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Problem 1:Anaerobic Condition
To cope with anaerobic conditions
during the high tide period, mangrove
plants have developed mechanism to
enhance gaseous exchange, and they
include:
Pneumatophores
They are erected aerial roots, which arise from cable roots and
they facilitate exchange of gases between the submerged roots
and the atmosphere (e.g., Avicennia marina).
They contain aerenchyma tissue (a specialized tissue with
large intracellular air spaces), which allows rapid
transportation of gases.
Knee Joints
They are roots that bend up like an arch and grow
above the substrate surface (e.g., Aegiceras
corniculatum Kandelia candel, Bruguiera
gymnorrhiza).
They are also rich in aerenchyma tissues, which
facilitate the transfer of air.
Lenticels
They are small pores for gaseous exchange, which are
present on pneumatophores, knee joints, and prop
roots.
Problem 2: Unstable Substratum
To cope with a soft unstable substratum, mangrove
plants have developed specialized root systems for
anchoring, they are:
Prop (or stilt) Roots
They are branched, looping aerial roots that arise from
the trunk and lower branches.
These supporting roots broaden the base and provide
stability (e.g., Kandelia candel, Bruguiera
gymnorrhiza, Aegiceras corniculatum).
Cable Roots
These roots spread horizontally and laterally just
below the soil surface, and anchor the plants firmly in
the soil (e.g., Avicennia marina).
Buttress Roots
These are a form of prop root, which thickens
unevenly to produce flattened blade-like supporting
structures and they function as flying buttresses (e.g.,
Heritiera littoralis; other Rhizophora species).
Problem 3:High Salinity (High NaCl
Concentration)
To cope with salinity, mangrove plants have developed
various mechanisms to regulate the salt concentration
in the plants, such as:
Salt Secretion
Salt glands are present in leaves, and they concentrate
and actively secrete the absorbed salts (e.g., Aegiceras
corniculatum, Avicennia marina).
Storage of Salt and Abscission
Excess salt is stored in vacuoles or older leaves, which
are then shed, thus reducing the salt content inside the
plants (e.g., Bruguiera gymnorrhiza, Lumnitzera
racemosa).
Balances Osmotic Potential
The plants can accumulate low molecular weight
carbohydrates to lower their osmotic potential, and so
enable plants to absorb water from a salty environment.
Salt Excluders
These physiologically prevent salt entering the root
xylem by an active pump mechanism (e.g., Aegiceras
corniculatum, Kandelia candel, Lumnitzera
racemosa).
Water Storage Tissue
The plants have xerophytic characteristics to reduce the intake of high
salinity water, e.g., water storage tissues, waxy thick cuticle, sunken
stomata and a hair surface.
Problem4:Establishment Problem
Due to the stressed environment conditions,
wave action and unstable substratum, survival
rates of seedlings are low. Mangrove plants
have evolved unique reproductive strategies.
Viviparous Reproduction
Seeds germinate inside the fruit without resting stage;
early development of seedlings occurs when seeds are
attached to the parent plant (e.g., Aegiceras
corniculatum, Kandelia candel, Bruguriera
gymnorrhiza).
This allows rapid growth and establishment of young
plants when they are finally detached from the plant
and come into contact with the substrate.
Shape of Propagules
Propagules (dropper) are elongated and have flesh
hypocotyls, so they float and disperse easily, but they
also easily anchor in an up-right position; once they
stick in mud, the already formed roots expand rapidly
(e.g., Kandelia candel, Aegiceras corniculatum,
Bruguiera gymnorrhiza).
Number of Seeds
Many seeds are produced annually to enhance the
reproductive success.
Adaptation for Non-viviparous Species
Heritiera littoralis produces large, woody fruits,
containing one to several seeds, which have spongy
outer layers that make them extra buoyancy.
Excoecaria agallocha and Acanthus ilicifolius have
an exploding capsule that releases numerous seeds.
Problem 5:Desiccation
To cope with the dry weather during exposure at low
tide, mangrove plants have developed the following
features:
Special Features of Leaves
Sunken stomata, epidermal hairs and scales, thick
waxy cuticle in leaf surface to reduce evaporation and
water loss (e.g., Kandelia candel, Avicennia marina,
Aegiceras corniculatum, Excoecaria agallocha).
Large Water Storage Tissues
Multi-layered water storage tissues to store water (e.g.,
Kandelal candel, Avicennia marina, Bruguiera
gymnorrhiza, Aegiceras corniculatum, Lumnitzera
racemosa)
White Tomentum or Scales at Lower
Leaf Surface
Reflect bright light and lower plant temperature to
reduce water loss (e.g., A. marina, H. littoralis).