Nowadays, due to the advantages of nanoparticles as nano-carriers and nano-pesticides, such as their small size, ease of use, capacity for long-term storage, and improvement of the efficiency of agricultural chemicals, the application of nanoparticles in agriculture is considerably more extensive [63]. The interaction between nanoparticles and plants has beneficial and harmful effects on the plant’s physiological morphology, plant development and yield of crops. The effects of nanoparticles on plants are related to plant species, use methods, dosage and concentration of nanoparticles.

The use of agricultural nanoparticles has improved the quality of plant products better than traditional pesticides, which has been proven by many researchers. Nanoparticles play an important role in plant growth and improving plant quality by increasing nutrient content, improving photosynthetic activity and metabolism [78] (Figure 1). Zinc oxide nanoparticles have been confirmed to participate in the synthesis and photosynthesis of plant chlorophyll and the formation of starch, thus increasing the concentration of soluble carbohydrates [79]. The use of ZnO nanoparticles can improve the antioxidant activity and chlorophyll content of cotton, increase the number and weight of cotton bolls per plant, and improve cotton fiber quality parameters such as uniformity and fiber strength [80]. ZnO NPs act on tomato plants and improve tomato yield by increasing the absorption of nutrients (phosphorus and zinc) [81]. Fe₃O₄ NPs can improve plant biomass and productivity by increasing the content of protein, nutrients, and carbohydrates in plants [81,82]. Sharifi et al. used Fe₃O₄ NPs to act on corn plants, and Armin et al. used Fe₃O₄ NPs to act on wheat plants. All their results confirmed the above conclusions [83,84]. In addition, when Fe₃O₄ NPs were used to treat wheat at the tillering stage, the number of wheat seeds increased significantly. Gao et al. found that TiO₂ NP increased the biomass accumulation of spinach by 60% [85]. In addition, CeO₂ NPs promoted stem elongation at 1–10 mg/L, and fruit weight significantly increased at 10 mg/L [86].

As an efficient environment-friendly photocatalyst, TiO₂ nanoparticles have been proven to be able to improve light absorption by improving the energy conversion of the light system, and have antibacterial activity after surface chemical modification, which can reduce the half-life of pesticides and promote seed germination and seedling growth [87]. In 2013, it was proven that TiO₂ nanoparticles could improve the photosynthetic efficiency of spinach. In recent studies, it was shown that TiO₂ NPs could promote wheat growth and increase wheat yield. At the same time, experiments have proven that these improvements are due to the fact that titanium dioxide nanoparticles promote cycling and linear phosphorylation to improve photosynthetic activity, thus increasing the supply of photosynthates and in turn increasing plant yield. Spraying TiO₂ NPs on the leaves improves the dry matter yield of barley because nanoparticles improve the photoreduction activity [88]. In addition, under the joint action of ZnO NPs and Si NPs, the yield, weight and sugar content of mango fruit increased [89]. The promotion effects of different kinds of nanoparticles on plant growth and seed germination are listed in Table 1.

Plant abiotic stress is the main problem that plants face in the process of growth due to drought, salt and heavy metal elements, which lead to the reduction of crop yield due to plant growth retardation. In order to alleviate these stresses, plants have evolved different defense mechanisms through physiological pathways. The application of nanoparticles can help plants to alleviate abiotic stresses. Due to environmental, climatic and other factors, the yield and biomass of crops in arid areas are significantly reduced compared with a normal environment. Sun et al. applied zinc oxide nanoparticles to treat corn and found that nanoparticles can improve the photosynthetic rate and chlorophyll content of plants under drought stress, which proved that nanoparticles have a mitigating effect on plant drought stress [59].

Inappropriate salinity in plant growth environments will lead to plant nutrition imbalance and slow plant growth [99]. The application of nanoparticles in agriculture can help improve the activity of enzymes involved in the salt tolerance mechanism in plants. The study showed that ZnO NPs alleviated the salt stress of cotton [100] and wheat [101]. The application of SiO₂ NPs on the leaves increased the elasticity and expansion of the cell wall of cucumber during the growth period and increased the accumulation of nitrogen and phosphorus elements in the leaves by reducing the loss of leaching process, thus reducing the content of Na, alleviated the salt stress on cucumber plants [66].

The existence of heavy metals is more harmful to plant growth, which will affect plant morphology, inhibit plant growth and stimulate plants to produce oxidative stress. In order to resist heavy metal stress, nanoparticles can improve the ability of antioxidant systems by regulating the concentration of heavy metal ions in soil, reducing the expression of heavy metal ion transport genes in plants, stimulate the production of defense substances (such as organic acids, root exudates and phytochelatin) to cope with the stress of heavy metal ions in plants [102].

Sharifan et al. used ZnO NPs and heavy metals Pb²⁺ and Cd²⁺ as the hydroponic culture system to culture lettuce (Lactuca sativa L. var. Longifolia), and detected the content of Pb and Cd in plant tissue. It was found that ZnO NPs significantly reduced the accumulation of Pb and Cd in the root of lettuce, which were 81% and 49%, respectively. Pb and Cd in the shoots of lettuce decreased by 44% and 30% [103], respectively. Yan et al. treated rice with ZnO NPs, proving that zinc oxide nanoparticles can improve rice growth and reduce the accumulation of arsenic in rice. The best effect was found when the concentration of ZnO NPs was 100mg/L. Compared with the control group, the concentration of As in the bud and root of rice seedlings decreased by 40.7% and 31.6%, respectively. Se and Si NPs reduced the absorption and accumulation of Cd and Pb in rice, thus reducing the lethal effect of heavy metals on plants. At the same time, the use of SiO₂ NPs in leaves helps to increase chlorophyll content and reduce the accumulation of Cd in rice [104].

In addition to many beneficial effects, the toxicity of nanoparticles in plants cannot be ignored. The toxicity of nanoparticles can damage plants in a variety of ways, for example, by stimulating plant oxidative stress, resulting in physical damage to plants, such as stomatal closure due to the aggregation of nanoparticles [6].

When nanoparticles are sprayed on plants through leaves at high concentrations, a large number of NPs will gather on the surface of leaves, which results in stomatal blockage and hinders the gas exchange and photosynthesis of plants (Figure 1). Some studies have shown that when the concentration of Zn NPs and Cu NPs exceeds the critical value, the plant growth rate will slow down and the leaves will turn yellow.

CuO NPs are toxic to H. sativum, and will reduce the photosynthetic rate of plants and inhibit the growth of roots and stems [105]. At the same time, nanoparticles transform in plants result in damage to cell structure and reduce the absorption and transportation of nutrients [106].

For polymer-based nanoparticles, the current research mainly discusses their toxic effects on plants in the process of application. For example, after treating the seeds of Lepidiumsativum with plastic particles with a size of 50 and 500 nm for 8 h, the germination rates decreased by 56% and 46%, respectively [51]. When ryegrass was exposed to PLA nanoparticles for 30 days, the germination rate decreased by 6% [107]. In contrast, the germination rate of wheat seeds and onions treated with PS NPs for 72 h had no effect [107,108]. Therefore, long-term exposure of plants to polymer nanoparticles will reduce the germination rate, which is mainly due to the reduction of nutrient absorption of plants due to the blockage of pores by nanoparticles.

In addition to the effects of polymer nanoparticles on plant seed germination, other studies have reported the effects on plant seedling growth. The length of the onion root decreased by 41.5% after polystyrene nanoparticles (50 nm) acted on the onion root for 72 h [109,110]. The length of the Lemna minor root also decreased under the effect of polyethylene nanoparticles. In Arabidopsis thaliana treated with polystyrene nanoparticles with different surface charges (PS-NH₂, PS-SO₃H), the fresh weight of Arabidopsis thaliana decreased by 50% on average. The plant length decreased by 15%, the root length decreased by 30% on average, and the higher the concentration of nanoparticles, the more obvious the inhibition of Arabidopsis thaliana seedlings is [49]. A better understanding of the effects of nanoparticles on plants can help assess their toxicity (Table 2).

The transport and accumulation of nanoparticles in plants will induce phytotoxicity and the interaction between nanoparticles and plants will lead to increased production of plant ROS, resulting in oxidative damage and genetic toxicity (Figure 1) [129,130]. Similarly to all aerobic organisms, plant cells activate ROS in response to environmental changes [131]. When ROS levels exceed defense mechanisms, cells are placed in a state of “oxidative stress”, causing unlimited damage to proteins, nucleic acids and lipids in cell membranes and inducing oxidative stress [132]. Plants can protect cells from the toxicity of reactive oxygen species (ROS) through antioxidant enzymes and antioxidants. Plants can protect cells and subcellular systems from the toxicity of active oxygen radicals by antioxidant enzymes and low molecular weight antioxidants [133]. Therefore, the research on oxidative damage caused by nanoparticles mainly focuses on the determination of antioxidant enzyme activity and ROS content. Some research and analysis, especially when nanoparticles act on plants at high concentrations, results in excessive accumulation of nanoparticles in plants and a large number of ROS that activate the antioxidant system [58,106,134]. As the concentration of plastic NPs increased, the activity of several representative antioxidant enzymes in rice roots increased, suggesting that the plant could stimulate a defensive response and remove the excessive accumulation of ROS [135]. Meanwhile, a higher concentration of nanoparticles will affect the level of plant endogenous hormones [58]. At present, there are also corresponding studies to evaluate the impact of nanoparticles on plant endogenous hormones [136]. For example, iron oxide nanoparticles with a concentration of 100 mg/L reduced the yield of Bt-transgenic cotton and led to an increase in hormone levels. While in the root of Bt-transgenic cotton, the hormone content decreased [137]. In addition, metal-based nanoparticles transform and release toxic metal ions in plants, which destroy DNA and protein in plants and inhibit normal cell metabolism [138].

In addition to inducing ROS production in plants and triggering hormonal changes in plants, nanoparticles are also genotoxic. Nanoparticles interact with biological macromolecules such as nuclei and lipids and exert genotoxicity, which can affect plant cell division [139]. For example, Ag NPs internalized in wheat root tips have been shown to interfere with normal cell division, inhibit DNA synthesis and lead to chromosome aberrations [140]. In addition, the mitotic index of onion cells exposed to PS NPs was significantly reduced and chromosomal aberrations and nuclear abnormalities were observed, leading to the destruction of genomic stability, demonstrating the genotoxicity of PS NPs [141].

In order to deal with the toxicity caused by nanoparticles, researchers have also developed corresponding solutions, in which the toxic substances released by nanoparticles can be effectively reduced by adding a surface coating. For example, adding an iron coating to the surface of zinc oxide nanoparticles can effectively reduce the release of zinc ions. Fe-coated zinc oxide nanoparticles have no inhibition on plant germination and pigment content in plants [142]. The method of reducing toxicity through encapsulation or surface modification of nanoparticles is also applicable to polymer nanoparticles [43].

Analyzing the toxicity of different nanoparticles as agricultural chemicals are helpful to determine the optimal concentration of nanoparticles in plant growth. As nontoxic, degradable and biocompatible compound nanoparticles, natural polymer nanoparticles can largely avoid the negative effects of metal-based, silicon-based and organic-based nanoparticles on plants and the environment [143]. Chitosan is the only positively-charged polysaccharide in nature, which has been reported as a nanoparticle material due to its antibacterial and antiviral properties. Because of its positive charge, chitosan nanoparticles enhance the affinity with the plant cell membranes and increase the reactivity of the plant systems. When chitosan-based nanoparticles are used as plant growth promoters to treat seeds and seedlings; it has been proven that they can improve plant nutrient absorption, chlorophyll content and photosynthesis rate. Three different sizes of chitosan nanoparticles (420 nm, 750 nm, 970 nm) were applied to plants. The results showed that plant chlorophyll increased by 61%, 81% and 61% and the photosynthetic rate increased by 29%, 59% and 72%, respectively [144]. The application of chitosan nanoparticles on wheat and barley has also been proven to promote plant growth [145].