| |
CO2 and light stimulate the growth - part 1 |
| By Ole Pedersen, Claus Christensen and Troels Andersen Poor growth in plant aquaria has usually been attributed to insufficient light over the tank and when asking the experts, the advice has always been to increase the light availability before any other action is taken. New research shows that this may be poor advice, in particular, for an aquarium without CO2 fertilisation |
| |
|
Even in modern textbooks, you may still meet the allegation that only one resource may limit plant growth at a time. This has also been known as the principle of Liebig. Justus Liebig was a famous German chemist that among many things worked with the nutrition of agricultural plants. He postulated that one and only one factor could limit plant growth at the time. It is unclear whether Liebig himself developed the ancestor to Figure 1, but the simplicity of the barrel, which is partly filled with water, has greatly contributed to maintaining this perception of resource limitation. For terrestrial plants this has been known to be wrong for several decades and also within the aquatic plant sciences co-limitation of resources has been an accepted principle for at least twenty years. Few aquatic studies have shown that the interacting effect of light and CO2 may translate from photosynthesis into effects on growth (Maberly 1985, Madsen and Sand-Jensen 1994). In this paper we show data from experiments where we have created co-limitation of CO2 and light, which in nature are the two main limiting factors for aquatic plant growth.
|
| |
  Figure 1 The figure shows the classical illustration of Liebig’s principle. In this particular case, the element Bor is limiting plant production and water is running out of the barrel when the growth limitation is reached.
|
| |
|
To understand how plants respond to incident light, the general light utilisation of an aquatic plant is schematically shown in Figure 2A. At very low light intensities, the incident light is insufficient to sustain a positive photosynthesis and the net oxygen budget of the plant is negative. In other words, the respiration processes exceed the photosynthesis. At a certain light level, however, the two processes equal each other and we have then defined the light compensation point of the plant. By illuminating the plant with still higher light intensities the photosynthesis is also positively stimulated in linear way. At high light, the resulting outcome from the photosynthesis becomes less until it finally levels out at a point where we have the maximum photosynthesis. From this point on, more light will not translate into a greater photosynthesis.
In nature, growth of aquatic plants is often limited by the availability of light. Light is efficiently absorbed by water itself where light energy is transformed into heat and if the water also contains some dissolved organic substances – for example humic acids in black waters – the light absorption from the water itself can be very efficient. The light absorption of water and of the substances dissolved herein sets the depth limit of aquatic plant growth in nature and the transparency of water can sometimes turn so bad that it excludes all submerged vegetation so that only floating and emergent plants can thrive. Since light is such an important competitive parameter, evolution has developed a very efficient system of light utilisation in submerged plants. If the plant has sufficient nutrients available, it often invests more energy into light capturing pigments carotenoids, Xanthophylls and more important, Chlorophyll. Chlorophyll is the green pigment that absorbs light and transforms it into chemical energy, which can be used for growth in the cells. By doing that the plant ensures that the light, which is actually reaching the plant surface, is absorbed and used for energy purposes instead of being merely transmitted through the plant tissue. It is also necessary to contain a lot of Chlorophyll to obtain a large maximum photosynthesis but a large pool of Chlorophyll is not of much use if the energy produced cannot be used to fix inorganic carbon into sugars and carbohydrates.
|
| |
Figure 2A and B The figure shows the theoretical relationship between light and photosynthesis (A) and CO2 and photosynthesis (B). In both situations, a saturation functions describes the relationship although the actual shape of the function differs.
|
| |
For both resources a compensation point is defined as the level where the net photosynthesis or growth is zero. Below this point, the plant cannot maintain its biomass. In a saturation curve, there is also a point at which an increase in resource availability does not result in increased photosynthesis. See the text for further explanation.
|
 CO2 and light stimulate the growth - part 2
CO2 and light stimulate the growth - part 3
CO2 and light stimulate the growth - part 4
Click here to send this page to a friend
|
|
|
