Stomata

Stomata are microscopic openings found mainly on the surfaces of plant leaves and, to a lesser extent, stems. Each stoma consists of two specialized guard cells that surround an opening called the stomatal pore. Stomata are most numerous on the abaxial (lower) leaf surface, though many species also have stomata on the adaxial surface or on other organs. They are present in many land plants, including ferns, mosses, gymnosperms, and flowering plants.

The primary function of stomata is to regulate gas exchange between the plant and the atmosphere. They allow carbon dioxide to enter for photosynthesis and enable oxygen and water vapor to exit. By controlling the size of the stomatal pore, plants balance the need for CO2 with the risk of excessive water loss through transpiration. Stomatal opening and closing are driven by changes in guard cell turgor pressure in response to environmental and internal signals.

Guard cell turgor is controlled by the movement of ions and water. Potassium ions and other solutes accumulate in guard cells to promote water uptake and pore opening, a process enhanced by blue light and photosynthetic activity. Drought, high atmospheric vapor pressure deficit, or the plant hormone abscisic acid (ABA) trigger ion efflux, water loss from guard cells, and pore closure. CO2 concentration inside the leaf also influences stomatal aperture, closing stomata when internal CO2 is high.

Stomatal density and distribution vary among species and in response to light, humidity, and other conditions. Their regulation is crucial for plant water use efficiency, temperature regulation, and overall photosynthetic performance.