The Vital Role of Nanoparticles in Enhancing Plant Growth and Development ^†

Abstract

Nanotechnology has facilitated various applications in food and agricultural industries due to the unique characteristics of nanoparticles (NPs), such as their large surface area, reactivity, tendency to agglomerate, ability to penetrate, and specific size and structure. The exploration of nanoparticles in agriculture is gaining interest because of their ability to minimize the use of chemical fertilizers and significantly enhance plant growth. Several studies have demonstrated NPs’ effects on plants, depending on their size, shape, concentration, coatings, self-aggregate tendency, and stability. Here, we provide an overview of the applications of recently developed nanoparticles, including metallic and non-metallic nanoparticles and carbon-based nanomaterials.

1. Introduction

Nanotechnology plays a vital role in improving methods for monitoring ecological conditions, managing pathogenicity, managing crop capacity (for photosynthetic respiration), increasing nutrient or pesticide absorption, and accelerating flowering and seed germination. NPs have proven to be highly beneficial for plant development [1]. The initial stages of plant growth and development begin with seed germination, followed by root elongation and shoot emergence [2]. Seed germination forms the foundation for the productivity, growth, and overall development of plants. Significant improvements in both germination rates and overall yield have been achieved with NPs-treated seeds [3]. Various studies have revealed that controlled concentrations and moderate doses of NPs greatly promote seed germination [4] and facilitate robust plant growth. Additionally, they have been shown to enhance photosynthetic efficiency and increase chlorophyll content [5,6,7], as well as improve water and fertilizer utilization (Figure 1). In this study, we present a summary of the applications of recently developed NPs, including metallic, non-metallic, and carbon-based nanomaterials, which significantly enhance plant development by improving growth, stress resistance, and nutrient uptake. AgNPs boost growth and protect mustard plants from pathogens [8], while copper NPs enhance stress tolerance in Arabidopsis [9]. Fe₂O₃NPs improve iron uptake and photosynthesis in soybeans [10], while AuNPs promote seed germination and growth in wheat [11]. Silicon NPs increase drought tolerance in tomatoes [12], while ZnONPs enhance growth and nutrient uptake in maize [13]. Despite their benefits, NPs can also exhibit toxic effects under certain conditions. Their beneficial or harmful impacts significantly depend on dosage, exposure duration, and the specific type of NPs used.

2. Nanoparticles for Enhancing Plant Growth and Development

2.1. Silver Nanoparticles (AgNPs)

AgNPs are currently the most extensively manufactured nanomaterial, and they are incorporated into a wide range of agricultural applications. Moderate doses of AgNPs have been shown to increase the level of chlorophyll and carbohydrate contents, improve photosynthetic efficiency in different plant species, and improve water and fertilizer utilization [14,15]. Shaikhaldein et al. (2020) studied the morphophysiological effects of AgNPs on Maerua oblongifolia (M. oblongifolia) in vitro. Different concentrations of AgNPs exhibited distinct effects on the morphological characteristics like shoot number, shoot length, fresh weight, dry weight, and leaf number of M. oblongifolia compared to the control group. Notably, 20 mg/L concentration of AgNPs increased the shoot length, fresh weight, and dry weight, along with the highest levels of chlorophyll a and chlorophyll b (Figure 2). Moreover, concentrations of 20, 30, and 40 mg/L of AgNPs resulted in the highest number of shoots (Figure 3). This demonstrates the concentration-dependent impact of AgNPs on the growth and development of M. oblongifolia [16].

Furthermore, in cowpea, Brassica sp. [17], fenugreek [18], and Lupinus termis [19], the germination percentage was notably higher in AgNP-treated plants compared to the control. AgNPs are also utilized as fungicides for preventing fungal diseases and are employed as agents to facilitate the ripening of fruits [20,21].

2.2. Zinc Oxide Nanoparticles (ZnONPs)

In agricultural practices, ZnONPs serve multiple functions, acting as fertilizers, pesticides, herbicides, and growth regulators [22,23]. Sarkhosh et al. (2022) investigated the effect of various concentrations of ZnONPs on the growth of Camelina sativa and Brassica napus L. (B. napus) crops. In the case of Camelina, the highest germination percentage was observed in seeds treated with 10 ppm ZnONPs compared to the control condition (Figure 4a), and for B. napus, the maximum germination percentage was recorded at a concentration of 5 ppm ZnONPs (Figure 4b) [24].

The application of ZnONPs demonstrated a significant enhancement in various physiological and growth factors of wheat [25]. Jankovskis et al. (2022) tested the effects of ZnONPs on wheat (Triticum aestivum L.), which significantly increased the chlorophyll b content at 1 mg/L and increased the leaf length at 4 mg/L; meanwhile, although root length increased at 1 mg/L, it decreased at 2 and 4 mg/L concentrations of ZnONPs (Figure 5) [26].

2.3. Copper Nanoparticles (CuNPs) and Copper Oxide Nanoparticles (CuONPs)

Various research studies have presented evidence demonstrating the antimicrobial application of CuNPs against a range of phytopathogens [27,28,29]. Ntasiou et al. (2021) synthesized and modified four CuNPs (CuNPs Type 1 to 4) by means of the wet chemistry approach. Among these, CuNPs Type 3 and CuNPs Type 4 demonstrated potent protective activity against both Fusicladium oleagineum and Colletotrichum acutatum. Their control efficacy values were notably higher as compared to the reference commercial compounds, copper oxide and copper hydroxide (Figure 6) [29].

Badawy et al. (2021) discovered that CuONPs improve the growth metrics of wheat plants [30]. The impact of CuONPs on Indian mustard (Brassica juncea L.) differed based on the application method. Faraz et al. (2022) applied CuONPs as a foliar spray on Brassica juncea plants, leading to augmented growth, improved antioxidant capacity, and an elevated photosynthetic rate. However, at higher concentrations, the effect on growth was found to be minimal [31]. In another work, irrigation with CuONPs raised the levels of macro- and micronutrients in the roots of L. sativa plants [32].

2.4. Iron-Based Nanoparticles

Iron-based NPs possess an intriguing characteristic in that they have the capacity to influence stomatal opening, germination, and seedling growth through various complex effects [33,34]. They also demonstrate the potential to shield plants from drought, potentially by mitigating the oxidative stress induced by water deficiency [35]. Additionally, treated plants exhibit heightened chlorophyll biosynthesis [36]. FeNPs have proven to be effective in the remediation of heavy metals in both soil and water [37,38]. Plaksenkova et al. (2019) studied the impact of Fe₃O₄NPs on rocket (Eruca sativa Mill.). They investigated the changes in morphology, chlorophyll contents, germination rate, genotoxicity, and level of miRNA expression at 1, 2, and 4 mg/L concentrations of Fe₃O₄NPs, which positively affected the growth and development of rocket plants. Fe₃O₄NPs significantly increased the germination percentage and shoot and root length. At a 4 mg/L concentration of Fe₃O₄NPs, the maximum germination rate (Figure 7a) and shoot length (Figure 7b) were observed. However, a decreased root length was observed at this concentration as compared to control. The chlorophyll a content and miRNA expression level increased at a 1 mg/L concentration (Figure 7c,d) [39].

When rice seeds (cv. Gobindabhog) were exposed to FeNPs during germination, they displayed heightened seedling growth, cell viability, and cell membrane integrity, higher chlorophyll contents, and increased metabolic activity [40]. Furthermore, in a study conducted by Khan et al. (2020), the impact of Fe₃O₄NPs on rice plants subjected to arsenic stress was examined. They observed that a lower concentration of Fe₃O₄NPs significantly lowered the arsenic levels and facilitated plant growth. Conversely, a higher concentration failed to produce the same effects. These studies indicated that the effects of Fe₃O₄NPs varied depending on their dosage [41].

2.5. Titanium Dioxide Nanoparticles (TiO₂NPs)

TiO₂NPs have been shown to have an impact on various aspects of plants, including growth cell division, callus induction, cell size, and hormone levels (specifically Gibberellins and cytokinins), resulting in a significant increase in treated Rosmarinus officinalis plants [42]. TiO₂NPs increased the levels of photosynthetic pigments in the wheat plants under drought stress [43]. The favourable effects of TiO₂NPs on seed germination have also been observed in chickpea (Cicer arietinum) and in fodder crops like berseem and oat [44,45]. Recently, Shah et al. (2021) reported a significant impact of TiO₂NP priming on seeds of maize (Zea mays L.) plants under salinity stress (Figure 8) [46].

2.6. Carbon-Based Nanomaterials

Carbon nanotubes (CNTs) have demonstrated a positive impact on tomato germination and seedling growth [47]. Additionally, research on six plant species including maize, soybean, rice, barley, tobacco, tomato and switchgrass demonstrated that single-walled carbon nanohorns (SWCNHs) enhanced the seed germination of these plant species [48]. Recently, Ren et al. (2020) discovered that the addition of single-walled carbon nanotubes (SWCNTs) to the plant vitrification solution significantly improved the relative survival rate in the cryopreservation of Agapanthus praecox embryogenic callus. The callus treated with SWCNTs exhibited elevated levels of antioxidants, including SOD, and POD, in comparison to the control group (Figure 9) [49].

2.7. Other NPs

Various studies have demonstrated the positive effect of NPs on seed germination, plant growth, and plant development. Studies have shown that silica nanoparticles (SiO₂NPs) enhanced the rate of germination of rice, thyme, broad bean, sunflower and lettuce [50,51,52,53,54]. Lemongrass (Cymbopogon flexuosus), known for its medicinal properties and essential oil production, experiences improved growth and yield with the application of silicon nanoparticles (SiNPs) [55]. Jayarambabu et al. (2016) demonstrated that magnesium nanoparticles (MgNPs) positively affect seed germination in maize plants [56]. Even though cerium is not considered an essential element for plants, it has been shown to have positive effects on various aspects including seed germination, plant growth, yield, as well as chlorophyll and saccharide content [57]. It inhibited the accumulation of cadmium [58]. Chitosan, a naturally occurring compound found in the exoskeleton of crustaceans, has been extensively studied for its effects on cell signalling molecules [59], film-forming ability, and antibacterial properties [60].

3. Toxic Effects of NPs

The above examples illustrate how NPs can positively influence plant growth and development, but NPs also show toxic effects. Both their toxic and beneficial impacts depend on factors like concentration, size, surface properties, and plant species. At low concentrations and under controlled conditions, NPs can enhance plant growth by improving nutrient uptake, increasing photosynthesis, and strengthening stress tolerance. However, at higher concentrations or with prolonged exposure, they may generate excessive reactive oxygen species (ROS), causing oxidative stress that damages cellular structures like membranes, proteins, and DNA. NPs can also interfere with water and nutrient absorption, disrupt enzyme activities, and release toxic metal ions, all of which impair plant growth [61,62,63].

To mitigate the toxic effects of NPs on plant growth and development, several strategies can be employed. Using NPs in controlled, low concentrations is essential to prevent overexposure. Surface modifications with biocompatible materials or the use of controlled-release systems can limit toxicity by reducing metal ion release and oxidative stress [64]. Soil amendments and phytoremediation plants can help to reduce NPs’ bioavailability in the soil. Additionally, antioxidant supplements can protect plants from ROS damage, while controlling NPs’ size and using biodegradable or eco-friendly NPs can further minimize long-term environmental effects [65]. These approaches can help to balance the positive uses of NPs in agriculture while reducing their negative effects on plant health and growth.

4. Conclusions

NPs offer several advantages over traditional fertilizers, making them a superior option for promoting plant growth through their targeted delivery, enhancing absorption and minimizing losses due to leaching. They also offer multifunctional capabilities by combining growth promotion, disease resistance, and stress tolerance, while improving soil quality by enhancing soil structure and microbial activity. This literature survey shows that controlled concentrations and moderate doses of NPs greatly promote plant growth. It is conceivable that the long-term use of NPs may impact the environment due to their accumulation in the soil [66,67,68]. Further exploration of NPs is required to enhance the growth of agricultural products and to minimize environmental challenges.