogen mineralization, denitrification, and nitrification which has led to the increase in ammonia volatilization and losses of nitrogen [71]. Soil nutrient cycling is affected by MPs/NPs through modulation of functional microbial communities and soil enzyme activities [72].

4. Entry and Transportation Modes of Microplastics into Plants

4.1. Entry Modes

4.1.1. Root Adsorption of Microplastics

Islam et al. [73] defined the root absorption mechanism (of microplastic) as a way by which (microplastic) solutes penetrate plant roots, blocking pore spaces and inhibiting water and nutrient uptake, as shown in Figure 3. It has been reported that polystyrene microplastics of 0.20–1.0 µm are accumulated and translocated into the shoots of raw vegetables through root adsorption [74]. It is reported that the meristematic region of roots is capable of absorbing ions (solutes/microplastics) more efficiently than any other part of the plant. In addition, root hair cells are reported by Alaoui et al. [75] to increase the adsorption mechanism of plants. The root hairs use the interception mode to accelerate the adsorption rate of MPs/NPs. Root interception explains how minerals (solutes) are taken up by contact with soil colloids on the surface of the roots. Plant roots produce secretions and mucus, which are capable of trapping nano or microparticles for easy adsorption into the roots [76]. After root adsorption, MPs/NPs are translocated to the stems for further adsorption into the plant tissues. It has been reported that roots are able to uptake MP particles of 1 µm size directly [77].

4.1.2. Crack-Entry Pathway of Microplastics into Plants

As diagrammatically presented in a portion of Figure 3, cracks or wounds found on plant leaves and the regions of the lateral root during the separation of cells automatically created a pathway for micro/nanoplastics to enter plants. It has been reported that a 2.0 µm size of micro/nanoplastic was able to penetrate the stele and was then transported to other parts through the xylem [78]. PMMA and other plastic components were found capable of entering the stele of the roots of both wheat and lettuce during lateral root emergence through wounds/cracks, which calls for alarm on plants whose edible parts enter the food chain [60]. It was also reported that polystyrene and polymethacrylate of submicron and nanometre sizes were able to enter the stele of lateral root emergence by the ‘crack-entry mode’ (Figure 3) and then be transported into the shoots [79]. One of the most conspicuous pollinators, known as Apis mellifera (honeybee) [80], has also been identified as a transporter of microplastics [81] which are capable of entering plants through the crack-entry mode. Foliar application, which is more of a direct application of microplastics, is where the plants absorb microplastic particles through the wounds, cuts, or cracks in leaves (Figure 3). For example, sprinkler watering or fertigation can distribute MPs/NPs into these wounds/cracks [82,83]. Lastly, Li et al. [84] and Sun et al. [85], in an experiment on the effective uptake of submicrometre plastics, reported that micrometre-sized polystyrene and polymethylmethacrylate entities penetrated the stele of both wheat and rice species by the crack-entry mode at the lateral root emergence sites (Figure 3).

4.1.3. Stomatal Entry of Microplastics into Plants

The stomata of a leaf are investigated to be an entry point to facilitate water transpiration, CO₂ gas, air humidity, abscisic acid, and phytopathogen endophytic colonization between the plant tissues and its environment [86]. Avellan et al. [87] reported that the stomata and cuticle of leaves serve as a pathway for micro and nanoplastic uptake. In a research study, PS nanoplastics at day 7 were seen in and out of the stomatal opening. Pathogens such as Pseudomonas syringae pv. of tomato are reported to enter leaves of tomato plants through the stomata [88]. This implies that micro and nanoplastic fragments can enter the opened stomata of leaves, as PS particles were seen in and around the opened stomata. Figure 3 shows nano/microplastic entering an opened stomata.

4.2. Transportation Modes

4.2.1. Transpiration Pull

Transpiration pull is reported by Annamalainathan et al. [89] and Bohra et al. [90] to bring mass upward flow that makes it possible to transport solutes to the shoots. As depicted in a portion of Figure 3, transpiration pull was reported to be a process that may allow microplastics to be syphoned into the plant through the xylem, at a nanoscale being more efficient than bigger particles [69]. It has been reported that roots of corn and soybeans can absorb phthalate acid esters (PAEs) from the soil solution directly and then transport them through the xylem to the leaf by the flow of transpiration pull and then accumulate within the edible parts [79,91,92]. High transpiration pull (Figure 3) of plants is reported to be the main driving force for micro/nanoplastics to be uptaken and transported into other parts of the plants [79]. Crops can absorb micro or nanoplastics in similar pathways to their tissues [69]. Bosker et al. [93] also reported that the translocation of microplastics depends on the shape, size, and chemical properties of the plastic. Therefore, the size and shape of the MPs would define how fast or slow the MP would be translocated to other tissues of the plant through transpiration pull.

4.2.2. Apoplastic Pathway

Apoplastic pathway, both in roots and leaves (Figure 3), is the transportation of substances (nutrients/solutes) along the gaps of the cell wall without entering the cell [94]. Force of transpiration is used to accomplish apoplastic transport [95]. It has been reported that 0.2 um microbeads were transported easily to the leaves and stems through the apoplastic pathway [96]. A study by Sun et al. (2020) on Arabidopsis thaliana showed that nanoplastics were able to enter the stele of plant through the apoplastic pathway [95]. Further research on wheat cells also revealed that plastics larger than 5 nm to 60 nm could not penetrate the cell wall pores and plasmodesmata, because cell wall pores have a size of 3.5–5.0 nm and intercellular plasmodesmata have a 50–60 nm size diameter, thereby the microplastics remained within the cell gaps [78]. Although transpiration pull is the force for apoplastic transport, it is reported that the Casparian strip hinders the penetration of pollutants, thereby allowing these pollutants to be retained in the cell gaps. Subsequently, micro and nanoplastics which are retained in the cell gaps are compelled to traverse the endodermis plasmalemma using the apoplastic pathway [78]. The apoplastic pathway is therefore reported to be a faster and a more suitable pathway than the symplastic pathway of plant solute transport.

4.2.3. Symplastic Pathway

Figure 3 depicts the movement of solutes (micro/nanoplastics) in a symplastic pathway in both roots and leaves. Uranin tracer used to demonstrate symplastic pathway in root tissues was reported to have a limited entry from the water column into the leaf tissues [97]. In the symplastic pathway, aquaporins or transporters are needed to move solutes, which makes this pathway somewhat difficult [98]. In the result of a study, the OsLsi6 transporter was needed to move silicon in a leaf sheath of a rice plant [98]. It is also reported that the roots of Halodule uninervis (narrowleaf seagrass) absorbed solutes from the soil through a symplastic route into the cortex, but entering the stele was impeded [99]. This is the reason why the symplastic pathway is known to be a slower and more difficult pathway in solute transport compared to the apoplastic pathway. Symplastic pathway is however noted to occur in both roots and leaves of plants.

4.2.4. Endocytosis

Endocytosis, as seen in Figure 3, could be defined as a cellular process by which (plant) cells absorb materials outside the cell wall/membrane by engulfing or internalizing the foreign material with the cell membrane [100,101]. Nano beads of size 20 nm were reported to be engulfed by cells known as BY-2 cells of tobacco through endocytosis [102]. BY-2 protoplast cells of tobacco are also reported to be able to uptake large-sized beads of 1000 nm [102]. It has been confirmed that the root cells of rice, sweet potato, corn, and peanut are able to absorb MPs and NPs through endocytosis [103,104]. Also, NPs are proven to aid in the rupture of cell walls, thereby increasing pore numbers. This opened pore is said to easily engulf NPs in the root region [103]. Investigation of NPs and maize seedlings showed that NPs entered the core of the seedling roots through endocytosis [105]. A report has also been made that, through endocytosis, the epidermal cells of roots can engulf PE sizes of 5 µm [103]. Endocytosis processes are therefore stipulated to be seen in all cells and tissues of the plant. A typical example is seen in Supplementary Table S2, where PS is accumulated in the root cell sap of Arabidopsis.

5. Effects of MPs on the Plants

5.1. Direct Effects of Microplastic on the Plants

Microplastics can penetrate plant tissues and cells, jeopardize the germination of seeds and root growth, inhibit plant height growth, reduce plant biomass, cause oxidative stress, decrease seed setting, inhibit nutrient and water uptake, decrease photosynthetic rate, alter the cell membrane, and then affect the plant-symbiotic relationship (Figure 4) [61,106,107]. Supplementary Tables S1 and S2 also show some direct effects of microplastics on plants, such as decreased biomass, inhibited germination, increased oxidative stress, and decreased growth of plants’ parts (root, stem, and leaves).

5.1.1. Phytohormones

Plant hormones are referred to as microscopic molecules that act as messengers to control plant stress, cell development, and the general growth of plants [108,109]. Auxin and cytokinin are reported to interplay and regulate the development and growth of lateral roots, crown roots, and root hairs [110]. However, it has been reported that MPs/NPs of PS were able to change the concentrations of auxin and cytokinin, which decreased the number of rootlets formed [111]. A study conducted by Zhou et al. [112] ascertained that PS-MPs inhibited the activities of jasmonate and lignin in rice plants, which resulted in altering the gene expression of the rice plant. Lignin content was observed to decrease in rice roots, which potentially weakened cell walls by the upregulation of the laccase-coding genes [113,114]. Salicylic acid has been reported to increase N uptake and improve the permeability of cells [115], but in another study, the levels of this particular hormone were decreased in the presence of PMMA and PS-MPs/NPs, thereby decreasing N uptake and inhibiting the cells’ permeability [116]. Also, gibberellins, which are the hormones responsible for regulating plant growth and development, were found to reduce endogenously in the presence of PMMA and PS micro/nanoplastic stress [116].

5.1.2. Photosynthesis

MPs have been reported to affect photosynthesis by inhibiting leaf growth, which hinders chlorophyll fluorescence. Supplementary Table S1 shows that chlorophyll in the cucumber leaves decreased when PS degradation released benzene. In Supplementary Table S2, chlorophyll content in soybean was reduced in the presence of PBAT, LDPE, and PLA. Microplastics decrease chlorophyll fluorescence and regulate photosynthetic pigments, which leads to the reduction in photosynthesis in plants [117]. This, in the long run, impedes protein synthesis as a result of decreasing chlorophyll a and b, thereby affecting photosynthesis and also interfering with the antioxidant mechanism for plant defence [78,117,118]. It has been confirmed that microplastic stress can reduce the photosynthesis of C3 plants through the inhibition of light use efficiency, as well as in C4 plants. The photosynthesis is reduced by preventing the carbon from being fixed [119,120]. The photosynthetic mechanism of leaves is reported to be disrupted by higher doses of MPs/NPs via the inhibition of protein synthesis [121,122]. For example, MPs/NPs are reported to hinder photosynthesis by impeding the functions of Cyt b6f and NADP⁺ enzymes in as much as decreasing electron transport [123]. With regards to PS, MPs/NPs have been reported to inhibit photosynthesis by causing oxidative damage to photosynthetic structures, the thylakoid membrane and chloroplast [82]. Anthocyanin pigments during photosynthesis absorb more protons, but MPs/NPs have been reported to affect photosynthesis by reducing the biosynthesis of anthocyanin [124]. Since MPs/NPs are capable of reducing photosynthesis, this means that nutrients absorbed would not be used efficiently to achieve the maximum growth and development of plants.

5.1.3. Nutrient Availability

Plants uptake nutrients supplied to them from the soil by their root hairs. However, the activity of the roots could be reduced by lignification. Lignification results from the hardening and thickening of the root’s cell walls [125,126]. MPs/NPs have been reported to cause lignification of roots and cell atrophy, thereby preventing the effective uptake and usage of nutrients supplied to plants [127]. It has been reported that the root produced exudates and mucilage, which served as a protection layer for the plants [87]; however, PS microbeads of 0.2 µm were seen to be trapped in the cells outside the root by the mucilage on the root hair [84]. This blockage could therefore obstruct the absorption and efficient usage of nutrients and water supplied to the plant. An experiment performed by Urbina et al. (2020) on maize and PE size of 0.0125 and 100 mg/L¹, discovered that nitrogen and water uptake by the maize plant was impaired by the PE present [128]. Also, Ma et al. (2022) [129] reported that the PS and PVC present in the rice plant inhibited the uptake of nutrients supplied. This nutrient uptake impairment affected the efficient usage of the nutrient supplied [129]. Again, MPs/NPs affected nutrient uptake and its effective usage by reducing the ratio of branching in the root of plants [130]. This reduced root branching ratio prevented the roots from up taking and using the nutrients far from their reach effectively. Liu et al. (2023) [131] discovered that PP-MPs/NPs reduced N uptake by increasing the production of ROS (Supplementary Table S1) and damaging the plasma membrane of the root cells. Figure 4 shows the direct effects of MPs on plants. Supplementary Tables S1 and S2 show how microplastics physically obstruct the availability of water and nutrients to plants.

5.1.4. Germination

Biodegradable plastics and synthetic fibres were reported to hinder rice seed germination [132]. It has been reported that MPs block the capsule of seeds thereby being able to reduce the growth of roots [93]. An experiment performed by Li et al. [133] reported that the exposure of plants to polymethyl methacrylate (PMMA) microplastics reduced germination index, length of the sprout, and root length. Gentili et al. [134] reported that seed germination and growth were delayed as a result of adsorbing 250 µm of PV. It has been reported that germination of seeds can be retarded by MPs/NPs causing obstruction to the stomata, thereby restricting the absorption of water and nutrients [105,135]. PS of 50 nm size has also been reported to reduce rice germination through oxidative damage [136]. PVC was also discovered to impede the germination of coriander seeds in a soil media experiment [137]. A study conducted by Pflugmacher et al. (2020) [138] on leachate of old and new PC particles also showed a decrease in germination of watercress seeds by absorbing dissolved poisons through the stomata. Therefore, it can be stipulated that the germination of plants can be reduced by MPs/NPs. Supplementary Tables S1 and S2 show how microplastics inhibit and decrease germination. PS used on Zea mays in hydroponic medium for 15 days showed abnormal root subcellular structure and abnormal cell morphology. Carbon metabolism and amino acid biosynthesis of wheat plant were altered when PS was introduced into the hydroponic medium for 21 days [139].

5.1.5. Yield and Quality

MPs/NPs are reported to affect the quality of crops by decreasing the nutritional quality as a result of reducing the flavour and taste of the crop [140,141]. The quality of Chinese cabbage was decreased by MPs/NPs reducing starch and soluble sugar by 30% and 25%, respectively [142]. Also, peanut quality was decreased in saturated fatty acid contents, reduced minerals (Mn, Ca, Zn, Fe, and Mg), and reduced contents of amino acids by MPs/NPs [104]. It was also reported that the quality of rice was reduced by a decrease in protein, amylose, amino acids, and soluble sugar contents in MPs/NPs [143]. Microplastic stress has been reported to reduce crop yield by altering plant development and overall growth [144]. The yield of Capsicum annuum L. was reduced by PE-MPs by 42.86% [145]. Ma et al. (2022) [129] reported that microplastic stress was able to cause the reduction in rice yield by reducing hemoglobin content and also inhibiting root activity and other tertiary tissues. Bio-MPs significantly decreased the fruit biomass of cauliflower [146]. PS of 80 nm size in soil reduced peanut weight, produced high quantity of peanut plants with empty shells, and then reduced the fruiting rate [104]. MP stress has been reported to cause a reduction in rice yield by destroying the tertiary structures of the rice plant [147]. An experiment by Lian et al. (2021) [82] reported that there was a decrease in plant height, fruit biomass, dry weight, nutrient quality, and growth of lettuce when foliar PS-NPs were sprayed on the experimental lettuce plant. The reduction in crop yield and quality by MPs/NPs can be related to plants not being able to use nutrients absorbed efficiently to maintain or increase the quality/yield of fruits produced by plants. Supplementary Tables S1 and S2 show the effects of microplastics on crops in terms of crop quality and yield (plant biomass).

5.2. Indirect Effects of Microplastics on the Plants

Microplastic stress indirectly affects plants, which subsequently affects the growth of the shoot and the roots of the plant. Some of these indirect negative effects can be seen on the physical and chemical soil properties and soil microorganisms as well [148,149] (Figure 4).

5.2.1. Soil Physical Properties

Soil porosity is always linked with bulk density and the root system of plants. Soil porosity and bulk density, however, indicate the productivity of a soil [150]. However, Souza MacHado et al. [17] discovered that soil bulk density, soil minerals, and soil aeration were decreased by adding different microplastics into the soil, thus reducing the quality of the soil (Figure 4). The reduction in soil bulk density (Figure 4) as a result of microplastic pollution increased the evaporation rate, which indirectly affected plant growth [22]. The structure of clay soil is tightly arranged with a high number of pores [151] but with high MPs/NPs concentrations, ineffective pores were formed, which caused weak water permeability [152,153]. The infiltration rate properties and water-holding capacity of soil have been reported to decrease with increasing MPs [154]. Desiccation cracks formed in soils are known to affect the permeability, strength, and stability of the soil [44]. However, it has been reported that desiccation cracks were formed when 10 mm and 5 mm plastics were added to the experimental soil [155,156]. The adsorption process of MPs can enhance reactive oxygen species (ROS) (Figure 4) production in algae, which causes a reduction in airflow and light penetration as a result of physical blockage by the MP/NP particles [157]. It is believed that microplastics in the soil block the channels (Figure 4) for water uptake as well as effective nutrient uptake by the plants [158]. All these effects of MPs/NPs on soil physical properties in one way or another prevent the effective uptake and usage of nutrients supplied to plants. Supplementary Table S2 summarizes the effects of MPs/NPs on the physical properties of soils.

5.2.2. Soil Chemical Properties

Chemical properties of soil can be adversely affected when MPs affect the EC, pH, and nutrient cycling of the soil [132,159]. MP/NP stress has been reported to affect soil quality by reducing soil organic matter [160,161]. MPs act as carriers for pollutants within the soil ecosystem, thereby altering the chemical composition of the soil [162]. PE and HDPE are reported to destabilize soil pH (Figure 4) by increasing or decreasing the pH of soils [163]. It has also been reported that HDPE and MPs from tyre debris reduced soil pH value significantly [164,165]. The change in pH by MPs/NPs can lead to the adsorption of certain heavy metals (Figure 4). In one experiment, the adsorption of cadmium (Cd) increased when the pH of the soil medium was increased by MPs/NPs [166]. PLA found in biodegradable plastics was reported to increase the concentrations of NO₃⁻ and NO₂⁻, which caused N losses after reducing NH₄ present in the soil [167,168]. NO₃⁻ (nitrate) and NO₂⁻ (nitrite) in high concentrations are reported to cause N losses [167]. A 30-day exposure of 5% MPs to soil increased the concentration of N₂O released. This was as a result of reducing NO₃⁻ by microorganisms, which was affected by the MPs [169]. Zhang et al. [170] found out that the toxic effects of microplastic concentration (MPVC, 1 µm) led to the adsorption of other heavy metals (microplastics + heavy metals, Figure 4), such as cadmium (Cd) (Supplementary Table S2), lead (Pb), cobalt (Co), nickel (Ni), zinc (Zn), and silver (Ag), whereby the adsorption of these heavy metals was able to destabilize soil chemistry [166]. As a result of the high adsorption potency of microplastics (Figure 4), there is an enhanced risk of microplastics negatively affecting the growth of plants, leading to an increase in health hazards [171]. Therefore, the changes in the nutrients of soils by MPs/NPs have the ability to make some soil nutrients supplied unavailable for effective uptake and usage by plants.

5.2.3. Soil Biological Properties

The diversity and abundance of microorganisms (earthworms, isopods, nematodes, fungi, bacteria, and microbes) that live in soil are reported to be affected by micro/nanoplastics [61]. The various kinds of soil bacteria involved in nitrogen fixation decreased (Figure 4) in the presence of MPs/NPs [172], which resulted in changes in soil pH (Figure 4) and soil NH₄⁺. Nematodes showed a decrease in growth when they were exposed to LPDE microplastics [173]. Mycorrhizal fungi are microorganisms that regulate soil stability by decreasing oxidative stress, enhancing the uptake of phosphorus, and antioxidating enzymes [174]. However, ZnO nanoparticles and microplastics were found to affect the arbuscular mycorrhizal fungal diversity as well as the community composition via a high-throughput sequencing mechanism [175]. Microplastics affect soil enzymatic processes (Figure 4). In an experiment, microplastics inhibited the fluorescein diacetate hydrolase and urease activities of microorganisms in the soil [172]. It has also been reported that microorganisms change in soil substrates and properties due to microplastic stress [176], because microplastics added to soil in an experiment decreased the activities of FDase, urease, CAT, and PO of microorganisms [76]. In a study where PE-MPs were tested on earthworms (Lumbricus terrestris), they were observed to have an increased mortality and decreased growth rate after 60 days [177]. It has also been reported that LPDE microplastic stress reduced the microbial activities of soil within 90 days [150]. These effects of micro and nanoplastics on microorganisms mean that decomposition, breakdown, and distribution of organic matter and nutrients would be affected, thereby affecting the availability, absorption, and efficient usage of nutrients supplied to plants.

6. Conclusions and Suggestions

6.1. Conclusions

In conclusion, as plastic production increases, its usage in agriculture increases as well; therefore, it is very crucial to understand how degraded plastics form micro/nanoplastics and how they are adsorbed by plants and then affect plants and the soil ecosystem. This review paper mainly focused on the mechanisms of how micro/nanoplastics enter plants and are transported within the plants. Although their effects on the aquatic ecosystem were not reviewed, the effects on the soil ecosystem were reviewed. Therefore, this paper reviewed the fragmentation of plastics, how MPs/NPs become embedded in the soil, the entry and transportation mode in plants, and their effects on crops (monocot and dicot species) and soil micro fauna. The results are summarized as follows.

The degradation palindrome of plastics is very fast and also acts as a carrier of pollutants in the soil. Whereas some novel bacteria (MED 1) capable of degrading micro/nanoplastics have been found, further research is required. The mechanisms and pathways through which micro/nanoplastics enter and move within plants were reported as root adsorption, endocytosis, apoplastic, and symplastic pathways which allow plants to uptake microplastics. Also, mechanisms such as crack-entry mode, stomatal entry, and transpiration pull were explained as a means of microplastic entry and movement within plants. Roots of raw vegetables were able to transport MPs of size 0.20–1.0 µm through root absorption. Transpiration pull also helped MPs to be syphoned through the xylem of plants. On day 7, MPs/NPs were seen in and around the stomatal openings of plants. BY-2 protoplast cells were able to engulf large beads of 1000 nm through endocytosis. For the symplastic pathway, it was difficult to transport MPs/NPs because aquaporins were needed to aid the transport of MPs/NPs in plants. MPs/NPs were seen to be capable of lignifying roots, which prevented effective uptake and usage of nutrients supplied. MPs were seen to block the seed capsule and roots of crops, thereby negatively affecting the uptake and efficient use of nutrients supplied as well as delaying and decreasing the rate of germination and growth of crops. The chlorophyll of cabbage leaves was decreased by microplastics which negatively affected photosynthesis. Effective root branching for nutrient uptake and usage was also seen to be impaired by PE. Nutrient uptake and its efficient usage were inhibited by microplastics, preventing the growth of almost every monocot and dicot crop reviewed. The bulk density, EC, and pH of soils were affected negatively after exposure to MPs/NPs. Also, some enzymatic activities of N-(leucine aminopeptidase), dehydrogenase, urease, and fluorescein diacetate hydrolase decreased, thereby affecting the quality of the soil ecosystem at large. Therefore, based on the evidence provided, it can be concluded that microplastics in the soil and uptaken by plants prevent nutrient uptake and nutrient use efficiently by crops (monocots and dicots) as well as affecting the crops in several physiological ways. Further research should be geared towards plastic degradation processes by microorganisms which would not be harmful to the environment. Also, research on how to replace plastic mulch with nature-based materials and still derive the benefits of mulching should be investigated.

6.2. Suggestions

The numerous advantages of plastic materials such as mulches on agricultural soils for crop production cannot be overlooked but various alternatives for mulching is needed to be investigated and used in order to mitigate the negative effects of microplastics on crops, specifically on their nutrient uptake and efficient use. Most research articles downloaded focused on just the toxicity of MPs/NPs; however, the following recommendations may need more information and investigation:

• The use of biochar as a mulch could further be investigated and used instead of plastics as mulch. Biochar adds several chemical and physical benefits to the soil and crops in a long-term usage but its ability to replace plastic mulch is needed to be investigated.
• Nature-based solutions for degrading microplastics without causing soil environment and plant pollution should be researched. Thus, novel soil microorganisms could be engineered to enhance micro/nanoplastics degradation.