Agriculture Reference
In-Depth Information
Fig. 14.3
Chromium toxicity to plants
walls and exhibits its toxic behaviour (Mishra et al.
1995
). The hydrated Cr(III) cat-
ion does not pass through the cell membrane, even at low pH (Cary et al.
1977
). The
more toxic nature of Cr(VI) may also be explained by its ability, being a strong oxi-
dizer, to cause oxidative damage to the cells. Oxidative damage caused by Cr(VI) to
outer root cells of bean plants is evident from electron microscopy studies (Vazquez
et al.
1987
). This may cause malfunctions in the uptake of mineral nutrients and wa-
ter leading to mineral nutrition deficiency (particularly chlorosis) of the rest of the
plant and eventually death (Vazquez et al.
1987
). Hauschild (
1993
) hypothesized
that due to strong oxidative damage of Cr(VI) to root cells, their efficient selective
mechanisms for control of inorganic uptake into the root may be destroyed, permit-
ting large amounts of Cr(VI) to enter the root passively. This may explain the higher
Cr(VI) uptake compared to Cr(III) and the apparent breakthrough of chromium to
the plant tops. If Cr(VI) is transported in the xylem in the same form, then oxida-
tive damage to leaf cells and the consequent chlorotic symptoms are to be expected
(Hauschild
1993
). Chromium toxicity symptoms have been correlated with elevated
levels (10-fold higher than the control) of putrescine in leaves (Hauschild
1993
).
Putrescine belongs to the group of aliphatic di- and poly-amines that act as antieth-
ylene compounds preventing senescence (Flores et al.
1989
). It is well document-
ed that putrescine levels increase in plants in response to various environmental
stresses (Hauschild
1993
). Based on these facts, Hauschild (
1993
) proposed a com-
mon mechanism of toxicity for the two chromium species as follows: Chromium
exposure causes root stress which results in, (a) induction of putrescine and other
stress compounds (e.g., chitinases); (b) reduced root growth, leading to reduced
shoot growth, water content, and leaf chlorosis (i.e., root-induced iron deficiency);
