Nanotechnology in Plant Growth Promotion and Protection. Группа авторов

oxygen species than both rutile and brookite. Brookite was the most inert out of the three (Lin et al. 2014; Wang and Fan 2014). However, amorphous TiO₂ material was found to be even more toxic than anatase toward human lung epithelial cells (Hsiao and Huang 2011).

Because of the relative inertness of brookite, the toxicity of anatase and rutile is more studied. The precise mode of action of anatase and rutile may differ. Anatase nanoparticles damaged the nucleus and cell membrane of algal cells whereas rutile nanoparticles were reported to cause damages to chloroplast and internal organelles (Iswarya et al. 2015). Considering these facts, two different studies have been performed to study the effect of such crystal phases on cucumber plants. The results obtained revealed a preferential translocation of rutile nanoparticles over their anatase form in fruits, leaves, and phloem (Servin et al. 2012, 2013). Although the mode of action is different for both anatase and rutile phase nanoparticles, their toxicity toward microscopic algae differs only at higher concentrations, and anatase was found to be more toxic (Iswarya et al. 2015). Yet, no difference in toxicity was found in the case of higher plants (Larue et al. 2012b). There are also studies that challenge the notion that anatase is more toxic to plants, such as in the case of Silva et al. (2016, 2017) where anatase was found to be less toxic than a mixture of anatase and rutile.

      Similarly, surface coating of nanoparticles can both increase or decrease toxicity toward organisms since the surface is the active region for nanoparticle interaction with biota and is determined by its chemical activity (Foltête et al. 2011; Wang and Fan 2014; Cox et al. 2016; Šebesta et al. 2019). The reports on comparing the influence of surface coating of TiO₂NPs toward plants are relatively scarce. Tan et al. (2017) synthsezied TiO₂NPs with a different surface coating such as unmodified, hydrophilic, and hydrophobic and evaluated their toxicity in basil (Ocimum basilicum). The findings recorded showed the differential toxicity for each type of nanoparticles in tested plants. Unlike hydrophobic TiO₂NPs, unmodified and hydrophilic TiO₂NPs had no negative effect on starch content and root elongation. The hydrophilic and hydrophobic TiO₂NPs significantly reduced the seed germination and only unmodified and hydrophobic were reported to significantly decrease the shoot biomass. There were also differences in uptake of important nutrients and coated TiO₂NPs had a greater effect on the nutritional quality of the plant (Tan et al. 2017). The surfactant in paints containing TiO₂NPs was proposed to ease their internalization in plants (Larue et al. 2014).

2.3 Pathways and Interaction of TiO₂NPs with Plants

Plants can be exposed to TiO₂NPs both intentionally and unintentionally. Usually, there are three major pathways through which different plants can be exposed to TiO₂NPs which include (1) foliar exposure, (2) root exposure, and (3) exposure to TiO₂NPs after application on seeds (Figure 2.1). These pathways significantly affect the interaction of TiO₂NPs with plants and their possible toxic or beneficial effects that these particles have on plants.

      2.3.1 Foliar Exposure

Upon foliar application, TiO₂NPs were taken up by leaves and were found in epidermis, parenchyma, and vascular tissues in both apoplastic and symplastic compartments (Larue et al. 2014). Several mechanisms have been proposed for underlying nanoparticle internalization. Both stomatal and cuticular pathways were suggested. TiO₂NPs were found at stomatal openings, in sub‐stomatal chambers and vacuoles of guard cells (Kurepa et al. 2010; Larue et al. 2014). In addition, TiO₂NPs were also reported to be entrapped in the cuticle thereby damaging it and the cell walls (Larue et al. 2014). Absorption through cell membranes probably occurred through endocytosis and TiO₂NPs were observed in endosomes (Kurepa et al. 2010; Larue et al. 2012a).

Figure 2.1 Three main ways of TiO₂ nanoparticle applications with their differences.

      2.3.2 Root Exposure

Roots of plants can be exposed to TiO₂NPs in both solid substrates such as soils and in liquid substrates where plants were grown hydroponically (Landa et al. 2012; Kořenková et al. 2017; Tan et al. 2017). Nanoparticles generally, and TiO₂NPs specifically tend to adsorb persistently and remain stuck to the root epidermis. Part of the detected TiO₂NPs in roots should always be considered adsorbed to the epidermal cells (Larue et al. 2016). Titanium (Ti) is a naturally occurring element in soils and an increase in Ti root accumulation was observed only in soils contaminated with 125 mg/kg of Ti and above (Tan et al. 2017). When applied on roots, TiO₂NPs were not significantly transported to shoots (Du et al. 2011; Larue et al. 2012a; Larue et al. 2016). Only a few studies are available demonstrating the translocation of TiO₂NPs from root to shoot (Servin et al. 2012, 2013; Larue et al. 2016; Kořenková et al. 2017), however, rutile being preferentially translocated to the leaves (Servin et al. 2012). Even 100 nm TiO₂ nanoparticles have been transferred from roots to leaves in Nicotiana tabacum (Ghosh et al. 2010). It was observed that the rate of TiO₂NPs translocation from roots to shoots is comparatively lower than some other nanomaterials, such as a copper oxide (CuO) and cerium oxide (CeO₂) nanoparticles (Perreault et al. 2014; Barrios et al. 2017). The inhibited translocation may be partially explained by the fact that many cell‐wall pores in plants have a small diameter. Vicia faba was reported to have an average diameter of cell wall pores of 10–14 nm and in some cases up to 20 nm (Hylmö 1955, 1958). Asli and Neumann (2009) reported an average cell wall pore diameter of 6.6 nm for corn (Zea mays). The nanoparticles may be diffused into root tissues through the intercellular spaces without directly entering the cells. Some of the smaller TiO₂NPs may be taken up by endocytosis through root hair (Ovečka et al. 2005).

      2.3.3 Seed Exposure

      TiO₂NPs were also used in the evaluation of their efficacy in seed germination of agriculturally important plants. Both laboratory and greenhouse trials found positive effects of TiO₂NPs on seed germination at various concentrations, however, they showed variations depending on plant species. TiO₂NPs are thus considered as priming agents (Haghighi and Teixeira da Silva 2014). The reported positive effects mainly include increased water absorption in spinach (Zheng et al. 2005) and flax (Clément et al. 2013) through an increase in length and weight of rape, tomato, and onion seedlings (Su et al. 2009; Haghighi and Teixeira da Silva 2014). Both time duration and concentration of nanoparticles are important factors when seeds were soaked in suspensions of TiO₂NPs (Su et al. 2009). Soaking of seeds is also more effective than direct application of nanoparticles to soil with seed planting (Haghighi and Teixeira da Silva 2014). The effect of TiO₂NPs on seed germination is concentration‐dependent, higher concentrations were found to have a negative effect on seed germination (Ruffini Castiglione et al. 2011). Higher concentrations might induce moisture stress and negatively affect water and oxygen uptake (Laware and Raskar 2014).

      2.3.4 Interaction of TiO₂NPs with Plants

      TiO₂NPs are considered to be plant‐growth stimulants (Liu and Lal 2015; Faraz et al. 2020; Kolenčík et al. 2020; Sun et al. 2020). The response of plants to these nanoparticles occurs on many levels. Physiologically, it was observed both positive and negative response in growth parameters like root and shoot length, dry and fresh weight, the content of chlorophylls, gluten and starch, and seed production (Zheng et al. 2005; Asli and Neumann 2009; Ruffini Castiglione et al. 2011; Larue et al. 2012b; Jaberzadeh et al. 2013; Raliya et al. 2015a,b).

      Leaf growth and transpiration may be affected via physical effects such as clogging which hinder root water transport (Asli and Neumann 2009) although there are few studies demonstrating that TiO₂NPs