Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 149

11 Anatomical Changes

In addition to growth effects, heavy metal exposure may produce more local effects
on certain plant structures. An example is in the leaves of Triticum aestivum, when
the plant is exposed to a 1 mM NiSO 4 solution. After exposure, this plant showed
decreases in the following structures: thickness of mesophyll cells, size of vascular
bundles, diameter of vessels present in the main and lateral vascular bundles, and
width of epidermal cells (Kovacevi et al. 1999 ). Similarly, the volumes of intercel-
lular spaces, palisade and spongy mesophylls in Brassica oleracea leaves decreased,
when grown in agar with 10-20 g m −3 NiSO 4 .7H 2 O (Molas 1997 ). Kravkina ( 2000 )
found that Ni-exposed Dianthus repens plants formed large leaf inclusions in meso-
phyll and bundle sheath cells. These were thought to result from complexation of Ni
with proteins.
Molas ( 1997 ) reported that Ni-stressed cabbage plants had a reduced number of
stomata per unit area, as well as fewer open stomata in leaves. In addition, Ni
deformed the stomata and increased the number of defective stomata in both adaxial
and abaxial leaf surfaces of cabbage plants. Nickel-treatment reduced the volume of
spongy and palisade mesophyll cells in comparison with controls in all tested plants.
However, the number of mesophyll cells on the same leaf cross sections was simul-
taneously increased. Compared to controls, the intercellular spaces of mesophyll
tissue increased at a low Ni concentration (5 mg L −1 ), but decreased at a high con-
centration (10 and 20 mg L −1 ). In 5 mg L −1 -treated plants, the number of chloroplasts
in mesophyll cells was higher than that in the control. A reduction of grana size and
an increase in the number of nonappressed lamellae of chloroplasts were also
observed to occur in cabbage plants. These anatomical modiications were accom-
panied by overall reduction in leaf chlorophyll content, indicating a reduction in the
functionality of light harvesting complexes of the leaf photosynthetic apparatus.
In Ni-treated Thlaspi japonicum plants grown on ultramaic soil, the Ni content
was highest (3,424 mg kg −1 ) in the lower epidermis, which also had numerous sto-
mata. Leaf edges and upper epidermis had the second highest Ni content, but these
had few stomata. The lowest Ni content occurred in mesophyll cells. Using a micro-
scope, dimethylglyoxime-staining disclosed a Ni-compound presenting as rod-
shaped crystals, and these appeared mainly in areas around stomata and projections
of the leaf edge. In addition, a considerable amount of Ni was found to be excreted
via the guttation luids (Mizuno et al. 2003 )
Heavy metals have been shown to alter stem tissue organization in some crop
plants. In particular, Ni caused disorganization of epidermal cells and distortion and
disintegration of root cortical cells of wheat and pigeon pea (Setia and Bala 1994 ;
Sresty and Rao 1999 ). Nickel may also reduce stem diameter, number of vascular
bundles and cell size in plant storage tissues. Moreover, exposure to Ni decreases cell
wall thickness in stem epidermal and hypodermal tissues (Setia and Bala 1994 ), and
decreases stem and root diameter in wheat cortical cells, when grown in sand culture.
The alteration in width and thickness of leaf midribs and the diameter of xylem
vessels of root, stem and leaves were observed in plants subjected to high Ni