Stoma

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In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. Air containing carbon dioxide and oxygen enters the plant through these openings where it gets used in photosynthesis and respiration. Waste oxygen produced by photosynthesis in the chlorenchyma cells of the leaf interior exits through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration.

Dicotyledons usually have more stomata on the lower epidermis than the upper epidermis. As these leaves are held horizontally, upper epidermis is directly illuminated. Less number of stomata on the upper epidermis can then prevent water loss.

Monocotyledons are different. For their leaves are held vertically, they will have the same number of stomata on the two epidermis.

If the plant has floating leaves, there will be no stomata on the lower epidermis as it can absorb gases directly from water through cuticle. If it is submerged leaf, no stomata will be present on both sides of it.

Stoma in Greek means "mouth".

Stoma Paradox

As the key reactant in photosynthesis, carbon dioxide, is found in the atmosphere, most plants require the stoma to be open during daytime. The problem is that during hot weather, this causes plants to lose huge amounts of water through transpiration. This problematic situation is known as the stoma paradox. A small group of plants evolved with a solution to the paradox; they open the stoma at night and store the carbon dioxide for use during the following day, allowing the stoma to close at dawn. These plants are known as CAM plants.

Stoma Opening and Closure

However, most plants do not have the above-said facility and must therefore open and close their stomata depending on their internal water levels.

When water levels are high and there is plenty of sunlight, blue light causes a phototropin to activate a proton pump. The proton pump removes protons (H⁺) from the guard cells flanking the stomata. This means that the cells' electrical potential becomes increasingly negative, and so an uptake of potassium ions (K⁺) occurs. This in turn causes a change in the osmotic pressure inside the cell, attracting water through osmosis. The increased water inside the cell results in greater turgor pressure, ie the cell becomes turgid (because the wall of the guard cell facing the stomata is less elastic than the other part of the wall, it bows inwards, so there is a greater gap between the two walls of the guard cells, and thus a large stomatal pore).

When the water uptake of the roots can no longer balance that lost through the leaves in transpiration (usually about midday), abscisic acid (ABA) is released. ABA binds to certain receptors in the guard cells' plasma membranes, which first raises the pH of the cytosols in the cells. This causes the chlorine (Cl^-) and inorganic ions to exit the cells. Second, it causes the release of calcium ions (Ca²⁺) from the cells' vacuoles in to the cytosols, which blocks the uptake of any further K⁺ into the cells. The loss of these solutes causes a reduction in osmotic pressure, thus making the cell flaccid and so closing the stomatal pores.

Viewing Stoma

The easiest way to view stomata on a leaf is to take a nail varnish impression of it.

1. Paint about one square centimeter of the underside of the leaf with transparent nail varnish.(or thin layer of PVA glue)
2. Allow to dry out thoroughly (takes a good 30 minutes).
3. Peel off and place on a microscope slide.

The stomata leave clearly visible impressions in the nail varnish. A graticule slide allows for the counting of how many stomata (per unit area) are on the leaf surface, a characteristic of physiological significance.da:Spalteåbning (plantedel) de:Stoma (Botanik) eo:Stomo lt:Žiotelė nl:Huidmond