Environmental Engineering Reference
In-Depth Information
Arbuscular mycorrhizal fungi participate in N
dynamics that relate in N cycling, plant growth
and ecosystem functioning (Miransari
2011
).
The reduction of NO
3
is of environmentally sig-
nifi cant concern. This has been accomplished
with the presence of AM fungi (Hodge and Fitter
2010
; Miransari and Mackenzie
2010
), which
absorb and transfer the N to the host under vari-
ous conditions (Liu et al.
2007
; Atul-Nayyar
et al.
2009
; Tian et al.
2010
). Symbiotic N
2
fi xa-
tion, the starting point in the N cycle, depends on
an adequate and steady supply of P to the root
and nodules (Barea et al.
1993
). The AM fungi
play an important role in enhancing growth, nod-
ulation and N
2
fi xation by legume crops symbi-
otic with nodulating bacteria. An increased N
2
fi xation of mycorrhizal crop plants both under
control (Kucey and Bonetti
1988
; Barea et al.
1989a
) and fi eld conditions (Barea et al.
1989b
;
Shivaram et al.
1988
) has been adequately
demonstrated.
species (see Augé
2001
). It has been shown that
an increased nutrient uptake mediated through
AM fungi could impart more resistance to
drought in mycorrhizal plants (Ruiz-Lozano
et al.
1995
). The increased uptake of P by AM
plants under drought conditions results in higher
yield than those without AM fungi (Smith and
Read
2008
). Therefore, improved P nutrition by
AM fungi during the periods of water defi cit has
been postulated as a primary mechanism for
enhancing host plant drought resistance under
water stress conditions (Subramanian et al.
2006
). In contrast, others consider that host plant
drought tolerance is independent of P uptake
stimulated by AM fungi (Davies et al.
1993
;
Augé et al.
1994
). In addition to P, mycorrhizal
plants can also absorb more N under drought
conditions resulting in increased growth and
yield (Tobar et al.
1994
; Subramanian et al.
2006
). One of the widely accepted mechanisms
of AM symbiotic infl uence on plant water rela-
tion involves the AM fungal effect on plant size.
The response of plants to mycorrhizal coloniza-
tion is often related to the direct infl uence of AM
fungus on plant size in conjunction with improved
P nutrition (Ebel et al.
1994
). However, mycor-
rhizal effect on metabolic changes (Subramanian
and Charest
1995
) and modifi ed N assimilation
pathways (Subramanian and Charest
1998
) as
shown earlier can also infl uence the size of host
plants. The AM fungi could therefore to a certain
extent replace genetic engineering and plant
breeding techniques (Xu et al.
2008
; Grover et al.
2011
) by modifying the crop plant physiology as
well as biochemical responses (Kohler et al.
2008
; Grover et al.
2011
) to stress tolerance. For
example, AM fungal association has been shown
to increase the stomatal conductance of mycor-
rhizal mutant bean (
Phaseolus vulgaris
) than
non-mycorrhizal under water defi cit condition
(Augé
2004
).
3.2
Plant Tolerance to Stresses
3.2.1 Abiotic Stresses
3.2.1.1 Water Relations
Water is an essential component for plant growth
which is affected by global climatic change.
Drought is one of the most important abiotic
stresses that limit the crop growth and yield in
agroecosystems in both arid and semiarid regions
(Feng et al.
2002
). The symbiotic association of
plants with AM fungi has been shown to enhance
plant tolerance to drought (Ruiz-Lozano et al.
2006
; Boomsma and Vyn
2008
). In arid regions,
minimum moisture content in plants is balanced
by an increased uptake of water by roots through
AM fungal hyphae (Khan et al.
2003
). Positive
infl uence of AM fungi in improving plant water
use effi ciency and sustaining drought has been
shown for wheat (Al-Karaki et al.
2004
), oats
(Khan et al.
2003
), corn (Subramanian et al.
1997
; Subramanian and Charest
1999
), soybean
(
Glycine max
) (Aliasgharzadeh et al.
2006
),
garden pea (
Pisum sativum
) (Quilambo et al.
2005
),
onion (
Allium cepa
) (Bolandnazar et al.
2007
),
tomato (Subramanian et al.
2006
) and other crop
3.2.1.2 Salinity
Salinity is one of the cosmopolitan threats to crop
production worldwide. Irrigation with groundwa-
ter and irrational use of easily soluble fertilizers
are main causes for salinity in agroecosystems
(Copeman et al.
1996
; Al-Karaki
2000
). It has
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