Plants developed on land before venturing under water and although aquatic plants are highly adapted to the underwater environment, many of their physical attributes can be traced back to their terrestrial ancestry.
Looking at the biology and structure of aquatic plants, helps us to understand why certain conditions are needed in the aquarium if we want to keep aquatic plants successfully.
The unique function that plants possess is the ability to obtain energy from sunlight, carbon dioxide and water, using the process of photosynthesis. Photosynthetic cells within the leaves and stem tissues contain pigments that trap light energy to break down the molecular structure of water (H2O) into hydrogen and oxygen. The hydrogen binds first to carbon dioxide and then oxygen to form glucose, which is a basic sugar and an important source of energy. Some oxygen is left over from this process and is released back into the water, where it is either used up by bacteria and animals for their respiration or release into the atmosphere at the water surface.
The glucose produces from photosynthesis is water soluble and, if stored in large quantities, will absorb water and enlarge the cells that contain it. Obviously, this is undesirable for plants, so the glucose is quickly converted into an insoluble starch compound and transported to various parts of the plant for storage, in most cases to the upper root area. Many plants store starch in tubes, rhizomes and bulbs. The starch can be easily converted back into glucose and transported around the plant when needed for energy or growth.
Factors affecting photosynthesis
A plant has very little control over the rate of photosynthesis is that occurs its cells. A number of environmental factors are responsible for the productivity of the photosynthesis cells and it is always the factor in least supply that limits the rate of photosynthesis. The aim in the aquarium is to remove the majority of constraints on photosynthesis to obtain the optimum level.
Higher rates of photosynthesis will encourage faster growth, reproduction and improved plant health. Light is the most obvious environmental factor, but temperature, carbon dioxide levels and nutrient availability also affect the rate of photosynthesis. In bright conditions, plant may receive more light than it needs to produce adequate amounts of glucose. At night, plants stop photosynthesis and only start again in daylight.
Respiration and oxygen levels
Respiration helps to break down food sources and release energy into the cells. During the process, oxygen is used up and carbon dioxide is released as a by-product. Respiration is a continual process that does not stop at night. Thus, photosynthesis stores food ‘energy’, whereas respiration releases energy.
In a heavily planted aquarium, respiration has a significant effect on oxygen levels within the tank. In any 24-hour period, plants release more oxygen through photosynthesis than they use up during respiration. Bacterial organism also use up oxygen continually through respiration. At night, a heavily planted aquarium can quickly use up oxygen until it is at such a low level that fish begin to suffer from oxygen deficiency. The problem is generally confined to heavily planted aquariums with little aeration or water movement, and be remedied by increasing oxygenation at night. Plants do not generally appreciate a high oxygen level in the aquarium because it diminishes their ability to obtain nutrients. This means that constant aeration is not beneficial in planted aquariums and should only be employed at night, when oxygen deficiencies may occur. The aim is to balance the needs of the plants and the fishes in a planted aquarium.
(Contents are taken from the book of MiniEncyclopedia Aquarium Plants)