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. 2019 Jul 29;13(7):712–719. doi: 10.1049/iet-nbt.2019.0060

Effects of spraying nano‐materials on the absorption of metal(loid)s in cucumber

Kerui Guo 1, Annan Hu 1, Kexiang Wang 2,3, Lingqing Wang 4, Dongheng Fu 1, Yi Hao 1, Yaoyao Wang 1, Arbab Ali 1, Muhammed Adeel 1, Yukui Rui 1,✉, Weiming Tan 5
PMCID: PMC8676231  PMID: 31573540

Abstract

This report investigates the spraying of nano‐silica and fullerene on cucumber leaves to expose their ability to reduce the toxicity and uptake of metal(loid)s. Cucumber seedlings were randomly divided into six treatment groups: 10 mg/L nano‐SiO2, 20 mg/L nano‐SiO2, 10 mg/L Fullerene, 20 mg/L Fullerene, 5 mg/L Fullerene + 5 mg/L nano‐SiO2, and 10 mg/L Fullerene + 10 mg/L nano‐SiO2. Nano‐silica‐treated plants exhibited evidence of the potential mitigation of metal(loid)s poisoning. Specifically, results showed that 20 mg/L of nano‐silica promoted Cd uptake by plants; comparatively, 10 mg/L of nano‐silica did not significantly increase the silicon content in plants. Both low‐concentration combined treatment and low‐concentration fullerene groups inhibited metal(loid)s uptake by plants. Scanning electron microscopy (SEM) was then used to observe the surface morphology of cucumber leaves. Significant differences were observed on disease resistance in plants across the different nano‐material conditions. Collectively, these findings suggest that both nano‐silica materials and fullerene have the potential to control metal(loid)s toxicity in plants.

Inspec keywords: soil pollution, cadmium, silicon compounds, surface morphology, fullerenes, toxicology, fertilisers, scanning electron microscopy, crops, spraying, nanoparticles, sorption, plant diseases, agricultural safety

Other keywords: cucumber leaves, nanosilica materials, fullerene, spraying process, metalloids absorption, toxicity, scanning electron microscopy, surface morphology, disease resistance, soil pollution, SiO2 , Cd

1 Introduction

The continuous and rapid industrialisation in China over recent years has led to increasingly serious metal(loid)s pollution in the soil. Metal(loid)s enter into the soil through a variety of avenues, including industrial waste discharge, improper use of pesticides and fertilisers, discharge of domestic sewage, dumping of municipal sludge, and mining of mineral deposits. Critically, metal(loid)s can enter plant systems, which form a hub of entry into wider human food chains. This not only causes a decline in crop yield and quality, but also leads to a host of diseases. Recent data show that the area of arable land in China that is contaminated by Cd, Hg, As, Cr, and Pb is ∼2000 × 104 Hm2; critically, the annual loss of food due to heavy metal pollution is ∼1000 × 104 t, contaminated grain is 1200 × 104 t per annum, and total economic loss is at least 2,883,506,343 USD per annum [1].

Nanomaterial research is an important and increasing area of interest for many scholars. As a result of this widespread interest, research and development into nano‐materials has undergone rapid changes. Dovetailing with these changes is the need for environmental‐friendly methods to manage metal(loid)s contamination in soil, which has become a global hot topic in environmental research. With the rapid development of environmental molecular science, the application of nano‐materials to environmental remediation has received increasing attention. Due to their large specific surface area and micro‐interface characteristics, nano‐particles enhance a variety of interfacial reactions including surface adsorption, specific adsorption, and enhanced oxidation‐reduction reaction of heavy metal ions and organic pollutants. These properties lend it an important, future role in the management of heavy metals and their pollution.

Given this, we sought to address today's increasingly serious metal(loid)s pollution in the soils of our arable land by using both novel nano‐materials and bio‐char to improve soil metal(loid)s as well as the passivation of soil. It is hoped that this approach will allow for more effective protection of arable land. Finally, we also measured plant metal(loid)s content as a way to ascertain how beneficial this approach is to crop protection.

Critical to these nano‐materials is silicon. The beneficial effects of silicon are manifold and include the following: First, resistance to biotic stressors like pests and diseases [2]; moreover, silicon is capable of reducing powdery mildew infections [3]. For instance, foliar spray applied to non‐silicon‐accumulated plants (e.g. cucumbers and grapes) has the same effect as fertilising roots [4]. Second, resistance to physical stressors. To this end, a large body of past work has confirmed that silicon can relieve physical stress. For instance, silicon can accelerate the repair of irradiated rice plants and increase their yield [5]. In addition, silicon can increase the lodging resistance of rice to the damage of typhoons [6]. Silicon can also increase the mechanical strength of stems, stem cell wall thickness, and increase the area of vascular bundles [7]. Third, resistance to chemical (e.g. metal) threats. Recent studies have shown that silicon plays an important role in increasing the resistance of plants to heavy metal toxicity. Silicon resources are very convenient and cheap to use; applying silicon‐based substances to reduce the heavy metal toxicity in plants has attracted increasing attention [8]. In many plants (e.g. rice, barley, broad beans, and pumpkin), silicon can relieve the toxic symptoms caused by excessive Mn; however, the mechanism of action is not the same for different plants.

Fullerene is widely used in electronics, solar cells and biomedicine. Fullerene was introduced into this experiment as another experimental object. In previous studies, fullerenes were often added directly to the soil or studied in the form of nutrient solutions. We hope to study the biological effects of foliar spraying on plants and whether it can inhibit or improve the absorption of heavy metals by plants. As there are few articles on the effect of fullerene on the absorption of heavy metals by rice, we introduce fullerene to explore its effect.

About this experiment, we mainly want to explore whether nano‐materials can change the absorption of heavy metals by plants based on foliar spray. As many experiments have been done before to add nano‐materials to soil or culture solution to study whether it will improve the absorption of metal(loid) in plants and achieve certain results. However, considering that in actual production, the method of spraying is simpler and cheaper than the method of adding nano‐materials to the soil. We hope to explore the effects of spraying C60 and SiO2 on plants through this experiment, and at the same time, we also study the effects of mixing them on plants, so as to pave the way for the choice of spraying materials in the future. Therefore, we conducted this study to explore its effects.

2 Materials and methods

2.1 Experimental materials

2.1.1 Experimental sample collection

Soil samples were taken from cadmium‐ and arsenic‐contaminated soils from Foshan, Guangdong (21°31'38’’N, 122°51'26’’E. Mar. 1st, 2017). Cultivated cucumber was strain Zhongnong No.16 (F1), which is a new hybrid cucumber with a wide range of planting in China. It is resistant to downy mildew, keratopathies, viruses, and other diseases. To ensure the germination rate prior to planting, cucumber plants were cultivated in an incubator for 24 h before being transplanted into the soil.

2.1.2 Treatment groups

There are nine pots in each treatment group and control group, and one cucumber seedling in each pot (300 g per pot of soil).

According to the study of Gnanamangai et al. [9], it was found that the application of 1.5 ppm (1.5 mg/L) Bionanocper had the greatest effect on the yield of tea. At the same time, Mamyandi et al. [10] chose the concentration of 1, 2, and 3 mg/L to study the foliar spraying of nano‐iron. It was found that all of them could affect the growth of plants. Therefore, the effect of higher spraying concentration on the growth and development of plants and the absorption of heavy metals was studied by raising the concentration a little on the basis of these study.

There are six treatment groups: 10 mg/L nano‐SiO2, 20 mg/L nano‐SiO2, 10 mg/L Fullerene, 20 mg/L Fullerene, 5 mg/L Fullerene + 5 mg/L nano‐SiO2, and 10 mg/L Fullerene + 10 mg/L nano‐SiO2. One control group.

Silicon is an essential nutritional element in rice, and carbon is of great significance to improve the ratio of carbon to nitrogen. We combine the two materials to hope that by combining the advantages of the two materials, we can explore whether the composite nano‐material reagent can improve the physiological and biochemical of plants or the absorption of heavy metals by plants.

Both the control group and the experimental group were in the same growth conditions. That is, the illumination time, irrigation amount, sowing time, and harvest time are the same, the only difference is that no foliar spray is applied.

2.1.3 Spray reagent preparation

The reagents used in the spray applications were divided into six types: 10 mg/L nano‐SiO2, 20 mg/L nano‐SiO2, 10 mg/L fullerene, 20 mg/L fullerene, 5 mg/L fullerene + 5 mg/L nano‐SiO2, and 10 mg/L fullerene + 10 mg/L nano‐SiO2. All solutions were prepared by using the listed equipment and were mixed to homogeneity using an ultrasonic oscillator (Hongkai instrument factory, Kunshan, China).

2.1.4 Rationale for silicon selection

Silicon is one of the most abundant elements in nature and most terrestrial and marine ecosystems have a biological silicon cycle. Moreover, many vascular plants have SiO2 deposits distributed in their bodies [11, 12]. The ability of plants to absorb silicon varies across different genotypes and environments and the amount of silicon found in plants is equivalent to that of calcium and magnesium content [13, 14, 15, 16]. Since silicon is ubiquitous, it is difficult to demonstrate the phytorequirement of silicon using the three principles of traditional nutrition's ‘essential nutrients’ approach. However, a large number of experimental results have shown the beneficial effects of silicon on the growth and development of many plants, including cucumber and rice [17, 18, 19].

2.1.5 Rationale for fullerene selection

The biological activity of fullerenes and their related compounds has attracted the interest of researchers. Preliminary studies have shown that fullerenes have unique effects with regard to their anti‐HIV properties as well as their ability to inhibit enzyme activity, affect DNA cleavage, and act as a photodynamic therapy [20]. As a result of these wide‐ranging properties, fullerenes and their related compounds have great potential for application in biochemistry, medicine, pharmacology, and other fields [20]. Interestingly, fullerenes also function as good adsorbents [21]; as a result, we sought to use fullerenes as a spraying material to examine its biological effects.

2.1.6 Nano‐material characterisation

The purity of C60 exceeded 99.9%, and the purity of SiO2 exceeded 99%. The particle size of SiO2 is 20 nm. The SEM and TEM characterisations of nano‐materials are showed in Figs. 1 and 2.

Fig. 1.

Fig. 1

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SEM images of nano‐materials. C60 (A,C); SiO2 (B,D)

Fig. 2.

Fig. 2

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TEM images of nano‐materials. C60 (A); SiO2 (B)

2.2 Experimental methods

Cucumber was planted in a greenhouse at a temperature of 23–25°C throughout the experimental period.

The experiment was from December 2, 2017 to January 12, 2018.

2.2.1 Measurements of biomass and root and shoot lengths

Plants were harvested and washed three times with tap water and three times with deionised water. They were then cut from the rhizome divisions to the aerial part to measure its height and subterranean parts, respectively. All measurements were made with a ruler. The above‐ground measurement standard was determined to be from the rhizome division to the stem and leaf bifurcation. The underground measurement standard was determined to be from the flattening of the underground section; specifically, from the division of the rhizome to the longest root apex of the root system. Sample wet weight was determined using an electronic analytical balance. The above‐ground and underground parts of the sample were then placed in paper bags in a drying oven and dried at 105°C for 30 min. Samples were t then dried at 65°C for 8 h. After drying, the dry weight was measured using an electronic analytical balance.

2.2.2 SEM assessment

One plant was selected from the treatment groups with the highest spraying concentration. Five samples with an area of ∼0.25 m2 were cut from the leaves and placed in a centrifuge tube containing 2.5% glutaraldehyde fixing solution. Samples were then stored at −40°C for five days. Two samples from each centrifuge tube were then selected – one for observing the front side and another for the back side of the leaf. Samples were then vacuumed and subjected to SEM.

2.2.3 Determination of plant hormonal content

Absisic acid (ABA), indole‐3‐acetic acid (IAA), isopentenyl adenosine (IPA), brassinolide (BR), gibberellic acid 3 (GA3), gibberellic acid 4 (GA4), trans‐Zeatin‐riboside (ZR), and Dihydrozeatin riboside (DHZR) were determined by the method of Hao Yi et al. [22]

2.2.4 Determination of plant elemental content

A dried sample was placed into a paper bag and stored at −20°C. Three ground parts were removed from each treatment group, ground, and prepared for analysis.

From each sample, 0.1 g sample was mixed with 2.0 mL HNO3, 1.0 mL HF, and 0.5 mL H2 O2, and digested using a microwave according to the aforementioned process. The digestive tube was then put on a hot plate for acid banding. After acid banding, the volume of the reaction was brought to 15 mL using purified water. The solution was further diluted 40 times after the digestion to allow for ICP‐MS measurements of As and Cd. The ICP‐MS operating parameters were set to the following: carrier gas flow (1.0 L/min), He gas flow (4.5 ml/min), RF power (1550 W), integration time (300 ms), and scan mode (3 Points). The blank solution was prepared simultaneously with the experimental samples.

The detection limit of As is 0.029 μg/L, the detection limit of Cd is 0.004 μg/L [23], and the detection limit of Si is 0.91 μg/L [24]. The spiked recovery of Cd is 86%‐104%, the spiked recovery of As is 102–113%, and the spiked recovery of Si is 93–109%.

2.2.5 Data processing

The results are expressed as the mean ± standard deviation of triplicate samples. All the data were analysed by one‐way analysis of variance with a Duncan's test using the SPSS 19.0 statistical software package for windows (SPSS, Chicago, IL, USA). Differences were considered statistically significant at p < 0.05.

3 Results and discussion

3.1 Representative cucumber leaf and plant images

Powdery mildew is a common disease that causes significant damage to a variety of host plants around the world [25], The Ascomycete fungi, Golovinomyces cucurbitacearum (formerly G. cichoracearum and Erysiphe cichoracearum), and Podosphaera xanthii (formerly Sphaerotheca fuliginea) have been reported to be the cause of cucumber powdery mildew [26].

We first examined the cucumber leaves from each treatment group, choosing the juvenile cucumber leaf as the plant sample to observe and analyse (Fig. 3 a). Our results indicated that the disease conditions on the surface of the leaves were not the same. Powdery mildew and cucumber downy mildew were found across all treatment groups. Due to the way the plants were initially sprayed, there was a lot of water attached to the blade. It should be noted that moist air, rain, and/or dew are necessary for the spore germination of this fungus. This is consistent with past reports, as high relative humidity promotes rice blast sporulation [27] and leads to disease propagation [28].

Fig. 3.

Fig. 3

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Representative images of cucumber leaves

(a) Representative images of the second true leaves of selected cucumber plants, (b) Images of the first true leaves of selected cucumber plants, and, (c) Representative images of cucumber juvenile leaves

To this end, the infections in the nano‐silica‐combined treatment group were more severe than those in the fullerene‐treated group. This was especially true in the high‐concentration‐combination treatment group. In the low‐concentration nano‐silica treatment group, yellowing and curling occurred at the edges of the leaves. By observing the first true leaf (Fig. 3 b), we found that the disease status remained severe in the nano‐silica and combined treatment groups. Importantly, the fullerene treatment group's disease status was severe when compared with other groups. In the combined treatment group, treatment with a high concentration still led to severe disease. In the nano‐silica treatment group, the two treatment conditions were comparable. As shown in Fig. 3 c in the second true cucumber leaf image, the experimental treatment group had different degrees of infection with downy mildew. This resulted in spot lesions. In the nano‐silica 10 mg/L combined with 10 mg/L Fullerene treatment group, the pathogenic area was observable in the images of downy, mildewed leaves. Notably, leaf growth in the fullerene‐treated group was faster than in other treatment groups and the pathogenic area was small.

To better understand this fullerene effect, we examined plant hormonal levels in plants. The cucumber plants shown in Fig. 4 were randomly selected and we concluded from these images that the plant body growth from the treatment groups was the same. This was also confirmed by analysing the root height and the plant height.

Fig. 4.

Fig. 4

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Selected images of cucumber plants

3.2 Analysis of cucumber plant fresh weight and plant height

Past work has shown that the interaction between nano‐materials and plants can influence plant growth [29, 30]. Given this, we next sought to determine whether nano‐materials affected plant growth. This was achieved by analysing plants' dry fresh weight (Fig. 5 a), root length (Fig. 5 c), and plant height (Fig. 5 c). Our results indicated that the spraying groups' dry weight was significantly different from that of the control group; however, there was no significant difference in plant height between the spraying groups and the control group.

Fig. 5.

Fig. 5

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Root length, plant height, fresh biomass, and dry biomass of cucumbers grown in solutions with different concentrations. The number of samples of each treatment is 3 and the meaning of the bars is ‘Error line’. The difference in letters represents a statistically significant difference (P < 0.05). Data are average of three replicates ± SE

3.3 SEM of plants

In the control group (Figs. 6 and 7 a), we observed dot‐like protrusions on both surfaces of the blade. The upper layer was also observed to be convex. There were obvious round, horny stripes, and the epidermis of the leaves had obvious folds in the stratum corneum. The stomata were relatively even. However, the guard cells and the surrounding epidermal cells had a clear decomposition formed by the stratum corneum in which the upper epidermis had fewer stomata with smaller openings. There were larger openings on the lower epidermis. The longitudinal axis of the stomatal opening was ∼9–10 μm with a horizontal axis of ∼3–4 μm. The openings were also circular in shape. The upper epidermis of the leaves was connected to the underlying cells with two, symmetrical inferior arcs that surrounded the epidermis.

Fig. 6.

Fig. 6

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Representative SEM images of the front side of the leaf

(a) Representative SEM image of the front side of the leaf of the control group, (b) Representative SEM image of the front side of the leaf of the nano‐silica 20 mg/L group, and, (c) Representative SEM image of the front side of the leaf of the joint processing 10 mg/L group

Fig. 7.

Fig. 7

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Representative SEM images of the backside of the leaf

(a) Representative SEM image of the backside of the leaf of the control group, (b) Representative SEM image of the backside of the leaf of the nano‐silica 20 mg/L group, (c) Representative SEM image of the backside of the leaf of the Fullerene 20 mg/L group, and, (d) Representative SEM image of the backside of the leaf of the joint processing 10 mg/L group

In the joint processing group (the 10 mg/L nano‐silica combined with 10 mg/L Fullerene treatment group) (Figs. 6 c and 7 d), we observed a pathogenic section on the backside of the leaf. The surface of the epidermal cells of the diseased section was smooth and showed the cells' morphology; the surface plant cuticle had fallen off, indicating that the cells in this area had become necrotic and were no longer able to perform normal biological activities. The remaining unaffected upper and lower epidermis were also raised when compared with the control group. However, we observed a difference in the state of the epidermis: in the surface of the island‐like projections, the plant cuticle was unevenly divided, and its size was random. The guard cells of the upper and lower epidermal stomata did not show significant decomposition when compared with their surrounding epidermal cells. The epidermis was surrounded by a disk‐shaped base which was surrounded by epidermal hairs.

In the group receiving SiO2 spray group (Figs. 6 and 7 b), we observed that the morphologies of the upper and lower surfaces of the blade were quite different. The upper epidermis had denser bulges than those of the control group and most had the appearance of round dots with irregular islands. The surface of the island‐shaped projections had a major axis >10 μm and showed uneven spots on most of the plant cuticle. The cells were also embedded within the epidermal cells. The lower surface plant cuticle was thin; epidermal cells were observed, but the boundary between cells was not obvious. Comparatively, the horny stripe was obvious. The stomata were linearly open and the pores had a long axis of ∼9–10 μm with a minor axis within 0–0.5 μm. The stomata were embedded within the epidermal cells.

In the C60‐treated group (Fig. 7 c), the plant cuticle of the upper surface of the leaf blade was not obvious; however, it was spotted. The plant cuticle streak was radiated and expanded. The stomatal density was larger than that of the control group and the stomata were closed. The long axis of the stomatal opening was ∼9–10 μm; the plant cuticle streak was evident in the stomatal guard cells, with a clear boundary between surrounding epidermal cells.

3.4 Determination of plant elemental content

Since the discovery by Wang and colleagues [31, 32] that the appropriate amount of exogenous silicon could enhance the tolerance of rice to cadmium, the beneficial effect of silicon on plant cadmium toxicity has been confirmed in a variety of other plants [17, 18, 19]. For instance, past work has shown that an important mechanism for silicon in alleviating plant poisoning is the reduction of cadmium absorption and transport by plants [32]. Given this, we measured the elemental metal(loid)s content in cucumber and found that there were significant differences in levels of Si and Cd in the silicon‐containing spray groups (Figs. 8 a and b). Among these, the 10 mg/L nano‐silica spray group had significantly lower Si content when compared with other treatment groups. However, our results indicated that while the 10 mg/L nano‐silica group did not have significantly increased silicon content in the plant, the Cd content of the nano‐silica reagent was significantly reduced when compared with that of the 20 mg/L‐treated group.

Fig. 8.

Fig. 8

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As and Cd concentration in plants. The number of samples of each treatment is 3 and the meaning of the bars is ‘Error line’. The difference in letters represents a statistically significant difference (P < 0.05)

We also compared the Cd content of the four silicon‐containing treatment groups relative to the control group. Our results indicated that the Cd content in the groups treated with high‐strength nano‐silica was significantly higher than that of the other treatment groups as well as the control group. Given this, we determined that the 20 mg/L treatment group resulted in excessive silicon content in the plants, but this increased concentration was unable to inhibit cadmium transport. Past work has shown that in plants – including cucumbers – repression of Cd absorption occurs during the process of transporting it from the underground [32, 33, 34]. Given this, we expected that the sprayed nano‐silica was transferred inside the plant and then transferred from the cucumber leaves to the rhizome junction; once there, it inhibited Cd absorption. A comprehensive analysis revealed that the treatment effect of the low‐concentration combined treatment group was the most efficacious, corresponding to a Cd concentration decrease of 61% relative to the control group. Therefore, the Cd content of the resulting fruit would be expected to be significantly improved; however, we did not specifically measure fruit metal(loid)s concentration and cannot make a definitive conclusion at this time. Currently, there are few experiments regarding the effect of fullerenes on the absorption of metal(loid)s by plants. As such, we are unable to make a conclusive determination regarding the relationship between combined treatment with fullerene and silicon and its effects on cadmium inhibition.

In the analysis of As in rice, we found that low concentrations of fullerenes significantly reduced the content of As (Fig. 8 c). At the same time, when analysing the Cd content in rice, we found that low concentrations of fullerenes can significantly reduce the absorption of Cd (Fig. 8 d) by plants, but the high concentration does not show this phenomenon. Due to the lack of previous studies on the absorption of heavy metals by fullerenes, the specific absorption mechanism needs to be further studied.

3.5 Determination of plant hormonal content

Plant hormones are a series of trace organic compounds produced in plants and play an important role in controlling plant growth and development [35, 36]. Plant hormones regulate many processes in plant growth and development, so the effect of nano‐tubes on plant hormones should be an important index of toxicity. Gibberellin (GA) is widely regarded as a regulator of plant growth and development, such as breaking dormancy, promoting germination [37, 38], stimulating stem elongation, and leaf expansion [39]. Indole‐3‐acetic acid (IAA) is the most common auxin in plants, which mediates plant growth and development, and also has the function of stress resistance, especially against stress from metals, such as aluminium and cadmium [40, 41, 42]; it can also alter the expression of relative genes to inhibit hypersensitive response [43]. Jasmonic acid (JA‐ME) can induce secondary metabolism of plants [44]. Indole propionic acid (IPA) is a plant growth regulator with auxin bioactivity, which can be absorbed by the roots, stems, leaves and flowers of plants. It has the function of promoting rooting, fruit setting and so on. Brassinosteroids (BR) plays a key role in the regulation of node cell proliferation, morphogenesis, apical dominance, leaf and chloroplast senescence, and gene expression. Intracellular BR biosynthesis or signal transduction defects often lead to abnormal cell proliferation, resulting in typical dwarf phenotype [45]. Desalination acid (ABA) is an important plant hormone that can participate in a variety of signal transduction pathways in plants [46], especially in the expression and inverse of ABA‐induced genes in terms of the adverse environmental effects. There are important interactions between environmental stresses, such as high temperature, low temperature, drought and other adverse conditions, which are called ‘adversity hormones’ [47]. Cytokinin (CTK) including ZR and DHZR plays an important role in the regulation of plant morphology, physiology and yield, and is one of the main factors regulating nitrogen absorption, transport, and metabolism [48].

Given this, we analysed the difference in hormonal content between the treatment groups to determine whether nano‐material application affected plant growth. An analysis of the underground portion of each plant revealed that the contents of JA‐ME (Fig. 9), ABA (Fig. 9), IPA (Fig. 9), and ZR (Fig. 9) in the control group were statistically significantly lower than those of the treatment groups. Further analysis revealed that GA4 content (Fig. 9) was statistically significantly different between the combined treatment and the control groups. However, the remaining treatment groups were not significantly different from the control group. Regarding GA3 (Fig. 9), low concentration of fullerenes and nano‐silica did not produce a significant difference from control levels; however, there was a significant difference in the remaining treatment groups between control groups. The content of DHZR (Fig. 9) was only different between the low‐concentration fullerenes and control groups; given this, it is clear that the nano‐materials had little effect on DHZR content [49]. Only the IAA content (Fig. 9) of the control group was significantly higher than the treatment groups. This is consistent with other studies [49], which have shown using nano‐materials results in differences in plant IAA levels.

Fig. 9.

Fig. 9

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Number of samples of each treatment is 3. Analyses of the hormonal content in each treatment group, including ABA, GA3, IPA, GA4, ZR, IAA, DHZR, JA‐ME, and HR. The experimental results showed that there was no significant difference in hormone content among the treatment groups, i.e. the nano‐material did not affect the growth of the plants

By analysing the above‐ground part of plants, we found a slight difference in ABA between the treatment groups in their aerial portions (Fig. 7); only the high concentration of fullerenes showed this difference. There was no significant difference in BR between the treatment groups, either above‐ or below‐ground. There was also no significant difference in IPA levels between the treatment groups (Fig. 9), which was also proved by Rui, M.M. et al. [50]. However, it is possible for hormone‐stabilised nano‐particles to act as ‘nanobullets’ to promote root growth and inhibit bacterial growth [51]. By combining our plant hormone results with those of plant height and plant dry weight, we found that while nano‐materials caused a change in plant hormonal content, it was not sufficient to affect plant growth, which was also proved by Yi Hao et al. [22].

4 Conclusions

Our experiments have found that the spraying of nano‐materials on the leaves does cause differences in biomass in plants, and this difference depends on the type of nano‐materials sprayed and the number of nano‐materials. Interestingly, when analysing the Cd content of cucumber, we found that 20 mg/L of nano‐silica did not reduce the absorption of Cd by cucumber, but made the cucumber rich in Cd, at this point we need to further explore the reasons. At the same time, we found that fullerene of 10 mg/L can significantly reduce the absorption of As and Cd by plants, which indicates that fullerene can slow down the toxicity of metals(loid)s. It also provides us with a way to solve soil Cd pollution, whether the method of foliar spray of nano‐materials on plants be used to enrich plants with Cd elements in soil? Thereby achieving the slowdown of Cd pollution. From the analysis of hormone content, it is not difficult to find that nano‐materials greatly affect the hormone content in cucumber, which leads to differences in the growth of cucumber. Some hormones are increased, and some hormones are reduced，whether this difference will be reflected in the yield of cucumbers remains to be confirmed by experiments. Based on the above points, we hope to further experiment on foliar spray, mainly focusing on the enrichment of heavy metals in soil by 20 mg/L of nano‐silica. Will spraying nano‐materials throughout the plant growth phase affect the final cucumber yield? If it has an impact, what is the direction of the impact? At the same time, we hope to study the plant toxicological effect of fullerene and its effect on the mechanism of metals(loid)s absorption by plants. This provides us with a new way of solving metals(loid)s pollution to a certain extent.

5 Acknowledgments

The project was supported by the National Key R＆D Program of China (2017YFD0801300 and SQ2017YFNC060064), the NSFC‐Guangdong Joint Fund (U1401234), and the National Natural Science Foundation of China (Grant No. 41371471).

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