Hydrophytic
Vegetation
• Definitions and depth zonation
• Survival strategies
• Role in the structure and function of wetlands
Hydrophytes
“Any macrophyte that grows in water or on a substrate
that is at least periodically deficient in oxygen as a result
of excessive water content; plants typically found in wet
habitats.²”
drier
wetter
The presence of hydrophytes is one of three elements
comprising the definition of a wetland.
2. Wetland Training Institute 1987. Field Guide for Wetland Delineation, Corps of Engineers
Manual
Evolution of Aquatic Plants
Unlike aquatic microflora, they are not “true children of the
water”.
“Their ancestors came out of the water and were
transformed into aerial organisms, then individual members
of these groups re-adapted to return to the water”. ³
3. Ruttner, Franz 1963. Fundamentals of Limnology, Univ. of Toronto Press. p. 179.
Structural Groups
Free-floating
Emergent
Floating-leaf
Submersed
Structural Groups
Emergents
Plants whose roots and basal
portions grow beneath the
surface of shallow water but
whose leaves and stems are
born primarily in the air.
Examples include bulrush,
cattails, arrowhead, rushes,
sedges, and many shoreline
plants. Depths -0.5m - 1.5m
Image from University of Florida
Aquatic, Wetland and Invasive Plant Information Retrieval System
Structural Groups
Plants whose
leaves float on
the water’s
surface but
their roots are
anchored in
the substrate.
Depths 0.5 m - 3 m
Floating-Leaf
Structural Groups
Free Floating
Plants that float with
most of their body
above the water’s
surface. Roots, if
present, hang free in
the water. Depths variable
but restricted to nonturbulent,
protected areas.
Water Hyacinth
Image from Univ. of Florida
Structural Groups
Submersed
Plants that spend their
entire life cycle, with
the possible exception
of flowering, beneath
the surface of the
water. Depths to 10 m
Hydrilla
Overview
Part II. Strategies for life under water:
1.
Oxygen exchange
2.
Photosynthesis
3.
Obtaining nutrients
4.
Structural support
Oxygen Exchange
“…as far as hydrophytes are concerned,
oxygen is a rare and precious commodity.”
Agnes Arber, 1920 Water Plants
Oxygen Exchange, continued
• Cells in root tissue respire, in terrestrial plants
oxygen is obtained from air spaces in the soil.
• Although oxygen may be present in the water column,
respiration by aquatic biota and by soil organisms,
together with slow diffusion of oxygen in water, results
in anoxic conditions in the soil
• In obligate anaerobes, anoxia leads to cell
death in 24 hours
Oxygen Suppy Strategies of Vascular Plants, cont.
• Structural adaptations
a. Aerenchyma
b. Special organs or responses
i. Adventitious roots
ii. Stem elongation
iii. Lenticels
iv. Pneumatophores
c. Pressurized gas flow
• Physiological adaptations
a. Anaerobic respiration
b. Malate production
Adventitious roots
Photo from Rolf Kyburz
Gaussia spirituana (palm growing on coral reef)
Pneumatophores
From www.nhmi.org
Bill Keogh, Photographer
Lenticels
www.pssc.ttu.edu/pss1411cd/PLANTID/ glossary/glossary.htm
Pear tree
Oxygen Exchange
Aerenchyma
Large intercellular
structures (pore
spaces) which
extend throughout
the entire plant and
allow for the
storage and
transport of gas to
the submerged
roots.
aka Lacunae
Image from University of Florida
Aquatic, Wetland and Invasive Plant Information Retrieval System
more on aerenchyma…..
• Development of aerenchyma in individual
plants stimulated by flooding
• Formed by increased cellulase activity (cell
lysis) or cell separation in cortex
• Pore space in submerged portions of plant as
high as 60% (compared to 7% in terrestrial
plants)
Jussiaea peruviana (tropical)
A. mud roots (m.r.) and the
adventitous roots (a.r.)
B. Transverse section of
submerged part of a stem to
show aerenchyma (a) which
develops from the phellogen
(pg). Also shown is the
phloem (ph), normal
cambium (c), xylem (xy).
From Arber, Agnes 1920 Water
Plants. Cambridge University
PressWater Plants, p.190.
Oxygen Exchange
Emergent and floating plants obtain
oxygen directly from the atmosphere
through stomata on the leaves.
Passive diffusion of oxygen along a
concentration gradient
Reverse flow due to concentration
gradient of CO2 and CH4
Figure from Brix, H. 1993. Macrophyte-Mediated Oxygen
Transfer in Weltands: Transport Mechanisms and Rates in
Constructed Wetlands for Water Quality Improvement,
Moshiri, ed. p. 393.
Oxygen Exchange
Convective flow of gas in
water lilies. Pressurized
gas transport is induced by
humidity and thermal
gradients.
Figure from Brix, H. 1993. Macrophyte-Mediated Oxygen Transfer in Weltands: Transport Mechanisms
and Rates in Constructed Wetlands for Water Quality Improvement, Moshiri, ed. p. 394.
Responses to Flooding
Flooding
Increased sensitivity
to gibberellic acid
Ethylene
buildup
Cellulase
activity
Petiole cell
elongation
Initiation of
adventitous roots
Increased
aerenchyma
Leaves reach
surface
Ethylene dissipates
Oxygen Exchange
Submersed plants must
obtain dissolved oxygen
from the water.
Leaves have high
surface area to volume
ratio, cuticle is absent
Eurasian watermilfoil
Image from Univ. of Florida
O2
Photosynthesis
• Submersed plants have their photosynthetic
maximum at lower light levels (ca. 15% full sun
or less)
• Light intensity is believed to be the limiting factor in
determining the maximum depth at which an aquatic
plant can survive (although for rooted plants it could be
gas transport)
• Compensation depth - where respiration exactly
equals photosynthesis (species specific)
Photosynthesis
•Not all submersed species are
equally adapted to low light:
Elodea densa optimum at 107 lux (0.3% full
sun) whereas Heteranthera dubia optimum at
6350 lux (18%)
•Accessory pigments allow for high
variability in spectral preferences :
Elodea densa died under light 480-630 nm
(yellow-green) whereas Heteranthera dubia
grew 3 times greater.
Getting Carbon for
Photosynthesis
Emergents, floating leaved
CO2

CO2 + H2O  H2CO3  H+ + HCO3-  H+ + CO32-
All submersed
Some submersed
use both
Obtaining
Nutrients
In rooted aquatic plants nutrient absorption is primarily
through the roots.
Obtaining Nutrients
Some foliar uptake
may occur, especially
in waters with high
nutrient
concentrations.
Submersed Plants
Free floating macrophytes
obtain nutrients directly
from the water through
foliar absorption and
through water roots.
The unrooted macroalga
Chara absorbs P equally
well in all parts (Littlefield and
Forsberg 1965)
Image from Univ. of Florida
Chara