Agriculture Reference
In-Depth Information
though the tendency to retain chromium in the roots seems to be common to all
plant species studied thus far by various researchers, there are quantitative differ-
ences among plant species in this regard. Leafy vegetables that tend to accumulate
iron (e.g., spinach, turnip leaves) appeared to be the most effective in translocating
chromium to the plant top. The leafy vegetables that do not accumulate relatively
high concentrations of iron in their leaves (e.g., lettuce, cabbage) are substantially
less effective in translocating chromium to their leaves (Zayed et al.
1998
; Cary
et al.
1977
).
4 Chromium Toxicity to Plants
Once entered into the plant, chromium leads to changes in the growth and devel-
opment pattern of the plant. The level at which chromium is phytotoxic depends
on several factors, including experimental conditions, plant species, soil charac-
teristics, and chromium species. Chromium phytotoxicity occurs at lower concen-
trations when chromium is supplied in hydroponic culture relative to chromium
supplied in soil. For example, Turner and Rust (
1971
) observed that the initial
symptoms of chromium toxicity on plants occurred with the addition of as little as
0.5 mg kg
−1
chromium to the nutrient culture and as much as 60 mg kg
−1
to the soil
culture. There is higher toxicity of chromium in solution culture because the chro-
mium supply is soluble and is thus available for plant uptake, whereas in soil major
portions of chromium become unavailable due to adsorption, reduction, and pre-
cipitation processes. Similarly, chromium is more toxic to plants growing on sandy
soils compared with peat soils due to the lower reducing and adsorbing capacity of
sandy soils (Mishra et al.
1995
). Visual symptoms of chromium toxicity in plants
are stunted growth, a poorly-developed root system, curled and discolored leaves
(Pratt
1966
), leaf chlorosis, narrow leaves (Hunter and Vergnano
1953
), chlorotic
bands on cereals (Kabata-Pendias and Pendias
1992
), yield reduction (Parr and Tay-
lor
1980
), and some plants may exhibit brownish-red leaves containing small ne-
crotic areas or purpling of basal tissues (Adriano
1986
; Hunter and Vergnano
1953
).
Immediate wilting and plant death has also been reported as a result of exposure to
very high levels of chromium (Parr and Taylor
1980
) (Fig.
14.3
).
4.1 Cr(VI) is More Toxic than Cr(III)
Chromium toxicity in plants is highly dependent on the chemical species of the ele-
ment. Cr(VI) has been reported to be more toxic to plants than Cr(III) in barley and
rape seed (Hauschild
1993
), wheat and buckwheat (Kleiman and Cogliatti
1998
).
Higher toxicity of Cr(VI) than Cr(III) has been explained by various hypotheses. In
one hypothesis, it has been proposed that at natural pH levels, Cr(VI), being water
soluble and of smaller size than the hydrated Cr(III) ion, readily penetrates cell
