Agriculture Reference
In-Depth Information
72-h treatment, the protein content in 0.1, 1, and 10 mg L
1
treatment groups
increased, while no significant changes were observed in groups exposed to 20 and
100 mg L
1
TiO
2
concentration. There were no significant changes for chlorophyll
a
in different levels of treatment groups and exposure times, while for chlorophyll
b
and carotenoids content, there were obvious decreases after treatment, and then
there were significant increases in treated groups after 3 and 4 days of exposure.
The chlorophyll
a
:
b
ratio increased when the concentration of nano-TiO
2
increased
after 24-h treatment. The malondialdehyde (MDA) content in all treatment groups
(0.1, 1, 10, 20, and 100 L
1
) increased after 4 h of treatment, compared to that of the
control group. They observed cell aggregation in TiO
2
treatment groups with SEM
images and light microscopy, as well as evidence of TiO
2
nanomaterial absorbance
on some cell surfaces in the high concentration treatment groups. The TEM images
of high dosage groups also supported NP aggregation on the cell surface. This could
prevent cellular exchange with the external milieu, for example, by sequestering
nutrients or altering pH or redox potential.
Ji et al. (
2011
) also showed that large aggregates of TiO
2
nanoparticles could
entrap the algal cells and consequently reduce the light and nutrient available to the
algal cells, inhibiting their growth. Aruoja et al. (
2009
) also showed that large nano-
TiO
2
aggregates could entrap algal cells, while the cultures with bulk TiO
2
always
contained free algal cells. Huang et al. (
2005
), for their turn, indicated that
Pseudokirchneriella subcapitata
also adsorbed nano-TiO
2
nanoparticles onto
their surface, being capable of carrying 2.3 times their own weight in TiO
2
particles,
and the kinetics and the extent of nano-TiO
2
adsorption on algae were extremely
dependent on pH (in this case, the maximum adsorption occurred at pH 5.5).
Song et al. (
2012
) examined the toxic effects of TiO
2
nanoparticles on aquatic
plants, evaluating the macrogrowth and microresponse of
Lemna minor
L. (duckweed) exposed to several concentrations of TiO
2
. Duckweed is a wide-
spread, free-floating aquatic macrophyte, a source of food for waterfowl and a
shelter for small aquatic invertebrates. They used 10-nm particles, in near
rhabditiform shape. The particle diameter of the nanoparticles in the culture
medium decreased with their increasing concentration in the media. Zeta potentials
of TiO
2
nanoparticles and bulk TiO
2
in the media were both negative. The zeta
potential absolute values of bulk TiO
2
were higher than those of nanoparticles in the
same concentration. The stability of culture media-added bulk TiO
2
was higher than
that added with the same concentration of TiO
2
nanoparticles. However, both
nanoparticles and bulk TiO
2
affected the growth of
L. minor
, but the effect of
bulk TiO
2
was not as obvious as that of TiO
2
nanoparticles, which increased
L. minor
growth in low concentrations but inhibited it in high concentrations.
L. minor
cells also accumulated more ROS when the plant was exposed to
nanoparticles than when exposed to the same concentration of bulk TiO
2
. The
plant cells increased antioxidant defense enzyme (POD, SOD, and CAT) activity
to eliminate the accumulated ROS in plant cells when the TiO
2
nanoparticle
concentration was lower than 200 mg L
1
in the culture media. The SOD activity
decreased when the TiO
2
nanoparticle concentration was higher than 200 mg L
1
and the plant cell encountered serious damage from 500 mg L
1
TiO
2
nanoparticle
