Conversely, foliar spray of nano Ce on Sorghum bicolor (L.)
plants under drought showed 31% increase in seed yield per
plant as compared to control (Djanaguiraman et al. 2018).
Conversely, Beta vulgaris when exposed to FeS 2 NPs
showed 47% increment in the yield (Das et al. 2016). Similar
results of increased root and shoot length and biomass were
observed in Lycopersicon lycopersicum and Cucumis melo
when grown with CoFe 2 O 4 and Fe 2 O 3 hydroponically. Some
ENPs are used against environmental stresses like heavy
metal contamination, drought, or some infectious diseases.
Triticum aestivum and Brassica juncea initially when grown
in soil contaminated with Pb, Zn, Cd, Cu, and As showed a
reduction in growth and biomass but supplementing the soil
with Fe 3 O 4 NPs showed improved growth (Konate et al.
2017; Praveen et al. 2018). Similarly, Imada et al. (2016)
showed that bacterial wilt in S. lycopersicum could be
controlled by the application of MgO NPs. Although ENPs
may either have a positive or negative impact on plants, the
response varies considerably with duration, dose, species,
and experimental conditions.
4.1.2 Effects of Engineered Nanoparticles
on Physiological Processes of Plants
Physiological responses of plants occured due to exposure of
ENPs which induce abiotic stress in plants. Plant biomass
indicates phytotoxicity which is mainly affected by a change
in photosynthetic efficiency due to change in photosystem I
(PS-I) or PS-II. Studies suggest that the higher concentrations of ENPs affect the photosynthesis, which causes suppression or death of crops (Perreault et al. 2014; Da Costa
and Sharma 2016). The exposure of 50 µg ml
−1 of MWCNT
to Z. mays showed a 10% increase in the photosynthetic rate,
whereas no change in G. max in hydroponics was observed
(Lahiani et al. 2017). Similar results of increased photosynthetic rate were observed in T. aestivum as nanochitins
caused an increase in stomatal conductance (Xue et al.
2017). Increased stomatal conductance results in increased
diffusion of external CO 2 in the pectin cavity which
enhances the CO 2 assimilation rate and hence photosynthetic
rate. Stomatal conductance also increases the transpiration
rate which causes movement of increase in water uptake,
eventually resulting in increased nitrogen and potassium
accumulation by T. aestivum plant exposed to 6 mg kg
−1 of
nanochitin (Xue et al. 2017). Similar results of increased
micronutrients (Cu, Fe, Mn, Mo, Ni, and Zn) in the seeds
have been shown by CNO treated C. arietinum which is due
to increased protein content (Tripathi et al. 2017a, b).
Metal-based NPs can affect the photosynthetic apparatus
of plants and its productivity, causing acute and chronic
effects (Arruda et al. 2015; Da Costa and Sharma 2016).
Hydroponically grown O. sativa in different concentrations
of Ag NPs showed a significant reduction in total chlorophyll and carotenoids due to peroxidation of chloroplast
membrane (Nair and Chung 2014a). The decrease in photosynthetic capacity may be due to suppression of fluorescence caused by a decrease in the quantum yield of PS-II and
electron transport chain inhibition (Matorin et al. 2013),
therefore, resulted in a significant decrease in reducing and
total sugar contents (Nair and Chung 2014b). In contrast to
previous studies, Al 2 O 3 NPs showed an increase in photosynthetic quantum yield of PS-II resulting in increased
photosynthesis and plant growth (Shenashen et al. 2017).
MoS 2 NPs treatments led to increased chlorophyll-a levels
suggesting it to be non-compromising with photosynthetic
process in rice (Sharma et al. 2020b). CeO 2 NPs exposure to
L. sativa has resulted in decreased chlorophyll content of
leaves. The reason for decreased chlorophyll is reduced
uptake of Fe which acts as an activator of key coenzyme for
synthesis of chlorophyll (Terry and Low 1982; Miller et al.
1984). Additionally, it also hampers electron transport during photosynthesis as Fe is an important constituent of
enzyme ferredoxin (Arnon 1965). A recent study showed
that the transfer of energy from PS-II to the Calvin cycle is
disrupted by CeO 2 by electron absorption from PS-II by, or
through reactions with reactive oxygen species
(ROS) (Conway et al. 2015). However, foliar application of
CeO 2 during drought condition improved photosynthetic
rate, PS-II quantum yield and stomatal conductance as
compared to plant growing in drought with CeO 2 exposure
(Djanaguiraman et al. 2018).
Iron (Fe) is an essential element for the synthesis of
chlorophyll, and its deficiency would reduce the rate of
photosynthesis (Briat et al. 2015). Cucumis melo exposed to
Fe 2 O 3 and Fe 3 O 4 initially showed a reduction in photosynthetic ability due to oxidative stress and increased in later
phase due to the absorption of Fe (Wang et al. 2019). The
presence of heavy metals might disrupt the pigment complex
or inhibit enzymes involved in the biosynthetic pathway of
chlorophyll. The addition of certain ENPs in soil contaminated with heavy metals showed improvement in photosynthetic activity. For instance, B. juncea grown in
arsenic-contaminated soil showed 36% improvement in
photosynthetic rate when supplemented with Fe 3 O 4 NPs
(Praveen et al. 2018). TiO 2 NPs were shown to increase the
Rubisco activity eventually improving photosynthesis (Sarmast and Salehi 2016; Ghoto et al. 2020). It also improves
the electron transport chain by increasing the number and
energy of electrons, improving ATP formation and photolysis of water and also by activating photochemical reactions
in the chloroplast (Hong et al. 2005; Mingyu et al. 2007).
However, detrimental effects of the photosynthetic rate have
also been reported due to a lack of stomatal regulation (Gao
et al. 2013) and a negative impact on the structure and
function of photosynthesis (Movafeghi et al. 2018). Similar
results were obtained in the case of CuO NPs which affect
the chlorophyll fluorescence increasing the dissipation of
210
A. Kumar et al.
plants under drought showed 31% increase in seed yield per
plant as compared to control (Djanaguiraman et al. 2018).
Conversely, Beta vulgaris when exposed to FeS 2 NPs
showed 47% increment in the yield (Das et al. 2016). Similar
results of increased root and shoot length and biomass were
observed in Lycopersicon lycopersicum and Cucumis melo
when grown with CoFe 2 O 4 and Fe 2 O 3 hydroponically. Some
ENPs are used against environmental stresses like heavy
metal contamination, drought, or some infectious diseases.
Triticum aestivum and Brassica juncea initially when grown
in soil contaminated with Pb, Zn, Cd, Cu, and As showed a
reduction in growth and biomass but supplementing the soil
with Fe 3 O 4 NPs showed improved growth (Konate et al.
2017; Praveen et al. 2018). Similarly, Imada et al. (2016)
showed that bacterial wilt in S. lycopersicum could be
controlled by the application of MgO NPs. Although ENPs
may either have a positive or negative impact on plants, the
response varies considerably with duration, dose, species,
and experimental conditions.
4.1.2 Effects of Engineered Nanoparticles
on Physiological Processes of Plants
Physiological responses of plants occured due to exposure of
ENPs which induce abiotic stress in plants. Plant biomass
indicates phytotoxicity which is mainly affected by a change
in photosynthetic efficiency due to change in photosystem I
(PS-I) or PS-II. Studies suggest that the higher concentrations of ENPs affect the photosynthesis, which causes suppression or death of crops (Perreault et al. 2014; Da Costa
and Sharma 2016). The exposure of 50 µg ml
−1 of MWCNT
to Z. mays showed a 10% increase in the photosynthetic rate,
whereas no change in G. max in hydroponics was observed
(Lahiani et al. 2017). Similar results of increased photosynthetic rate were observed in T. aestivum as nanochitins
caused an increase in stomatal conductance (Xue et al.
2017). Increased stomatal conductance results in increased
diffusion of external CO 2 in the pectin cavity which
enhances the CO 2 assimilation rate and hence photosynthetic
rate. Stomatal conductance also increases the transpiration
rate which causes movement of increase in water uptake,
eventually resulting in increased nitrogen and potassium
accumulation by T. aestivum plant exposed to 6 mg kg
−1 of
nanochitin (Xue et al. 2017). Similar results of increased
micronutrients (Cu, Fe, Mn, Mo, Ni, and Zn) in the seeds
have been shown by CNO treated C. arietinum which is due
to increased protein content (Tripathi et al. 2017a, b).
Metal-based NPs can affect the photosynthetic apparatus
of plants and its productivity, causing acute and chronic
effects (Arruda et al. 2015; Da Costa and Sharma 2016).
Hydroponically grown O. sativa in different concentrations
of Ag NPs showed a significant reduction in total chlorophyll and carotenoids due to peroxidation of chloroplast
membrane (Nair and Chung 2014a). The decrease in photosynthetic capacity may be due to suppression of fluorescence caused by a decrease in the quantum yield of PS-II and
electron transport chain inhibition (Matorin et al. 2013),
therefore, resulted in a significant decrease in reducing and
total sugar contents (Nair and Chung 2014b). In contrast to
previous studies, Al 2 O 3 NPs showed an increase in photosynthetic quantum yield of PS-II resulting in increased
photosynthesis and plant growth (Shenashen et al. 2017).
MoS 2 NPs treatments led to increased chlorophyll-a levels
suggesting it to be non-compromising with photosynthetic
process in rice (Sharma et al. 2020b). CeO 2 NPs exposure to
L. sativa has resulted in decreased chlorophyll content of
leaves. The reason for decreased chlorophyll is reduced
uptake of Fe which acts as an activator of key coenzyme for
synthesis of chlorophyll (Terry and Low 1982; Miller et al.
1984). Additionally, it also hampers electron transport during photosynthesis as Fe is an important constituent of
enzyme ferredoxin (Arnon 1965). A recent study showed
that the transfer of energy from PS-II to the Calvin cycle is
disrupted by CeO 2 by electron absorption from PS-II by, or
through reactions with reactive oxygen species
(ROS) (Conway et al. 2015). However, foliar application of
CeO 2 during drought condition improved photosynthetic
rate, PS-II quantum yield and stomatal conductance as
compared to plant growing in drought with CeO 2 exposure
(Djanaguiraman et al. 2018).
Iron (Fe) is an essential element for the synthesis of
chlorophyll, and its deficiency would reduce the rate of
photosynthesis (Briat et al. 2015). Cucumis melo exposed to
Fe 2 O 3 and Fe 3 O 4 initially showed a reduction in photosynthetic ability due to oxidative stress and increased in later
phase due to the absorption of Fe (Wang et al. 2019). The
presence of heavy metals might disrupt the pigment complex
or inhibit enzymes involved in the biosynthetic pathway of
chlorophyll. The addition of certain ENPs in soil contaminated with heavy metals showed improvement in photosynthetic activity. For instance, B. juncea grown in
arsenic-contaminated soil showed 36% improvement in
photosynthetic rate when supplemented with Fe 3 O 4 NPs
(Praveen et al. 2018). TiO 2 NPs were shown to increase the
Rubisco activity eventually improving photosynthesis (Sarmast and Salehi 2016; Ghoto et al. 2020). It also improves
the electron transport chain by increasing the number and
energy of electrons, improving ATP formation and photolysis of water and also by activating photochemical reactions
in the chloroplast (Hong et al. 2005; Mingyu et al. 2007).
However, detrimental effects of the photosynthetic rate have
also been reported due to a lack of stomatal regulation (Gao
et al. 2013) and a negative impact on the structure and
function of photosynthesis (Movafeghi et al. 2018). Similar
results were obtained in the case of CuO NPs which affect
the chlorophyll fluorescence increasing the dissipation of
210
A. Kumar et al.