Hydrophytic Vegetation

1
Hydrophytic Vegetation

• Definitions and depth zonation

• Survival strategies

• Role in the structure and function of wetlands

2
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
3
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.
4
Structural Groups
Free-floating
Emergent
Floating-leaf
Submersed
5
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
6
Structural Groups Floating-Leaf
Plants whose leaves float on the waters surface
but their roots are anchored in the substrate.
Depths 0.5 m - 3 m
7
Structural Groups Free Floating
Plants that float with most of their body above
the waters surface. Roots, if present, hang
free in the water. Depths variable but restricted
to nonturbulent, protected areas.
Water Hyacinth Image from Univ. of Florida
8
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
9
Overview
Part II. Strategies for life under
water 1. Oxygen exchange 2. Photosynthesis 3
. Obtaining nutrients 4. Structural support
10
Oxygen Exchange
as far as hydrophytes are concerned, oxygen is
a rare and precious commodity.
Agnes Arber, 1920 Water Plants
11
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

12
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

13
Adventitious roots
Photo from Rolf Kyburz
Gaussia spirituana (palm growing on coral reef)
14
Pneumatophores
From www.nhmi.org Bill Keogh, Photographer
15
Lenticels
www.pssc.ttu.edu/pss1411cd/PLANTID/
glossary/glossary.htm
Pear tree
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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
17
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)

18
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.
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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.
20
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.
21
Responses to Flooding
Flooding
Ethylene dissipates
22
Oxygen Exchange
Submersed plants must obtain dissolved oxygen
from the water.
Leaves have high surface area to volume ratio,
cuticle is absent
O2
Eurasian watermilfoil Image from Univ. of Florida
23
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)

24
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.

25
Getting Carbon for Photosynthesis
CO2 ?? CO2 H2O ? H2CO3 ? H HCO3- ? H
CO32-
Emergents, floating leaved
Some submersed use both
All submersed
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Obtaining Nutrients
In rooted aquatic plants nutrient absorption is
primarily through the roots.
27
Obtaining Nutrients Submersed Plants
Some foliar uptake may occur, especially in
waters with high nutrient concentrations.
28
Chara
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