microscopy (TEM) study confirmed that NPs maintained
their morphology and size during accumulation in various
plant parts (Raliya et al. 2016). Following a well-known
apoplastic pathway, TiO 2 -NPs may diffuse in the spaces
present between the cell wall and plasma membrane (Lin
et al. 2009). Gonzalez-Melendi et al. (2008) reported some
NPs in extracellular space and within some cells in Cucurbita plants. Aggregates of TiO 2 -NPs tend to accumulate in
endodermal cells due to presence of Casparian strips which
act as barrier for apoplastic movement of NPs (Larue et al.
2012a; Patrick et al. 2015). For efficient translocation, NPs
that are following apoplastic pathway must enter into the
symplast of the cell to reach to vascular tissues (xylem and
phloem). Ultra-small TiO 2 -NPs disturbed the structural
integrity of microtubular networks of plasmodesmata indicating its symplastic movement in Arabidopsis (Wang et al.
2011). Further, binding of TiO 2 -NPs with array of carrier
proteins such as aquaporins helps in accomplishment of their
internalization into cell (Rico et al. 2011; Patrick et al.
2015).
2.2 Uptake and Translocation of TiO 2
Nanoparticles Through Leaves
In addition to root pathway, TiO 2 -NPs can also enter the
plants by means of leaves through foliar spray. The entry of
TiO 2 -NPs through aerial parts may involve stomata, trichomes, hydathodes, lenticels and cuticle wounds or they may
directly penetrate through the foliar cells as found in tomato
leaves; then, translocated to other plant tissues along with the
sugar and nutrients through phloem (Nair et al. 2010; Raliya
et al. 2015a). The current studies regarding the explained
mechanisms are scanty, but many researchers are working
toward it. Wang et al. (2013) reported that upon application of
100 mg l
−1 of TiO 2 -NPs on leaves, the recovery rate of TiO 2 -
NPs translocated into leaf, stem and roots was observed
61.25%, 33.30% and 5.45%, respectively. This led them to
assume that uptake of TiO 2 -NPs was mediated via stomata.
Similar studies were also reported where TiO 2 -NPs got distributed from leaves to other parts in lettuce (Larue et al.
2011). Studies showed that foliar applications of TiO 2 -NPs at
reproductive stage increased pigment content and photosynthesis in maize (Morteza et al. 2013). However, antagonistic
effects of TiO 2 -NPs were also reported, for instance, application of TiO 2 -NPs decreased net photosynthetic rate in long
raceme elm (Gao et al. 2013). A size-dependent uptake of
TiO 2 -NPs in Triticum and rapeseed leaves was also reported
by many researchers (Kurepa et al. 2010; Larue et al.2012b;
Chichiriccò and Poma 2015).
Overall, it is concluded that some morphological and
chemical dissimilarities among plant species, such as difference in hydraulic conductivity and pore size of cell wall
may manipulate translocation and accumulation of NPs
(Judy et al. 2012). However, more research is needed to
understand the mechanisms of TiO 2 -NPs uptake and their
intracellular accumulation and distribution.
3 Impacts of TiO 2 Nanoparticles
3.1 In Ecosystem
Large-scale use of TiO 2 -NPs in consumer products contribute their exposure to both biotic (flora and fauna) as well
as abiotic factors (soil, air, water) of environment. Recent
estimate shows 13.8% to soil, 18.5% to water and 2.2%
release of TiO 2 -NPs in soil, water and air, respectively (Tan
et al. 2018). Boxall et al. (2007) anticipated their concentration to be 24.5 mg l
−1 for water and 1030 mg kg
−1 for
soil. It is clear that TiO 2 -NPs present in a significant amount
in each compartment of ecosystem which acts as the sink for
them. Plants being a primary producer are an important
component of any food chain and occupy first trophic level.
Moreover, they provide first point of entry of TiO 2 -NPs
through which they may further transferred to different
trophic levels of food chain occupied by consumers like
invertebrates and vertebrates and ultimately affect its functioning (Federici et al 2007; Blaise et al. 2008; Binh et al.
2015; Cox et al. 2016; Tripathi et al. 2017; Tan et al. 2018).
Some of the impacts of TiO 2 -NPs on different components
of ecosystem are listed in Table 1.
The excessive accumulation of TiO 2 -NPs in soil badly
affects its composition, quality and fertility by inhibiting soil
microbial enzyme activities and diversity (Du et al. 2011;
Simonin et al. 2016; Tan et al. 2018). Contrary to this,
Menard et al. (2011) reported alteration and improvement in
water properties due to aggregation, partition and increased
suspended particulate matter on TiO 2 -NPs exposure in water
bodies. High concentrations of TiO 2 -NPs in air may further
amalgamate with other environmental pollutants (Shah et al.
2017). In addition, they are also reported to affect various
biotic communities by altering their biological processes and
growth (Sharma 2009; Roh et al. 2010; Lapied et al. 2011;
Menard et al. 2011; Shi et al. 2013; Hou et al. 2019). Ranjan
and Ramalingam (2016) reported inhibition in bacterial
growth on exposure to TiO 2 -NPs due to production of
reactive oxygen species (ROS) and alternation of membrane
integrity. TiO 2 -NPs are observed to inhibit growth and colonization in fungi (Markowska-Szczupak et al. 2011).
Mosses or bryophytes also respond to TiO 2 -NPs and are
considered as good TiO 2 -NPs accumulator, thus can be
utilized for monitoring pollution (Motyka et al. 2019).
Inhibition in fertility, sustainability, enhancement in
mortality and apoptotic frequency are some of prominent
effects of ecotoxicity observed in fishes and invertebrates on
54
R. Singh et al.
their morphology and size during accumulation in various
plant parts (Raliya et al. 2016). Following a well-known
apoplastic pathway, TiO 2 -NPs may diffuse in the spaces
present between the cell wall and plasma membrane (Lin
et al. 2009). Gonzalez-Melendi et al. (2008) reported some
NPs in extracellular space and within some cells in Cucurbita plants. Aggregates of TiO 2 -NPs tend to accumulate in
endodermal cells due to presence of Casparian strips which
act as barrier for apoplastic movement of NPs (Larue et al.
2012a; Patrick et al. 2015). For efficient translocation, NPs
that are following apoplastic pathway must enter into the
symplast of the cell to reach to vascular tissues (xylem and
phloem). Ultra-small TiO 2 -NPs disturbed the structural
integrity of microtubular networks of plasmodesmata indicating its symplastic movement in Arabidopsis (Wang et al.
2011). Further, binding of TiO 2 -NPs with array of carrier
proteins such as aquaporins helps in accomplishment of their
internalization into cell (Rico et al. 2011; Patrick et al.
2015).
2.2 Uptake and Translocation of TiO 2
Nanoparticles Through Leaves
In addition to root pathway, TiO 2 -NPs can also enter the
plants by means of leaves through foliar spray. The entry of
TiO 2 -NPs through aerial parts may involve stomata, trichomes, hydathodes, lenticels and cuticle wounds or they may
directly penetrate through the foliar cells as found in tomato
leaves; then, translocated to other plant tissues along with the
sugar and nutrients through phloem (Nair et al. 2010; Raliya
et al. 2015a). The current studies regarding the explained
mechanisms are scanty, but many researchers are working
toward it. Wang et al. (2013) reported that upon application of
100 mg l
−1 of TiO 2 -NPs on leaves, the recovery rate of TiO 2 -
NPs translocated into leaf, stem and roots was observed
61.25%, 33.30% and 5.45%, respectively. This led them to
assume that uptake of TiO 2 -NPs was mediated via stomata.
Similar studies were also reported where TiO 2 -NPs got distributed from leaves to other parts in lettuce (Larue et al.
2011). Studies showed that foliar applications of TiO 2 -NPs at
reproductive stage increased pigment content and photosynthesis in maize (Morteza et al. 2013). However, antagonistic
effects of TiO 2 -NPs were also reported, for instance, application of TiO 2 -NPs decreased net photosynthetic rate in long
raceme elm (Gao et al. 2013). A size-dependent uptake of
TiO 2 -NPs in Triticum and rapeseed leaves was also reported
by many researchers (Kurepa et al. 2010; Larue et al.2012b;
Chichiriccò and Poma 2015).
Overall, it is concluded that some morphological and
chemical dissimilarities among plant species, such as difference in hydraulic conductivity and pore size of cell wall
may manipulate translocation and accumulation of NPs
(Judy et al. 2012). However, more research is needed to
understand the mechanisms of TiO 2 -NPs uptake and their
intracellular accumulation and distribution.
3 Impacts of TiO 2 Nanoparticles
3.1 In Ecosystem
Large-scale use of TiO 2 -NPs in consumer products contribute their exposure to both biotic (flora and fauna) as well
as abiotic factors (soil, air, water) of environment. Recent
estimate shows 13.8% to soil, 18.5% to water and 2.2%
release of TiO 2 -NPs in soil, water and air, respectively (Tan
et al. 2018). Boxall et al. (2007) anticipated their concentration to be 24.5 mg l
−1 for water and 1030 mg kg
−1 for
soil. It is clear that TiO 2 -NPs present in a significant amount
in each compartment of ecosystem which acts as the sink for
them. Plants being a primary producer are an important
component of any food chain and occupy first trophic level.
Moreover, they provide first point of entry of TiO 2 -NPs
through which they may further transferred to different
trophic levels of food chain occupied by consumers like
invertebrates and vertebrates and ultimately affect its functioning (Federici et al 2007; Blaise et al. 2008; Binh et al.
2015; Cox et al. 2016; Tripathi et al. 2017; Tan et al. 2018).
Some of the impacts of TiO 2 -NPs on different components
of ecosystem are listed in Table 1.
The excessive accumulation of TiO 2 -NPs in soil badly
affects its composition, quality and fertility by inhibiting soil
microbial enzyme activities and diversity (Du et al. 2011;
Simonin et al. 2016; Tan et al. 2018). Contrary to this,
Menard et al. (2011) reported alteration and improvement in
water properties due to aggregation, partition and increased
suspended particulate matter on TiO 2 -NPs exposure in water
bodies. High concentrations of TiO 2 -NPs in air may further
amalgamate with other environmental pollutants (Shah et al.
2017). In addition, they are also reported to affect various
biotic communities by altering their biological processes and
growth (Sharma 2009; Roh et al. 2010; Lapied et al. 2011;
Menard et al. 2011; Shi et al. 2013; Hou et al. 2019). Ranjan
and Ramalingam (2016) reported inhibition in bacterial
growth on exposure to TiO 2 -NPs due to production of
reactive oxygen species (ROS) and alternation of membrane
integrity. TiO 2 -NPs are observed to inhibit growth and colonization in fungi (Markowska-Szczupak et al. 2011).
Mosses or bryophytes also respond to TiO 2 -NPs and are
considered as good TiO 2 -NPs accumulator, thus can be
utilized for monitoring pollution (Motyka et al. 2019).
Inhibition in fertility, sustainability, enhancement in
mortality and apoptotic frequency are some of prominent
effects of ecotoxicity observed in fishes and invertebrates on
54
R. Singh et al.