Agriculture Reference
In-Depth Information
brane potential is disturbed by the arsenates of Pb and Zn (Barlian-Aidid and Oka-
moto
1992
) e.g. low Zn concentration in the plasma membrane of roots of
Zea mays
and enhances the H
+
ATPase activity by competing with Mg (Axelsen and Palmgren
2001
), but 3 mM of Zn is inhibitory. Whereas lead does not activate the ATPase
but depolarizes the membrane potential slowly (Kennedy and Gonsalves
1987
). In
Lemma gibba
arsenate depolarization is dependent on the phosphate level (Ulrich-
Eberius et al. 1989), and in
Impatients balbaniana
stem sections Pb (0.5 mM) xy-
lem parenchyma membrane potential is depolarized (Barlian-Aidid and Okamoto
1992
). Recent studies have confirmed the phosphate and arsenate competition at
the uptake level (Clements and Munson
1947
) e.g. in
Oryza sativa
(Abedin et al.
2002
),
Halcustantus
(Hartley-Whitaker et al.
2001a
,
b
),
L. gibba
(Ulrich-Eberius
et al.
1989
),
Brassica juncea
(Pickering et al.
2000
) and pteris ferns (Zhao et al.
2002
). Uptake metabolism of As in plants has been reviewed by Meharg and Hart-
ley-Whitaker (
2002
). Negative charged root cells absorb anions instead of moving
them in to apoplastic space of the root cortex (Clarckson
1996
). Uptake of arsenic
is characterized as proton anion co-transport in
Lemna gibba
(Ulrich-Eberius et al.
1989
). Uptake system is shared by arsenate and phosphate in higher plants (Abedin
et al.
2002
), mycorrhizae (Sharples et al.
2000
) and bacteria (Bruins et al.
2000
),
and the further details are being investigated (Meharg and Hartley-Whitaker
2002
).
Accumulation of arsenate follows the Michaelis-Menten kinetics in which concen-
tration range coincides with the level of activity of the high affinity phosphate up-
take (Sharples et al.
2000
; Abedin et al.
2002
). In micro-organisms, two types of
arsenate transporters have been recognized operating in the pumping arsenite either
into the vacuole or in efflux from cells (Rosen
1999
; Ali et al.
2009
). It means that
accumulation, uptake and toxicity, varies within and between plant species and in
general more the As in soil higher will be the concentration in plants (Banejad and
Olyaie
2011
).
ArsenicToleranceandToxicity
Contaminated and naturally enriched soils will be used for agriculture in future
with higher concentrations of one or more elements (Abedin et al.
2002
). Met-
al resistance enhancement in crop varieties is important only as long as the food
plants with metal concentration do not exceed health levels. The variable response
of crop plants to soil toxicants extends itself to their nutrient efficiency (Aniol and
Gustafson
1989
). Tolerance “represents a genotype environment interaction” (Mac-
nair
1993
) and the plants have been divided into two groups such as accumulators
or excluders (Baker
1987
; Tangahu et al.
2011
). According to Aniol and Gustafson
1989
many crop plants are accumulators. Excluder plant reduces the elements up-
take, Baker (
1987
). Exclusion capacity in higher plants is poor or absent (Ernst,
1976
), while as in bacteria (Nies and Silver
1995
) and some mycorrhiza (Sharples
et al.
2000
) are able to efflux toxicants. Tolerance is under genetic control, although
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