Classification, uptake, translocation, and detection methods of nanoparticles in crop plants: a review

Abstract

Nanotechnology offers a viable solution to enhancing agricultural sustainability by supporting seed germination and crop growth. Besides augmenting crop yields and mitigating the effects of abiotic stresses on crops, nanoparticles (NPs) can also serve as carriers to amplify the effectiveness of fertilizers and pesticides by facilitating their gradual or highly targeted release. However, application of NPs in agricultural systems can result in interaction with crops and pave the way for transfer along the food chain, which raises concerns for consumers. To address these matters, this article provides a comprehensive review of the tracking of various NPs delivered to crops through different uptake pathways. Three types of NPs (including inorganic, carbon-based, and organic NPs) are highlighted for uptake, transport, and their impact on crop growth in agricultural crops. NPs can enter the plant via seeds, roots, and leaves, while the translocation of NPs mainly involves the apoplastic and symplastic pathways. The influence of NPs on plant growth highly depends on the specific plant species as well as the type, size, charge, and concentration of NPs. NPs emit fluorescence through inherent photoluminescence or in combination with fluorescent dyes, which enables monitoring of their distribution within the crop. The benefits and challenges of NPs are discussed, establishing signposts for future research that will enable NPs to contribute to precise and sustainable agriculture.

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He-Yi Zhang

He-Yi Zhang received his Bachelor's degree in Agricultural Engineering from China Agricultural University. He is currently pursuing a Master's degree at China Agricultural University under the supervision of Assoc. Prof. Wen-Hao Su. His research interests mainly involve visualizing the traces of nanoparticles in plants to develop crop identification methods and intelligent robots for weed control.

Wen-Hao Su

Wen-Hao Su is an associate professor in agricultural intelligent equipment engineering at the department of agricultural engineering, China Agricultural University. Prof. Su got his PhD degree in Biosystems and Food Engineering, from University College Dublin (UCD), National University of Ireland. He previously worked at University of Birmingham (UK), University of California-Davis (UCD), United States Department of Agriculture, and University of Minnesota, focusing on the development of advanced sensing and automation technologies for agricultural and biological systems, aiming to embed smart technologies into sustainable agricultural production and management.

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Environmental significance

Application of NPs in agricultural systems can result in interaction with crops and pave the way for transfer along the food chain, which raises concerns for consumers. It is necessary to fully characterize the potential risks of NPs to avoid negative impacts on the environment, plant health and human health. To address these matters, this article provides a comprehensive review of the tracking of various NPs delivered to crops through different uptake pathways. The benefits and challenges of NPs are discussed, establishing signposts for future research that will enable NPs to contribute to precise and sustainable agriculture.

1. Introduction

According to United Nations statistics, the current global population surpasses 8 billion, which is predicted to reach 9.7 billion by 2050.¹ Population growth has caused an increase in the demand for food.² Moreover, the reduction of arable land and the impact of climate change pose a serious threat to sustainable agricultural production. Nanoparticles (NPs), particles ranging in size from 1 to 100 nm in one or more dimensions, are considered to play a crucial part in solving these challenges in the future.¹ Compared to bulks of NPs and molecular counterparts, NPs exhibit unique physical and chemical properties that have recently gained interest and attention from researchers.² In recent decades, a considerable number of NPs have been synthesized and applied in cultivation, including promoting seed germination,^3,4 improving growth of the crops,^5,6 enhancing the availability of fertilizers and pesticides^7,8 and mitigating abiotic stresses in crops.^9,10 These studies have demonstrated the various uses of NPs in agriculture, which have gradually increased the significant scientific interest in crop science.¹¹

With the introduction of nanotechnology into modern agriculture, traditional agricultural strategies are shifting towards precision farming to ensure cost-effective and sustainable forms of food security.¹² The classification of NPs is a prerequisite for studying the effect of NPs on crops as the same types of NPs have similar properties. Usually, researchers divide NPs into organic, inorganic, and carbon-based NPs.^1,13 When the plant is exposed to the NPs, it will absorb the NPs through seeds, roots, and leaves as well as translocate them to other parts internally via apoplastic and symplastic pathways. Although applying NPs could increase crop yield, the accumulation of NPs in plants may adversely affect crops and may be transferred along the food chain, therefore it is necessary to understand whether NPs pose a risk to humans and the environment.¹⁴ Research focusing on the traces and effects of NPs in crops is still in the developmental stage. The application of NPs requires a thorough understanding of the mechanisms by which NPs are taken up and transported. However, tracking the different NPs entering the crop is difficult and requires proper monitoring strategies. The method of detecting NPs varies depending on the type of NPs. Fluorescence provides a powerful tool for tracking and imaging NPs in plants. In addition to relying on their own photoluminescence to generate fluorescence, NPs can also bind to functionalized probes.¹⁵ NPs with photoluminescence can be referred to as luminescent NPs (LNPs), including mainly semiconductor quantum dots, carbon dots, single-walled carbon nanotubes, graphene quantum dots and nanodiamonds.¹⁶ Apart from tracing their location in the planta using a range of analytical and imaging techniques, it is possible to track whether they escape from the plant into the soil or even to quantify them.¹⁷ A detailed understanding of the transport pathways of NPs will help researchers to better explore their mode of action and assess the possible hazards of NPs.¹⁸

This paper provides a comprehensive review of the tracking and influence of different NPs delivery to crops based on different uptake pathways. There are many previous review articles related to this topic. For example, Su et al. review critical plant morphological and physiological indices for NP transport, and examine the efficacy of various delivery methods for NPs (foliar application, root application, and feeding/injecting directly into plant tissue) with an emphasis on NP transport efficiency throughout the entire plant.¹⁹ Gao et al. described uptake and translocation of NPs in plants, and summarized impacts of NPs on plants.²⁰ Wang et al. focused on the effect of size, surface charge and chemical composition of NP on the absorption and transportation in leaf and root through different ways.²¹ Shrivastava et al. highlighted existing as well as promising developments in the area of ENPs detection and quantifications in soil–plant system.²² Unlike the aforementioned literature, the uptake and translocation of three types of NPs (inorganic, carbon-based, and organic NPs) in plants are highlighted in this article. The mechanisms and differences in uptake (seeds, roots, and leaves) and transport pathways (apoplastic and symplastic pathways) are analysed and compared to understand how NPs enter the plant and are transported to other regions. The effect of NPs on plants depends upon the plant species and the physicochemical properties of NPs (such as type, size, shape, and charge of NPs). Methods of NPs detection after uptake and translocation (direct observation, analyses of elemental content, and novel technologies) are summarized. The photoluminescence of NPs and the fluorescence produced by NPs in combination with fluorescent dyes provide an effective way to traces of NPs in crop plants. This paper compiles the latest literature in the field of crop NPs technology and identifies the gaps in current research and future directions. The content of this article is expected to guide the application of NPs in agricultural production.

2. Methods: literature search and analysis

The literature search was conducted in three electronic databases, which are Web of Science, Ovid, and ProQuest, on 9 October 2023. The databases selected covered literature from a wide range of fields including agricultural sciences, environmental sciences, biology, and chemistry to meet the needs of the review. Four search terms, nanoparticle, plant, uptake, and translocation, were used to search all fields to prevent missing relevant literature. Each search term is linked by the Boolean operator ‘AND’. The search period was restricted to 2011–2023. After importing the retrieved literature into Endnote X9 and removing duplicates, the screening and eligibility stages were carried out by checking the title, abstract and full text to select literature that matched the topic of the paper. Articles were selected based on the following criteria: (1) the article was experimental and published in a peer-reviewed journal; (2) the article was written in English; (3) the article examined traces of NPs; (4) the research was conducted on plants that were crops; and (5) NPs were not used as tracers. After careful review, a database of papers on the study of nanoparticle uptake and transport in crops was created. In addition, citations contained in the references of the retrieved literature that were considered relevant were also included in the database. Ultimately, a total of 84 papers are available in the database which can be viewed in the supplementary information. The search process was displayed as “Preferred Reporting Items for Systematic Reviews and Meta-Analyses” (PRISMA 2020) flow diagram, as shown in Fig. 1.²³ The researchers scrutinized the collected literature and extracted relevant information to gain insight into the multidisciplinary knowledge synthesis of NP traces in agricultural crops.

Fig. 1 Preferred reporting items for systematic reviews and meta-analyses flow chart (PRISMA 2020)²³ of this review.

3. Classification of NPs

NPs are classified into three types based on their chemical composition: inorganic, carbon-based, organic NPs. Inorganic NPs include metal-based, ceramic, and semiconductor NPs. Pure metal NPs using gold (Au), silver (Ag), copper (Cu) and iron (Fe) as matrix are often referred to as the first-generation nanomaterials because they are passive nanostructures that can only be applied to project-specific materials after synthesis.²⁴ Pure metal NPs are the foremost nanomaterials due to their early discovery and versatility, where different synthesis methods have been developed, including chemical, physical, and biological, such as electrodeposition process, laser ablation, and preparation with bacteria and fungus.^25,26 The substrates of metal oxide NPs mainly include copper oxide (CuO), zinc oxide (ZnO), and iron oxides (FeO, Fe₂O₃, Fe₃O₄). In contrast to micron-sized and bulk semiconductor materials, semiconductor NPs offer a wealth of quantum phenomena, unique size-dependent material properties and a widely tunable energy bandgap.²⁷ Quantum dots made from silicon (Si), zinc sulphide (ZnS), cadmium sulphide (CdS) and cadmium selenide (CdSe) are one type of semiconductor NPs that have size-dependent photoluminescence properties. Even identical materials can emit different colors of light at different sizes, making them useful for analysis and imaging in biology and chemistry.^16,28 Ceramic NPs, as their name implies, consist mainly of metal oxides such as aluminum (Al) and titanium (Ti), carbides and silica.²⁹ Due to the numerous promising characteristics of ceramic NPs, such as high heat resistance and chemical inertness, they have been used as pesticide delivery agents in plants.³⁰ Carbon-based NPs are engineered NPs that can be particles, aggregates, or agglomerates.³¹ Carbon-based NPs include but are not limited to carbon dots, carbon nanotubes, fullerenes, diamonds, graphene, carbon nanofibers.³² As scientists continue to research the synthesis of novel NPs, the variety of carbon-based NPs is ever-growing.³¹ Carbon-based NPs applied to plants are absorbed and translocated to different parts of their tissues. Since a fraction of the absorbed carbon-based NPs may promote physiological changes in plants, more is focusing on their application to plants. Organic NPs are made of proteins, lipids, polymers, or other organic compounds.³³ The organic NPs applied in the study are mainly composed of polymers, hence the organic NPs of interest in this paper can also be referred to as polymeric NPs. These NPs are colloidal structures consisting of active compounds entrapped within or surface-adsorbed onto polymeric core, which can be further classified into either nanocapsules or nanospheres.³⁴ Given the biocompatibility, controlled release features and non-toxicity of organic NPs, they are prospective delivery systems for pesticides in plants.³⁵ In addition to this, NPs can be categorized into water-soluble and water-insoluble NPs based on solubility in water. Water-soluble NPs exist in particle and ion states in aqueous solutions, suggesting that the effects of NPs on plants may result from particles or ions, or a combination of both. Copper ions, an essential micronutrient, promote mung bean growth at low concentrations. In a previous study, water-insoluble copper nanoparticles exhibited toxicity to mung bean seedlings.³⁶ In another study, zinc ions in aqueous solutions of zinc oxide NPs inhibited root elongation in lettuce seedlings, whereas water-insoluble dispersions of SiO₂ and TiO₂ NPs had no effect on root elongation.³⁷ This demonstrates the difference in the mechanisms by which the two affect plants.

4. Pathways of NPs uptake and translocation in plants

In recent years, nano-fertilizers, nano-herbicides, and nano-pesticides have been developed to help improve crop growth and increase the yield of agricultural products.³⁸ These nanoscale products can be applied to the aerial parts or the roots of the plant, which then interact with the plant (absorbed and transported to all parts of the plant).³⁹ The main methods of application are foliar spraying, hydroponic treatment, biolistic gun, injection and seed incubation with NPs.⁴⁰ According to the principles of these methods, the uptake of NPs by plants is summarized as foliar uptake, root uptake and seed uptake, as shown in Fig. 2. Regardless of the method used, the applied NPs will first form deposits on or adhere to the surface of application site before being taken up by the plant.¹⁹ NPs used for foliar and root treatments can promote crop growth by facilitating nutrient uptake. Foliar treatments increase the efficiency of insecticides, while root treatments increase the plant's resistance to environmental stresses. Unlike foliar and root treatment, at the seed stage, NPs have been synthesized for use as nano-priming agent for enhancing seed germination and plant growth. Table 1 summarizes the advantages and disadvantages of the three treatments for NPs uptake by crops to help in choosing the appropriate treatment method during agricultural production. The pathways for uptake and translocation of NPs in plants are described in detail below.

Fig. 2 Different application modes and translocation pathways of NPs in plant. (A) Leaf structure and passway of NPs in leaf. (B) Root structure and passway of NPs in root. (C) Transportation of NPs in vasculature.

Table 1 Advantages and disadvantages of foliar, root and seed treatments

Treatment	Advantages	Disadvantages
Foliar	Rapid uptake by plants	Potential phytotoxicity
	Targeted delivery of nutrients or pesticides to specific plant parts	Limited systemic effects as NPs may not distribute evenly throughout the plant
	Enhanced effectiveness of pesticides through improved adhesion and penetration	Regulatory concerns regarding toxicity and environmental impact
Root	Enhanced nutrient uptake by plant roots.	Limited root absorption for some nanoparticles
	Reduced nutrient leaching and improved fertilizer efficiency	Potential interactions with soil components, affecting mobility and bioavailability
	Increased plant resilience to environmental stresses	Uncertain regulatory status and compliance requirements
	Contributes to sustainable agriculture by reducing fertilizer usage	Long-term effects on soil health and ecosystem functioning require further study
Seed	Improved seed germination rates and seedling vigor	Potential seed damage or inhibition of germination with improper formulation
	Protection against seedborne and soilborne pathogens	Concerns about nanoparticle residues accumulating in soil and water ecosystems
	Reduction in the need for chemical pesticides and fertilizers	Regulatory requirements for safety assessments and compliance
		Cost-effectiveness compared to conventional seed priming

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4.1. Foliar treatment

To better understand how NPs are absorbed by plants through the leaves, it is necessary to understand the leaf structure of plants first. Although different plant species have different leaf structures, the leaf surface usually has stomata and trichome. The leaf surface is mostly covered by a lipophilic cuticle, as shown in Fig. 2(A).⁴¹ The pathways by which foliar applied NPs enter the leaf are unknown, but the hydrophilic pathway through the leaf surface polar apertures (stomata and trichomes) and the hydrophobic pathway through the cuticle are two potential modes of penetration.⁴² Eichert et al. studied the penetration of fluorescent polystyrene NPs into leaves of Vicia faba L.⁴³ They found that the NPs were absorbed by the leaves as well as fluorescent signals were detected in the stomata, which indicated that the NPs entered the leaves through the stomata. Larue et al. investigated the internalization of silver NPs after foliar exposure to silver NPs in lettuce.⁴⁴ The results showed that silver NPs were observed between the guard cells and in the sub-stomatal chambers, demonstrating that silver NPs enter the plant through the stomata. Carbon dioxide concentration, light intensity, temperature, and water pressure in plant influence the opening and closing of stomata to regulate the gas and water exchange between the plant and the environment.⁴⁵ Theoretically, the average size of stomata is 10 μm, which explains why NPs can be absorbed by plants through stomata. Most of the stomata are located on the abaxial leaf surface, covering only 5% of the leaf area.⁴⁶ The small area, the location of stomata and the opening and closing of stomata with the environment reduce the uptake efficiency of this pathway. Trichomes, known as leaf hairs, play an important role in performing various protective functions and exchanging gases and fluids at the leaf-atmosphere interface.⁴⁷ Valletta et al. found that fluorescently labeled polymer NPs had been internalized into non-glandular trichomes, when grape leaves were immersed in NP suspension.⁴⁸ It is still not clear whether the trichomes are part of the NP uptake routes or they only accumulate NPs.

The other absorption pathway is through the cuticle covering the leaf surface. This pathway is also considered to be an important pathway as it has a larger area compared to the stomata. The cuticle has very small channels, which limits the size of NPs passively absorbed into the plant to <5 nm.⁴² Foliar application of ZnO NPs with different coatings to wheat and sunflower, respectively, was carried out by Read et al.⁴⁹ The results showed that the cuticle of the plant leaves was the main route of uptake. However, characterization of the NPs used in this study revealed particle sizes of 40–50 nm, all of which were larger than 5 nm. The larger NPs were able to be absorbed by the plants through the cuticle probably due to the surfactant effect of the NP coating. The cuticle is a waxy porous layer while the surfactant can dissolve the epidermal wax, thus promoting the uptake of NPs by the leaves.^50,51 The reason why such NPs, which are larger than the cuticle pores, are absorbed by plants through the cuticle has not been well explained, so more intensive studies on the role of the foliar cuticle in NP absorption are needed.

NPs entering the leaf via the two methods mentioned above transport to vasculature through several barrier (epidermis, palisade mesophyll, and spongy mesophyll) after passing through the cuticle. The NPs are transported to the bundle sheath cells that are connected to companion cells by either the apoplastic or symplastic pathway, and further to sieve-tube elements (STE) of the phloem. The apoplastic pathway is the passage through the cell wall pore followed by diffusion into the space between the cell wall and the plasma membrane or through the intercellular space without crossing the cell membrane.⁵² The symplastic pathway is a mode of transport from the interior of one cell to the interior of another through plasmodesmata.⁵³ Currently, there is no consensus as to which is the major route of NPs transport.⁵⁴ NPs reaching bundle sheath cells transport to vasculature through plasmodesmata. Then, NPs can be transported through the phloem to the roots, internalized with water and nutrients, and then transferred through the xylem to the aerial parts.⁵⁵ The vasculature, consisting of xylem (apoplast) and phloem (symplast), determines the extent of NP translocation, making it essential to understand about the vasculature.^46,56 The vasculature is the tube-like structure that connects the plant roots to the leaves, as shown in Fig. 2(C). The xylem channels allow the upward flow of water absorbed by the roots, while the phloem permits the transport of photosynthetic products from their source (photosynthetic organs producing sugars and amino acids) to their sink (developing tissues).⁵⁴ The sieve tubes of the phloem have a higher local concentration of photosynthates compared to the surrounding xylem cells, leading to an osmotic exchange between xylem and phloem.⁵⁷ The transport of NPs takes place during the osmotic exchange. A more detailed understanding of the transport after foliar uptake is essential to improve the efficiency of NP transport.

4.2. Root treatment

When NP suspensions are used for hydroponics or solutions containing NPs are used for irrigation of plants in soil, the plants absorb the NPs through their roots. The applied NPs first aggregate in the roots of the plant, as shown in Fig. 2(B). The amount of aggregation is related to the surface charge of the NPs. Normally, the root cap of a plant is protected by a layer of border cell mucilage consisting of negatively charged root secretions, so that positively charged NPs are more likely to aggregate in the root or root cap and in the mucilage layer.⁵⁸ NPs accumulated in the roots must penetrate the cell wall and the plasma membrane of the epidermal layer to enter the roots.⁵⁹ Although the root surface has a cuticle similar in composition and function to the leaf surface, the root surface is not completely covered by the cuticle. Consequently, NPs can be directly exposed to these areas so as to contact the epidermal layer.⁵⁶ The cell walls of the epidermal layer of plant roots have a sophisticated apparatus that reduces the entry of unwanted or plant-harmful substances, while allowing the passage of essential nutrients and water.⁶⁰ Due to these special effects of cell walls, they have very small size exclusion limits. Studies have shown that the pore size of plant cell walls is typically 3–8 nm, which is smaller than many NPs.⁶¹ However, more recent studies have demonstrated that the particle size of NPs is not a decisive factor in their uptake by plant roots. Taylor et al. used gold NPs to study the uptake and translocation of NPs in Arabidopsis thaliana.⁶² Although the NPs they used were larger than the pore size of the cell wall, they found gold NPs in the root cytoplasm and shoots, demonstrating that the roots took up the NPs and internalized and translocated the NPs to the shoots. One possibility is that the NPs may disrupt or induce the cell wall to form large size pores, thus allowing the larger NPs to penetrate.⁶³ It has recently been shown that when plant roots receive stress, pectin is often reduced by stress-induced reactive oxygen species (ROS) and peroxidases. If ROS-levels remain high OH˙-radicals are formed which leads to cell wall loosening and larger cell wall pores.⁶⁴ There are two basic translocation pathways for NPs after uptake by roots, the apoplastic and symplastic pathways, which are similar for foliar application. Several hypotheses have been proposed for the transport of NPs across the membrane via aquaporins, interconnected ion channels and endocytosis. Ion channels are only available for specific ions, while aquaporins are usually less than 1 nm in diameter, which is theoretically unexplained.⁶⁵ Hence, the most likely transmembrane transport pathway is endocytosis. However, little is known about the theory of endocytosis in plant cells. The transport of NPs from one cell to another cell relies on plasmodesmata. Although cells are separated by cell walls, plasmodesmata can connect neighboring cells for intercellular communication. In the current study, only gold and silver NPs were found in the plasmodesmata.^52,66 As a result, there is still insufficient evidence for the intracellular transport of NPs via plasmodesmata. More evidence that NPs can be transported across plasmodesmata should be provided in the future. NPs pass through the epidermis and travel through the cortex and endodermis via the above-mentioned pathways, where they are eventually blocked by the casparian strip. The casparian strip has been shown to isolate the polar domains and prevent lateral diffusion.⁶⁷ However, it has been demonstrated that NPs can enter the xylem through root tips that have not formed a casparian strip or lateral root junctions where the casparian strip is disconnected.^68,69 After crossing the casparian strip, NPs eventually enter the vascular tissue (xylem), which are then transported with other materials upwards to the stem and leaves in the aerial part of the plant.⁵⁹

4.3. Seed treatment

Seeds can also be used as a medium for NPs uptake by plants. As seed treatment is very convenient, it has been used in many studies.⁷⁰ Specifically, this method uses NP suspensions that soak the seeds to germinate them or uses NPs as a coating for the seeds. In the former case, the seeds are soaked in the solution, while in the latter case they are dried after immersion to form the corresponding coating. Most of the NPs remain on the seed coat, which may or may not be absorbed by the seed.⁷¹ The absorbed NPs penetrate through the seed coat and enter the seed interior.⁷² Khodakovskaya et al. used carbon nanotubes to study seed germination and plant growth.⁷³ They found that carbon nanotubes could penetrate the seed coat of plants. Duran et al. observed that NPs absorbed by seeds can be found in the primary root.⁷⁴ It is not clearly understood whether the NPs are taken up by the seed prior to root growth or by the roots during root growth. Currently, seed treatments with NPs are mainly used for seed priming or seed germination enhancement. Xin et al. treated maize with polysuccinimide polymeric NPs which improved the germination of maize seeds.⁷⁵ They suggested that NPs may promote seed germination by penetrating the seed coat to create new water permeable pores, thereby promoting water uptake by the seeds. Moreover, it is not only the transportation of NPs that promotes seed germination and crop growth. Mahakham et al. observed more ROS production as well as up-regulation of aquaporin genes in germinating seeds of nanopriming treatment.⁷⁶ This suggests that both ROS and aquaporins play important roles in promoting seed germination. Rai-Kalal et al. found that ZnO nanopriming significantly increased photosynthetic pigment content in wheat plants. This contributed to improved overall primary photochemistry in NP plants, which directly correlated with improved biomass accumulation.⁷⁷ More studies are required to explore the movement of NPs after their penetration into the seeds.

5. Traces of NPs in crops and their effects on crop growth

Plant uptake is considered an important transport and exposure pathway for NPs.⁷⁸ Plant uptake of the element above a certain dose may cause toxicity.^79,80 The uptake, translocation, and accumulation effects of NPs in plants vary considerably due to differences between plants (species and age) and NP characteristics (species, size, and applied concentration).⁸¹ It is necessary to fully characterize the potential risks of NPs to avoid negative impacts on the environment, plant health and human health. Fig. 3 shows the traces of different NPs in the crop.

Fig. 3 Traces of NPs in crops. (A) Transmission electron microscopy of cross sections of roots containing FeNPs (yellow arrows indicate FeNPs).⁸² (B) Scanning electron microscope (SEM) images of leaf surfaces (bright white patches represent adherent graphene quantum dots modified with positively charged amino functional groups).⁸³ (C) Confocal images of stems and roots of lettuce after carbon dots (CDs) treatment.⁸⁴ (D) Fluorescence microscopy images of CDs in maize leaves (arrows show CDs).⁸⁵

Tables 2–4 show the uptake, transport, and impact of crops on inorganic, carbon-based, and organic NPs, respectively. Plants can take up NPs and transport them systemically through root, seed, and foliar treatments. Internalization of NPs by plants may cause phytotoxicity. Understanding the uptake and transport of NPs is therefore fundamental to studying their toxic effects on plants. It has been reported that AgNPs were able to translocate from roots to shoots of soybean and wheat.^86,87 Afzal et al. studied the transport of foliar absorbed FeO NPs in rice (Oryza sativa L.) which revealed that FeO NPs were absorbed by rice leaves and biodistributed throughout the plant by the vasculature.⁸⁸ Hala et al. proposed TiO₂ NPs as a medium for seed-priming.⁸⁹ They studied the translocation of NPs in pomegranate seeds observing that TiO₂ NPs could translocate cross the seed coat into the seed embryo. Recently, studies have focused on the use of mesoporous silica NPs as delivery systems for pesticides and fungicides in plants. Pan et al. found that bimodal mesoporous silica NPs can transport pesticide molecules for delivery to plants with excellent biological safety.⁹⁰ Kou et al. treated tomato and lettuce seeds using CDs, showing that CDs were able to translocate to roots and stems.⁸⁴ Ristroph et al. found that ZNPs could be taken up by soybean roots and biodistributed in the plant.⁹¹ Factors that influence absorption and transport are plant species, NP type, NP size, NP charge, and processing method. Malejko et al. found that radish, carrot, and lettuce all absorbed AuNPs, but the uptake was ranked from highest to lowest: radish, carrot, lettuce.⁹² Likewise, the translocation in each crop was different. This suggested that plant species affects nanoparticle uptake and transport. Yilmaz et al. found greater deposition of AgNPs with smaller sizes in the shoots and roots of maize seedlings, demonstrating the impact of nanoparticle size on uptake and transport.⁹³ Similarly, Xu et al. found that the smaller the size of the MSN, the easier it was to be absorbed and transmitted by the cucumber roots.⁹⁴ Surface modification can change the charge of the NPs, thereby affecting the uptake of the NPs by the crop. Milewska-Hendel et al. found that AuNPs could not enter Barley's roots, which may be due to their modification of the AuNPs surface using polyethylene glycol.⁹⁵ Zhu et al. found that positively charged NPs were more evenly distributed within wheat leaves and that smaller size NPs were more likely to enter the crop.⁹⁶ Afzal et al. found that foliar treatment enabled rice to absorb more NPs than root treatment.⁹⁷

Table 2 Summary of inorganic NPs uptake, translocation, and effects in crop plants

NPs	Size	Morphology	Plant species	Exposure method	Concentration	Treatment medium	Uptake and translocation	Detection technique	Effects	Ref.
“—” indicates that the information is not available. Abbreviations: R—reference; AgNP–PVP—silver nanoparticles coated with polyvinylpyrrolidone; AgNP–CTAB—silver nanoparticles coated with cetyltrimethylammonium bromide; AgNPs—silver nanoparticles; MS—Murashige and Skoog; ICP-MS—inductively coupled plasma mass spectrometry; SERS—surface-enhanced Raman spectroscopy; AAS—atomic absorption spectrometer; AuNPs—gold nanoparticles; ATR-FTIR—attenuated total reflectance-Fourier transform infrared spectroscopy; CuO NPs—copper oxide nanoparticles; ICP-OES—inductively coupled plasma optical emission spectroscopy; FeNPs—iron nanoparticles; SEM—scanning electron microscope; EDS—energy dispersive X-ray spectroscopy; FeO NPs—ferrous oxide nanoparticles; FM—fluorescence microscopy; Fe2O3 NPs—hematite nanoparticles; VSM—vibrating sample magnetometer; Fe3O4 NPs—magnetite nanoparticles; TEM—transmission electron microscope; XRD—X-ray diffraction; CLSM—confocal laser scanning confocal microscopy; ZnO NPs—zinc oxide nanoparticles; n-ZnO—nanosized zinc oxide; ZnO NFs—ZnO nanoflowers; ZnO QDs—zinc oxide quantum dots; TiO2 NPs—titanium dioxide nanoparticles; XRF—X-ray fluorescence; BMMs—bimodal mesoporous silicas; FL-MSNs—fluorescent mesoporous silica nanoparticles; MSNs—mesoporous silica nanoparticles.
AgNPs	6 nm	Spherical	Annual ryegrass (Lolium perenne L.)	Root treatment	0, 1, 5, 10, 20, 40 mg L^−1	NP suspension	Internalized AgNPs	ICP-OES	Inhibited growth and caused cell damage at high doses (10, 20, 40 mg L^−1)	99
AgNPs	20–50 nm	Circular	Soybean (Glycine max)	Root treatment	5 mg L^−1	NP suspension	Transported from roots to shoots	SEM, EDS	Compromised the root integrity of soybeans	87
AgNPs	35 ± 15 nm	Round	Candy leaf (Stevia rebaudiana B.)	Root treatment	0, 12.5, 25, 50, 100, 200 mg L^−1	Gelling medium	Deposited between the intercellular spaces and reached leaves	FM	Stimulated growth at low dose while inhibited at high doses (50, 100, 200 mg L^−1)	102
AgNP–PVP	80 nm	—	Tobacco (Nicotiana tabacum L.)	Root treatment	25, 50, 100 μM	MS medium	Absorbed by roots and translocated to leaves	ICP-MS	Lead the induction of oxidative stress	98
AgNP–CTAB	40 nm	—	Tobacco (Nicotiana tabacum L.)	Root treatment	25, 50, 100 μM	MS medium	Absorbed by roots and translocated to leaves	ICP-MS	Lead the induction of oxidative stress	98
AgNPs	56 nm	—	Maize (Zea mays L.)	Root treatment	7.5 ppm	Hoagland solution	Accumulated in shoots	SERS	Inhibited crop growth	93
AgNPs	30 nm	—	Maize (Zea mays L.)	Root treatment	1 mg kg^−1	Soil medium	Taken up by roots	AAS	Found no significant effects	103
AuNPs	5 nm	—	Tobacco (Nicotiana tabacum L.)	Root treatment	100 mg L^−1	NP suspension	Presented throughout the plant tissue cross section	ICP-MS	—	104
AuNPs	2 nm	—	Rice (Oryza sativa L.)	Root treatment	0, 50 μM	NP suspension	Translocated into plant stems and leaves from roots	ICP-MS	—	105
AuNPs	5 nm	Spherical	Barley (Hordeum vulgare L.)	Root treatment	0, 50 μg ml^−1	Hoagland solution	No AuNPs entered the roots from the external solution	TEM, SEM	—	95
AuNPs	90.0 ± 4.5 nm	—	Carrot (Daucus carota), radish (Raphanus sativus L.), lettuce (Lactucca sativa L.)	Root treatment	0, 500 mg	Soil medium	Taken up by all plants, but varied among species	ICP-MS	—	92
AuNPs	20–30 nm	—	Broad bean (Vicia faba L.)	Root treatment	0, 20, 50, 100, 200 mg L^−1	NP suspension	Absorbed by roots and transported to the aboveground part	AAS	Had the toxicity performance	106
AuNPs	1–5 nm	Spherical	Parsley (Petroselinum crispum)	Root treatment	0, 1, 5, 10, 50, 100, 200 mg L^−1	Soil medium	Taken up by roots and translocated to leaves	ICP-MS	Increased the content of pigments at low concentrations (0, 1, 5, 10 mg L^−1)	107
CuO NPs	39 ± 3 nm	Spherical	Leaf mustard (Brassica juncea)	Root treatment	0, 200, 500, 1000, 1500 mg L^−1	Hoagland solution	Traveled from roots to shoot and up to the leaf	XRD	Accumulated reactive oxygen species at high doses (1000, 1500 mg L^−1)	108
CuO NPs	Around 46.1 nm	—	Rape (Brassica napus L.)	Root treatment	10, 50, 100 mg kg^−1	Soil medium	Translocated from roots to leaves	ICP-OES	Damaged roots	109
CuO NPs	30–50 nm	—	Barley (Hordeum sativum L.)	Root treatment	300, 2000, 10 000 mg kg^−1	Soil medium	Accumulated in roots and transferred to stems	AAS	Impeded the development and productivity	110
FeO2 NPs	20–30 nm	Spherical	Rue (Ruta graveolens)	Root treatment	50 μM	Hoagland solution	Accumulated in plant roots	AAS	Increased Fe bioavailability	111
Fe2O3 NPs	—	—	Rice (Oryza sativa L.)	Root treatment	50 mg L^−1	NP suspension	Accumulated in symplast and apoplast of roots	ICP-MS	Alleviated cadmium accumulation	112
Fe2O3 NPs	10 ± 3.2 nm	—	Rice (Oryza sativa L.)	Root treatment	0, 2, 20, 200 mg L^−1	NP suspension	Penetrated the cell membrane and translocated to the shoots	ICP-MS, TEM	Decreased abscisic acid and indole-3-acetic acid concentrations in the roots	113
Fe2O3 NPs	14 nm	Spherical	Barley (Hordeum vulgare L.)	Root treatment	50, 100, 200 mg L^−1	NP suspension	Absorbed by roots and reached leaves through stem	ICP-OES, VSM	Improved germination, plant growth, and biomass	114
Fe3O4 NPs	12 nm	Spherical	Barley (Hordeum vulgare L.)	Root treatment	50, 100, 200 mg L^−1	NP suspension	Absorbed by roots and reached leaves through stem	ICP-OES, VSM	Improved germination, plant growth, and biomass	114
Fe3O4 NPs	10 nm	—	Common bean (Phaseolus vulgaris L.)	Root treatment	0, 1000, 2000 mg L^−1	NP suspension	Absorbed by the roots, transported to stems and leaves	ICP-OES, VSM	Detected no toxicity visually	115
Fe3O4 NPs	17 nm	—	Hemp (Cannabis sativa L.)	Root treatment	50, 100, 200, 500 mg L^−1	NP suspension	Dispersed throughout root cells	ICP-MS, TEM	Promoting hemp growth	116
Fe3O4 NPs	1–4 nm	Polydisperse	Mung bean (Vigna radiata)	Root treatment	10, 100 mg L^−1	Hoagland solution	Absorbed by roots and translocated to shoots	ICP-MS	Had favorable effect on the photosynthetic parameters	117
Fe3O4 NPs	25 nm	Agglomerate	Barley (Hordeum vulgare L.)	Root treatment	1, 10, 20 mg L^−1	NP suspension	Penetrated roots and translocated to leaves	CLSM	Increased the level of genotoxicity	118
ZnO NPs	386 ± 18.3 nm	—	Wheat (Triticum aestivum L.)	Root treatment	100, 200, 400, 800 mg kg^−1	Soil medium	Transported to endodermis and vascular cylinder	CLSM	—	119
ZnO NPs	<40 nm	—	Radish (Raphanus sativus L.)	Root treatment	0, 0.1, 10, 100, 1000 mg L^−1	NP suspension	Absorbed by the seedlings	AAS	Shown toxic effects on root length, shoot length, and fresh weight	120
ZnO NPs	10–20 nm	Polydisperse	Mung bean (Vigna radiata)	Root treatment	10, 100 mg L^−1	NP suspension	Absorbed by roots and translocated to shoots	ICP-MS	Observed no apparent benefit or toxicity	117
ZnO NPs	70 nm	—	Red perilla (Perilla frutescens)	Root treatment	50, 100, 200 mg L^−1	Moss medium	Translocated to leaves	AAS	Increased plant biomass at low doses (50, 100 mg L^−1)	121
ZnO NPs	37 ± 2 nm	Face-centered cubic	Rice (Oryza sativa L.)	Root treatment	10, 50, 100, 500 mg L^−1	Hoagland solution	Biodistributed throughout the plant	AAS	Promoted plant protein and sugar content at low doses (10, 50 mg L^−1)	88
ZnO NPs	20–30 nm	Spherical	Rice (Oryza sativa L.)	Root treatment	10, 20, 50, 100 mg L^−1	NP suspension	Translocated from roots to shoots	ICP-MS	Decreased as accumulation	122
ZnO NPs	51 ± 18 nm	—	Pea (Pisum sativum L.)	Root treatment	100, 200 mg L^−1	Hoagland solution	Clustered in the roots with limited transit	AAS	Decreased photosynthetic pigments	123
TiO2 NPs	14 nm	—	Wheat (Triticum aestivum L.)	Root treatment	10 g L^−1	NP suspension	Accumulated in seedlings	XRF	Induced increased root elongation	124
TiO2 NPs	20–160 nm	—	Tomato (L. esculentum)	Root treatment	0, 20 μg mL^−1	Soil medium	Absorbed and accumulated in roots	ICP-OES	Decreased the leaf dry matter	125
TiO2 NPs	32–171 nm	Agglomerate	Wheat (Triticum aestivum L.)	Root treatment	0, 10, 100, 1000 mg L^−1	Hoagland solution	Translocated to shoots from roots	ICP-MS	Increased abscisic acid content	126
TiO2 NPs	30 nm	—	Radish (Raphanus sativus L.)	Root treatment	0, 5 mg L^−1	NP suspension	Translocated up to aboveground organs	ICP-MS	—	127
TiO2 NPs	35.7 ± 1.2 nm	Spherical	Wheat (Triticum aestivum L.)	Root treatment	0, 5, 10, 30 mg L^−1	NP suspension	Taken up by roots	ICP-MS	Shifted the trend of ionic flux	128
TiO2 NPs	—	—	Spinach (Spinacia Oleracea L.)	Root treatment	—	Soil medium	Translocated from roots to shoots	SEM, EDS	Enhanced photosystem II	129
TiO2 NPs	23.0 ± 1.6 nm	Spherical	Fenugreek (Trigonella foenum-graecum L.)	Root treatment	0, 50, 100 mg L^−1	Hoagland solution	Transported to stem and leaves	ICP-OES	Damaged root cellular membranes	130
SiO2 NPs	—	—	Rice (Oryza sativa L.)	Root treatment	0, 50 μg ml^−1	NP suspension	Observed an upward translocation to the shoot system	FM	Detected no negative effects on germination	131
SiO2 NPs	30 ± 10 nm	—	Cotton (Gossypium hirsutum L.)	Root treatment	0, 10, 100, 500, 2000 mg L^−1	NP suspension	Transported from roots to shoots via xylem sap	ICP-OES, TEM	Decreased plant height and biomass	132
SiO2 NPs	13.3 ± 0.1 nm	Spherical	Wheat (Triticum aestivum L.)	Root treatment	0, 10, 50, 100 mg L^−1	NP suspension	Taken up by roots	FM	Caused root damage	133
MSNs	20 nm	Spherical	Lupin (Lupinus Linn)	Root treatment	0, 0.1 mg mL^−1	MS medium	Taken up by roots and translocated in the vascular system	CLSM	Had no effect on seed germination and radicle length	134
MSNs	20 nm	Spherical	Wheat (Triticum aestivum L.)	Root treatment	0, 0.2 mg mL^−1	NP suspension	Penetrated the root cell wall, entered to the vascular tissue, and transported to the aerial parts	CLSM, TEM	Observed no negative effects on seed germination	135
MSNs	15 nm	Spherical	Cucumber (cucumis sativus L.)	Root treatment	0, 0.3 mg mL^−1	NP suspension	Entered plant and translocated upward to other parts	CLSM	—	94
FeNPs	10–80 nm	Chain, spherical, semi-spherical or ellipsodial	Spring wheat (Longchun 29)	Seed treatment	5, 10, 15 mg L^−1	NP suspension	Absorbed by seeds	TEM, EDS	Enhanced growth performance	82
CuO NPs	79.9 nm	—	Soybean (Glycine max L.)	Root and foliar treatment	75, 300 mg kg^−1	Soil medium	Absorbed through the medium, but limited transit	ICP-OES	Found plant growth-promoting effect	136
BMMs	20–50 nm	Spherical	Tomato (Solanum lycopersicum L.)	Root and foliar treatment	0, 5 mg mL^−1	NP suspension	Absorbed and translocated in plants	FM	—	90
AgNPs	15–100 nm	Spherical	Wheat (Triticum aestivum L.)	Seed treatment	0, 8 mg L^−1	NP suspension	Accumulated in roots and translocated to aerial parts	AAS	Reduced root growth significantly	86
AuNPs	10 nm	—	Chickpea (Cicer arietinum L.)	Seed treatment	0, 2.15 mg mL^−1	NP suspension	Transferred from root system to upper part	ATR-FTIR	—	137
CuO NPs	<50 nm	Spherical	Buckwheat (F. esculentum)	Seed treatment	0, 50, 500, 2000, 4000 mg L^−1	NP suspension	Entered the endodermis of buckwheat root cells	TEM	Inhibited root growth at high doses (2000, 4000 mg L^−1)	138
CuO NPs	42.80 nm	—	Cucumber (Cucumis sativus L.)	Seed treatment	50, 100, 200 mg kg^−1	Soil medium	Translocated to shoots and leaves	ICP-OES	Stimulated cucumber growth	139
Fe2O3 NPs	—	—	Chickpea (Cicer arietinum L.)	Seed treatment	0, 0.022, 1.1 g L^−1	NP suspension	Transported through the shoot and localized in the leaves	ATR-FTIR, ICP-OES	Increased plant growth and fruit production	140
Fe3O4 NPs	14 nm	—	Soybean (Glycine max L.)	Seed treatment	50, 100 mg L^−1	Hoagland solution	Accumulated in plant roots	VSM, XRD	Found positive effects on root hair formation	141
ZnO NPs	30 ± 10 nm	—	Rice (Oryza sativa L.)	Seed treatment	25, 50, 100 mg L^−1	NP suspension	Transferred from seeds to seedling	TEM, EDS	Had no effects on seed germination	142
ZnO NPs	15–40 nm	—	Wheat (Triticum aestivum L.)	Seed treatment	50, 100, 150, 200 mg L^−1	NP suspension	Accumulated in roots and shoots	AAS	Decreased α-amylase activity at high doses (150, 200 mg L^−1)	143
TiO2 NPs	45.14 nm	Irregular	Pomegranate (Punica granatum L.)	Seed treatment	0, 40 mg L^−1	NP suspension	144	TEM	—	89
SiO2 NPs	17.23 nm	AgglomerateIrregular	Arugula (Eruca sativa)	Seed treatment	0, 100, 250, 500, 1000 μg L^−1	NP suspension	Found in root, shoot and leaf tissues	EDS	Improved plant biometrics and physiology
CuO NPs	18.51 ± 1.56 nm	—	Lettuce (Lactuca sativa L.)	Foliar treatment	0, 1 mg L^−1	NP suspension	Entered leave via the stomata	ICP-MS	—	145
CuO NPs	40–200 nm	—	Lettuce (Lactuca sativa L.)	Foliar treatment	100, 1000 mg L^−1	NP suspension	Absorbed by leaves and translocated to roots	AAS	Triggered oxidative burst	146
FeO NPs	27 ± 2 nm	Face-centered cubic	Rice (Oryza sativa L.)	Foliar treatment	10, 50, 100, 500 mg L^−1	NP suspension	Biodistributed throughout the plant	AAS, FM	Inhibited plant growth at high doses (100, 500 mg L^−1)	88
Fe2O3 NPs	34.48 nm	Spherical	Watermelon (Citrullus lanatus L.)	Foliar treatment	10, 50, 100, 500 ppm	Aerosol	Passed through the shoots and reached the roots	ICP-OES	—	147
Fe2O3 NPs	20.2 ± 11.3 nm	Spherical	Pomelo (Citrus maxima)	Foliar treatment	20, 50, 100 mg L^−1	NP suspension	Absorbed by leave, but no downward transported from shoots to roots	AAS	Observed no phytotoxicity at lower concentrations (20 and 50 mg L^−1)	148
n-ZnO	13.3 ± 0.3 nm	Spherical	Alfalfa (Medicago sativa L.)	Foliar treatment	30, 60, 90 mg L^−1	NP suspension	Found in vacuoles of the mesophyll cells	ICP-MS, TEM	Upregulated alfalfa growth under heat stress	149
ZnO NFs	18.1 ± 0.7 nm	—	Lettuce (Lactuca sativa L.)	Foliar treatment	—	NP suspension	Trapped on the leaves	FM	Showed higher photosynthetic parameters	150
ZnO NPs	40 nm	Spherical	Wheat (Triticum aestivum L.)	Foliar treatment	2 mmol L^−1	NP suspension	Distributed homogeneously on and within the leaf	CLSM	—	96
ZnO QDs	—	Spherical	Tomato (Solanum lycopersicum L.)	Foliar treatment	0, 1, 5, 15, 25, 50 mg L^−1	NP suspension	Translocation from shoots to roots via phloem	CLSM	Enhanced seedling fitness and leaded to higher biomass	151
ZnO QDs	3.76 nm	Mono-disperse	Lettuce (Lactuca sativa L.)	Foliar treatment	0, 50, 100, 200, 500 mg L^−1	NP suspension	Absorbed by root	CLSM	Improved biomass and nutritional quality at low doses (0, 50, 100, 200 mg L^−1)	152
FL-MSNs	—	—	Tomato (Solanum lycopersicum L.)	Foliar treatment	0, 22.5 g ai hm^−2	NP suspension	Entered tomato plant tissues from root effectively.	FM	—	153
FL-MSNs	—	—	Pakchoi (Brassica chinensis L.)	Foliar treatment	0, 22.5 g ai hm^−2	NP suspension	Found in various parts of the plant.	FM	—	154
MSNs	20–50 nm	Spherical	Rice (Oryza sativa L.)	Foliar treatment	0, 0.1 mg mL^−1	NP suspension	Translocated to different parts of the rice plants	CLSM	—	155

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Table 3 Uptake, translocation, and influence of carbon-based NPs in crop plants

NPs	Size	Morphology	Plant species	Exposure method	Concentration	Treatment medium	Uptake and translocation	Detection techniques	λ ex/λem (nm)	Influence	Ref.
“—” indicates that the information is not available. Abbreviations: R—reference; CDs—carbon dots; MWCNT—multi-walled carbon nanotubes; SWCNTs—single-walled carbon nanotubes; NH2-GQDs—graphene quantum dots modified with positively charged amino functional groups; OH-GQDs—graphene quantum dots modified with negatively charged hydroxyl functional groups; TEM—transmission electron microscope; FM—fluorescence microscopy; CLSM—confocal laser scanning confocal microscopy; ATY-FTIR—attenuated total reflectance-Fourier transform infrared spectroscopy; EDS—energy dispersive X-ray spectroscopy; SEM—scanning electron microscope.
CDs	2–6 nm	Spherical, or semi-spherical	Pumpkin	Root treatment	0, 200, 400 and 800 mg L^−1	Hoagland solution	Entered the leaf cells	FM	—	Triggered the defense mechanisms	156
CDs	2 nm	—	Maize (Zea mays L.)	Root treatment	1, 5, 10 mg L^−1	NP suspension	Passed through the whole plant	FM	558/580	Raised photosynthetic pigments	85
CDs	1.5–2.7 nm	—	Tomato (Solanum lycopersicum L.), lettuce (Lactuca sativa L.)	Root treatment	0.033, 0.066, 0.132 mg mL^−1	NP suspension	Translocated to root and stem	CLSM	289/410	Improved absorbance of mineral elements and photosynthesis	84
MWCNTs	37.84 nm	Long tubes	Malabar spinach (Basella alba)	Root treatment	50, 100, 150, 200 μg mL^−1	NP suspension	Absorbed and accumulated in leaves	EDS	—	Increased plant protein and chlorophyll content	157
CDs	3.07 ± 0.55 nm	—	Maize (Zea mays L.)	Seed treatment	250, 500, 1000, 2000 mg L^−1	NP suspension	Translocated in maize	FM	330–380/—	High concentration (1000, 2000 mg L^−1) inhibited the growth.	101
CDs	4–6 nm	Spherical	Mung bean	Seed treatment	0.02, 0.04, 0.12 mg mL^−1	NP suspension	Translocated to stems and leaves.	LSCFM, TEM	405/—	Enhanced the photosynthesis	158
Fullerol	5.0 ± 0.7 nm	—	Bitter melon (Momordica charantia)	Seed treatment	0.943, 4.72, 9.43, 10.88, 47.2 nM	NP suspension	Translocated to stem, petiole, leaf, and flower	FTIR	—	Increased biomass yield, fruit yield and phytomedicine content in fruits	159
SWCNTs	—	Bundle	Chickpea (Cicer arietinum L.)	Seed treatment	1.25 mg mL^−1	NP suspension	Transferred from root system to upper part	ATR-FTIR	—	—	137
CDs	3.25 ± 0.58 nm	Tori-spherical	Rice (Oryza sativa L.)	Foliar treatment	300 μg mL^−1	NP suspension	Absorbed by leaves and evenly distributed	TEM	—	Increased chlorophyll content and shoot length	160
CDs	3.8 nm	Spherical	Tobacco (Nicotiana tabacum)	Foliar treatment	0.01, 0.05, 0.1, 0.3 mg mL^−1	NP suspension	Absorbed by root and transported to stems and leaves	CLSM	420/680	Promoted plant growth and strengthened photosynthesis	161
NH2-GQDs	13.0 ± 1.8 nm	—	Maize (Zea mays L.)	Foliar treatment	10 mg L^−1	NP suspension	Translocated to stems and roots	FM, SEM	326/415	Impaired the photosynthesis	83
OH-GQDs	14.1 ± 2.3 nm	—	Maize (Zea mays L.)	Foliar treatment	10 mg L^−1	NP suspension	Translocated to stems and roots	FM, SEM	300/393	Impaired the photosynthesis	83

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Table 4 Organic NPs uptake, translocation, and influence in crop plants

NPs	Size	Morphology	Plant species	Exposure method	Concentration	Growth medium	Uptake and translocation	Detection techniques	Fluorescent labeling	λ exc/λem (nm)	Influence	Ref.
“—” indicates that the information is not available. Abbreviations: R—reference; ZNPs—zein nanoparticles; FITC—fluorescein isothiocyanate; FLNPs—fluorescent lignin nanoparticles; FM—fluorescence microscopy; FZNP–D—fluorescent zein nanoparticles synthesized with didodecyldimethylammonium bromide; LNPs—lignin-graft-poly(lactic-co-glycolic) acid nanoparticles; LNCs—lignin nanocapsules; FY088—fluorol yellow 088; PSI-NPs—polysuccinimide nanoparticles; PS—polystyrene; CLSM—confocal laser scanning confocal microscopy; Py-GC/MS—pyrolysis gas chromatography–mass spectrometry.
ZNPs	135 ± 3 nm	Mono-disperse	Soybean (Glycine max L.)	Root treatment	0.88 and 1.75 mg mL^−1	Hoagland solution	Biodistributed within the plant	—	FITC	485/535	—	91
LNPs	113.8 ± 3.4 nm	Spherical	Soybean (Glycine max L.)	Root treatment	0.02, 0.20, 2.00 mg mL^−1	NP suspension	—	—	—	—	High doses increased root biomass	162
LNCs	200–250 nm	—	Eragrostis tef	Root treatment	—	NP suspension	Translocated to leaves.	FM	FY088	450/515	—	163
PS nanospheres	Around 80 nm	—	Rice (Oryza sativa L.)	Root treatment	40 mg L^−1	NP suspension	Translocated from roots to aerial parts	CLSM	Nile blue	620/680	—	164
PS nanoplastics	63.82 ± 0.57 nm	—	Wheat (Triticum aestivum L.)	Root treatment	100 mg L^−1	Hoagland solution	Translocated to stem, leaf, and root	FM	—	620/680, 480/525	Affected carbohydrate metabolism	165
PS nanoplastics	33.9–58.8 nm	—	Cucumber (Cucumis sativus)	Root treatment	50 mg L^−1	NP suspension	Absorbed by root and transported to upper part	Py-GC/MS	—	—	—	166
FLNPs	52.9 ± 0.2 nm	—	Soybean (Glycine max L.)	Seed treatment	5, 10 mg mL^−1	NP colloids	Entered between seed coat and cotyledons	FM	FITC	480/535	Plant viability was not affected	167
FZNP-D	138.8 ± 0.9 nm	—	Soybean (Glycine max L.)	Seed treatment	5, 10 mg mL^−1	NP colloids	Entered under the seed coat	FM	FITC	480/535	Plant viability was not affected	167
PSI-NPs	14.2 ± 0.5 nm	—	Maize (Zea mays L.)	Seed treatment	50, 100, 200, 400, 800 mg L^−1	NP suspension	—	—	—	—	Improved seedling growth	75

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The impact of NPs on crops varies depending on NP type, size, and concentration. Košpić et al. treated tobacco roots with AgNPs with different surface coatings found that AgNP–PVP treatment increased superoxide dismutase (SOD) activity and proline content, while AgNP–CTAB activated more catalase (CAT) and ascorbate peroxidase (APX).⁹⁸ Different types of NPs have different effects on different indicators of plants. Yin et al. found that smaller AgNP particles (6 nm) more strongly affected plant growth than similar concentrations of larger (25 nm) particles.⁹⁹ Smaller AgNPs with higher surface areas can better interfere with cell membrane function by directly reacting with the membrane and allowing many atoms localized at the surface to interact with cells. Generally, NPs have a hormesis effect on crops, which means that they promote crop growth at low concentrations and inhibit crop growth at high concentrations. Biocompatible carbon-based NPs and readily biodegradable polymeric NPs have higher critical concentrations than metal-based NPs. Castro-González et al. found that low doses (12.5, 25 mg L⁻¹) of AgNPs simulated the growth of candy leaf, while high doses (50, 100, 200 mg L⁻¹) inhibited the growth.¹⁰⁰ Chen et al. found that CDs at 250 and 500 mg L⁻¹ showed no toxicity to maize, while 1000 and 2000 mg L⁻¹ CDs significantly reduced the fresh weight.¹⁰¹

6. Methods for the detection of NPs in crops

Understanding the mechanisms of uptake and translocation of NPs in plants requires the detection of NPs in crop plants. The suitable methods for detecting NPs vary depending on the type of NPs due to the different properties of NPs and the complexity of plant composition. Typically, the translocation and distribution of NPs in plant tissues can be studied by direct observation using SEM or TEM. Both methods can obtain high resolution images that are suitable for elemental analysis by complementary techniques (energy dispersive X-ray spectroscopy).¹⁶⁸ However, they are destructive, incompatible with wet and liquid samples, which may require coatings. For wet or liquid samples improvements can be made using wet scanning electron microscopy (WetSEM) or scanning transmission electron microscopy (STEM). Moreover, the uptake and translocation of metal-based NPs can be studied by analyzing metal elements in plants by energy dispersive X-ray spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), atomic absorption spectrometry (AAS), and X-ray diffraction (XRD), as shown in Table 5. EDS has the advantage of being able to generate the elemental composition of individual particles, but the method is biased towards heavy elements.¹⁶⁹ ICP-OES and ICP-MS can detect the composition of tens of elements simultaneously with high sensitivity. The latter uses a collision cell to eliminate interferences which provides better selectivity than the former. However both destructive methods require acid digestion.¹⁷⁰ AAS also has high sensitivity but requires a pre-concentration step prior to analysis.¹⁷¹ The pre-processing operations of ICP-OES, ICP-MS and AAS extend the detection time. Unlike other metal-based NPs, magnetic NPs can be detected by vibrating sample magnetometer (VSM) for the presence of NPs. When NPs are very few in quantity, the detection accuracy of VSM is higher than that of XRD, whereas VSM is limited to the detection of magnetic NPs.¹⁷² Ju et al. used magnetic particle spectrometry (MPS) to study the uptake and transport of iron oxide nanoparticles in garden cress.¹⁷³ Like VSM, the MPS method, although sensitive and effective, is destructive and can only detect magnetic nanoparticles. Although numerous methods of elemental analysis have been widely used to track the translocation of metal NPs, the technique is unable to determine the form of the element.¹⁷⁴ However, Malejko et al. suggested that ICP-MS techniques can also detect and analyze NPs taken up by plants without altering the original morphology of the analyte, but this requires the use of specific digestion procedures.⁹² This suggests the significance of using orthogonal techniques to characterize the fate and distribution of nanoscale materials in complex biological matrices. Other inorganic NPs, carbon-based NPs, and organic NPs cannot be easily detected by analyzing elemental content. Researchers prefer to use fluorescent dyes to label NPs. The labelled NPs can produce unique fluorescent signals, which can be detected by FM or CLSM to indicate the distribution of NPs, as shown in Fig. 3. This method of detecting fluorescence is non-destructive and fast, which makes it more advantageous than the previous method. Researchers have also developed novel detection methods. For example, Candan et al. used attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) combined with support vector machine (SVM) and principal component analysis (PCA) to detect the uptake and distribution of AuNPs and CNTs in crops, which were shown to be effective.¹³⁷ Mehmet et al. propose for the first time to detect AgNPs in different tissues of crops by surface-enhanced Raman spectroscopy (SERS) method.⁹³ They are confident that in the future this new approach will allow them to monitor not only NPs in plants, but also their fate, chemical transformations, and their creative applications such as delivery systems and imaging agents. The new pyrolysis gas chromatography–mass spectrometry (Py-GC/MS) analysis method developed by Li et al. offers new possibilities for screening nanoplastics in plants.¹⁶⁶ Exploring the uptake and distribution of NPs in plants will become more explicit as the number of research methods gradually increases.

Table 5 Strengths and shortcomings of NPs detection methods

Detection method	Strengths	Shortcomings
Abbreviations: SEM—scanning electron microscope; TEM—transmission electron microscope; EDS—energy dispersive X-ray spectroscopy; ICP-OES—inductively coupled plasma optical emission spectroscopy; ICP-MS—inductively coupled plasma mass spectrometry; AAS—atomic absorption spectrometry; XRD—X-ray diffraction; VSM—vibrating sample magnetometer; MPS—magnetic particle spectrometry; FM—fluorescence microscopy; CLSM—confocal laser scanning confocal microscopy; ATR-FTIR—attenuated total reflectance-Fourier transform infrared spectroscopy; SERS—surface-enhanced Raman spectroscopy; Py-GC/MS—pyrolysis gas chromatography–mass spectrometry.
SEM	High-resolution	Destructive
	Suitable for elemental analysis by complementary techniques (EDS)	Incompatible with wet and liquid samples, which may require coatings
TEM	High-resolution	Destructive
	Suitable for elemental analysis by complementary techniques (EDS)	Incompatible with wet and liquid samples, which may require coatings
EDS	Generation of the elemental composition of individual particles	Biased towards heavy elements
ICP-OES	Detection of the composition of tens of elements simultaneously with high sensitivity	Destructive
		Requirement for acid digestion
ICP-MS	Detection of the composition of tens of elements simultaneously with high sensitivity	Destructive
		Requirement for acid digestion
	Provision of better selectivity than ICP-OES
AAS	High sensitivity	Requirement for pre-concentration step prior to analysis
		Long detection time
XRD	Provision of structural information	Requirement for a large amount of sample 1–3% weight limit
VSM	Ability to distinguish between different magnetic behaviors	Destructive
		Detection of magnetic nanoparticles only
MPS	Sensitive	Destructive
	Effective	Detection of magnetic nanoparticles only
FM	Direct observation	Need for fluorescent dye labelling
	Non-destructive	Understanding the wavelengths of excitation light and filters for dyes
CLSM	Direct observation	Need for fluorescent dye labelling
	Non-destructive	Understanding the wavelengths of excitation light and filters for dyes
ATR-FTIR	Little or no sample preparation required	Overlapping between NPs and other substances
	Fast analysis
SERS	High sensitivity and selectivity	Not suitable for the determination of many complex samples
		Need for pretreatment
Py-GC/MS	Sensitive to complex environments and low concentrations	Risk of misinterpretation, because different polymers have similar pyrolysis products

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7. Discussion

This paper summarizes the classification, uptake, translocation, and detection of NPs in agricultural crops. NPs have been shown to interact with and affect crop growth by detecting indicators related to crop growth and its fruit nutrition. Some of the NPs beneficial to crops have been proposed as nano-fertilizers or nanocarriers. Fluorescence detection has also proven to be an effective means of tracking NPs in crops. The applications of NPs in agriculture are currently focused on promoting seed germination and crop growth, delivering agrochemicals, and alleviating the accumulation of harmful elements in plants.

7.1. Advantages of NPs

As new NPs are synthesized, their application areas are expanding. NPs can be used for the controlled and targeted release of agrochemicals or as carriers to deliver specific substances to specific parts of plants.¹⁷⁵ The application of NPs as a carrier can increase efficiency while sustaining release compared to traditional methods of applying agrochemicals directly to the crop. In the case of pesticides, they can be loaded into the core of the NPs, thus reducing the metabolites of the pesticide, or protecting it from photodegradation.^176,177 In the case of fertilizers, slow release can improve nutrient utilization efficiency by preventing the premature conversion of plant-needed nutrients to forms that are not available to plants, such as gases.¹⁷⁸ In addition, suitable concentrations of non-toxic NPs have a growth-promoting effect on plant, so they can act as nano-fertilizers on crops.

The previous study demonstrated that crops can take up NPs through seeds, roots and leaves and transport them systematically in the crop, which then interacts with the crop. The data on the effect of NPs on plants are diverse and insufficient, making it difficult to draw general conclusions. What is certain is that it is related to plant species and the physicochemical properties of the NPs (such as type, size, and charge of NPs). In addition, most NPs have a dose effect on the crop, with high concentrations inhibiting crop growth and low concentrations promoting it. The threshold concentrations of biocompatible carbon-based NPs and readily biodegradable polymeric NPs are higher than those of metal-based NPs, which means that the dose must be selected according to the different species when using the corresponding NPs. This provides insight into the future application of NPs to agricultural systems.

Tracking of NPs in crops is necessary to reveal the principles of nanoparticle-crop interactions. Direct observation and analysis of elemental content are common testing techniques for tracking NPs. The latter is done by comparison with a control group which is not exposed to NPs. If the test part of the plant exposed to NPs contains more of the corresponding element, it proves that the part can absorb NPs or that NPs can be transferred from the absorbing part to the test part. However, since it is not possible to analyse the morphology of the NPs in the plant, it is not possible to determine whether the absorbed NPs are converted to the ionic state. If the morphology is to be analysed, a specific digestion procedure is required, which adds to the difficulty of the test. Another method uses the fluorescence produced by NPs for tracking, which has the advantages of simple operation, adjustable fluorescence, fast detection speed, and good reproducibility. Specifically, NPs produce fluorescence by photoluminescence and fluorescent dye labelling. Combined with laser confocal scanning microscopy or fluorescence microscopy, the position of fluorescent NPs in the crop can be quickly obtained. In addition, different doses of NPs produce different fluorescence intensities, so fluorescence intensity can be used for quantitative measurements. If fluorescence is detected in time after treatment, the path of NPs in the crop can be demonstrated. This provides a powerful means of tracing the pathways of NPs through the crop.

7.2. Challenges of NPs in crop plants

Many challenges still exist when applying nanoparticles to crops. Firstly, although the macroscopic mechanisms of NPs entry into crops are understood, there is still much confusion about the microscopic pathways, such as the ability of the leaf cuticle to absorb NPs larger than the pores of the cuticle, and the translocation of NPs from seeds to roots. Although advanced tools, such as confocal laser scanning microscopy and transmission electron microscopy, can be combined with fluorescence from NPs to perform in-depth studies on the visualization of NPs absorbed by plants, they fail to address the above issues. The size of NPs affects crop uptake and transportation. Small NPs lead to efficient NP transport.¹⁹ The mechanism of the effect of size on transport efficiency is unclear. In addition, studies on the pathways of NPs penetrating seeds are lacking. Secondly, metal-based NPs that enter the plant undergo morphological changes, which is a complex process. There are many unresolved issues in this process, such as the factors affecting the transformation process, the location of transformation and the toxicity of transformation products. Thirdly, NPs enter the crop and are present in edible parts, thus moving up the food chain to the next trophic level and eventually passing to humans. There is no research to prove that NPs are harmless to humans, raising concerns about exposure to crops infected with NPs. Fourthly, although NPs can promote crop growth, some NPs are too expensive to be of sufficient benefit to farmers. The replacement of chemical fertilizers with such NPs to increase crop yields will increase the cost which is not friendly to farmers. Cost-effective synthesis methods should be developed in the future to facilitate the use of NPs in agriculture. Finally, the concept of hormesis has also received much attention in nanotoxicology and nano-risk assessment. Different NPs with various physicochemical properties play an important role in determining the dose–response relationship.¹⁷⁹ The exact molecular mechanisms of hormesis in exposure to NPs are unclear. The resulting changes in nutritional status and the expression of genes regulating cellular pathways such as proliferation, photosynthesis and hormone signaling cannot be determined.

7.3. Future trends

In the context of the popularity of nanotechnology in agriculture, attention should be paid in the future to the duration of NPs presence in the plant and their destination. The quantitative assessment of uptake and translocation of NPs in plants is a very important issue, as the appropriate dose is crucial for plant growth and human health. The effect of NPs on crops is related to the plant species, but the information is no comprehensive. If it can be demonstrated that a specific NP has a promotive or no impact on crop growth at a certain concentration, but has an inhibitory effect on weeds, it means that this NP can be used as an herbicide. Future research should conduct detailed controlled experiments to study the effects of different concentrations of NPs on different kinds of plants. In addition to being an effective method for tracking the translocation of NPs in plants, the fluorescence of NPs could be developed for novel applications, such as using it as a new crop signaling or tracking their location in the soil. Crop signaling is a new concept proposed by Su et al. for distinguishing weeds from crops: a machine-readable fluorescent signal applied to plants.^180,181 According to previous studies, CDs emit fluorescence that can be tuned according to their size, with most of them emitting blue fluorescence.¹⁸² The chlorophyll fluorescence of the leaves consists of two maxima in the red (near 685–690 nm) and far-red (near 730–740 nm) regions.¹⁸³ The fluorescence of CDs at appropriate size (for example, green or yellow) is not disturbed by chlorophyll fluorescence which promotes crop growth. By introducing NPs into the crop before entering the field, the machine can distinguish between crops and weeds by filter and excitation light. This offers a new way of thinking about precision agriculture. Weeding in this manner requires that the crop be pre-soaked in a solution containing fluorescent NPs so that the crop absorbs the NPs and fluoresces. Planting the treated crops in the soil may cause NPs to spill out which may enter the external environment. Fluorescence technology offers researchers the possibility to track NPs in the soil. Before NPs can be used in agricultural systems, it is important to figure out their impact on the environment, including whether they move through the soil to other plants and whether they persist in the soil. In addition to the three uptake methods mentioned in the article, researchers have identified other sites for introducing NPs into plants. Su et al. used a pneumatic injection apparatus to inject AgNP suspensions into clementine mandarin trunks.¹⁸⁴ Compared to other methods (foliar application, branch feeding, and soil drenching), trunk injection can easily deliver large amounts of AgNPs into citrus trees. Sembada et al. proposed a new method to deliver particles up to 110 nm into plants by cutting tomato seedling stems.¹⁸⁵ These plants could successfully regenerate adventitious roots and grow normally after particle introduction. These new methods were able to increase the transport efficiency or the size of the transported particles. In the future, the uptake pathway of nanoparticles is not limited to roots and leaves. Further research is needed to fully develop and optimize new methods for various applications in plant biology.

8. Conclusions

In this critical review, a classification of NPs suitable for agriculture is presented, with a summary and critique of recent studies on the uptake and translocation of different kinds of NPs (inorganic, carbon-based, and organic NPs) in agricultural crops. NPs can be taken up by the seeds, roots, and leaves with transport through the apoplastic pathway and the symplastic pathways. The results of recent studies showed that, in general, NPs can be absorbed and transported by plants; however, the effects of NPs on crops vary depending on NPs types, sizes, concentrations, plant species. Generally, NPs have a dose effect on crops, which means that they promote crop growth at low concentrations and inhibit crop growth at high concentrations. Biocompatible carbon-based NPs and readily biodegradable polymeric NPs have higher critical concentrations than metal-based NPs. Silicon dioxide NPs and polymeric NPs can be used as delivery systems for agrochemicals. Methods for detecting NPs in crops include direct observation (TEM, SEM, FM, and CLSM), analysis of elemental content (EDS, ICP-OES, ICP-MS, AAS, and XRD), and novel techniques (SERS, ATR-FTIR coupled with computer-based techniques and Py-GC/MS). Using fluorescence can track NPs in plants, which primarily includes fluorescence generated by the NPs' own photoluminescence as well as fluorescence generated by binding with fluorescent tags. NPs have great prospects for application in agriculture, where relevant experiments need to be continued to develop them as herbicides and novel crop signaling. Given the increasing research on the use of NPs for crops, it is expected that in the future, they will be more widely used in agricultural systems to improve crop yields, which will contribute to precision farming and sustainable agriculture.

Author contributions

Wen-Hao Su: conceptualization, methodology, resources, writing – review & editing, supervision, project administration, funding acquisition. He-Yi Zhang: investigation, writing – original draft, visualization. All authors read and approved the submitted version.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (32101610).

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