256
were absorbed by plant leaves but were not translocated from exposed leaves to
fresh leaves or other plant parts (Birbaum et al. 2010). Accumulation of CeO 2 and
ZnO NPs was observed in cucumber plants. NPs have been noticed to adhere on
plant roots without blocking their water channels. However, both NPs did not affect
their growth, gas exchange, and chlorophyll content (Zhao et al. 2013). ZnO NP
exposure has also no effect on the overall vegetative growth of cowpea (Vigna
unguiculata) (Wang et al. 2013a).
Exposure of aluminum (Al) NPs has no effect on the growth of California red
kidney beans (Phaseolus vulgaris) and rye grass (Lolium perenne). However, aluminum accumulation was observed only in rye grass plants (Doshi et al. 2008).
Pumpkin plants (Cucurbita maxima) also showed absorption, translocation, and
accumulation of iron oxide (Fe 3 O 4 ) NPs in roots and leaves from aqueous medium
with no observed side effect on their growth and development (Zhu et al. 2008).
Stimulatory Effect of NPs on Seed Germination and Plant
Vegetative Growth
Exposure of carbon nanotubes (CNTs), fullerenes, and graphene to rice seeds
improved the seed germination rate and seedling growth compared to nontreated
seedlings (Remya et al. 2012). Similarly, exposure of CNT has induced 200% increment in the fruit yield and vegetative growth of tomato (Husen and Siddiqi 2014).
TiO 2 NP exposure has, likewise, documented improvement in the seed germination
and growth of spinach plants (Zheng et al. 2005; Hong et al. 2005a; Lei et al. 2007;
Su et al. 2007; Zheng et al. 2007; Su et al. 2008a; Xuming et al. 2008).
Influence of NPs on plant root growth has been reported as an important stress
indicator. Root elongation is beneficial for plant growth and development, while
decrease in root length negatively affects plant growth and hence indicates toxicity.
Nonfunctionalized SWCNTs have been documented to induce root elongation in
onion and cucumber plants (Canas et al. 2008). Root elongation was evident in
Brassica juncea on MWCNT exposure. However, CNTs did not affect their seed
germination (Gajanan et al. 2010). Likewise, MWCNTs were found to increase the
seed germination and shoot–root length of Onobrychis arenaria (Smirnova et al.
2012). Oxidized MWCNTs were also noticed to promote the growth of wheat
plants. The roots showed faster growth; however, no variation in seed germination
rate and stem elongation was observed. Further, a considerable increment in plant
vegetative biomass was also evident (Wang et al. 2012a). TiO 2 NP exposure has
been documented to promote the process of root elongation in oilseed rape, kidney
bean, and lettuce plants (Song et al. 2013).
Iron (Fe) NP exposure was reported to enhance the root length of Arabidopsis
thaliana plants (Kim et al. 2014). Interestingly, graphene ribbons have been documented to increase the germination of 2-year-aged wheat seeds by 15%. Further, the
developed plants showed better root differentiation and resistance to oxidative stress
compared to untreated plants (Hu and Zhou 2014). CeO 2 NP exposure enhanced the
root–shoot length and biomass accumulation in Coriandrum sativum plants. The
V. Kumar et al.
were absorbed by plant leaves but were not translocated from exposed leaves to
fresh leaves or other plant parts (Birbaum et al. 2010). Accumulation of CeO 2 and
ZnO NPs was observed in cucumber plants. NPs have been noticed to adhere on
plant roots without blocking their water channels. However, both NPs did not affect
their growth, gas exchange, and chlorophyll content (Zhao et al. 2013). ZnO NP
exposure has also no effect on the overall vegetative growth of cowpea (Vigna
unguiculata) (Wang et al. 2013a).
Exposure of aluminum (Al) NPs has no effect on the growth of California red
kidney beans (Phaseolus vulgaris) and rye grass (Lolium perenne). However, aluminum accumulation was observed only in rye grass plants (Doshi et al. 2008).
Pumpkin plants (Cucurbita maxima) also showed absorption, translocation, and
accumulation of iron oxide (Fe 3 O 4 ) NPs in roots and leaves from aqueous medium
with no observed side effect on their growth and development (Zhu et al. 2008).
Stimulatory Effect of NPs on Seed Germination and Plant
Vegetative Growth
Exposure of carbon nanotubes (CNTs), fullerenes, and graphene to rice seeds
improved the seed germination rate and seedling growth compared to nontreated
seedlings (Remya et al. 2012). Similarly, exposure of CNT has induced 200% increment in the fruit yield and vegetative growth of tomato (Husen and Siddiqi 2014).
TiO 2 NP exposure has, likewise, documented improvement in the seed germination
and growth of spinach plants (Zheng et al. 2005; Hong et al. 2005a; Lei et al. 2007;
Su et al. 2007; Zheng et al. 2007; Su et al. 2008a; Xuming et al. 2008).
Influence of NPs on plant root growth has been reported as an important stress
indicator. Root elongation is beneficial for plant growth and development, while
decrease in root length negatively affects plant growth and hence indicates toxicity.
Nonfunctionalized SWCNTs have been documented to induce root elongation in
onion and cucumber plants (Canas et al. 2008). Root elongation was evident in
Brassica juncea on MWCNT exposure. However, CNTs did not affect their seed
germination (Gajanan et al. 2010). Likewise, MWCNTs were found to increase the
seed germination and shoot–root length of Onobrychis arenaria (Smirnova et al.
2012). Oxidized MWCNTs were also noticed to promote the growth of wheat
plants. The roots showed faster growth; however, no variation in seed germination
rate and stem elongation was observed. Further, a considerable increment in plant
vegetative biomass was also evident (Wang et al. 2012a). TiO 2 NP exposure has
been documented to promote the process of root elongation in oilseed rape, kidney
bean, and lettuce plants (Song et al. 2013).
Iron (Fe) NP exposure was reported to enhance the root length of Arabidopsis
thaliana plants (Kim et al. 2014). Interestingly, graphene ribbons have been documented to increase the germination of 2-year-aged wheat seeds by 15%. Further, the
developed plants showed better root differentiation and resistance to oxidative stress
compared to untreated plants (Hu and Zhou 2014). CeO 2 NP exposure enhanced the
root–shoot length and biomass accumulation in Coriandrum sativum plants. The
V. Kumar et al.