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protein moieties of tubules on NP exposure were responsible for observed phytotoxicity (Wang et al. 2011). Graphene phytotoxicity has been identified in tomato, cabbage, and red spinach. Graphene induced the formation of a long primary root and
several smaller lateral roots. This enhanced the further absorption of graphene.
Graphene enhanced the production of H 2 O 2 and thus induced cell death (Begum
et al. 2011). Exposure of Ag NPs has led to their accumulation in roots and leaves
of A. thaliana. Ag NP accumulation has induced accumulation of anthocyanin in
plants. Anthocyanin accumulation increased the ROS generation in plants and
hence, oxidative stress. ROS generation and the physical presence of Ag NPs were
collectively responsible for the change in chlorophyll structure, reduced antioxidant
potential, and downregulation of aquaporins (Qian et al. 2013). MWCNTs have also
been reported to induce ROS-mediated oxidative stress in red spinach. MWCNTs
have changed the morphology and internal structure of spinach roots. This alteration in root architect has enhanced the MWCNT uptake. This has further magnified
the toxicity responses in red spinach (Begum and Fugetsu 2012). C 60 NPs have
induced ROS-mediated dysfunctioning of PS II in L gibba. C 60 NPs impaired the
chloroplast functionality that disturbed the electron transfer between photosynthetic
electron transport chain complexes (Santos et al. 2013). Similarly, exposure of graphene oxide sheets was found to impair the glutathione redox system of V. faba
plants in a concentration-dependent manner. At 100, 200, and 1600 mgL
−1
, graphene oxide induced oxidative stress to plants and increased the level of oxidized
glutathione. However, 400 and 800 mgL
−1
of grapheme oxide reversed the conditions of oxidative stress and increased the reduced glutathione level (Anjum et al.
2013). CeO 2 NP exposure on rice seedlings has documented enhanced ROS production. ROS has induced lipid peroxidation and electrolyte leakage that has further
increased H 2 O 2 generation. Chlorophyll a photodegradation and enhanced membrane damage was also observed in NP-treated seedlings. Oxidative stress was also
evident from lower activity of antioxidant enzymes, namely, GPOX, APOX, and GR
enzymes (Rico et al. 2013a).
Exposure of various metal and metal oxide NPs has reduced the dry weight of
tomato plants. NP-mediated oxidative stress and membrane damage in tomato
plants were responsible for the induced changes (Antisari et al. 2015).
Likewise, CeO 2 NPs also reduced the fresh weight of fruits and nonreducing
sugars and antioxidant potential of cucumber plants. CeO 2 NPs have been noticed to
alter the starch, nonreducing sugars and antioxidant content of cucumber. CeO 2 NPs
enhanced the sucrose content of treated plants. Increased content of starch indicates
stress (Zhao et al. 2014).
As mentioned earlier, reduction in plant root length on NPs’ exposure demonstrates their phytotoxicity on plants. Free OH group on the surface of alumina NPs
has been documented to induce oxidative stress-mediated inhibition of root elongation in maize, C. sativus, G. max, B. oleracea, and D. carota. Surface modification
of alumina NPs with phenanthrene was observed to reduce the toxic effect on root
elongation. Similar reduction in root elongation was obtained in the presence of
DMSO, a free hydroxyl radical scavenger (Yang and Watts 2005). Fullerene exposure was found to disrupt the tobacco cell wall. Fullerene increased the glycosyl
V. Kumar et al.
protein moieties of tubules on NP exposure were responsible for observed phytotoxicity (Wang et al. 2011). Graphene phytotoxicity has been identified in tomato, cabbage, and red spinach. Graphene induced the formation of a long primary root and
several smaller lateral roots. This enhanced the further absorption of graphene.
Graphene enhanced the production of H 2 O 2 and thus induced cell death (Begum
et al. 2011). Exposure of Ag NPs has led to their accumulation in roots and leaves
of A. thaliana. Ag NP accumulation has induced accumulation of anthocyanin in
plants. Anthocyanin accumulation increased the ROS generation in plants and
hence, oxidative stress. ROS generation and the physical presence of Ag NPs were
collectively responsible for the change in chlorophyll structure, reduced antioxidant
potential, and downregulation of aquaporins (Qian et al. 2013). MWCNTs have also
been reported to induce ROS-mediated oxidative stress in red spinach. MWCNTs
have changed the morphology and internal structure of spinach roots. This alteration in root architect has enhanced the MWCNT uptake. This has further magnified
the toxicity responses in red spinach (Begum and Fugetsu 2012). C 60 NPs have
induced ROS-mediated dysfunctioning of PS II in L gibba. C 60 NPs impaired the
chloroplast functionality that disturbed the electron transfer between photosynthetic
electron transport chain complexes (Santos et al. 2013). Similarly, exposure of graphene oxide sheets was found to impair the glutathione redox system of V. faba
plants in a concentration-dependent manner. At 100, 200, and 1600 mgL
−1
, graphene oxide induced oxidative stress to plants and increased the level of oxidized
glutathione. However, 400 and 800 mgL
−1
of grapheme oxide reversed the conditions of oxidative stress and increased the reduced glutathione level (Anjum et al.
2013). CeO 2 NP exposure on rice seedlings has documented enhanced ROS production. ROS has induced lipid peroxidation and electrolyte leakage that has further
increased H 2 O 2 generation. Chlorophyll a photodegradation and enhanced membrane damage was also observed in NP-treated seedlings. Oxidative stress was also
evident from lower activity of antioxidant enzymes, namely, GPOX, APOX, and GR
enzymes (Rico et al. 2013a).
Exposure of various metal and metal oxide NPs has reduced the dry weight of
tomato plants. NP-mediated oxidative stress and membrane damage in tomato
plants were responsible for the induced changes (Antisari et al. 2015).
Likewise, CeO 2 NPs also reduced the fresh weight of fruits and nonreducing
sugars and antioxidant potential of cucumber plants. CeO 2 NPs have been noticed to
alter the starch, nonreducing sugars and antioxidant content of cucumber. CeO 2 NPs
enhanced the sucrose content of treated plants. Increased content of starch indicates
stress (Zhao et al. 2014).
As mentioned earlier, reduction in plant root length on NPs’ exposure demonstrates their phytotoxicity on plants. Free OH group on the surface of alumina NPs
has been documented to induce oxidative stress-mediated inhibition of root elongation in maize, C. sativus, G. max, B. oleracea, and D. carota. Surface modification
of alumina NPs with phenanthrene was observed to reduce the toxic effect on root
elongation. Similar reduction in root elongation was obtained in the presence of
DMSO, a free hydroxyl radical scavenger (Yang and Watts 2005). Fullerene exposure was found to disrupt the tobacco cell wall. Fullerene increased the glycosyl
V. Kumar et al.