Environmental Engineering Reference
In-Depth Information
been estimated that more than 250 million ha of
groundwater irrigated lands is salinized, of which
ten million ha is abandoned annually (Codevasf
2011
). Salt deposition in the soil results in hyper-
ionic and hyperosmotic stresses (Evelin et al.
2013
). The presence of excess salts in the soil
solution may limit the growth of an organism due
to specifi c ion toxicity or osmotic stress. These
factors tend to differ in relative importance
depending on the species and concentration of
ions involved, as well as the tolerance of the
organism in question (Brownell and Schneider
1985
). Salinity may affect certain stages of the
life history of an organism more compared to
other stages. Salt stress induced decline in crop
productivity results from its negative impact on
plant growth and development (Giri et al.
2003
;
Mathur et al.
2007
). Arbuscular mycorrhizal
fungi have been shown to increase crop yield
under saline soils (Daei et al.
2009
). Nevertheless,
results on the infl uence of salinity on AM forma-
tion and function are often contradictory. Some
studies have shown that soil salinity reduces root
colonization by AM fungi and increases plant's
mycorrhizal dependency (Tian et al.
2004
; Sheng
et al.
2008
). In contrast, it has also been shown
that AM colonization either remains unaffected
(Yamato et al.
2008
) or even increased under salt
stress (Aliasgharzadeh et al.
2001
). An increased
soil salinity has also been shown to adversely
affect the production of extraradical hyphae of
AM fungal strains that are sensitive to salinity
(Juniper and Abbott
2006
; Evelin et al.
2009
).
The extent to which salinity reduces AM coloni-
zation depends on the stage of the association
such that inhibition is more prominent during
early stages of the symbiosis development than
during the later stages (McMillen et al.
1998
).
For example, salinity inhibited early colonization
of roots by
Gigaspora decipiens
more than by
Scutellospora calospora
(Juniper and Abbott
2006
). It has been shown that AM fungi alleviate
salt stress in some plants through modifi cations
in physiological mechanisms (see Evelin et al.
2009
; Porcel et al.
2012
). However, the adjust-
ment of osmotic potential by settling down of
soluble sugars in mycorrhizal fungal parts has
been suggested to protect the plant from salinity
(Soliman et al.
2012
). For instance, trehalose in
spores and extraradical mycelium enables AM
fungi to colonize host plants even under high
salinity (Schubert et al.
1992
). Several studies
have reported that salt stress induces modifi ca-
tions in plants even at ultrastructure levels
(Yamane et al.
2004
; Miyake et al.
2006
; Andrea
and Tani
2009
). Recently, Evelin et al. (
2013
)
showed that the ultrastructural changes in
AM-inoculated fenugreek (
Trigonella foenum-
graecum
) plants exposed to four different levels
of salt were less than non-mycorrhizal plants.
Studies have also shown that some AM fungi are
able to adapt to different environmental condi-
tions better than others (Stahl and Christensen
1991
). Thus, the varied observations reported by
different workers may partly refl ect the differ-
ences between the fungi used and their ability to
adapt to various environments. Nevertheless,
most of studies examining mycorrhiza and soil
salinity to date have not considered these
differences.
Arbuscular mycorrhizal fungal-mediated salt
stress tolerance has been shown for crops like
chilli (
Capsicum annuum
) (Çekiç et al.
2012
),
Chinese milk vetch (
Astragalus sinicus
) (Peng
et al.
2011
), pepper (
Piper nigrum
) (Turkmen
et al.
2008
; Kaya et al.
2009
), fenugreek (Evelin
et al.
2012
), corn (Sheng et al.
2008
,
2011
), bajra
(
Pennisetum glaucum
) (Borde et al.
2011
),
tomato (Hajiboland et al.
2010
) and clover
(
Trifolium alexandrinum
) (Gharineh et al.
2009
).
Like drought stress, an increased P uptake medi-
ated by AM fungi has been suggested to alleviate
saline stress (Tian et al.
2004
). However, in cer-
tain cases, saline tolerance of mycorrhizal plants
appears to be independent of P concentration
(Feng et al.
2002
). Both differences in the ability
between AM fungi to obtain P from the soil and
their ability to adapt to changing edaphic condi-
tions (del Val et al.
1999
) could reason for varied
sensitivities of AM fungi to salinity. Therefore, it
might be expected that an isolate originating
from saline soil would have a higher adaptability
and a greater capacity to promote plant growth
under saline stress.
Search WWH ::
Custom Search