A Combined Effect of Mixed Multi-Microplastic Types on Growth and Yield of Tomato

1. Introduction

Recent studies on emerging pollutants highlight the critical need to investigate compost, a common soil enhancer and carrier of microplastics (MPs) in the soil. MPs are defined as plastic particles ranging from 1 mm to 5 mm in diameter and are ubiquitous in ecosystems (Auta et al., 2017). They can act as vectors for contaminants (Alimi et al., 2018; Yu et al., 2022), contribute to soil pollution, and cause damage to soil structure and fauna (Ng et al., 2018). MPs can also affect plant health by blocking root cells and nutrient flow (Zhang et al., 2022b; Giorgetti et al., 2020; Zhou et al., 2021), particularly when absorbed by stems and leaves (Gao et al., 2019). In addition, recent studies have highlighted the phytotoxicity of MPs on various plants, including mung beans (Vigna radiata L.) (Chae & An, 2020), broad beans (Vicia faba L.) (Jiang et al., 2019), rice (Oryza sativa L.) (Liu et al., 2022), corn (Zea mays L.) (Zhang et al., 2022a), lettuce (Zeb et al., 2022), Garden cress (Lepidium sativum L.) (Bosker et al., 2019), wheat (Triticum aestivum L.) ( Zhu et al., 2022), and tomato (Lycopersicum esculentum L.) (Shi et al., 2022).

MPs in soil can negatively affect plant development and food production by interfering with nutrient absorption and possibly acting as vectors of other contaminants (Yu et al., 2022) because their abundance in agricultural lands can be linked to municipal solid waste (MSW) compost. Compost is increasingly recognized as a significant route for introducing MPs into soil ecosystems (Edo et al., 2022; Vithanage et al., 2021). During the composting process, plastic debris from various sources, including household waste and agricultural plastics is broken down into MPs (Rujnić-Sokele & Pilipović, 2017). These MPs-contaminated composts are subsequently spread onto agricultural fields and gardens, facilitating their entry into agricultural soil environments (Weithmann et al., 2018). This pathway does not only introduce MPs directly into the soil but also affects soil properties and microbial communities, potentially altering nutrient cycles and soil health (de Souza Machado et al., 2019). Understanding the extent and impact of MPs in compost is essential for developing strategies to mitigate their spread and protect soil ecosystems (Bläsing and Amelung, 2018).

Tomato is a vegetable crop belonging to the Solanaceae family and is globally recognized as one of the most extensively cultivated fruits (Panno et al., 2021). It is consumed in a variety of forms, including fresh, cooked, and processed. Tomato fruits are commonly processed into various food products such as soups, sauces, and ketchup, as well as dried forms like powders and flakes, and utilized in food additives and preservatives such as pulp, juice, puree, pickles, and paste. This versatility and its nutritional benefits have led to a continuous increase in tomato production (Chaudhary et al., 2018; Oprea et al., 2022). In 2022, tomatoes were recorded as the most widely grown vegetable, with a global production exceeding 186 million tons, representing a 1.08% increase from 2021 (Food Agriculture Organization of the United Nations-Statistic Division (FAOSTAT), 2023).

While there has been an increase in studies examining the effects of MPs on various plants, the literature on the phytotoxic effects of MPs on the physiological and morphological parameters, and the productivity quality of tomato plants remains limited. Most prior research on the uptake of plastics has focused on polystyrene (PS) plastic including mung bean (Chae & An, 2020), broad bean (Jiang et al., 2019), rice (Liu et al., 2022; Dong et al., 2021a; Wu et al., 2022; Zhou et al., 2021), corn (Zhang et al., 2022a), lettuce (Dong et al., 2021b), and wheat (Taylor et al., 2020; R. Li et al., 2023; J. Zhu et al., 2022), potentially introducing bias, given that other plastic types are present in soil and compost. In soil environments, polyethylene (PE) and polypropylene (PP) are some of the most prevalent (Hu et al., 2022), warranting further investigation into the phytotoxic effects of these plastic types. Notably, the World Health Organization (WHO) categorized PE as carcinogenic in 2017 (Teng et al., 2022). A significant portion of plastic waste in MSW consists of PP, PS, and PE (Rani et al., 2023). Although there are studies suggesting that plant roots can uptake MPs, the translocation to plant stems and tissues may not be consistent and may depend on factors such as plastic-type, size, and exposure duration (Ceshin et al., 2023). Further research is needed to understand the uptake, translocation, and phytotoxicity of MPs across different plant species, as the presence of MPs has been shown to cause significant changes in plant biomass, tissue elemental content, and soil microbiome activities (de Souza Machado et al., 2019). This will allow scientists to identify sustainable management practices since little is known about the clean-up of MP contamination, coupled with the intricacies of MP extraction, identification, and characterization (Adelugba & Emenike, 2023). The soil environment and organic compost usually contain a combination of MPs from various plastic types including PE, PS, and PP (Vinathage et al., 2022; Scopetani et al., 2022). Hence, this study investigated the impact of a combination of three common MP types in compost as contaminants and examined the effects on tomato plant performance and fruit quality in a greenhouse environment. The interactions between compost, soil, and MPs, and their influence on the growth and development of tomato plants were explored.

2. Materials and Methods

2.1. Study Location

The research was conducted in the greenhouse located within the Department of Plant, Food, and Environmental Sciences, Faculty of Agriculture, at Dalhousie University in Truro, Nova Scotia, Canada. The study took place from December 2023 to July 2024.

2.2. Microplastics

PE-MPs, PP-MPs, and PS-MPs were obtained from household plastic materials, packaging bags, food packaging containers, and drinking cups with specific identification codes labelled PE, PP, and PS. The plastics were commercially bought from Walmart Supercenter, Truro, NS, Canada, and were cut into particle sizes ranging from 1 to 5 mm, equal weights of the sizes were homogenized for the treatments.

2.3. Plant Material and Growing Media

Surface soil (0-20 cm) was obtained from the demonstration garden at the Dalhousie University Agricultural Campus(45ͦ 22’15” N, 63ͦ15’26” W). The collected soil was screened for rocks and other unwanted materials and stored in the greenhouse until application. Municipal solid waste (MSW)-generated compost was sourced from the Colchester Balefill and Composting facility (Kemptown, NS, Canada), which upcycles organic waste into compost for agricultural and economic purposes. The profiles of the compost and soil used in this study are presented in Table 1 and Table 2, respectively.

Tomato cultivar ‘Scotia’ seeds were obtained from Halifax Seeds (Halifax, Canada). Seeds were sown in a 32-cell pack filled with Pro-Mix® BX (Premier Tech Horticulture, Québec, Canada), and the seedlings were grown for 30 days in a growth chamber. The chamber maintained a day/night temperature of 25 ℃, with 16/8 h d⁻¹ illumination, 300 µmol m⁻².s⁻¹ light intensity, and 70% relative humidity. Once the seedlings reached the third to fourth true-leaf stage, uniform seedlings were transplanted into 6.52 L experimental pots, with one seedling per pot. Each experimental pot contained 2 kg of a soil-compost mixture (3:1), with MPs (1% w/w) at 10 cm depth. The growth medium was climate-hardened for one week before applying the treatments and transplanting, which was performed under greenhouse conditions at a day/night temperature of 28/20℃ and 70% relative humidity with a 16-hour photoperiod. Supplemental lighting was provided by a 600 W HS2000 high-pressure sodium lamp with NAH600.579 ballast (P.L. Light Systems, Beamsville, Canada) throughout the growing period.

2.4. Experimental Design and Treatment

The experimental design used was a completely randomized design with two (2) treatments, i.e., control (soil and sieved compost without spiked MPs) and M-MPs (soil and sieved compost with 1% w/w PE-MPs, PS-MPs and PP-MPs) at an equal ratio. The MSW compost was sieved through a 1 mm, 8-inch full-height sieve mesh (Advantech - W.S. Tyler Company, OH, U.S.A) to eliminate potential MPs in compost samples. The sieved compost was added to the soil before the M-MPs were added into the growth media at 10 cm depth. The treatment was applied during the preparation of the growing medium, and regular watering was maintained to field capacity throughout the study. The pots were rearranged biweekly on the bench to mitigate any unforeseen environmental variations within the greenhouse.

2.5. Determination of Plant Growth and Yield

Following the methodology outlined by Ofoe et al. (2022), plant growth parameters were evaluated 50 days after transplanting (DAT). Plant height was measured from the stem collar to the highest leaf tip using a measuring ruler, and the main stem diameter or girth was assessed at 10 cm above the stem collar with vernier calipers (Mastercraft®, Ontario, Canada). Intracellular carbon dioxide concentration (Ci), net photosynthetic rate (A), transpiration rate (E), and stomatal conductance (gs) were measured from four fully expanded leaves per plant using an LCi portable photosynthesis system (ADC BioScientific Ltd., Hoddesdon, UK). Additionally, chlorophyll fluorescence indices, such as maximum quantum efficiency (F_v/F_m) and potential photosynthetic capacity (F_v/F_o), were measured on the same leaves using a chlorophyll fluorometer (Optical Science, Hudson, NH, USA). Chlorophyll content was quantified with a chlorophyll meter (SPAD 502-plus, Spectrum Technologies, Inc., Aurora, IL, USA). The total fresh weight of ripe tomato fruits per plant was measured using a portable balance (Ohaus Navigator®, ITM Instruments Inc., Sainte-Anne-de-Bellevue, QC, Canada). The equatorial and polar diameters of the harvested tomato fruits were measured with a digital Vernier caliper.

2.6. Fruit Analysis

At the time of harvest (75DAT), nine ripe fruits, a representative in size and colour were randomly selected and surface sterilized with 70% ethanol. The pericarp was carefully excised from the longitudinal section of each fruit using a sterile scalpel blade. The excised pericarp was immediately frozen in liquid nitrogen and stored at -80℃, while the remaining fruits were preserved at -20℃ for subsequent analyses. The frozen fruits were thawed at room temperature, and the total soluble solids (TSS) were measured with a handheld refractometer (Atago, Japan). The ripe fruits were cut, placed in clear Ziploc bags, and manually squashed to determine TSS. The tomato fruit juice was collected in a 50 mL beaker, with 500 µL used for TSS measurement and expressed as degrees Brix (ͦ Brix). The fruit juice pH, total dissolved solids, and electrical conductivity were measured using a pH/EC/TDS/Temp portable meter (Hanna Instrument, Woonsocket, Rhode Island, USA). The elemental composition of the tomato fruits was analyzed at the Nova Scotia Department of Agriculture Laboratory Services in Truro, NS, Canada, using inductively coupled plasma mass spectrometry (PerkinElmer 2100DV, Wellesley, Massachusetts, USA) (Donohue et al., 1992; Ofoe et al., 2022).

2.7. Biochemical Analysis

2.7.1. Carotenoid Content

Carotenoid content in the fruit was assessed by finely grinding the fruit pericarp following the methodology described by Lichtenthaler (1987). A 0.2-g sample of the ground pericarp was homogenized in 1.5 mL of 80% acetone within a sterile 2 mL Eppendorf tube. The homogenate was then centrifuged at 15,000 ×g for 15 min. The absorbance of the supernatant was measured at wavelengths of 646.8 nm, 663.2 nm, and 470 nm corresponding to chlorophylls a, b, and carotenoid, respectively, with 80% acetone used as the blank. The total carotenoid content, chlorophyll a (chl a), and chlorophyll b (Chl b) were calculated using the following equations:

Carotenoid (µg/mL) = (1000*A470) - 1.8*(Chlorophyll a) - 85.02*(Chlorophyll b) /198

(1)

{Chl a} {µg/mL} = 12.25*A{663.2} - 2.79*A{646.8}

(2)

{Chl b} {µg/mL} = 21.50*A{646.8} - 5.1*A{663.2}

(3)

The total carotenoid content was expressed as µg g^-1 fresh weight (FW) of the sample.

2.7.2. Total Ascorbate Content

The determination of the total ascorbate content was determined following the procedure outlined by Ofoe et al. (2022), which is a modified version of the method originally developed by Ma et al. (2008). Approximately 0.2 g of ground fruit pericarp was homogenized in 1.5 mL of ice-cold freshly prepared 5% trichloroacetic acid (TCA). The resulting mixture was then vortexed for 2 min and subsequently centrifuged at a speed of 12,000 × g for 10 min at a temperature of 4℃. A volume of 100 µL of the supernatant was carefully transferred into a new tube and 400 µL of 150 mM phosphate buffer was added. Following this, 100 µL of 10 mM dithiothreitol (DDT) was added to the mixture and vortexed for a duration of 30 s. To initiate the colour development, a reaction mixture comprising 400 µL of 10% (w/v) trichloroacetic acid (TCA), 400 µL of 44% orthophosphoric acid, 400 µL of 4% (w/v) α, α-dipyridyl in 70% ethanol, and 200 µL of 30 g/L ferric chloride (FeCl₃) was added. The resulting mixture was then incubated in a water bath at a temperature of 40℃ for 60 min. Subsequently, the absorbance of the solution was measured at a wavelength of 525 nm. Total ascorbate content was determined by using a standard ascorbic acid curve and was expressed as µmol g^-1 FW.

2.7.3. Total Phenolics Content

The total phenolic content (TPC) was quantified by adapting the Folin-Ciocalteau assay method (Ainsworth and Gillespie, 2007; Ofoe et al., 2022). Ground fruit pericarp weighing approximately 0.2 g was mixed with 1.5 mL of ice-cold 95% methanol and left to incubate in darkness at room temperature for 48 h. Following this, the mixture was centrifuged at 15,000 × g for 15 min, after which 100 µL of the supernatant was mixed with 200 µL of 10% (v/v) Folin-Ciocalteau reagent. The resulting solution was vortexed for 5 min, then mixed with 800 µL of 700 mM sodium carbonate (Na₂CO₃), and allowed to incubate in the dark at room temperature for 2 h. The absorbance of the supernatant was then measured at 765 nm against a blank. TPC was determined using a gallic acid standard curve and reported as mg gallic acid equivalents per gram of fresh weight (mg GAE g^-1 FW).

2.7.4. Total Flavonoid Content

The quantification of total flavonoid content (TFC) was determined following the colorimetric method outlined by Chang et al. (2002). A 0.2 g sample of ground fruit pericarp was homogenized in 1.5 mL of ice-cold 95% methanol, followed by centrifugation at 15,000 × g for 15 min. Subsequently, 500 µL of the supernatant was combined with a reaction mixture consisting of 1.5 mL of 95% methanol, 0.1 mL of 10% aluminum chloride (AlCl₃), 0.1 mL of 1 M potassium acetate and 2.8 mL of distilled water. The resulting mixture was incubated at room temperature for 30 min, after which the absorbance was measured at 415 nm against a blank devoid of AlCl₃. TFC was determined using the quercetin standard curve and expressed as a as µg quercetin per gram of fresh weight (µg g^-1 FW).

2.7.5. Soluble Sugar Content

The determination of the total sugar content of the tomato fruits was carried out following the procedure described by Ofoe et al. (2022), which was a modified version of the method originally developed by Dubois et al. (1956). Approximately 0.2 g of ground fruit pericarp was homogenized in 10 mL of 90% ethanol. The resulting mixture was then placed in a water bath at a temperature of 60℃ for 60 min. Subsequently, the final volume of the mixture was adjusted to 5 mL using 90% ethanol and subjected to centrifugation at 12,000 × g for 3 min. Then, 1 mL aliquot of the supernatant was transferred into a thick-walled glass test tube containing 1 mL of 5% phenol and thoroughly mixed. To initiate the reaction, 5 mL of concentrated sulfuric acid (H₂SO₄) was added to the mixture, which was then vortexed for 20 s and incubated in the dark for 15 min. The mixture was allowed to cool to room temperature and the absorbance was measured at a wavelength of 490 nm against a blank. Finally, the total sugar content was determined using a standard sugar curve and expressed as µg of glucose g^-1 FW.

2.7.6. Total Protein Content and Peroxidase Enzyme Activity

The methodology followed to assess the fruit protein content and antioxidant enzyme activity was described by Ofoe et al. (2022). A 0.2 g of the ground sample was mixed with 1.5 mL of ice-cold extraction buffer containing 50 mM potassium phosphate buffer (pH 7.0), 1% polyvinylpyrrolidone (PVP), and 0.1 mM ethylenediamine tetraacetic acid (EDTA). The mixture was then homogenized and centrifuged at 15,000 × g for 20 min at a temperature of 4℃. The resulting supernatant, representing the crude enzyme extract, was transferred to a new sterile 2 mL microfuge tube on ice. Subsequently, 1 mL of Bradford’s reagent was added to the new tube containing the crude enzyme extract, vortexed, and left for 5 min at room temperature before measuring the protein content. A standard curve of bovine serum albumin (BSA) ranging from 100 to 500 µg mL-1 was used to estimate the protein content (Bradford, 1976).

For the determination of peroxidase (POD, EC 1.11.1.7) activity, pyrogallol was used as the substrate, following the method described by Change and Maehly (1955) and modified by Ofoe et al. (2022). The reaction mixture consisted of 100 mM potassium-phosphate buffer (pH 6.0), 5% pyrogallol, 0.5% H₂O₂, and 100 µL of the crude enzyme extract. After incubation at a temperature of 25℃ for 5 min, the reaction was halted by adding 1 mL of 2.5 N sulfuric acid (H₂SO₄₎. The absorbance was then measured at a wavelength of 420 nm against double-distilled water (ddH₂O) as the blank. It is noteworthy that one unit of POD forms 1 mg of purpurogallin from pyrogallol in 20 s at pH 6.0 and a temperature of 20℃.

2.8. Statistical Analysis

The data collected from this study were analyzed using the two-sample t-test on Minitab version 21 (Minitab, Inc., State College, Pennsylvania, USA). Anderson-Darling normality test was first used to ascertain the data normality. Due to the non-normality of some of the data (Anderson-Darling, p < 0.05), a Mann-Whitney non-parametric approach was followed (Lamaro et al., 2023). The parameters analyzed were expressed as their median value and their errors were expressed as the interquartile range divided by the square root of the number of observations from five replicates (n = 5).

3. Results

3.1. Physiological Parameters

The results of the chlorophyll content, transpiration rate, photosynthetic rate, sub-stomata CO₂ concentration, and stomatal conductance showed that the M-MPs in the soil had no significant (p > 0.05) effect on all physiological attributes measured on the tomato plants. Although the outcomes of both the treatment and the control were comparable, slight differences were noted. Chlorophyll content, transpiration rate, photosynthetic rate and stomata conductance of tomato plants in MP-spiked soil were increased by ca. 5.2%, 16.7%, 25.8% and 20.8% respectively compared to the control (Table 1). Conversely, the Ci value of the tomato plant decreased by ca. 3.23% compared to the control experiment.

3.2. Morphological Response of Tomato to Mixed Microplastics (M-MPs)

There were no significant (p > 0.05) effects recorded for the plant height, number of fruits produced, number of leaves, total fruit weight, total root length, root surface area, root volume, root tips, fruit polar diameter (PD) and equatorial diameter (ED) but there was a significant (p < 0.05) effect on the stem girth (Table 2). However, no significant (p > 0.05) difference was observed when M-MPs was compared to the control. The plant height, number of leaves, number of fruits, root length, root surface area, and root volume increased non-significantly (p > 0.05) by ca. 0.5%, 23.7%, 11.8%, 13.8%, 0.6% and 28.3% respectively with M-MPs treatment compared to the control (Table 2). Conversely, M-MPs caused a slight reduction in total fruit weight, fruit PD and ED, and root tips by ca. 0.1%, 4.4%, 7.6% and 13.7% respectively compared to the control. In addition, the M-MPs in the soil significantly (p < 0.05) caused a 15.0% reduction in the stem girth of the tomato plant.

3.3. Biochemical Activity and Fruit Quality

The biochemical analysis carried out on the harvested tomato fruits revealed significant (p < 0.05) effects of M-MPs treatment MPs on carotenoid content, total flavonoid, sugars, total protein, total ascorbate and peroxidase activity. However, there was no significant (p > 0.05) difference between M-MPs treatment and control on the total phenolic content of tomato fruits. Notably, M-MPs caused an increase in the total protein, total phenolics, total ascorbates, and peroxidase activity by ca. 11.1%, 11.5%, 38.4% and 30.2% respectively compared to the control (Table 3). Also, the carotenoids, flavonoids and total sugars were reduced in tomato fruits harvested from M-MPs in soil by ca. 12.6%, 42.3% and 21.7% respectively, compared to the control. Furthermore, the tomato fruit analysis showed a significant (p < 0.05) difference in TSS, but no statistically significant difference (p > 0.05) was recorded for the TDS and electrical conductivity. Despite this, M-MPs generally caused an increment in all three parameters, TSS (ca. 5.6%), TDS (ca. 9.1%) and EC (ca. 20.0%). In addition, our data on the fruit nutritional values revealed that the presence of the M-MPs {PE-MPs, PS-MPs, and PP-MPs (1:1:1; 1% w/w)} in soil enhanced all the analyzed macronutrients (nitrogen {N}, calcium {Ca}, potassium {K}, magnesium {Mg}, phosphorus {P}, sodium {Na}) including DM. However, there were some variations in the micronutrient values as M-MPs tomato fruits had higher B and Fe but lower Cu, Mn and Zn values (Table 4).

4. Discussion

The continuous increase in food demand and the drive for achieving food security and zero hunger has focused on green and environmentally friendly soil management practices such as applying organic composts to agricultural lands to improve soil quality and optimized plant growth, yield, and fruit quality. Hence, farmers and agriculturalists are drawn to the purity and quality of compost as a soil enhancer. In this study, though the contamination of M-MPs had no statistically significant effect on the physiological parameters of the Scotia tomato plant. The treatment slightly caused an increase in SPAD, transpiration rate, photosynthetic rate and stomata conductance of the plants. This finding contradicts the results of Wang et al. (2022), who reported that MPs reduced the SPAD content of lettuce leaves following foliar application of MPs (Table 1). Specifically, polyethylene microplastics (PE-MPs) have been shown to induce stress and alter enzymatic activity, thereby disrupting chlorophyll synthesis. Consequently, this disruption led to a reduction in chlorophyll content in duckweed (Lemna minor). It also exacerbated the toxic effects of cadmium in a co-contamination study (Tunali et al., 2020; Zhang et al., 2021; Yang et al., 2023). Furthermore, PE-MPs have been linked to the reduction of chl a and b in maize leaves (Sun et al., 2023). In a recent study on soybean (Glycine max) plants, a concentration of 0.1% PE-MPs resulted in a 6.05% increase in SPAD value (Lian et al., 2022). It is possible that the M-MPs treatment did not induce water stress, thereby limiting the production of reactive oxygen species (ROS) that would have disrupted the chlorophyll molecules.

The Fv/Fo ratio shows a plant's potential photosynthetic capacity under optimal conditions, while Fv/Fm shows the maximum quantum efficiency yield, which indicates how efficiently absorbed light is used in photosynthesis, particularly in photosystem II (PSII) (Strasser et al., 2010; Li. Z et al., 2020). In this study, both F_v/F_m and F_v/F_o were slightly reduced in tomato leaves grown in soil contaminated with M-MPs (Table 1). This suggests that the combination of PE-MPs, PS-MPs and PP-MPs may decrease the photosynthetic capacity of tomatoes, potentially due to their impact on leaf size, chlorophyll content, or enzymatic activity within the photosynthetic pathway. The F_v/F_m values in this study were slightly below 0.8, however, the optimal values are expected to be in the range of 7.0 – 8.0 (Ritchie, 2006). A reduction in F_v/F_m typically indicates compromised photosynthetic activity, potentially linked to stress (Murchie and Lawson, 2013). This suggests that both the treatment and control plants may have experienced slight stress, which could include some factors such as light stress, heat stress, or nutrient deficiencies.

The regulation of stomatal conductance and transpiration rate is essential for plants to efficiently control gas exchange and water use in response to environmental conditions, which in turn affects their ability to regulate temperature and perform photosynthesis (Urban et al., 2017; Ofoe et al., 2022). This study demonstrated that M-MPs slightly improved gs and E. This increase in E aligns with a rice-MPs interaction study using 3mg/L PS-MPs but contradicts with gs and SPAD which experienced a 33.6% and 34.9% decline respectively (Ma et al., 2022). Although there is limited direct research on the effects of M-MPs on the physiological traits of tomato plants, it can be hypothesized that the presence of microplastics may enhance photosynthesis and nutrient uptake in tomato plants and provide some protection against stress. This is because plants often reduce stomatal conductivity as an adaptive strategy to manage water loss, heat stress, and other related climatic stressors (Ofoe et al., 2022). Conversely, increased stomatal conductance and transpiration can also lead to greater water loss.

Similarly, the photosynthetic rate (A) showed a significant increase with M-MPs treatment. This finding contradicts the results of Ma et al. (2022), who reported that PS-MPs reduced the photosynthetic rate by 31.5%. The reduction was less pronounced at a lower concentration of PS-MPs (0.5 mg/L), with a decrease of only 11.8%. The results also contrast with those of Li. Z et al. (2020), observed a decline in photosynthetic pigments in cucumber (Cucumis sativus) leaves exposed to PS nanoparticles (100 nm and 700 nm). These discrepancies might be attributed to the nano size and concentration of the plastic particles, which could facilitate the absorption of PS-NPs into plant leaves. Additionally, the presence of PE-MPs and PP-MPs may have mitigated the negative effects of MPs on tomato plants. This suggests that the 1% w/w concentration used in this study might have contributed to the observed enhancement in the photosynthetic rate of tomato plants. In the same study on rice, the internal CO₂ concentration (Ci) decreased with 3 mg/L PS-MPs, a finding that is consistent with the slight reduction in Ci observed with M-MPs treatments in this study.

Agricultural scientists and farmers are interested in plant productivity i.e. fruit yield, and the present study demonstrated that M-MPs has no significant effect on tomato fruit yield and size (including fruit weight, fruit ED and PD). Given that the physiological activities of the plants were unaffected by M-MPs, it could be hypothesized that the tomato yield from the treatment is comparable with the control experiment. As seen in Table 2, comparable fruit weights were recorded, M-MPs slightly increased the number of fruits but caused a decline in the fruit size including fruit weight, fruit ED and PD. This result aligns with a recent study that reported a reduction in the number of tomato fruits produced by PET-MPs and PVC-MPs pollutants (Dainelli et al., 2023). Outstandingly, M-MPs caused a significant decline in the tomato stem girth. The stem girth is the circumferential measurement of the stem that could be linked to the thickness of the plant, stamina and vigour. The importance of the stem girth in tomato is mostly evident at the fruiting stage where the plant requires more support for the weight of the fruits. Thinner stems could bend, break and cause damage to plant vascular tissues. Also, the stem girth is principal in the translocation of water and nutrients (De Schepper and Steppe, 2010). This conforms with a recent study on the ability of PP-MPs to reduce the translocation of macronutrients in tomato plants (Shorobi et al., 2023). M-MPs treatment had negligible effects on the tomato plant height confirming Dainelli et al. (2023) that PET-MPs and PVC-MP had insignificant effects on plant growth.

Similarly, the root morphology demonstrated negligible differences though the M-MPs spiked plants slightly improved the root length, surface area and volume (Table 2). Contradictorily, it caused a reduction in the number of root tips produced. The inversely proportional relationship between the root volume and root tips is suggested to be a devised means for the plant to make up for adequate nutrient uptake from the soil. Zhuang et al. (2023) reported that MPs have the potential to significantly obstruct root growth in cucumber plants, but this study demonstrated that the effects of MPs in soil can be inconsequential to root growth in tomato plants. Contrary to the effect of PE-MPs on strawberry (Fragaria ananassa) roots (Pinto-Poblete et al., 2023), PS-MPs (0.3 mm) have been reported to significantly increase the root profile of cucumber plants (Li. Z et al., 2021).

This study was monitored until fruit harvesting, and this allowed for the evaluation of some antioxidant compounds in tomato fruits to understand how plastic contaminants in soil could affect fruit quality. The carotenoid and flavonoid content measured in tomato fruits harvested from growth media containing the combination of MPs was drastically lower than those in the control group, with a statistically significant effect. On the contrary, M-MPs significantly increased the ascorbate content, total protein content, POD activity and TSS. These POD activity changes contradict the effect of PVC and PS on cucumber. PVC and PS-MPs were reported to decrease the POD activity in cucumber plants (Zhuang et al., 2023). However, in the presence of a combination of PE-MPs, PS-MPs and PP-MPs, there was a percentage increase in TDS, and EC. These parameters are important in determining the quality of fruits, this suggests that the combination of PE-MPs, PS-MPs and PP-MPs in soil could have a positive contribution to improving food security and safety. However, due to the decline in carotenoids and flavonoids caused by M-MPs treatment, it is difficult to place the combinative effect of PE-MPs, PS-MPs and PP-MPs on fruit quality on a positive scale since carotenoids and flavonoids are part of the antioxidants and health benefit strength of tomato (Ofoe et al., 2022). This result conforms with (Dainelli et al., 2023) which reported a decline in lycopene and phenolics. This reduction may be associated with MP types that have a greater propensity to generate oxidative stress in plants. The colouration of fruit is dependent on its carotenoid concentration, which is associated with pigmentation in fruits (Giusti et al., 2023), and prevention against oxidative stress (Ofoe et al. 2022), hence critical to customer preference, and human and plant health. Contrary to (Dainelli et al., 2023) report on PET-MPs, M-MPs-treated plants produced fruits with increased TSS. TSS is regarded as a key factor considered by consumers (Malundo et al., 1995).

Another significant effect was recorded on the POD of tomato fruits from M-MPs treated plants. Peroxidase activity is closely related to the fruit’s quality and resistance to stress and pathogens since it is associated with physiological and biochemical processes in plants (Fang & Kao, 2000). POD also possesses reactive oxygen species (ROS)-scavenging abilities that protect the cells against oxidative stress (Ofoe et al., 2022). These have been reported to be connected to the prevention of cancers, atherosclerosis and inflammatory diseases (Chaudhary et al., 2018; Nowak et al., 2018; Das, 2014; Ofoe et al., 2022). Other notable increments observed in M-MPs fruits include TDS and EC. While EC can be associated with fruit quality (flavour and firmness) and sugar content (Banti, 2020), it could also be linked to worsening biochemical compounds (do Carmo et al., 2024), calcium deficiency, causing blossom end rot on fruits (Banti, 2020). This was experienced during this study though no data was collected on this deficiency. EC has also been connected to the interference of some essential nutrient uptake (do Carmo et al., 2024).

While the number of fruits made up for the smaller size, both the farmers' and consumers’ satisfaction are only partially included in these results. This study anticipated an increase in the antioxidants in the microplastics-spiked fruits since there were negligible effects on the plant's physiological and overall growth. This inconsistency could be linked to the expression of stress since the plants store up these phytochemical compounds as an adaptation strategy to stress. An increase in TDS, sugar, and proteins in tomato fruits has been reported to be associated with tomato plant response to salinity stress (Athar et al., 2022; Yin et al., 2010). Consequently, it is plausible that although M-MPs-spiked soil did not significantly alter the overall physiology and morphology of the tomato plants, it instigated the plants to store up these biochemicals in the fruits.

The nutritional composition of fruits is a key factor in determining the quality of fruits, and the elemental content of the M-MPs-spiked tomato fruits aligns with the phytochemical contents which make up a small percentage of a fruit's dry matter, yet they are essential for enhancing the quality and nutritional value of vegetables. Despite their minor presence in the overall composition, these minerals play a crucial role in contributing to the health benefits and flavour profiles that vegetables offer (Abou Chehade et al., 2018; Ofoe et al., 2022). Some potential reasons might include (1) soil pH status and deficiency in Cu, Mn, and Zn; (2) antagonistic interactions as a result of high levels of P availability; (3) contaminations of heavy metals such as lead (Pb) or cadmium (Cd) in the soil could inhibit the uptake of essential metals; (4) organic compounds in polymers may interfere with the availability of these micronutrients (undetermined). In a recent study, 0.2% of PP-MPs caused an increase in Fe levels while PVC-MPs decreased the Fe content in roots. PE-MPs also increased the Ca levels (Colzi et al., 2022). PP-MPs and PE-MPs also increased Cu, Mn, and Zn levels in the field pumpkin (Cucurbita pepo) roots contrary to this study where the M-MPs decreased Cu, Mn, and Zn in tomato fruits (Colzi et al., 2022). Though it was anticipated that the presence of MPs would increase the Cu availability as reported in Pinto-Poblete et al. (2023), the combination of PS-MPs and PP-MPs to the treatment could be a plausible explanation for the contradictory effect that was observed.

5. Conclusions

The effect of combined PE, PS, and PP MPs on the growth, yield, and biochemical properties of tomato plants were evaluated in the present study. These MP types are prevalent in compost which is commonly used as a soil enhancer. Although the mixture of these MPs did not significantly affect the physio-morphological traits and yield parameters of the tomato ‘Scotia’ plants, some outstanding changes were observed in the biochemical activities and tomato fruit quality. Specifically, the M-MPs treatment led to an increase in the total ascorbate, total protein content, peroxidase activity and total soluble solids indicating an adaptive stress response in the tomato plants. However, a noticeable reduction was observed in the carotenoid and flavonoid content of the tomato fruits. This raised concerns about the potential impact of microplastics on the nutritional quality and antioxidant capacity of fruits. In summary, these findings suggest that while microplastics in soil may not drastically harm the growth or yield of tomatoes, their presence could alter fruit quality by reducing the key antioxidants. It is recommended that future studies should focus on the soil with long-term exposure to microplastics, varying concentrations of the different microplastic types, smaller MP sizes (< 1 mm), and the response of different plant species be assessed. Furthermore, as MPs continue to accumulate in the soil, their potential effects on plant health and food quality must be prioritized to mitigate the possible risks associated with food and nutrition security..