Agriculture Reference
In-Depth Information
play a role in their response to MWCNTs, yet how the crop morphology and
anatomy affect the plant responses to MWCNTs is not clear.
14.3.2 Phyto-Effect of Silver Nanoparticles
While AgNPs have been frequently applied as antimicrobial agents in agricultural
practices, their impact on agricultural crops is not extensively evaluated. Musante
and White (
2010
) compared the phytotoxicity of AgNPs and bulk Ag to squash and
found that AgNPs decreased the biomass accumulation and transpiration of squash
by 66-84 % compared with bulk Ag. The authors noticed that Ag ion concentration
was 4.4-10 times higher in AgNPs solution than bulk Ag solution, but the role of
ionic Ag was not explicitly investigated. A later study indicated that AgNPs are
phytotoxic to Italian ryegrass and the observed toxicities such as the inhibited root
hair development, vacuolated and collapsed cortical cells, and broken root caps
were not observed when plants were exposed to the same concentrations of ionic
Ag, suggesting that AgNPs exerted unique toxic effects on plant growth (Yin
et al.
2011
). Lee et al. (
2012
) reported that AgNPs are toxic to mugbean and
sorghum in agar medium, but their phytotoxicity to both plant species was greatly
reduced in the soil medium at the same concentrations. Mechanistic investigations
suggested that the underlying mechanisms of AgNPs phytotoxicity to these two
species differed in different media. The authors attributed the phytotoxicity of
AgNPs to the release of ion in agar media, while in the soil medium, the ion release
became less important, possibly due to the adsorption of ion and nanoparticles by
soil particles. Another study showed that AgNPs-treated water hyssop (a herb) did
not display any morphological abnormalities; however, the anatomical structure of
root and shoot tissues was affected (Krishnaraj et al.
2012
). These authors also
noticed heightened expressions of some antioxidant enzyme activities such as the
peroxidase and catalase activities, but the induction of these oxidative stress
regulation proteins was not different from the effect of AgNO
3
. The observation
was supported by another study which showed that both AgNPs and Ag ions
induced the expression of redox regulation enzymes in salad rocket; however,
some proteins related to the endoplasmic reticulum and vacuole were uniquely
altered by AgNPs, providing further evidence that phytotoxicity of AgNPs cannot
be fully explained by the release of ions (Vannini et al.
2013
). In conclusion, the
debate on the nature of AgNPs toxicity to plants appeared to favor that AgNPs exert
their unique effect on plant growth and metabolism, and even though ionic release
may explain part of the observed toxicity, the release of Ag ion cannot fully account
for the observed toxicity of AgNPs. It is important to indicate that all these studies
were conducted through root exposure. The only foliar exposure of AgNPs to plants
showed that foliar exposure of lettuce to up to 1,000 mg/L AgNPs did not result in
any toxicity on lettuce growth (Larue et al.
2014
). The authors found that some
AgNPs were internalized in the leaf tissues and washing did not effectively remove
Ag content.
