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Troels Andersen1, Claus Christensen1 and Ole Pedersen2
is article deals with light—light
quality, light quantity and illumination time—and how plants use light.
It explains how aquatic plants can
survive and even grow in light environments receiving much less energy
than the equivalent terrestrial ecosystems. Finally, the article challenges
the view that only light is essential for
the growth of aquatic plants.
Light Quality
In nature, plants receive light
energy from the sun. Sunlight,
which we interpret as white, is in
reality composed of all possible
colors (Figure 1). When sunlight
is split into its basic colors, as it
happens in the rainbow with water
droplets or in a prism, all the various colors can be recognized. ere
is a continuous transition from
Figure 1. Relative absorption as a function of wavelengths for the different pigments in plants.
26 The Aquatic Gardener
the short wavelengths in the violet
and blue spectrum, over green
and yellow to red, which has long
wavelengths. Outside this visible
spectrum, we have UV light that
has very short and energy rich
wavelengths, and infrared light that
has long wavelengths and is basically heat radiation.
However, plants are able to use
visible light only from about 400
to 700 nm and this range is termed
Photosynthetically Active Radiation
(PAR). Primarily, red and blue light
are used in photosynthesis while
green light is reflected or transmitted and thus, plants appear green
because green light is not absorbed.
Plants capture light by means of
pigments, which absorb light of
different colors depending on the
pigment in question (Figure 1). All
plants have Chlorophylla, many
have Chlorophyllb, while only few
have Chlorophyllc. e three chlorophylls have very different absorption spectra, i.e. they absorb light
of different colors and thus, they
may complement each other in the
process of light harvesting. Carotenoids is another group of pigments
that can absorb blue-green light
where chlorophylls are inefficient
(carotenes are orange as we know
them from carrots where they
don’t play any role in light absorption). Not all higher plants have
carotenoids, whereas most algae
do and thus, algae may become a
nuisance if the light source over the
aquarium contains too much green
Jan Ole Pedersen
Light—The Driving Force for Growth
of Aquatic Plants
Figure 2. Pogostemon stellata is an example
of a plant that has high light demands for
obtaining the beautiful red coloration.
and yellow-green light. In that case,
this additional light only benefits
the algae.
Our visual interpretation of the
plants’ colors is determined by the
light which is reflected by the plants.
Most aquarium plants are green
since they reflect the light that they
cannot use in photosynthesis. However, some plants have spectacular
colors (yellow, orange and red)
and such plants require extra light
because they reflect a large proportion of light which could, in fact,
have been used in photosynthesis
(Figure 2). Considering that plants
primarily use blue and red light,
it may be tempting to use a light
source where green light has been
Volume 20 Number 2
27
reduced to a minimum (for examwhat most aquarium plants face
ple GroLux fluorescent tubes from
when they are transferred from the
Sylvania) but�����������������������������
such sources of light
well-illuminated nursery, where
dramatically changes
the colors of
most plants are produced emergent,
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the plants and hence, the thrill of
to the low-light environment in the
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the
planted aquarium! Without
aquarium. As a consequence, many
������������������������������������
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green light, the colors of leaves applants lose their terrestrial leaves
����������������������������������
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pear grayish and tame.
and new ones are formed. ese
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new leaves are much better adapted
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Light Quantity
to light
harvesting under low light
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Light intensity is an expression
in the aquarium, where it becomes
��������
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of how� ��������������������������
much light (energy) reaches
important to capture every single
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a given surface and in natural sciphoton that reaches the leaf surface.
��������������������������������������
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ences, light intensity is measured in
Because most aquaria are
under���������������������������������������
��������������������������
� ���������
µmol photons per square meter per
illuminated,
many
plant
aquarists
�����������
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���������������������������������
�
second (µmol m-2 s-1). In the aquar�������������������������������������
will gain a positive experience if
ium hobby, Lux has traditionally
������������������������������������
better lightning is achieved. Many
been used to measure light because
������������������������������������������
colored plants do not achieve their
quantum sensors are extremely
�������������������������������������
full glow unless we provide them
�����������������������������������������
expensive devices, whereas lux can
with high light intensities. At
���������������������������������������
be measured by an old-fashioned
lower light, they may either lose
����������������������������������������
light meter used in photography.
their spectacular colors completely
���������������������������������������
As a rule of thumb, 1 µmol m-2 s-1
or they appear less colorful. For
���������
is equivalent to 55 Lux in the PAR
example, Echinodorus barthii may
spectrum but this transformation is
develop many large, dark red leaves
��������������������������������������������
not accurate since the Lux scale has
under high light, whereas under
��������������������������������������
been developed to suit the eye’s senlow light it resembles the more
sitivity and thus, it is not the same
�����������������������������������
common
Echinodorus osiris.
���������������������������������
for all color combinations.
Various sources of light may be
��������������������������������������
In nature, many aquatic plants
used over the planted aquarium
���������������������������������������
are found at places where they
and it is a science on its own and
����������������������������������������
receive direct sunlight (2000 µmol
beyond the scope of this article to
�����������������������������������
m-2 s-1) at least part of the day.
analyze each and every possible so�������������������������������������
Not even plants growing in shade
���������������������������������������������
lution. We will focus on fluorescent
�������������������������������������
receive less than about 200 µmol
����������������������������������
tubes because they are the most
-2 s-1 at noon. In comparison, a
m��������������������������������
��������������������������������������
economic and efficient in terms of
��������������������������������������
very well-illuminated aquarium
������������������������������
useful light per Watt consumed.
������������������������������������
receives about 80 – 100 µmol m-2
������������������������������������������
Unfortunately, a large proportion
��������������������������������������
�������������
s-1. is is a dramatic reduction in
of the light that is emitted by the
������������������������������������
��������������������������������
energy supply, but it is nevertheless
fluorescent tubes never reaches the
���������������������������������������� ���������������������������������������
�
��28
The Aquatic Gardener
���
plants. Light is emitted in all directions and only beams that hit the
water surface almost perpendicular
penetrate the water surface, while
the remaining light is reflected (Figure 3, upper panel). Use of cover
glass causes an even larger proportion of light to be wasted. However,
an ideal reflector increases the proportion of light that penetrates the
water surface considerably because
the reflector collects all beams and
reflects them in parallel bundles
(Figure 3, lower panel).
e depth of the tank is the
most important factor controlling
how much light is reaching the
bottom once the light has successfully penetrated the surface. e
light intensity decreases dramatically with distance to the lamp.
For example, if 50% of the light
Figure 3. A simple illustration showing the importance and function of using a
reflector for optimizing light intensity in the aquarium.
30 The Aquatic Gardener
remains at a depth of 10", then
only 25% will reach that area at a
depth of 20". Most of this reduction is due to the fact that the light
beams are not totally parallel and
thus, much light is scattered on
its way to the bottom. Also, light
is absorbed by colored substances
(humic acids absorbing mainly red
and infrared light) dissolved in the
water and by particles suspended
in the water (mostly microscopic
algae and detritus absorbing mainly
blue light). In conclusion, much of
the useful wavelengths have been
filtered before the light reaches the
bottom of the aquarium.
e temperature inside the
fluorescent tubes is of great importance for the amount of light
emitted. e higher or lower the
temperature from the optimum,
the less light is actually emitted.
e optimum temperature is about
38ºC and at 60ºC, most fluorescent tubes emit only 25% less light
compared to the optimum operating temperature. Equally important
is the type of fluorescent tubes.
Basically, the old T8 type is much
less efficient compared to the newer
T5. A T5 tube may emit at least
50% more light per Watt consumed
and part of it is due to the fact that
the temperature is much lower in
those tubes.
Finally, you should always consider the placement of the plants in
relation to the light source. Using a
panel with fluorescent tubes cover-
ing the tank results in only 25% of
light in the corners compared to the
center of the tank. Consequently,
plants that require high light should
never be place in corners or along
the edges where the light intensity
is much lower than in the center.
Illumination Time
Most aquarium plants come
from the tropics with a typical day
length of 10 – 14 hours. e plants
follow this rhythm, which can be
observed in for example Cabomba,
which folds in the leaves in the
shoot apex at the time just prior to
switching off the light. It is probably even more important to respect
that plants need a dark period to
“rest.” If they are not given this dark
period, they develop symptoms of
stress. ey use the dark period to
transform energy-rich compounds
formed in photosynthesis to more
complicated molecules, a process
that eventually leads to new growth.
e optimum illumination
time is approximately 12 hours for
most plants. Any additional light
does not really benefit the higher
plants, whereas algae are always able
to capitalize on the extra energy
provided. On the other hand, a
significantly short period of illumination has an adverse effect on
the plants. ey simply do not get
enough energy and they start losing
leaves, particularly the lower ones
(Figure 4). However, because of the
relatively large starch deposits in
Volume 20 Number 2
31
Claus Christensen
higher plants they are able to withstand periods of really low light.
is fact is often used in the battle
against algae, where a prolonged
period of darkness may kill the
algae, because they have few energy
deposits, whereas the higher plants
survive.
Acclimation to Low Light
Intensity
Claus Christensen
Aquatic plants are well-acclimated to life under water morphologically (Figure 5) as well as
physiologically. Underwater leaves
are often compared to shade leaves
of terrestrial plants. e leaves
are thinner than normal and they
also contain fewer cell layers (in
extreme instances, the lamina only
contain two layers of cells). e
Figure 4. The impact of day length, here illustrated by photos from Bonito, Brazil showing the
same location with identical physical conditions except day length.
32 The Aquatic Gardener
cuticle, which is the protective layer
that prevents evaporation of water
vapor, is almost completely reduced
in most aquatic leaves. Further, the
chloroplasts containing the light
absorbing pigments are placed in
the outer cell layers. Measurements
have shown that 80% – 100% of
all pigments are located in the outer
cell layers, which for some plants
make up a minor proportion of
the entire leaf. Finally, the thylakoid membranes are also reduced
(see the “Did You Know?” box on
page 35) such that each individual
pigment molecule receives a larger
proportion of light. e affect of all
these morphological modifications
is that light is used more efficiently
and self shading is significantly
reduced.
Figure 5. Morphological adaptation in a submerged leaf from Zostera marina showing the reduced cuticle, reduced number of cell layers and the chloroplast arranged in the outer cell layers.
Volume 20 Number 2
33
Claus Christensen
Did You Know ...
Light is the energy source in photosynthesis where water and inorganic carbon is transformed to energy rich sugar and oxygen:
light
↓
6H2O + 6 CO2 → 6C6H12O6 + 6O2
Figure 6. Submerged and emerged Cryptocoryne wendtii from the same location in Sri Lanka
showing dramatic changes in leaf structure and coloration.
Synergy Between Various
Essential Resources
Often, light is referred to as the
most important resource for plants’
photosynthesis and growth, and
this article also adds to the fact that
light is a central resource. However, in this process other resources
should not be neglected. is is
particularly important in cases
where light is already a limiting
resource, as is most often the case in
the planted aquarium. Under these
circumstances, it is important that
other resources are readily available
while the plant is acclimating to
low light. Carbon dioxide (CO2) is
an excellent example of a resource
that can stimulate photosynthesis
and growth even under severe light
limitation.
In the next issue of TAG, we will
explain more about CO2 and how
carbon stimulates the growth of
submerged aquatic plants in nature
as well as in the planted aquarium.
Footnotes
1 Tropica
Aquarium Plants,
Mejlbyvej 200, DK8250 Egaa,
Denmark.
2 Freshwater Biological Laboratory, University of Copenhagen,
Helsingørsgade 51, DK3400
Hillerød, Denmark.
For other articles, plant information, and details of the new AquaCare line of
aquatic plant fertilizers and substrates, visit the Tropica Aquarium Plants web site
at www.tropica.com.
34 The Aquatic Gardener
In this process, light energy is
transformed and fixed into chemical energy, which later may be
used in other metabolic processes
in the plant. Consequently, the
sugar is primarily used directly to
synthesize cellulose and starch,
which are important elements in
plant growth.
The chloroplasts contain stacks of
membranes that are termed thylakoid membranes. These stacks
are usually reduced in aquatic
leaves to achieve a better light
efficiency at low light intensities.
In the thylakoids, light is captured by pigments (mainly chlorophylls),
which are placed as “antennae” on an energy center termed the light
reaction center. Antennae and light reaction center is called a light
harvesting complex. The number of antennae containing chlorophyll
may vary from about 300 units to more than 1000 units, and measurements have shown that light harvesting increases linearly up to about
1000 units.
Once a pigment has captured a photon (the light particle), the energy
is channeled to the light reaction center and when enough energy
is present, an electron is released and channeled through a series of
energy rich compounds (illustrated by the tilted "z" on the figure). The
energy is used to split water into hydrogen and oxygen and to melt
six carbon dioxide molecules into a sugar molecule. The Z-scheme,
the antenna and the light reaction center is termed the photosynthetic unit.
Volume 20 Number 2
35