A Brief Summary of Transpiration
The function of the stomata is ultimately linked to the process of transpiration in plants.
Transpiration is the evaporation of water from parts of plants, including the leaves, roots, stems, and flowers.
Transpiration is essential in transporting water from the roots to the other parts of the plant. This water then complements both the plant’s photosynthetic requirement of water, and the structural integrity of the plant. Evaporation of water from leaves and stems assists the plant in drawing more water out of the soil through water’s properties of cohesion and adhesion. Guard cells are the structures in angiospermophytes that are responsible for controlling the rate of transpiration.
The Role and Structure of the Stoma
A stoma (plural stomata) is a pore located in the epidermis layers of leaves and stems. These pores provide the pathway responsible for the majority of transpiration. Water contained within the leaf evaporates through these pores, escaping into the atmosphere.
Each stoma is surrounded by two guard cells, which are responsible for regulating the aperture (diameter) of the stomatal opening. These guard cells open and close in response to certain stimuli, including abscisic acid, in order to maintain the plant’s homeostasis. A smaller stomatal aperture results in a reduced rate of transpiration. A wider aperture results in an increased rate of water loss.
Mechanisms Behind the Opening and Closing of Stomata
The stomata can be opened by moving more fluids into the guard cells, causing them to expand; similar to the inflation of a balloon with air. The inner walls of the guard cells are resistant to this expansion, causing the cell to curve when its fluid content increases. The pressure of the cells contents against the cell’s wall is referred to as turgidity. When the guard cells are turgid (a significant pressure placed on the cell wall by the cell’s contents), they curve away from each other, opening the stomata. To close the stomata, they lose fluids and become flaccid. The curve of the guard cell decreases, and the stomata is closed.
The mechanism behind the increase in turgidity is based upon an osmotic gradient. When the guard cells are stimulated to open the stomata, potassium (K+) ions are actively transported into the cell. The water potential of the cell is decreased, causing water to flow via osmosis into the cell, increasing the cell’s turgidity.
Similarly, when the guard cells are stimulated to close the stomata (via abscisic acid, for example), the potassium (K+) ions are actively transported outside of the cell, increasing the cell’s water potential, causing water to move out of the cell through osmosis.
Causes of the Opening and Closing of Stomata
In some plants, there is an intrinsic circadian rhythm which influences the opening and closing of stomata. The term circadian rhythm describes a roughly 24-hour cycle. Even when some plants remain in constant conditions, the stomata can open and close based solely upon the time of day.
Carbon dioxide concentrations are a significant stimulus in the opening and closing of stomata. When carbon dioxide concentrations in the leaf’s internal air spaces are low, the stomata open to allow external carbon dioxide to enter. When these concentrations are high, the stomata close.
The amount of water available to a plant may also affect the opening and closing of stomata. When the amount of water available to a plant is low, the stomata close to prevent water loss through transpiration. This has the side-effect that carbon dioxide cannot easily enter the leaf, slowing the rate of photosynthesis.