Environmental Engineering Reference
In-Depth Information
whereas Pb, Cd, K and Cl were accumulated in vascular bundles and collenchymas,
Pb and Cd in mesophyll and Zn in epidermis. Kachenko et al. (
2008
) reported a
significant accumulation of Ni in the epidermis of hyperaccumulating shrub
Hy-
banthus floribundus
.
Stomatal density, size and orientation are sensitive to heavy metal toxicity.
Kastori et al. (
1992
) showed a significant increase in stomatal density on both ad-
axial and abaxial leaf surfaces under heavy metal toxicity, in particular Cd, Cu and
Zn. Stomatal density was more responsive to Cd stress, followed by that of Cu and
Zn, while the least by Pb stress. Additionally, the stomatal size also decreased as
a result of heavy metal stress. Molas (
1997
) reported a significant reduction in the
stomatal density, as well as density of open stomata in
Brassica oleracea
. In addi-
tion, deformation of stomatal complexes has also been reported under nickel stress.
Anjana et al. (
2006
) and Gostin (
2009
) were of the strong view, that cadmium
causes reduction in the size of stomata and their frequency on the adaxial and ab-
axial sides of leaves. However, according to de Silva et al. (
2012
), the only plant
response specific to metal stress was decreasing trends of stomatal density and chlo-
rophyll content. Reduction of the size of stomata and their frequency can lead to
more negative impact on transpiration, photosynthesis and gas exchange, because
in the presence of metal the most of the stomata are closed.
4
Mechanism of Heavy Metal Tolerance
Physiological and biochemical mechanisms are of heavy metal tolerance has gained
considerable insight during the last few decades. Plants employ specific strategies
to tolerate noxious levels of heavy metals in the soil (Kochian et al.
2002
). Most of
the plants have developed complicated strategies for the acquirement of relatively
unavailable micronutrients like Zn, Mn, Cu, Fe and Ni from soil.
The mechanisms of tolerance to metal stress in plants may range from exclusion
of toxic metals, and inclusion and accumulation at inert places, which may vary
from species to species (Raskin and Ensley
2000
). Prasad (
1995
) discussed five
possible mechanisms that enable plants to tolerate heavy metals: binding of metals
to cell wall, reduced transport, active efflux, compartmentalization, and chelation.
Metal tolerance or accumulation can be enhanced by production of binding pro-
teins and peptides in plants. High specificity of these peptides or proteins for more
toxic metals like Cd, Hg and Pb instead of less toxic Zn and Cu will be of great
significance in plants (Ryu et al.
2003
).
On the basis of accumulation of heavy metals, plants can be broadly divided
into three categories, first that can accumulate Cu or Co, second Zn, Cd or Pb and
the third Ni accumulators (Raskin et al.
1994
). However, tolerance level in plants
can be grouped into sensitive, resistant excluder, tolerant non-hyperaccumulator,
and hypertolerant hyperaccumulator species, each with specific physio-anatomical
and molecular mechanisms for their resistance/tolerance to metal toxicity. Plant re-
sponses against toxic effects of heavy metals are regulated in a process called metal
