275
smaller in size and induced more phytotoxicity than PVP-Ag NPs (Yin et al. 2012).
Likewise, release of Zn ions from ZnO NPs on interaction with root exudates and
the physical interaction of ZnO NPs with plant roots were responsible for the toxicity on rapeseed seedlings (Kouhi et al. 2014). Likewise, Cu NPs and Ag NPs have
reduced the biomass of C. pepo plants. Ion dissolution from the NPs was considered
one but not the sole reason for induced phytotoxicity (Stampoulis et al. 2009).
In contrast, Cu NPs have toxic effect on P. radiatus and T. aestivum due to nanometer size of Cu. Cupric ion released from Cu NPs was not responsible for toxicity
(Lee et al. 2008). Similarly, ZnO NPs inhibited the seed germination as well as root
elongation in ryegrass and corn (Lin and Xing 2007; Lin and Xing 2008). Silica,
palladium (Pd), Au, and Cu NPs have inhibitory effect on root growth (Shah and
Belozerova 2009). Inhibition of root elongation in rape and wheat on exposure of
Yb 2 O 3 , Gd 2 O 3, La 2 O 3, and CeO 2 NPs occurred during different stages of seed germination (Ma et al. 2010). While exposure of TiO 2 NPs has induced hindrance in root
and shoot growth of wheat seedlings (Mahmoodzadeh et al. 2013). These studies
reported that some NP-based mechanism is responsible for the phytotoxicity.
Further, the ions released from NPs did not cause any toxicity to the tested plants,
suggesting that the nanometric size of particles was responsible for toxicity. Ag NPs
induced toxicity by altering the architect of root cell-like vacuolated and collapsed
cortical cells and broken epidermis and root cap. Direct interaction of Ag NPs with
root cells and interaction of Ag ions generated by Ag NPs with biotic receptors were
responsible for the toxicity to roots (Yin et al. 2011).
Further, change in ionic form of cerium CeO 2 NPs was found to induce phytotoxicity to lettuce seeds. It was found that the conversion of Ce (IV) to Ce (III) form
was responsible for the decreased growth of lettuce seedlings on exposure. Enhanced
lipid peroxidation, altered SOD activity, and membrane damage were also observed
in lettuce seedlings on CeO 2 NP exposure (Cui et al. 2014). Exposure of CuO NPs
was inducing Cu accumulation in the shoots and roots of P. vulgaris plants.
Accumulation of excess Cu was responsible for their growth retardation.
Supplementation of CuO exposed plants with ZnO induced reduction in the accumulated levels of Cu. ZnO treatment thus counteracted the phytotoxic effect of CuO
NPs (Dimkpa et al. 2015).
Au NPs has been observed to induce size-dependent necrosis in tobacco leaves
Interestingly, 3.5 nm NPs entered the plant vasculature through roots. NPs induced
toxicity by affecting transport capabilities of vascular tissues. Bigger-sized, 18 nm
NPs were not able to enter the plant root. These NPs accumulated outside the roots
and hence did not induce toxicity (Sabo-Attwood et al. 2012). Likewise, accumulation of Ag NPs in vascular tissues was reported to inhibit seed germination and
seedling growth of rice (Thuesombat et al. 2014). MWCNTs were reported to
induce alteration in xylem architecture of red spinach, lettuce, rice, and cucumber
plants. Differential xylem architecture was considered responsible for the phytotoxicity (Begum et al. 2012).
Exposure of fullerenes was found to alter the hormonal distribution, cell division, mitochondrial activity, and microtubular organization in A. thaliana seedlings
(Liu et al. 2010). The finely agglomerated MWCNTs were found toxic to A.
10 Phytoresponse to Nanoparticle Exposure
smaller in size and induced more phytotoxicity than PVP-Ag NPs (Yin et al. 2012).
Likewise, release of Zn ions from ZnO NPs on interaction with root exudates and
the physical interaction of ZnO NPs with plant roots were responsible for the toxicity on rapeseed seedlings (Kouhi et al. 2014). Likewise, Cu NPs and Ag NPs have
reduced the biomass of C. pepo plants. Ion dissolution from the NPs was considered
one but not the sole reason for induced phytotoxicity (Stampoulis et al. 2009).
In contrast, Cu NPs have toxic effect on P. radiatus and T. aestivum due to nanometer size of Cu. Cupric ion released from Cu NPs was not responsible for toxicity
(Lee et al. 2008). Similarly, ZnO NPs inhibited the seed germination as well as root
elongation in ryegrass and corn (Lin and Xing 2007; Lin and Xing 2008). Silica,
palladium (Pd), Au, and Cu NPs have inhibitory effect on root growth (Shah and
Belozerova 2009). Inhibition of root elongation in rape and wheat on exposure of
Yb 2 O 3 , Gd 2 O 3, La 2 O 3, and CeO 2 NPs occurred during different stages of seed germination (Ma et al. 2010). While exposure of TiO 2 NPs has induced hindrance in root
and shoot growth of wheat seedlings (Mahmoodzadeh et al. 2013). These studies
reported that some NP-based mechanism is responsible for the phytotoxicity.
Further, the ions released from NPs did not cause any toxicity to the tested plants,
suggesting that the nanometric size of particles was responsible for toxicity. Ag NPs
induced toxicity by altering the architect of root cell-like vacuolated and collapsed
cortical cells and broken epidermis and root cap. Direct interaction of Ag NPs with
root cells and interaction of Ag ions generated by Ag NPs with biotic receptors were
responsible for the toxicity to roots (Yin et al. 2011).
Further, change in ionic form of cerium CeO 2 NPs was found to induce phytotoxicity to lettuce seeds. It was found that the conversion of Ce (IV) to Ce (III) form
was responsible for the decreased growth of lettuce seedlings on exposure. Enhanced
lipid peroxidation, altered SOD activity, and membrane damage were also observed
in lettuce seedlings on CeO 2 NP exposure (Cui et al. 2014). Exposure of CuO NPs
was inducing Cu accumulation in the shoots and roots of P. vulgaris plants.
Accumulation of excess Cu was responsible for their growth retardation.
Supplementation of CuO exposed plants with ZnO induced reduction in the accumulated levels of Cu. ZnO treatment thus counteracted the phytotoxic effect of CuO
NPs (Dimkpa et al. 2015).
Au NPs has been observed to induce size-dependent necrosis in tobacco leaves
Interestingly, 3.5 nm NPs entered the plant vasculature through roots. NPs induced
toxicity by affecting transport capabilities of vascular tissues. Bigger-sized, 18 nm
NPs were not able to enter the plant root. These NPs accumulated outside the roots
and hence did not induce toxicity (Sabo-Attwood et al. 2012). Likewise, accumulation of Ag NPs in vascular tissues was reported to inhibit seed germination and
seedling growth of rice (Thuesombat et al. 2014). MWCNTs were reported to
induce alteration in xylem architecture of red spinach, lettuce, rice, and cucumber
plants. Differential xylem architecture was considered responsible for the phytotoxicity (Begum et al. 2012).
Exposure of fullerenes was found to alter the hormonal distribution, cell division, mitochondrial activity, and microtubular organization in A. thaliana seedlings
(Liu et al. 2010). The finely agglomerated MWCNTs were found toxic to A.
10 Phytoresponse to Nanoparticle Exposure