in tomato plants in all concentrations except at 1000 mg
kg
−1 . TiO 2 -NPs treatment (up to 250 mg kg
−1 ) of soil
increases the length of root at low concentration but the
higher concentrations do not show any significant variations
in the root growth. Asli and Neumann (2009) reported that
TiO 2 -NPs treatments (30 and 1000 mg l
−1
) promote the
leaves growth in stunted maize plant.
On the contrary, the study of Da Costa and Sharma
(2015) pointed toward phytotoxicity of TiO 2 -NPs on Oryza
sativa and observed a decrease in seedling growth on
exposure to TiO 2 -NPs at 1000 ppm. Song et al. (2013)
observed the inhibitory effects of TiO 2 -NPs on germination
and seedling growth of Brassica napus, Lactuca sativa L.
and Phaseolus vulgaris. Additionally, TiO 2 -NPs application
under different experimental conditions negatively affects
the growth and development of some plants like Allium
cepa, Lemna minor, Oryzae sativa, Hordeum vulgare and
Zea mays by altering physiological and biochemical processes (Table 2).
3.2.2 Phytotoxicity at Physiological Level
The impacts of TiO 2 -NPs application on plants at the
physiological level could be inspected by observing the
chlorophyll content, nutrients uptake, transpiration rate,
photosynthetic rate, stomatal conductivity and alteration of
hormones. TiO 2 -NPs regulate the activities of all enzymes
related to nitrogen metabolism, including nitrate reductase,
glutamine synthase, glutamate dehydrogenase and glutamine–pyruvic transaminase, etc. which facilitate the
absorption of active nitrogen in plants in form of nitrate and
help in the conversion process of inorganic nitrogen into
organic nitrogen in form of protein and chlorophyll molecules, and improve the rate of photosynthesis that could
ultimately reflect as improved biomass and dry mass of
treated plant (Yang et al. 2007; Mishra et al., 2014). TiO 2 -
NPs treatment at low concentration facilitate the absorbance
of minerals which in turn promotes the chlorophyll formation and activation of key enzymes for carbon fixation, but at
high dose TiO 2 -NPs produce ROS under light which would
disrupt the membrane structure, therefore, reduce photosynthesis leading to reduced biomass in spinach (Zheng et al.
2005).
Raliya et al. (2015b) reported that foliar spray of
10 mg l
−1 concentration of TiO 2 -NPs on 14-day-old Vigna
radiata plants significantly improve chlorophyll content and
total soluble protein content in leaves by 446.4 and 94%,
respectively. TiO 2 -NPs treatment also enhanced chlorophyll
content in Solanum lycopersicum (Raliya et al. 2015b),
Linum usitatissimum (Aghdam et al. 2016), Triticum aestivum (Rafique et al. 2018), Phaseolus vulgaris (Ebrahimi
et al. 2016) and Hordeum vulgare (Janmohammadi et al.
2016) too. It is reported that appropriate doses of TiO 2 -NPs
protect chlorophyll from degradation and increase its
synthesis by reducing H 2 O 2 content in the cells and also
enhance seed oil and protein contents (Aghdam et al. 2016;
Ebrahimi et al. 2016). Due to photocatalytic properties,
TiO 2 -NPs can stimulate photosynthetic efficiency by
enhancing light absorption capacity and they can also alter
profiling of biochemicals such as amino acid and fatty acids
(Rui et al. 2018). Zheng et al. (2005) observed that the rutile
TiO 2 -NPs showed photo-oxidation–reduction reactions
which could accelerate the electron transport and the transformation from electric energy to active chemical energy like
ATP, promoting the activity of the rubisco activase, and
therefore, enhancing the photosynthetic activity. In Hordeum
vulgare, foliar application of TiO 2 -NPs reported to improved
defense mechanism, and increase biosynthesis of phytohormones, photosynthetic pigments and photosynthetic efficiency (Janmohammadi et al. 2016).
Seeds of Solanum lycopersicum soaked in TiO 2 -NPs
solution also showed an enhancement of net photosynthesis
(PSII activity), transpiration and conductance (Qi et al.
2013). Some studies reported the positive impacts of TiO 2 -
NPs on chlorophyll content in algal system, including Picochlorum sp. (Hazeem et al. 2016), Scenedesmus sp. and
Chlorella sp. (Sadiq et al. 2011). Gao et al. (2013) demonstrated that the treatment of anatase-TiO 2 -NPs reduced
photosynthetic rate and transpiration rate by reducing the
photosynthetic efficiency of mesophyll cells of leaves not by
regulating stomatal activity. In Zea mays, TiO 2 -NPs exposure remarkably decreased the transpiration rate by reducing
cell wall pore size and root hydraulic conductivities in a
concentration-dependent manner (Asli and Neumann 2009).
In addition, TiO 2 -NPs can affect the membrane integrity
which consequently modifies the uptake mechanism of water
as well as of nutrients. Apart from this, TiO 2 -NPs are
reported to be augmenting the performance in plants grown
under various stresses, which will be discussed later.
The risks of TiO 2 -NPs to plants due to its high sensitivity
toward cytotoxic and genotoxic effects, clogging of pores
and barriers in apoplast stream leads to interruption in
nutrients uptake, and thus, causes toxicity (Mattiello et al.
2015; Santos Filho et al. 2019). Despite, TiO 2 -NPs’ treatment in seedling augment growth and developmental processes by increasing light absorption, chlorophyll content
and photosynthesis in treated plants (Yang et al. 2007;
Raliya et al. 2015b). Further, Tumburu et al. (2015) provided
a genetic basis and observed that TiO 2 -NPs treatment
increased the expression of various transcripts responsible
for root development and cell differentiation.
3.2.3 Phytotoxicity at Biochemical Level
The mechanism of plant responses to TiO 2 -NPs exposure
could be better assessed by observing ROS generation,
oxidative damages H 2 O 2 content, malondialdehyde
(MDA) level, electrolyte leakage and enzymatic and
Interaction of Titanium Dioxide Nanoparticles …
59
kg
−1 . TiO 2 -NPs treatment (up to 250 mg kg
−1 ) of soil
increases the length of root at low concentration but the
higher concentrations do not show any significant variations
in the root growth. Asli and Neumann (2009) reported that
TiO 2 -NPs treatments (30 and 1000 mg l
−1
) promote the
leaves growth in stunted maize plant.
On the contrary, the study of Da Costa and Sharma
(2015) pointed toward phytotoxicity of TiO 2 -NPs on Oryza
sativa and observed a decrease in seedling growth on
exposure to TiO 2 -NPs at 1000 ppm. Song et al. (2013)
observed the inhibitory effects of TiO 2 -NPs on germination
and seedling growth of Brassica napus, Lactuca sativa L.
and Phaseolus vulgaris. Additionally, TiO 2 -NPs application
under different experimental conditions negatively affects
the growth and development of some plants like Allium
cepa, Lemna minor, Oryzae sativa, Hordeum vulgare and
Zea mays by altering physiological and biochemical processes (Table 2).
3.2.2 Phytotoxicity at Physiological Level
The impacts of TiO 2 -NPs application on plants at the
physiological level could be inspected by observing the
chlorophyll content, nutrients uptake, transpiration rate,
photosynthetic rate, stomatal conductivity and alteration of
hormones. TiO 2 -NPs regulate the activities of all enzymes
related to nitrogen metabolism, including nitrate reductase,
glutamine synthase, glutamate dehydrogenase and glutamine–pyruvic transaminase, etc. which facilitate the
absorption of active nitrogen in plants in form of nitrate and
help in the conversion process of inorganic nitrogen into
organic nitrogen in form of protein and chlorophyll molecules, and improve the rate of photosynthesis that could
ultimately reflect as improved biomass and dry mass of
treated plant (Yang et al. 2007; Mishra et al., 2014). TiO 2 -
NPs treatment at low concentration facilitate the absorbance
of minerals which in turn promotes the chlorophyll formation and activation of key enzymes for carbon fixation, but at
high dose TiO 2 -NPs produce ROS under light which would
disrupt the membrane structure, therefore, reduce photosynthesis leading to reduced biomass in spinach (Zheng et al.
2005).
Raliya et al. (2015b) reported that foliar spray of
10 mg l
−1 concentration of TiO 2 -NPs on 14-day-old Vigna
radiata plants significantly improve chlorophyll content and
total soluble protein content in leaves by 446.4 and 94%,
respectively. TiO 2 -NPs treatment also enhanced chlorophyll
content in Solanum lycopersicum (Raliya et al. 2015b),
Linum usitatissimum (Aghdam et al. 2016), Triticum aestivum (Rafique et al. 2018), Phaseolus vulgaris (Ebrahimi
et al. 2016) and Hordeum vulgare (Janmohammadi et al.
2016) too. It is reported that appropriate doses of TiO 2 -NPs
protect chlorophyll from degradation and increase its
synthesis by reducing H 2 O 2 content in the cells and also
enhance seed oil and protein contents (Aghdam et al. 2016;
Ebrahimi et al. 2016). Due to photocatalytic properties,
TiO 2 -NPs can stimulate photosynthetic efficiency by
enhancing light absorption capacity and they can also alter
profiling of biochemicals such as amino acid and fatty acids
(Rui et al. 2018). Zheng et al. (2005) observed that the rutile
TiO 2 -NPs showed photo-oxidation–reduction reactions
which could accelerate the electron transport and the transformation from electric energy to active chemical energy like
ATP, promoting the activity of the rubisco activase, and
therefore, enhancing the photosynthetic activity. In Hordeum
vulgare, foliar application of TiO 2 -NPs reported to improved
defense mechanism, and increase biosynthesis of phytohormones, photosynthetic pigments and photosynthetic efficiency (Janmohammadi et al. 2016).
Seeds of Solanum lycopersicum soaked in TiO 2 -NPs
solution also showed an enhancement of net photosynthesis
(PSII activity), transpiration and conductance (Qi et al.
2013). Some studies reported the positive impacts of TiO 2 -
NPs on chlorophyll content in algal system, including Picochlorum sp. (Hazeem et al. 2016), Scenedesmus sp. and
Chlorella sp. (Sadiq et al. 2011). Gao et al. (2013) demonstrated that the treatment of anatase-TiO 2 -NPs reduced
photosynthetic rate and transpiration rate by reducing the
photosynthetic efficiency of mesophyll cells of leaves not by
regulating stomatal activity. In Zea mays, TiO 2 -NPs exposure remarkably decreased the transpiration rate by reducing
cell wall pore size and root hydraulic conductivities in a
concentration-dependent manner (Asli and Neumann 2009).
In addition, TiO 2 -NPs can affect the membrane integrity
which consequently modifies the uptake mechanism of water
as well as of nutrients. Apart from this, TiO 2 -NPs are
reported to be augmenting the performance in plants grown
under various stresses, which will be discussed later.
The risks of TiO 2 -NPs to plants due to its high sensitivity
toward cytotoxic and genotoxic effects, clogging of pores
and barriers in apoplast stream leads to interruption in
nutrients uptake, and thus, causes toxicity (Mattiello et al.
2015; Santos Filho et al. 2019). Despite, TiO 2 -NPs’ treatment in seedling augment growth and developmental processes by increasing light absorption, chlorophyll content
and photosynthesis in treated plants (Yang et al. 2007;
Raliya et al. 2015b). Further, Tumburu et al. (2015) provided
a genetic basis and observed that TiO 2 -NPs treatment
increased the expression of various transcripts responsible
for root development and cell differentiation.
3.2.3 Phytotoxicity at Biochemical Level
The mechanism of plant responses to TiO 2 -NPs exposure
could be better assessed by observing ROS generation,
oxidative damages H 2 O 2 content, malondialdehyde
(MDA) level, electrolyte leakage and enzymatic and
Interaction of Titanium Dioxide Nanoparticles …
59