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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, ���������������������������������������� the plants and hence, the thrill of to the low-light environment in the ����������������������������� ������������������������������������� the planted aquarium! Without aquarium. As a consequence, many ������������������������������������ ������������������������������������ green light, the colors of leaves applants lose their terrestrial leaves ���������������������������������� ��������������������������������������� pear grayish and tame. and new ones are formed. ese ������������������������������������� ��������������������������� new leaves are much better adapted ���������������������������������������� ���������������������������������� Light Quantity to light harvesting under low light ������������������������������������������� �������������������������������������� Light intensity is an expression in the aquarium, where it becomes �������� ��������������������������������������� of how� �������������������������� much light (energy) reaches important to capture every single ��������������������������������������� �������������������������������������� a given surface and in natural sciphoton that reaches the leaf surface. �������������������������������������� ������������������������������������� ences, light intensity is measured in Because most aquaria are under��������������������������������������� �������������������������� � ��������� µmol photons per square meter per illuminated, many plant aquarists ����������� ��������������������������� ��������������������������������� � 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