Stomatal Opening and Closing
Stomata are functional unit of the epidermis serving the exchange of gases between the intercellular spaces of the plant and its surrounding. They are especially common and of characteristic shape at the epidermis of the leaf's underside of most species. Their development differs from plant group to plant group, but unequal cell divisions are always involved. A functional unit consists of the guard cells themselves that contain nearly always chloroplasts and of their neighbouring subsidiary cells that are usually devoid of chloroplasts.
Mechanism and Regulation of Stomata Movements
The exchange of oxygen and carbon dioxide in the leaf (as well as the loss of water vapour in transpiration) occurs through pores or stomata (singular = stoma).
Normally stomata open when the light strikes the leaf in the morning and close during the night.
The immediate cause is a change in the turgor of the guard cells. The inner wall of each guard cell is thick and elastic. When turgor develops within the two guard cells flanking each stoma, the thin outer walls bulge out and force the inner walls into a crescent shape. This opens the stoma. When the guard cells lose turgor, the elastic inner walls regain their original shape and the stoma closes.
Factors affecting Stomatal Movement
Guard cells can emit water in three different directions:
- Into the neighbouring subsidiary cells
- Into the respiratory cavity that is a part of the intercellular system lying beneath the guard cells.
Light and Carbon Dioxide
The stomata of most plant species are closed in darkness. Light stimulates opening. The action spectrum is similar to that of photosynthesis. Blue light is especially effective. The stomata of CAM-plants, like Crassulaceans, are open during the night. They depend on the accumulation of carbon dioxide during the night. These plants store the carbon dioxide as malate or aspartate and feed it into the Calvin cycle during daytime. Open stomata would cause intolerable transpiration losses in the areas that CAM-plants live.
A low concentration of carbon dioxide (in the respiratory cavity) causes the stomata to open. A high concentration leads to their closing. Photosynthesis starts with the first light of the day, because enough carbon dioxide has been accumulated. Photosynthesis takes place in guard cells too, since they contain chloroplasts in contrast to the subsidiary cells. This activity again is related to the rise of the osmotic value and thus also to the opening of the stomata.
Controlling cycles regulating the movements of stomata. A guard cell is depicted schematically.
On what is the rise or lowering of the osmotic value based? Good evidence exists that light exerts its effect mainly by decreasing the intercellular and intracellular concentration of carbon dioxide. Carbon dioxide is faster consumed than supplied due to the process of photosynthesis. This causes an intercellular deficit close to the stomata's opening. The photosynthetic activity within the guard cells leads to a decrease of the intracellular level of carbon dioxide and this causes water to be drawn from the subsidiary cells.
It turned out that the water-uptake is preceded by an uptake of potassium ions. Potassium ions are actively pumped (by a potassium pump) from the subsidiary cells into the vacuoles of the guard cells. At the same time, anions (chloride, malate) accumulate within the vacuoles. Protons are given off to the subsidiary cells.
The actual importance of the potassium pump for the guard cell movement is best demonstrated with a fungal toxin called Fusicoccin (from the fungus Fusicoccum amygdali). This toxin activates the potassium pump. If consequently, the toxin is applied to the stomata, then the loss of water becomes higher than its supply, resulting in withering. The biological advantage for the fungus lies in the open stomata since they are, beside wounds, the only places where its hyphes can penetrate the leaf tissue. The total water balance is hardly or not at all influenced, as long as the effect of Fusicoccin remains a local one. In principle, one open stomata is enough for a hype.
Stomata tend to open more with an increase in temperature and close with a decrease in temperature. In some plant species, stomata remain closed even under continuous light at OoC. However, if the temperature is increased, stomatal opening in these species increases. At temperatures higher than 30oC there is a decline in stomatal opening in some species.