electron transfer (Falco et al. 2011). NPs also affect the amount of photosynthetic
pigments. CuONPs and AgNPs reduced carotenoids and chlorophylls content in
soybean, mustard, and rice (Nair and Chung 2014), while in mung bean plants only
reduction in chlorophyll content occurred (Nair et al. 2014). Similarly, ZnONPs at
concentrations of 125, 250, and 500 mgÁL
-1 decreased chlorophyll content in Pisum
sativum (Mukherjee et al. 2014). SiO 2 NPs enhance the activity of carbonic
anhydrase, the enzyme supplying CO 2 to the Rubisco and synthesis of photosynthetic pigments, which may improve photosynthesis (Siddiqui et al. 2014; Xie et al.
2012). In turn, AgNPs induced a sharp decrease in photosynthetic performance due
to a change in the maximum quantum yield for primary photochemistry and electron
transport activity in the green algae Chlorella vulgaris and Dunaliella tertiolecta.
The chlorophyll a fluorescence yield decreased strongly with elevated
concentrations of AgNPs, which may be explained by the inhibition of electron
transport at the donor side of the PS II (Oukarroum et al. 2012).
It has also been shown that TiO 2 NPs have an influence on light absorption and
electron transfer due to the photocatalytic and thermal conductivity properties of this
nanomaterial (Rezaei et al. 2015; Rico et al. 2015). Some studies indicate that
electron transfer between metal NPs can increase photosynthetic performance by
inducing the efficiency of chemical energy production in photosynthetic systems.
The chlorophyll molecule in the reaction center binds to the AuNPs and Ag
nanocrystals, thus creating a new hybrid system that can produce ten times more
excited electrons, thanks to plasmon resonance and fast electron-hole separation
(Govorov and Carmeli 2007). Such mechanisms can thus help in the design of
artificial light-harvesting systems.
A number of reports are available which show phytotoxicity of NPs caused by the
generation of reactive oxygen species (ROS), which result in oxidative stress, lipid
peroxidation, damage to DNA and proteins, and apoptosis-related changes (Arruda
et al. 2015; Kim and Ryu 2013; Panda et al. 2011). Interaction of NPs with the
cellular components – namely, chloroplasts, mitochondria, peroxisomes, and plasma
membranes – may result in the formation of ROS, such as singlet oxygen (
1 O 2 ),
superoxide radical (O 2
•À ), hydroperoxy radical (HO 2
• ), hydrogen peroxide (H 2 O 2 ),
and hydroxyl radical (OH
•
). ROS molecules are continually removed
nonenzymatically or enzymatically by a complex antioxidant system including the
following enzymes: catalase (CAT), ascorbate peroxidase (APX), superoxide
dismutase (SOD), guaiacol peroxidase (GPX), glutathione reductase (GR),
dehydroascorbate reductase (DHAR), and low-molecular-weight ascorbate and
reduced (GSH) and oxidized (GSSG) glutathione. The literature indicates that NPs
modulate both enzymatic and nonenzymatic antioxidants. However, it still remains
unclear how the chemical properties of NPs induce ROS formation in plants. The
effects of interactions of NPs with antioxidant enzymes reported by numerous
studies seem to be irregular and in some cases contradictory. Oxidative stress in
plants exposed to metal NPs was caused by 10-nm-sized ZnONPs at a concentration
of 500 ppm in P. sativum (Mukherjee et al. 2014) and by (PVP-coated) AgNPs of
20 nm in size in A. thaliana (Kaveh et al. 2013). Exposure to NiONPs caused higher
activity of SOD and CAT, as well as higher GSH content and lipid peroxidation in
18
A. Gorczyca et al.
pigments. CuONPs and AgNPs reduced carotenoids and chlorophylls content in
soybean, mustard, and rice (Nair and Chung 2014), while in mung bean plants only
reduction in chlorophyll content occurred (Nair et al. 2014). Similarly, ZnONPs at
concentrations of 125, 250, and 500 mgÁL
-1 decreased chlorophyll content in Pisum
sativum (Mukherjee et al. 2014). SiO 2 NPs enhance the activity of carbonic
anhydrase, the enzyme supplying CO 2 to the Rubisco and synthesis of photosynthetic pigments, which may improve photosynthesis (Siddiqui et al. 2014; Xie et al.
2012). In turn, AgNPs induced a sharp decrease in photosynthetic performance due
to a change in the maximum quantum yield for primary photochemistry and electron
transport activity in the green algae Chlorella vulgaris and Dunaliella tertiolecta.
The chlorophyll a fluorescence yield decreased strongly with elevated
concentrations of AgNPs, which may be explained by the inhibition of electron
transport at the donor side of the PS II (Oukarroum et al. 2012).
It has also been shown that TiO 2 NPs have an influence on light absorption and
electron transfer due to the photocatalytic and thermal conductivity properties of this
nanomaterial (Rezaei et al. 2015; Rico et al. 2015). Some studies indicate that
electron transfer between metal NPs can increase photosynthetic performance by
inducing the efficiency of chemical energy production in photosynthetic systems.
The chlorophyll molecule in the reaction center binds to the AuNPs and Ag
nanocrystals, thus creating a new hybrid system that can produce ten times more
excited electrons, thanks to plasmon resonance and fast electron-hole separation
(Govorov and Carmeli 2007). Such mechanisms can thus help in the design of
artificial light-harvesting systems.
A number of reports are available which show phytotoxicity of NPs caused by the
generation of reactive oxygen species (ROS), which result in oxidative stress, lipid
peroxidation, damage to DNA and proteins, and apoptosis-related changes (Arruda
et al. 2015; Kim and Ryu 2013; Panda et al. 2011). Interaction of NPs with the
cellular components – namely, chloroplasts, mitochondria, peroxisomes, and plasma
membranes – may result in the formation of ROS, such as singlet oxygen (
1 O 2 ),
superoxide radical (O 2
•À ), hydroperoxy radical (HO 2
• ), hydrogen peroxide (H 2 O 2 ),
and hydroxyl radical (OH
•
). ROS molecules are continually removed
nonenzymatically or enzymatically by a complex antioxidant system including the
following enzymes: catalase (CAT), ascorbate peroxidase (APX), superoxide
dismutase (SOD), guaiacol peroxidase (GPX), glutathione reductase (GR),
dehydroascorbate reductase (DHAR), and low-molecular-weight ascorbate and
reduced (GSH) and oxidized (GSSG) glutathione. The literature indicates that NPs
modulate both enzymatic and nonenzymatic antioxidants. However, it still remains
unclear how the chemical properties of NPs induce ROS formation in plants. The
effects of interactions of NPs with antioxidant enzymes reported by numerous
studies seem to be irregular and in some cases contradictory. Oxidative stress in
plants exposed to metal NPs was caused by 10-nm-sized ZnONPs at a concentration
of 500 ppm in P. sativum (Mukherjee et al. 2014) and by (PVP-coated) AgNPs of
20 nm in size in A. thaliana (Kaveh et al. 2013). Exposure to NiONPs caused higher
activity of SOD and CAT, as well as higher GSH content and lipid peroxidation in
18
A. Gorczyca et al.