Arsenic Toxicity and Tolerance Mechanisms in Plants: An Overview - Crop Improvement: New Approaches and Modern Techniques - page 366

arsenic- binding species such as iron and aluminum. With the increase in pH proton-
ated sites allow the arsenic to become more mobile.
Arsenic does not have the capability to form a strong association with calcium
(calcite) at higher pH. According to Woolson ( 1983 ) under high arsenic concentra-
tions this association may be found where calcium is the secondary preference over
aluminum. Acidic conditions i.e. lower pH allows the calcite to be dissolved and the
arsenic is released.
The other aspect that affects the arsenic availability is soil texture (Adriano
2001 ). Soil surface area is affected by soil texture e.g. textured soils (silt and clays)
have much more surface area than sandy soils therefore they are more reactive.
Higher amounts of trace elements are likely to be retained by fine textured soils
as compared to sandy soils (Chen et al. 1999 ). Fine textured soils have higher cat-
ion enhance capacity (CEC) apart from increased surface area. Higher retention of
cationic species like copper is caused due to higher cationic enhancement capacity
(Chen et al. 1999 ).
High organic matter (OM) is found in finer textured soils with a higher CEC,
compared to sandy soils with low CEC. Often, higher organic matter leads to higher
cationic enhance capacity. In fine textured soils the pH dependent charge condi-
tions also favours the organic matter accumulation and retention. Retention of both
anionic and cationic species increases in organic matter because cations bridge with
the iron and aluminum, resulting in anion retention and the dissociation of organic
matter complexes in response to change in pH.
Sandy textured soils increases the arsenic toxicity in plants and arsenic mobil-
ity compared to soils with the clayed textures (Jacobs and Keeney 1970 ; Adriano
1986 ). Presence of aluminium and iron oxides play a vital role in the ability of soil
to retain arsenic (Adriano 2001 ; Jacobs and Keeney 1970 ). Moreover, concentra-
tions of iron and phosphorus influences arsenic concentrations in florida soils (Chen
et al. 2002 ). In sand grains the phophorus with clay coatings increases the retention
of elements as compared to bare quartz grains (Harris et al. 1987a , b ). Metal oxides
and alumino-silicates are coating compounds that have higher affinity for trace ele-
ments such as arsenic soil horizons (i.e, albic horizons in spodosols) that have been
exposed to extreme weathering and leaching. And the weathering results in sand
grains that are exposed to clay coatings (Harris et al. 1987a , b ). Rhue et al. ( 1994 )
found that these horizons are able to retain these clay coatings and they exhibit
higher retention as compared to those that did not retain their coatings.
ArsenicUptake
Plants metabolize the elements through the plasma membrane of the roots. Electro-
chemical potential is created by H + -ATPases across the membrane (Kennedy and
Gonsalves 1987 ; Palmgren 2001 ). Membrane potential is depolarized by plasma
membrane and acidifies the cytoplasm by the excess of positively charged ions
passing through it (Cumming and Taylor 1990 : Axelsen and Palmgren 2001 ). Mem-