274
reduced. Reduced antioxidant content of seedlings showed that the NPs were imposing stress. Hence, CeO 2 NPs were creating nutritional deficit in the treated rice
seedlings (Rico et al. 2013b).
Likewise, CeO 2 NP exposure considerably reduced the uptake of molybdenum
ions by soybean. As a result, the activity of nitrogen fixation enzymes, nitrate reductases, and sulfite oxidases was reduced. So, CeO 2 NPs decreased the nitrogen uptake
and nitrogen assimilation of treated plants. Similarly, exposure of ZnO NPs has
induced hyperaccumulation of zinc micronutrient in the leaves of soybean. Zn in
higher amounts binds with the proteins and displaces other metal ions from the
binding sites of proteins. Hence, these NPs have negatively affected the nutritional
and commercial value of soybean plants (Peralta-Videa et al. 2014). Ag NPs and
FeO NPs have a negative effect on the T. repens plant growth through growth inhibition of AMF. FeO NPs did not affect the growth of AMF; instead, FeO NPs were
observed to bind glomalin glycoprotein secreted from AMF. Glomalin controls the
fluxes of water, gases, and nutrients in soil. So, FeO NP exposure decreases the
vegetative growth and antioxidant profile of plants (Feng et al. 2013). Exposure of
Ag NPs was observed to delay the vegetative growth of A. thaliana by inducing
temporary development arrest and reducing the absorption of nitrogenous nutrients
by plants (Geisler-Lee et al. 2014).
Citric acid-capped CeO 2 NP treatment was reported to reduce the Ce content and
increase the absorption of micronutrients in R. sativus. As a result, the citric acidcapped CeO 2 NPs increased the root biomass, while bare CeO 2 NPs reduced root
and shoot biomass due to accumulation of more Ce (Trujillo-Reyes et al. 2013).
Si NPs induced toxicity due to adsorption of micro- and macronutrient on the NP
surface, thus reducing the nutrition availability for A. thaliana. The nutrient adsorption and phytotoxicity of NPs was dependent on pH and surface silanol moieties. No
phytotoxicity was observed when the pH of the exposure medium was adjusted to
5.8 and silanol moieties were removed from the Si NP surface (Slomberg and
Schoenfisch 2012). CeO2 NPs decreased the nutritional quality of kidney bean
(P. vulgaris) seeds. The seed produced from CeO 2 NP-treated plants possessed
lower amounts of nutrient storage (phaseolin) and carbohydrate metabolism (lectins). Further, the adverse effects on seeds were more pronounced on exposure of
CeO 2 NPs through organic matter soil-rich soil than soil containing low organic
matter (Majumdar et al. 2015).
Overall Effect on Vegetative Growth Through Other Mechanisms
La 2 O 3 NPs were considered toxic to cucumber plants. Plant roots have been known
to exude out organic acids in their vicinity. Interaction of acetic acid released from
cucumber roots with the La 2 O 3 NPs was enhancing its dissolution and its bioavailability and hence induced toxicity to plants (Ma et al. 2011). Likewise, Ag NPs
interacted with P. americana plant cells to release Ag ions. Ag ion-cell-Ag NP interactions were responsible for the observed phytotoxicity. Size and surface coating
were also responsible for the toxicity of Ag NPs. GA-stabilized Ag NPs were
V. Kumar et al.
reduced. Reduced antioxidant content of seedlings showed that the NPs were imposing stress. Hence, CeO 2 NPs were creating nutritional deficit in the treated rice
seedlings (Rico et al. 2013b).
Likewise, CeO 2 NP exposure considerably reduced the uptake of molybdenum
ions by soybean. As a result, the activity of nitrogen fixation enzymes, nitrate reductases, and sulfite oxidases was reduced. So, CeO 2 NPs decreased the nitrogen uptake
and nitrogen assimilation of treated plants. Similarly, exposure of ZnO NPs has
induced hyperaccumulation of zinc micronutrient in the leaves of soybean. Zn in
higher amounts binds with the proteins and displaces other metal ions from the
binding sites of proteins. Hence, these NPs have negatively affected the nutritional
and commercial value of soybean plants (Peralta-Videa et al. 2014). Ag NPs and
FeO NPs have a negative effect on the T. repens plant growth through growth inhibition of AMF. FeO NPs did not affect the growth of AMF; instead, FeO NPs were
observed to bind glomalin glycoprotein secreted from AMF. Glomalin controls the
fluxes of water, gases, and nutrients in soil. So, FeO NP exposure decreases the
vegetative growth and antioxidant profile of plants (Feng et al. 2013). Exposure of
Ag NPs was observed to delay the vegetative growth of A. thaliana by inducing
temporary development arrest and reducing the absorption of nitrogenous nutrients
by plants (Geisler-Lee et al. 2014).
Citric acid-capped CeO 2 NP treatment was reported to reduce the Ce content and
increase the absorption of micronutrients in R. sativus. As a result, the citric acidcapped CeO 2 NPs increased the root biomass, while bare CeO 2 NPs reduced root
and shoot biomass due to accumulation of more Ce (Trujillo-Reyes et al. 2013).
Si NPs induced toxicity due to adsorption of micro- and macronutrient on the NP
surface, thus reducing the nutrition availability for A. thaliana. The nutrient adsorption and phytotoxicity of NPs was dependent on pH and surface silanol moieties. No
phytotoxicity was observed when the pH of the exposure medium was adjusted to
5.8 and silanol moieties were removed from the Si NP surface (Slomberg and
Schoenfisch 2012). CeO2 NPs decreased the nutritional quality of kidney bean
(P. vulgaris) seeds. The seed produced from CeO 2 NP-treated plants possessed
lower amounts of nutrient storage (phaseolin) and carbohydrate metabolism (lectins). Further, the adverse effects on seeds were more pronounced on exposure of
CeO 2 NPs through organic matter soil-rich soil than soil containing low organic
matter (Majumdar et al. 2015).
Overall Effect on Vegetative Growth Through Other Mechanisms
La 2 O 3 NPs were considered toxic to cucumber plants. Plant roots have been known
to exude out organic acids in their vicinity. Interaction of acetic acid released from
cucumber roots with the La 2 O 3 NPs was enhancing its dissolution and its bioavailability and hence induced toxicity to plants (Ma et al. 2011). Likewise, Ag NPs
interacted with P. americana plant cells to release Ag ions. Ag ion-cell-Ag NP interactions were responsible for the observed phytotoxicity. Size and surface coating
were also responsible for the toxicity of Ag NPs. GA-stabilized Ag NPs were
V. Kumar et al.