224
and of the swamp cypress ( Taxodium distichum ).
These are very porous vertical outgrowths of
lateral roots which project for several centimetres above the soil surface where they are furnished with lenticels. Each pneumatophore
carries a cluster of thin absorptive roots just
below the soil surface. In the case of the mangrove Avicennia germinans , new layers of surface roots grow as sediment accumulates.
15.1.2 Stress Tolerance: Morphology,
Anatomy and Oxygen
Diffusion
The plant anatomists noted that aquatic plants are
sometimes unusually porous. Their tissues contain enormous intercellular spaces. These often
form regularly arranged chambers. Physiologists
now refer to such tissues as aerenchyma. This
term was originally coined to describe a rather
specialised porous tissue arising secondarily
from an epidermal or cortical layer. Roots in
waterlogged soils are similarly porous. The airspace formation is environmentally inducible.
Thus, rice roots grown in waterlogged soil contained 45 % airspace, almost twice as much as
those in freely drained soil. Early workers
believed that the role of pore space was to supply
oxygen to buried organs embedded in ‘asphyxiating mud’. However, the continuity of the airspace
and the dependence of root upon shoot for oxygen supply were not convincingly demonstrated
until about 40 years ago (Armstrong 1980 ).
The use of tracers, such as
15
O 2 , has shown that
even in dry-land plants, oxygen can diffuse to the
root system. This oxygen not only supports root
transpiration but may also leak to the exterior in
anaerobic conditions. The radical oxygen loss is
demonstrable with redox dyes, such as reduced
indigo carmine which becomes blue in contact
with oxygen. Also, the dissolved oxygen may be
measured by a suitable electrometric technique.
Some wetland plants, particularly those of
oligotrophic soils, are apparently xeromorphic. This
was originally interpreted as a response to ‘physiological drought’. This concept has long been
abandoned. But there is some experimental support
now for the suggestion that a reduced transpiration
rate will show the movement of reduced toxins to
the root surface and permit more effective oxidative detoxifi cation of Fe
2+
.
15.1.3 Metabolic and Biochemical
Responses to Flooding
In extreme conditions, the wetland plants may
have to adjust with long periods of total anoxia.
Thus, fl oating-leaved and reed swamp species,
which lose their leaves during winter, must
resume growth and produce considerable new tissue before reaching a copious oxygen supply.
Laing suggested that tolerance of anoxia
might be provided by the ability to ferment carbohydrate. It is now known that many plants convert glycolytically produced pyruvate to ethanol
via acetaldehyde. Further, the accumulation of
shikimic acid and end products of glycolysis
other than ethanol has been interpreted as an
adaptive waterlogging-tolerance mechanism
(Crawford 1976 ). Ethanol is believed to be toxic
at high concentration. Hence, it is likely that a
further adaptation to survival of waterlogging
will be either tolerance of ethanol or provision
for its loss by leakage in solution or through the
intercellular spaces as vapour.
The growth-controlling metabolism of plants
may be changed by fl ooding. Normal plant tissues synthesise a small amount of ethylene from
methionine via intermediates in a cyclic process.
Low concentration of ethylene interacts with
other growth-controlling substances as a necessary part of normal metabolism. Surplus ethylene
escapes through the intercellular spaces and
stomata (Drew et al. 1979).
15.1.4 Mineral Nutrition of Wetland
Vegetation
15.1.4.1 Rainfed and Groundwater
Systems: Contrasting
Environments
Nutrient availability in wetlands differs very
widely. Rainfed pits are probably the world’s most
15 Wetland Flora, Plankton, Productivity, Fauna and Fishes
and of the swamp cypress ( Taxodium distichum ).
These are very porous vertical outgrowths of
lateral roots which project for several centimetres above the soil surface where they are furnished with lenticels. Each pneumatophore
carries a cluster of thin absorptive roots just
below the soil surface. In the case of the mangrove Avicennia germinans , new layers of surface roots grow as sediment accumulates.
15.1.2 Stress Tolerance: Morphology,
Anatomy and Oxygen
Diffusion
The plant anatomists noted that aquatic plants are
sometimes unusually porous. Their tissues contain enormous intercellular spaces. These often
form regularly arranged chambers. Physiologists
now refer to such tissues as aerenchyma. This
term was originally coined to describe a rather
specialised porous tissue arising secondarily
from an epidermal or cortical layer. Roots in
waterlogged soils are similarly porous. The airspace formation is environmentally inducible.
Thus, rice roots grown in waterlogged soil contained 45 % airspace, almost twice as much as
those in freely drained soil. Early workers
believed that the role of pore space was to supply
oxygen to buried organs embedded in ‘asphyxiating mud’. However, the continuity of the airspace
and the dependence of root upon shoot for oxygen supply were not convincingly demonstrated
until about 40 years ago (Armstrong 1980 ).
The use of tracers, such as
15
O 2 , has shown that
even in dry-land plants, oxygen can diffuse to the
root system. This oxygen not only supports root
transpiration but may also leak to the exterior in
anaerobic conditions. The radical oxygen loss is
demonstrable with redox dyes, such as reduced
indigo carmine which becomes blue in contact
with oxygen. Also, the dissolved oxygen may be
measured by a suitable electrometric technique.
Some wetland plants, particularly those of
oligotrophic soils, are apparently xeromorphic. This
was originally interpreted as a response to ‘physiological drought’. This concept has long been
abandoned. But there is some experimental support
now for the suggestion that a reduced transpiration
rate will show the movement of reduced toxins to
the root surface and permit more effective oxidative detoxifi cation of Fe
2+
.
15.1.3 Metabolic and Biochemical
Responses to Flooding
In extreme conditions, the wetland plants may
have to adjust with long periods of total anoxia.
Thus, fl oating-leaved and reed swamp species,
which lose their leaves during winter, must
resume growth and produce considerable new tissue before reaching a copious oxygen supply.
Laing suggested that tolerance of anoxia
might be provided by the ability to ferment carbohydrate. It is now known that many plants convert glycolytically produced pyruvate to ethanol
via acetaldehyde. Further, the accumulation of
shikimic acid and end products of glycolysis
other than ethanol has been interpreted as an
adaptive waterlogging-tolerance mechanism
(Crawford 1976 ). Ethanol is believed to be toxic
at high concentration. Hence, it is likely that a
further adaptation to survival of waterlogging
will be either tolerance of ethanol or provision
for its loss by leakage in solution or through the
intercellular spaces as vapour.
The growth-controlling metabolism of plants
may be changed by fl ooding. Normal plant tissues synthesise a small amount of ethylene from
methionine via intermediates in a cyclic process.
Low concentration of ethylene interacts with
other growth-controlling substances as a necessary part of normal metabolism. Surplus ethylene
escapes through the intercellular spaces and
stomata (Drew et al. 1979).
15.1.4 Mineral Nutrition of Wetland
Vegetation
15.1.4.1 Rainfed and Groundwater
Systems: Contrasting
Environments
Nutrient availability in wetlands differs very
widely. Rainfed pits are probably the world’s most
15 Wetland Flora, Plankton, Productivity, Fauna and Fishes