Environmental Engineering Reference
In-Depth Information
concentrations. High Ni exposure decreased stem parenchymatous cell area, leaf
midrib tissue, and pith-cortex of roots in wheat. Dimensions of stem vascular bun-
dles were also affected, and root xylem vessels were reduced in number, as was the
density of stomata on abaxial leaf surfaces. Such reduced cell size in stems, roots
and leaves may be a direct effect of metal-induced inhibition of cell elongation
(Kovacevi et al.
1999
).
12
Conclusions
Nickel, at low concentrations, acts as a micronutrient and effectively enhances the
growth of many crop plants. It has many essential roles in plants: it is a constituent of
many metalloenzymes such as urease, some Ni-containing superoside-dismutases
(SOD), NiFe hydrogenases, methyl coenzyme M reductase, carbon monoxide dehy-
drogenase, acetyl coenzyme-A synthase, hydrogenases, and RNase-A. Therefore, Ni
deiciencies result in reduced urease activity and perturbance of N assimilation and
SOD activity, thereby reducing scavenging of superoxide free-radical. Another impor-
tant role for Ni in plants is its contribution to phytoalexin synthesis, and hence plant
stress resistance. In bacteria and fungi, Ni participates in important metabolic reac-
tions such as hydrogen metabolism, methane biogenesis, and acetogenesis. Therefore,
Ni deiciency produces several plant growth symptoms and metabolic effects that
pertain to senescence, N metabolism, and Fe uptake. Ni-deicient plants may develop
chlorosis in the newly emerged leaves, thereby causing meristematic necrosis.
Most soils are not Ni-deicient. However, Ni often causes plant toxicity. When
affected by Ni exposure, plants show reduced shoot and root growth, poor branch-
ing development, deformed structures abnormal lower shape, decreased biomass
production, leaf spotting, mitotic root tip disturbances, inhibition of germination,
and induction of Fe deiciency leading to chlorosis and foliar necrosis. Other Ni
toxicity symptoms include reduced nutrient absorption by roots, and decreases in
root development, and metabolism, along with photosynthesis and transpiration
inhibition. Nickel can replace Co and certain other heavy metals at metalloenzyme
active sites and disrupt their functioning. All of these toxic effects reduce agricul-
tural crop yields.
The main toxic effects caused by Ni exposure affect germination, growth, photo-
synthesis, nutrient accumulation, and crop yield. However, little research has been
conducted on changes to membrane permeability, water relation parameters, and
patterns of Ni-induced accumulation of solutes (e.g., proline). All of these topics are
ones that require further research investigation. In addition, not one report exists in
the literature on how Ni affects glycinebetaine, or how it potentially may alter plant
growth regulators. These also constitute fertile ground for new research work.
Although Ni-induced modiications in the cellular structure of leaves, stems, and
roots are well known, new research is needed to further explore whether such modi-
ications confer Ni tolerance to plants. New research in these areas promises to
improve the mechanistic understanding of how plants tolerate Ni toxicity.
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