Nanotechnology in Plant Growth Promotion and Protection. Группа авторов
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5 Faculty of Chemistry, Brno University of Technology, Brno, Czech Republic
6 Nanotechnology Centre, VŠB Technical University of Ostrava, Ostrava, Czech Republic
7 Department of Soil Science and Geology, Faculty of Agrobiology and Food Resources, Slovak University of Agriculture in Nitra, Nitra, Slovakia
2.1 Introduction
To date, titanium dioxide (TiO2) mainly in its nanoforms was found to be one of the most useful and effective materials. Various promising properties of TiO2 nanoparticles (TiO2NPs) such as its semiconductor and photocatalyst nature and other beneficial characteristics for plants attracted a great deal of attention for their applications in agriculture (Prasad et al. 2014, 2017; Wang et al. 2016; Mattiello et al. 2018; Kolenčík et al. 2019a). Nowadays, a variety of nanomaterials are increasingly used in different sectors including agriculture because of their unique or favorable properties arising from their minuscule size. Nanomaterials are defined as materials which have at least one dimension between1 and 100 nm. The definition includes a subgroup of nanoparticles with a defined size of 1–100 nm in three dimensions (CODATA‐VAMAS Working Group On the Description of Nanomaterials and Rumble 2016; Šebesta and Matúš 2018). This definition is sometimes broadened and includes slightly larger particles that display nanomaterial behavior in organisms (Cox et al. 2016; Kolenčík et al. 2019b).
Engineered TiO2NPs are synthesized with one of several crystalline structures (most commonly anatase, rutile, or brookite), each of them having unique properties (Macwan et al. 2011). Hydrolysis of titanium salts in an acidic solution is the most commonly used method for the synthesis of TiO2NPs (Mahshid et al. 2007). Moreover, it was proposed that the structure, size, and shape of TiO2NPs can be controlled by the use of chemical vapor deposition (Li et al. 2002) or nucleation from sol–gel synthesis (Liao and Liao 2007). TiO2NPs (and other TiO2 nanomaterials) are often coated with aluminum, silicon, or polymers to adjust their photo‐stability and agglomeration/aggregation (Carlotti et al. 2009; Labille et al. 2010).
Nanomaterials made with TiO2 have broad use in nearly all human activities. They are used in various products like paints, cementitious composites, catalytic coatings, plastics, paper, pharmaceuticals, and sunscreen having applications including packaging, commercial printing inks, other cosmetics, toothpaste, and food (Weir et al. 2012; Li et al. 2018; Baranowska‐Wójcik et al. 2020). Besides, TiO2 nanomaterials are extensively used as photo‐catalysts in many chemical processes at the industrial level (Lan et al. 2013) and also used in photovoltaic cells (Gong et al. 2017). In addition, properties possess by TiO2 nanomaterials are suitable for a variety of environmental and biomedical applications such as water purification, photocatalytic degradation of pollutants, biosensing, antimicrobial coatings, and drug delivery (Mahlambi et al. 2015; Han et al. 2016; Jarosz et al. 2016; Yan et al. 2017; George et al. 2018). In analytical chemistry, TiO2 nanomaterials are used for extraction and detection of elements, and inorganic and organic compounds (Matúš et al. 2009; Hagarová et al. 2012a,b,c; Hagarová et al. 2013; Hagarová 2017, 2018; Gavazov et al. 2019; Nemček and Hagarová 2020).
Along with all these potential uses the applicability of TiO2NPs in agriculture has been assessed in the past few years and it was observed that these nanoparticles play a pivotal role in the enhancement of plant growth, plant seed protection and enhanced germination, crop disease control (Servin et al. 2015), degradation of pesticides, pesticide residue detection (Aragay et al. 2012), and aforementioned water purification (Kumar and Bansal 2013; Prasad et al. 2014, 2017; Reddy et al. 2017). Considering these facts, the present chapter aimed to explore how the properties and modes of interaction of TiO2NPs with plants affect the growth, health, and yield of plants and especially crops. Moreover, the properties of TiO2NPs that affect the biology of plants are discussed. Positive effects of TiO2NPs are also briefly discussed. In addition, gaps in our understanding are described by proposing four areas of research that need to be studied in the foreseeable future. We believe that the present chapter will definitely help biotechnologists, agronomists, and food technologist to realize the value of TiO2 nanomaterials application.
2.2 Properties of TiO2NPs Important for Biological Interaction
The methods used for the synthesis can profoundly change the properties of TiO2NPs that affect their biological interaction. Among the various properties, size and shape of TiO2NPs, their crystal structure, and surface coating are some of the most important properties found to affect the interaction of nanoparticles with other systems. Furthermore, the above‐mentioned properties also affect the surface area of nanoparticles, as well as their agglomeration/aggregation properties, generation of reactive oxygen species, and their ability to react with cell structures upon contact.
The size of the TiO2NPs has a twofold effect on the interaction with organisms. (1) Lower size corresponds to the higher surface area that can interact with the surfaces of organisms and thus negatively or positively affect the organism to a greater extent. Smaller nanoparticles were also observed to attach more easily to cell walls (Lin et al. 2014). The higher surface area can also generate more reactive oxygen species under sunlight illumination and thus more negatively affect the organism (Kim et al. 2014). An increase in malondialdehyde, another reactive compound was also observed to increase with decreasing size of TiO2NPs (Lin et al. 2014). (2) Particles of smaller size more readily pass through the epidermis and other membranes via pore structures. Thus, they can enter deeper and in larger concentrations into the organism and interact more strongly with plants' vital parts (Larue et al. 2012b). Also, it was reported that the pattern of accumulation in plant tissues may change depending on the size of nanoparticles. Larue et al. (2012a) observed that 14 nm TiO2NPs can easily accumulate in root parenchyma, whereas, 25 nm TiO2NPs accumulated primarily in the vascular cylinder. Overall, the smaller the size of the nanoparticles, the higher the toxicity observed at the same concentration (Wang and Fan 2014; Sun et al. 2015). There is a size limit to which nanoparticles have an effect on plants. Positive effects, such as germination and root elongation have been observed in smaller nanoparticles, but the same effect was not observed in larger particles and bulk TiO2. (Zheng et al. 2005; Larue et al. 2012a,b).
In plants, two thresholds for root exposure were suggested by Larue et al. (2012a) when experimenting on wheat: (1) TiO2NPs having a threshold diameter of 140 nm and above do not accumulate in roots, and (2) TiO2NPs having a threshold diameter of 36 nm can be accumulated in root parenchyma, but cannot translocate to plant parts above the ground. This threshold proposed for wheat can serve as an approximation for root uptake thresholds of other plants although there were some variations dependent on plant species (Larue et al. 2012b). When applied on leaves, there was a size‐exclusion limit higher than 100 nm in lettuce and they were internalized in parenchymatic tissues (Larue et al. 2014).
Not only the size of the TiO2 nanomaterials, but their shape is also very important. Nanospheres of TiO2 were found to be less toxic than other shapes, such as nanorods, nanowires, nanotubes, and nanobelts (Porter et al. 2012; Silva et al. 2013; Yeo and Nam 2013; Wang and Fan 2014; Landa et al. 2016). One of the proposed reasons is a higher surface area of the elongated shapes nanomaterials like nanorods and other similar shapes. Nevertheless, a study by Hsiao and Huang (2011) demonstrated that nanorods with the same surface area as that of TiO2 nanospheres showed higher toxicity, proposing the possible reason is that the area in contact with cells of an organism is more important than the whole surface area. However, some studies performed on plants show little to no difference between bulk TiO2, TiO2 nanospheres, and nanowires (Landa et al. 2016).
The surface of the TiO2NPs is strongly affected by their crystal structure and different crystal phases of TiO2 display varying properties. Four different crystal phases of TiO2 were synthesized in form of nanoparticles, that is, amorphous, anatase, rutile, and brookite. There was a consensus that the anatase phase