314
C. Marquina
from different parts of the plants were taken after 24 and 48 h, from the sections
drawn in (Fig. 13.6a, and fixed for conventional light microscope analysis.
Big amounts of nanoparticles (in the form of a black staining corresponding to
nanoparticle aggregates) were observed in samples of root tissue collected after 24 h
(see Fig. 13.6b, c). These observations led to the conclusion that nanoparticle application by immersing the roots into the nanoparticle solution is faster, more reliable
and efficient (in terms of the amount of nanoparticles) than application by pulverization or injection [52, 54]. This conclusion is common to the four crops analyzed
in this study. Although in all cases nanoparticles were easily detected in the xylem
vessels, some differences were observed depending on the species. Pea roots accumulated higher contents of nanoparticles than sunflower or wheat, for example. This
difference still remained after 48 h of exposure to the nanoparticle fluid, suggesting
that pea roots could be more permeable to nanoparticle penetration. Looking for
the presence of nanoparticles in roots not exposed directly to the suspension, the
characteristic black deposit was detected within the central cylinder of roots located
diametrically opposite to the treated roots. Therefore, the nanoparticles had moved
there probably through the phloem and using the source-sink pressure gradient [72].
This is also in good agreement with the observations in pumpkin plants with respect
to the radial transport of nanoparticles from cell to cell [52, 54].
The translocation of the nanoparticles into the aerial parts of the plant was also
studied, taking tissue samples from the plant crown 24 and 48 h after nanoparticle
application. After 24 h the black deposit was observed in the xylem vessels of the
four crops as shown in Fig. 13.7a–d, which means that the nanoparticles had quickly
moved most likely by the transpiration stream.
As in the case of the roots, there were differences between species. Pea and wheat
showed a high concentration of nanoparticles in the vascular tissues of the crown,
whereas the black staining was less intense in tomato and sunflower. In the case of
sunflower, it seems that the nanoparticle uptake through the roots is much slower
than in the other species, and for that reason there is a lower accumulation after
24 h of nanoparticle treatment. In addition, in this case the nanoparticle suspension
seems to be more restricted to the vascular tissues than in the other species. The
observation of subsequent upper part sections confirmed that nanoparticles reached
most of them also after 24 h of exposure to the suspension. The samples taken 48 h
after nanoparticle application (see Fig. 13.7e–j) showed no significant differences
from crop to crop, as an intense accumulation of nanoparticles was detected in all the
cases. According to the microscopy analysis, the nanoparticles moved also towards
the leaves and leaf petioles. Another striking result is that large accumulations of
nanoparticles were detected in the leaf trichomes of the wheat plants, but not in the
other three crop species. As mentioned in previous sections, the same was observed
in pumpkin plants [52, 54], although not in such a high amount. This fact would
confirm the secretory function of the trichomes [73], being this a putative detoxifying
pathway. Different behavior regarding accumulation and excretion of heavy metals
has been reported for different plant species [74], suggesting that such differences can
also be found when working with metal nanoparticles. The reasons for the observed
differences between crops are unclear, but they should be related to the physiology
C. Marquina
from different parts of the plants were taken after 24 and 48 h, from the sections
drawn in (Fig. 13.6a, and fixed for conventional light microscope analysis.
Big amounts of nanoparticles (in the form of a black staining corresponding to
nanoparticle aggregates) were observed in samples of root tissue collected after 24 h
(see Fig. 13.6b, c). These observations led to the conclusion that nanoparticle application by immersing the roots into the nanoparticle solution is faster, more reliable
and efficient (in terms of the amount of nanoparticles) than application by pulverization or injection [52, 54]. This conclusion is common to the four crops analyzed
in this study. Although in all cases nanoparticles were easily detected in the xylem
vessels, some differences were observed depending on the species. Pea roots accumulated higher contents of nanoparticles than sunflower or wheat, for example. This
difference still remained after 48 h of exposure to the nanoparticle fluid, suggesting
that pea roots could be more permeable to nanoparticle penetration. Looking for
the presence of nanoparticles in roots not exposed directly to the suspension, the
characteristic black deposit was detected within the central cylinder of roots located
diametrically opposite to the treated roots. Therefore, the nanoparticles had moved
there probably through the phloem and using the source-sink pressure gradient [72].
This is also in good agreement with the observations in pumpkin plants with respect
to the radial transport of nanoparticles from cell to cell [52, 54].
The translocation of the nanoparticles into the aerial parts of the plant was also
studied, taking tissue samples from the plant crown 24 and 48 h after nanoparticle
application. After 24 h the black deposit was observed in the xylem vessels of the
four crops as shown in Fig. 13.7a–d, which means that the nanoparticles had quickly
moved most likely by the transpiration stream.
As in the case of the roots, there were differences between species. Pea and wheat
showed a high concentration of nanoparticles in the vascular tissues of the crown,
whereas the black staining was less intense in tomato and sunflower. In the case of
sunflower, it seems that the nanoparticle uptake through the roots is much slower
than in the other species, and for that reason there is a lower accumulation after
24 h of nanoparticle treatment. In addition, in this case the nanoparticle suspension
seems to be more restricted to the vascular tissues than in the other species. The
observation of subsequent upper part sections confirmed that nanoparticles reached
most of them also after 24 h of exposure to the suspension. The samples taken 48 h
after nanoparticle application (see Fig. 13.7e–j) showed no significant differences
from crop to crop, as an intense accumulation of nanoparticles was detected in all the
cases. According to the microscopy analysis, the nanoparticles moved also towards
the leaves and leaf petioles. Another striking result is that large accumulations of
nanoparticles were detected in the leaf trichomes of the wheat plants, but not in the
other three crop species. As mentioned in previous sections, the same was observed
in pumpkin plants [52, 54], although not in such a high amount. This fact would
confirm the secretory function of the trichomes [73], being this a putative detoxifying
pathway. Different behavior regarding accumulation and excretion of heavy metals
has been reported for different plant species [74], suggesting that such differences can
also be found when working with metal nanoparticles. The reasons for the observed
differences between crops are unclear, but they should be related to the physiology