Agriculture Reference
In-Depth Information
Fig. 2.6
Succulent leaves in
Lumnitzera
and
Excoecaria
species
groves thrive best in muddy coastal plains where adequate fresh water supplies
from river discharge with ample nutrients are available.
Each mangrove species is associated with an optimum salinity (Snedaker
1978
).
Each species occupies the salinity zone to which it is best suited and is best adapted
physiologically. There are mainly two mechanisms of salt regulation (Scholander
1968
).
3.1 Salt Exclusion
Rhizophora
and other members of family Rhizophoraceae have a well-developed
mechanism of ultra-filtration in their roots enabling only selective absorption of
ions while extracting water from the soil. They may retain a low internal salinity by
means of salt-excluding mechanisms in the roots.
3.2 Salt Excretion
Avicennia
sp.,
Aegiceras
sp., and several other species have salt glands on their
leaves which secret salt.
Lumnitzera
and
Conocarpus
have analogous structure
to salt glands. Sodium chloride concentration in the xylem sap of these species is
about 10 times greater than that in exclusion type. The ions which are excreted by
these glands are mostly sodium and chloride. Though salt-excreting species allow
more salt into the xylem than the non-salt-excreting species, but still they exclude
about 90 % of the salt (Scholander et al.
1962
; Azocar et al.
1992
). Salt excretion is
an active process as evidenced by ATPase activity in the plasmalemma of the excre-
tory cell (Drennan et al.
1992
).
Salt accumulation is also another salt-regulatory mechanism found in species of
Lumnitzera
and
Excoecaria
which accumulate salts in leaf vacuoles and become
succulent (Fig.
2.6
). In some species, salt concentration can also be reduced by
