When there is a low concentration of CO2 in the environment, the opening width of the stomata is at its largest. When the concentration of CO2 increases, the stomata gradually close.
Carbon dioxide is one of the three major greenhouse gases responsible for global warming. The others are methane and ozone. Long-lived carbon dioxide stays in the atmosphere for hundreds or even thousands of years, while methane only lasts 12 to 17 years and ozone lasts just 10 to 16 days. The amount of carbon dioxide in the atmosphere has increased since the beginning of industrialization in the 1700s because humans burn fossil fuels like oil and coal which contain carbon atoms bonded with hydrogen and oxygen molecules. This gives them less carbon/hydrogen/oxygen than ordinary air molecules and so they leave the fuel source and look for other things to bind themselves with. One method plants use to bind themselves with carbon dioxide is through photosynthesis where they take atmospheric carbon dioxide and water and use them to make sugar which plants need to grow and live.
Photosynthesis is how plants create energy from the sunlight received by their leaves. During photosynthesis, green plants use the gas carbon dioxide and water vapor from the atmosphere to produce new organic material called glucose.
Plants with fewer stomata will have an advantage and will be more prevalent during high-carbon-dioxide periods. When carbon dioxide levels are low, plants require a large number of stomata to scrape together enough carbon dioxide to survive. With increasing levels of carbon dioxide, however, some species close their stomata permanently or use other mechanisms to acquire the gas they need.
The evidence for this hypothesis comes from studies of fossil plants that were exposed to different levels of carbon dioxide in the atmosphere. The number of stomata on the leaves of these plants can be counted and compared today with the number of stomata present on their relatives living now. If it turns out that less water is lost through the leaf surface and the plant can gather enough carbon dioxide into its cells then there will be less need for many stomata.
Studies like this one have been done on several species of fossil plants from different time periods right up until the present day. They show that at high levels of carbon dioxide our current plants would be expected to have reduced numbers of stomata. However, some plants appear to have kept the same number of stomata even under high levels of atmospheric carbon dioxide. This means that they may have found another way to acquire enough carbon dioxide to survive. Others have increased the number of stomata on their leaves to help them absorb more carbon dioxide.
Stomata are the exterior entrances to the interior air space. Oxygen, carbon dioxide, and water vapor are all present. When there was a lot of photosynthesis, oxygen would accumulate. This is why plants have roots that go down into the soil where there is oxygen, and leaves that face up towards the sun where there is not enough oxygen.
The way plants obtain energy from sunlight is through the process of photosynthesis. During this process, green pigment in the chloroplasts uses light energy from the sun to combine carbon dioxide with water to produce carbohydrates. The result is organic material such as straw or wood. Carbon dioxide from the atmosphere passes into the plant through its roots and becomes part of the carbohydrate store available to it. Oxygen from the air enters through the stomata and can be absorbed by the leaves or released by the plants in the form of vapour. The presence of oxygen is what makes air space necessary for plants to function properly. Without air space, none of these processes could take place because there wouldn't be enough oxygen available to fuel them.
Plants need oxygen to live but they also produce toxic chemicals when they use too much of it. They must protect themselves against these toxins by keeping their environment clean by using enzymes and bacteria.
In vascular plants, leaf stomata are the primary mechanism of gas exchange. The greater the number of stomata per unit area (stomata density), the more CO2 can be taken in and the greater the amount of water that may be discharged. As a result, increased stomata density can considerably increase the possibility for behavioral control over water loss rate and CO2 absorption.
Stomatal density is the average number of stomata per unit area. It can be calculated by dividing the total length of stomata on a leaf by the total area of one side of the same leaf. For example, if you counted 100 stomata on one side of a leaf and they were all the same length, then the stomatal density would be 100 stomata per square inch (6.45 stomata per mm).
The term "stoma" is derived from the Greek word for mouth, which refers to the opening between each pair of leaves. Thus, a "stoma" is an individual opening in a plant's skin through which gases enter or leave the plant's body. Stomata are found on both vegetative and reproductive organs of flowering plants. However, only those on the lower surface of leaves interact with air flow and thus form a significant barrier to water loss.
Leaves evolve ways to conserve water while still allowing it to absorb carbon dioxide from the atmosphere. Some plants reduce their water consumption by closing their stomates during dry periods.
High light intensity during growth increased stomatal frequency, although the length of the stomatal hole changed very little. The number of stomata per unit area was more than twice as high in high-light leaves as in low-light leaves. As a result, total stomatal conductance was higher in high-light plants.
Stomatal density is defined as the average number of stomata per unit area. In general, higher stomatal densities are associated with smaller stomatal holes and thus less gas exchange surface area for any given plant. However, under certain conditions this relationship may be reversed. One such condition is when there is greater stomatal opening in response to greater light levels (as seen in some species of Pimelea). Under these circumstances, we would expect to see higher stomatal densities in lower light environments.
The third factor that influences stomatal density is genetics. Some species of plants, including many grasses, have stomata only on the lower side of the leaf while others, such as most angiosperms, have them on both sides. Genetic studies have shown that leaves with stomata on both surfaces tend to have higher stomatal densities than those with stomata limited to one side only. This is probably because it provides more contact with water and other substances needed for gas exchange to take place.