Agriculture Reference
In-Depth Information
(MWCNTs). SWCNTs are still very expensive to synthesize and are less relevant to
current discussion. As for the interactions of MWCNTs with plants, the literature
generally reports positive effect of MWCNTs on plants. Miralles et al. (
2012
) found
that industrial grade MWCNTs up to 2,560 mg/L of MWCNTs with 640 mg/L of
catalyst impurities (Al
2
O
3
) did not display any negative effect on wheat and alfalfa;
instead, they improved the growth of both species. Both Raman mapping and
transmission electron microscopic images revealed that the root of alfalfa and
wheat were not damaged in the presence of this high concentration of MWCNTs.
Interestingly, these authors noticed that the impurities in MWCNTs boosted the
beneficial impact of MWCNTs. Similarly, exposure of mustard plant seeds to low
levels of MWCNTs resulted in higher germination rate and seedling growth due to
the improved moisture content in mustard plant seeds and enhanced water uptake
by the seedlings (Mondal et al.
2011
). These authors further showed that MWCNTs
with high oxygen content on the surface were more effective in stimulating mustard
seed germination and seedling growth. For instance, the authors reported that
MWCNTs with high content of oxygen could achieve the same stimulative effect
on mustard growth at one-tenth of the concentration of unfunctionalized MWCNTs,
but the mechanisms were not provided. A later study provided some insights on the
observed phenomenon in which the authors showed that functionalized MWCNTs
with high oxygen content on CNT surface were better dispersed in liquid solution
and their dispersion state correlated well with the expression of water channel
protein: aquaporin (Villagarcia et al.
2012
).
In contrast, Begum and Fugetsu (
2012
a) reported that after 15 days of hydro-
ponic exposure to 1,000 mg/L of MWCNTs, the growth of red spinach was strongly
inhibited. The root and leaf morphology was also adversely affected and the
induction of reactive oxygen species (ROS) was suggested as a main mechanism
for MWCNTs phytotoxicity. In a separate study, these authors greatly expanded the
plant species to determine the differential responses of different plant species to
MWCNTs. Their results showed that after exposure to 1,000 and 2,000 mg/L of
MWCNTs, the growth of red spinach and lettuce was drastically inhibited and the
root membrane integrity severely damaged (Begum et al.
2012
b). Rice and cucum-
ber were also affected but the extent of impact was less severe compared to spinach
and lettuce. Chili and soybean displayed no signs of toxicity after exposure to these
high concentrations of MWCNTs for 15 days. Clearly,
the phyto-effect of
MWCNTs is species dependent.
In summary, the literature reported seemingly contradictory results concerning
the impact of MWCNTs on agricultural crops. However, these contradictions could
be reconciled if the results are examined closely. Most negative reports concerning
the phyto-effect of MWCNTs came from studies with high concentrations of
MWCNTs, while the enhancive observations derived primarily from studies
applied low concentrations of MWCNTs (
100 mg/L). Tiwari et al. (
2013
) dem-
onstrated that the beneficial effect of MWCNTs on the growth and water content of
maize was only observed at 20 mg/L or lower concentrations of MWCNTs and
beneficial impact diminished at higher concentrations. Plant species undoubtedly
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