with their differences."/>
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 promote water uptake (Zheng et al. 2005; Clément et al. 2013; Jaberzadeh et al. 2013). Promotion of root growth may therefore happen not only because of the improved conditions by TiO2NPs but also as an avoidance mechanism arising from the stress caused by nanoparticles (Barrena et al. 2009; Feizi et al. 2013b).
Treatment with TiO2NPs was shown to protect chloroplasts from aging with prolonged stability of chloroplast under illumination. Suggested mechanisms involve a significant increase in catalase, peroxidase, and superoxide dismutase, a decrease in the concentration of reactive oxygen species and the content of malondialdehyde. Moreover, at the molecular level, the reduction rate of ferricyanide, the rate of oxygen evolution in chloroplasts, and the rate of noncyclic photophosphorylation activity of chloroplasts were found to be higher. In addition, treatment with TiO2NPs also showed improvements in chloroplast coupling, and activities of Mg2+‐ATPase and chloroplast coupling factor I (CF1)‐ATPase on the thylakoid membranes (Hong et al. 2005). Similarly, the overall increase in chlorophyll concentration was also observed (Servin et al. 2013; Raliya et al. 2015a).
However, the application of TiO2NPs at high concentrations was found to be toxic. The decreased growth was accompanied by a lowered mitotic index, increase in reactive oxygen species, antioxidant activity, and genotoxicity (Rafique et al. 2015). In microalgae, the genotoxicity is connected with the absence of an intact nucleus (Dalai et al. 2013). Ghosh et al. (2010) simply reported DNA damage in N. tabacum.
TiO2NPs also affect the uptake and homeostasis of essential elements. It was proposed that the accumulation of essential elements like Cu and Fe was significantly higher under influence of pristine TiO2NPs (Tan et al. 2017). Kužel et al. (2003) reported a similar effect on homeostasis in plants grown with dissolved Ti4+ citrate. The proposed mechanism involved suggested that Ti causes apparent Fe deficiency resulting in upregulation of transport of divalent ions and accumulation of Fe, Zn, and possibly also Cu. A higher conversion of inorganic nitrogen to its organic form was observed in spinach (Yang et al. 2006) and a higher accumulation of K and P was also observed in cucumber treated with TiO2NPs (Servin et al. 2013).
2.4 Effect of Different Concentrations of TiO2NPs on Plants
Both dissolved elements and some nanoparticles, including TiO2NPs, have concentration‐dependent behavior. The concentration range for positive or negative effects may be largely affected by the size and surface of particles and the means of application. Different plant species are also more or less tolerant of different concentrations of TiO2NPs. There were few general trends already well established for dissolved elements, a similar pattern was observed in the case of TiO2NPs. The application of low concentrations does not show any observable positive effects. At a certain higher concentration range positive effects show up. However, a further increase in concentration induces toxicity. The toxicity is often dependent on concentration and higher concentrations lead to higher toxicity (Kořenková et al. 2017). There are also some nano‐specific behaviors. The higher concentrations of TiO2NPs may induce enhanced aggregation of particles and the increased size of aggregates may lead to lower toxicity in hydroponic experiments (Clément et al. 2013; Kořenková et al. 2017). In a hydroponic experiment conducted by Kořenková et al. (2017), the concentration of TiO2NPs used between 150 and 600 mg/L led to a significant reduction in root length with concentration. However, at 1000 mg/L, the length increased as compared to plants grown at 400 and 600 mg/L.
Experimental design, such as the choice between hydroponic growth and growth in soil or solid substrate, the pathway of application, and its timing change the concentration range of TiO2NPs at which they have positive or negative effects on plants. Application of nanoparticles on leaves and hydroponic growth tend to induce response at lower
