Agriculture Reference
In-Depth Information
transferring the salts into senescent leaves or by storing them in the bark or the
wood (Tomlinson
1986
). With increase in water salinity, some species restrict their
water use in order to achieve greater tolerance (Ball and Passioura
1993
). In addi-
tion to these direct regulatory mechanisms, mangroves may also accumulate or syn-
thesize other solutes to regulate and maintain osmotic balance (Werner and Stelzer
1990
; Popp et al.
1993
). Some species such as
Aegiceras corniculatum
,
Aegialitis
annulata
and
Laguncularia racemosa
store mannitol and proline (Polania
1990
),
Avicennia marina
stores glycine betaine, asparagines and stychyose (Ashihara et al.
1997
) and
Sonneratia alba
synthesizes purine nucleotides that facilitates tolerance
to salt load of 100 mM sodium chloride (Akatsu et al.
1996
). Scholander et al.
(
1964
) have demonstrated that in order to facilitate water flow from roots to leaves,
the water potential at the leaves is held lower (−2.5 to −6.0 MPa) than in the roots
(−2.5 MPa). Recent studies also show that mangroves can restrict cytosolic salt
contents not only by ultra-filtration (Zheng et al.
1999
; Wang et al.
2002
; Aziz et al.
2001
; Khan et al.
2001
), but also by other means such as salt accumulation and ion
sequestration (Mimura et al.
2003
; Kura-Hotta et al.
2001
). Salt-controlling strat-
egies in mangroves are similar to those in glycophytes, but probably mangroves
could exclude or sequester salt ions more efficiently (Shan et al.
2008
). Many man-
grove species (e.g.
Kandelia obovata
,
Avicennia marina
(Zhao et al.
1999
; Suarez
et al.
2006
)) can accumulate inorganic ions and use them as osmolytes to main-
tain osmotic and water potential. This characteristic confers a survival advantage
to these species in a saline environment (Tomlinson
1986
). Shan et al. (
2008
) have
shown that while sequestering excessive ions into vacuoles, mangroves could also
accumulate organic osmolytes in cytoplasm to get osmotic equilibrium across the
tonoplast. Organic osmolytes of mangroves mainly include hydroxyl compounds,
free amino acids (especially Proline), polysaccharide (e.g. starch), etc. Oku et al.
(
2003
) studied the relevance of lipid composition to salt tolerance in propagules
of
Kandelia candel
and
Bruguiera gymnorhiza
planted with varied salt concentra-
tions. This study result shows that salt stress specifically modulated the terpenoid
concentrations in mangroves, whereas phospholipid and fatty acid compositions in
both species are not changed with respect to varying salinity.
Salinity increases biosynthesis and accumulation of ABA, which modulates
physiological reactions in plant response to salinity (Zhao et al.
1991
; Montero
et al.
1997
; Gomez-Cadenas et al.
1998
). It has been documented that ABA induces
the expression of antioxidant genes encoding Cu/Zn-superoxide dismutase (Cu/
Zn-SOD) (Guan and Scandalios
1998
). Calmodulin (CaM), a ubiquitous calcium-
binding protein, regulates the activity of a variety of enzymes and proteins that
confers salt tolerance (Li et al.
2009
). Yang and Poovaiah (
2002
) demonstrated the
role of CaM in regulating H
2
O
2
homeostasis, i.e. CaM down-regulated H
2
O
2
levels
in plants by stimulating the catalytic activity of catalase. Li et al. (
2009
) recently
studied the correlation between ABA, CaM and antioxidant defense in
Bruguiera
gymnorhiza
and
Kandelia candel
and found that elevated ABA and CaM concentra-
tion under short-term and long-term salt treatment may up-regulate the activity of
antioxidant enzymes in the two mangrove species, thus avoiding excess ROS pro-
