| Where to find duckweeds
Wetlands and ponds are the most common sites to find
duckweeds, but
other quiet bodies of water may harbor them. Duckweeds may also
be
found on the fringes of larger lakes and in quiet backwaters and
sloughs
cut off from mighty rivers. Since duckweeds may be brought
in by migratory birds, there is no necessity that a site
communicate
with another body of water having duckweeds.
Duckweeds of every genus, Spirodela, Landoltia,
Lemna, Wolffia
and Wolffiella are found world-wide, although each species has certain
areas and climates where it is particularly well-adapted.
Environmental Conditions for Growth: Like other
plants,
duckweeds have specific mineral
requirements.
Each species and clone grows best under specific environmental
conditions
(day and night temperature, day-length, geographic latitude and
elevation
etc.). However, these optimal conditions are broad generalities
--
duckweeds aren't fussy.
|
Above: Huntley
Meadows, a natural wetland in Fairfax County, Virginia.
Below: Duckweed populations often grow among the reeds and other
emergent plants in shallow, slow-moving water. |
Surviving Harsh
Conditions:
Environmental Stress
Actively growing duckweed plants are sensitive to freezing and to
drought.
Duckweeds can tolerate a degree of salinity,
but
are unable to grow in salt marshes.
Cold and Drought
Certain duckweeds can prosper and grow in chilly (but not freezing)
conditions, particularly at night. However, during vegetative
growth
all duckweeds are sensitive to frost. Duckweeds survive freezing
as turions or seeds,
which
sink to the bottom after they are released from the mother fronds.
Dormant buds and seeds allow many species of duckweeds to survive
periods
that would otherwise be lethal. Some species can form dormant
buds
(turions), which are
resistant
to frost (but not to drought). Many species can flower
and set seed. Duckweed
seeds are resistant to cold and drought. Not all species of
duckweed
are known to flower. Those species may have lost the ability to
flower,
or perhaps we do not yet know the conditions which will cause them to
flower.
Mud and silt can protect duckweeds: When their water
dries
up, duckweeds may become stranded on mud. Duckweed fronds are
drought
sensitive, but they will survive on a surface of wet mud. The
small
immature fronds and buds hidden
inside
the mother frond dry up last and have greater resistance to
drought.
These fronds and buds will grow out and re-establish duckweed plants if
they are rewatered before it is too late. Winter-dormant seeds
and
turions resting on the bottom are insulated from the cold and drying
conditions
on the surface. Silt and sediments on the bottom may cover these
dormant strucures and help protect them until good growing conditions
return.
Salinity (Salt) Stress. Another
stress factor is salinity, the concentration of sodium chloride in the
water or soil. Worldwide, salinity is a major factor in what
plants
can grow where. Salinity can be high because of the presence of
sea
water, as in tidal marshes and estuaries, or because of leaching from
saline
soils at inland locations. Duckweeds, like many plants can
tolerate
a small amount of salt. Small amounts are even stimulatory for
growth
(figure). However, above a threshold (>60 mM NaCl), growth is
begins
to be inhibited. At yet higher concentrations, duckweeds cannot
survive.
Figure: Growth of Lemna minor in diluted sewage
with added amounts of salt. Error bars are +/- standard error.
Growth conditions starting with three replicates of 10
plants/each were: seven days at 20 C, 2700 lux from CW
fluorescent
lamps, in 100 mL 2% swine waste in 125 mL bottles with loose-fitting
clear
plastic lids. Regraphed from Stanley, RA and Madewell, CE (1976)
"Chemical tolerance of Lemna minor L." Tenn. Valley Authority,
Muscle
Shoals, Al. Circular Z-72, 17 pp. (Polycopy).
Almost all of the major food crops grown on land are salt sensitive
and will not complete a full life cycle above 100 mM NaCl. For
comparison,
full strength sea water is approximately 550 mM NaCl. Barley and
the beet are two exceptions, crop plants with a high salinity
tolerance.
Sugar beet will survive in 2/3-strength sea water. Plants that
can
tolerate higher levels of salt, up to that in seawater, are called
halophytes.
Halophytes deal with high salinity in a variety of ways:
exclusion,
excretion and osmotic balance. Different plants have evolved
combinations
of these mechanisms.
Links for more information on salt stress:
- Mechanisms of salinity tolerance, [ link
].
- Research on biochemical mechanisms of salt excretion, [ link
].
- Genetic engineering to make plants more salt-tolerant,
[ link
].
- Coverage on all types of plant stress, [ plantstress.com
].
Experiments you can try:
- Compare the growth of different duckweed species under specific
environmental
conditions.
- Study the effect of drought on duckweeds: What happens when
they
are stranded on mud? How dry can they get before they die?
- How long can duckweeds survive in the cold? Which species
can
survive
cold best?
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Revised: December 10, 2005