Stoma

A stoma, which is the plural form stomata, is a microscopic pore found in the epidermis of leaves, stems, and other plant organs. These tiny openings are essential for life because they control the rate of gas exchange between the atmosphere and the internal air spaces of a leaf.

The term usually refers to the entire stomatal complex. This complex includes the pore itself, known as the stomatal aperture, and a pair of specialized cells called guard cells. Through these pores, carbon dioxide enters the plant for photosynthesis. At the same time, oxygen produced during photosynthesis can exit.

Water vapour also moves through these pores into the air in a process called transpiration.

The movement of the stoma is driven by the guard cells that border the aperture. When environmental conditions like high light intensity and high humidity are favorable, a proton pump moves protons (H+) out of the guard cells. This makes the electrical potential inside the cells increasingly negative. This change triggers potassium voltage-gated channels to open, allowing potassium ions (K+) to enter the cells. To balance this, negative ions like chloride or organic malate also enter.

This increase in solute concentration lowers the water potential inside the cells. As a result, water enters the guard cells through osmosis. This increases the volume and turgor pressure of the cells. Because of cellulose microfibrils that restrict their width, the guard cells can only elongate. They bow apart, which opens the stomatal aperture.

Plants must carefully manage this process because they cannot gain carbon dioxide without losing water. Most plants use an enzyme called RuBisCO to fix carbon dioxide in mesophyll cells. However, RuBisCO has a low affinity for carbon dioxide and can also fix oxygen, which wastes energy through photorespiration. To get enough carbon dioxide, RuBisCO often requires wide stomatal apertures, which leads to high water loss. Some plants use an alternative enzyme called phosphoenolpyruvate carboxylase (PEPcase) to help. PEPcase has a higher affinity for carbon dioxide, which allows for narrower apertures and less water loss. This is especially useful when light is plentiful but water is limited.

Different plant groups have evolved unique strategies for managing their stomata. Most plants, often called C3 plants, open their stomata during the day. However, a group of mostly desert plants known as C.A.M. plants (crassulacean acid metabolism) use a different method.

C.A.M. plants open their stomata at night when water evaporates more slowly. They use PEPcase to fix carbon dioxide and store the products in large vacuoles. The next day, they close their stomata and release the stored carbon dioxide to RuBisCO. This method allows them to survive in environments where water is severely limited.

Stomata are also categorized by their location on the plant. Plants with stomata on both the upper and lower leaf surfaces are called amphistomatous. Those with stomata only on the upper surface are epistomatous or hyperstomatous. Most trees are hypostomatous, meaning they have stomata only on the lower leaf surface.

In vascular plants, the number and size of stomata vary widely. For example, dicotyledons usually have more stomata on the lower surface. Monocotyledons, such as maize or onion, may have a similar number on both sides. Floating leaves often have stomata only on the upper epidermis, while submerged leaves may lack them entirely.

From a developmental standpoint, stomata arise from protodermal cells through specific patterns. An asymmetrical cell division creates a small cell called a meristemoid. This meristemoid divides further to become a guard mother cell, which then undergoes a symmetrical division to produce the two guard cells.

This patterning is controlled by signaling components like EPF (Epidermal Patterning Factor) and the YODA protein. Mutations in these genes can change how stomata grow. For instance, a mutation in the TMM gene can cause a "Too Many Mouths" condition where stomata cluster together. Conversely, a mutation in the SPCH gene can prevent stomata from developing at all.

Evolutionary history suggests that stomata appeared in land plants by the middle of the Silurian period. They likely evolved alongside the waxy cuticle, a trait that helped early plants survive on land. While there is little evidence in the fossil record, scientists believe they may have evolved from the conceptacles of alga-like ancestors. Today, scientists use specialized instruments to measure how stomata respond to the environment. By calculating the transpiration rate and the humidity gradient, they can determine stomatal resistance. This helps us understand how plants manage their water use efficiency in a changing world.