A stoma (plural stomata) is a microscopic pore in the outer tissue of a plant, bordered by two specialized guard cells. Stomata connect internal air spaces with the surrounding atmosphere. In vascular plants, changes in their aperture regulate the entry of carbon dioxide for photosynthesis and the escape of water vapor through transpiration. The term sometimes denotes the pore alone and sometimes the pore together with its guard cells; a stomatal complex may additionally include associated subsidiary cells. (pmc.ncbi.nlm.nih.gov)
Structure and distribution
Stomata occur principally on leaves, but also on stems and other aboveground organs. Each pore opens into an internal air space connected with the spaces among photosynthetic tissues. Guard cells are specialized epidermal cells whose shape and mechanical properties allow changes in cell volume to alter the aperture. Many plants have kidney-shaped guard cells; grasses typically possess dumbbell-shaped guard cells associated with subsidiary cells. These surrounding cells contribute to the mechanical and physiological operation of the complex. (pmc.ncbi.nlm.nih.gov)
Distribution varies among species and organs. Leaves with stomata restricted to the lower surface are hypostomatous, whereas those with stomata on both surfaces are amphistomatous. Two-sided gas exchange can shorten diffusion pathways within a leaf and reduce some limitations imposed by the external boundary layer, but it also changes the relationship between carbon uptake and water supply. Stomatal size, density, and placement therefore represent distinct anatomical dimensions rather than interchangeable measures of gas-exchange capacity. (pubmed.ncbi.nlm.nih.gov)
Gas exchange and water balance
Gases move through stomatal pores by diffusion. Opening a pore facilitates carbon dioxide entry but simultaneously permits water vapor to escape from moist internal surfaces. This coupling creates a fundamental physiological trade-off: greater access to carbon dioxide can support carbon assimilation, while increased transpiration can deplete the plant’s water supply. Stomatal control connects photosynthetic activity with water transport through the xylem and with the hydraulic condition of the leaf. (pmc.ncbi.nlm.nih.gov)
Stomatal conductance expresses how readily gas passes through the stomatal pathway. It depends on pore aperture as well as anatomical properties such as stomatal number and size. Anatomical maximum conductance is not the same as operating conductance: a leaf with numerous stomata may nevertheless exchange little gas when its pores are nearly closed. Transpiration also depends on the vapor-pressure difference between the leaf and surrounding air, so identical apertures need not produce identical rates of water loss. (pmc.ncbi.nlm.nih.gov)
Opening and closing
Stomatal movement results from changes in guard-cell osmotic conditions and turgor. During opening, guard cells accumulate potassium ions and other solutes, causing water uptake and increased volume. The geometry and properties of their walls translate this swelling into pore enlargement. Solute release and subsequent water loss reduce turgor and promote closure. Guard-cell metabolism supplies both osmotic compounds and the energy required for transport. (pmc.ncbi.nlm.nih.gov)
Blue light activates guard-cell photoreceptors called phototropins. Their signaling pathway stimulates proton pumps in the cell membrane, establishing an electrical driving force for potassium uptake through ion channels. Red-light responses are closely associated with photosynthetic activity in guard cells and surrounding tissues. These pathways help coordinate pore opening with the availability of light for carbon assimilation. (pmc.ncbi.nlm.nih.gov)
The plant hormone abscisic acid, or ABA, promotes closure and inhibits opening during water stress. Its signaling network regulates anion and potassium transport, reducing guard-cell solute content and volume. Guard cells also integrate carbon dioxide signals and other environmental inputs, rather than responding to a single stimulus independently. (pmc.ncbi.nlm.nih.gov)
Daytime opening is common but not universal. Plants using crassulacean acid metabolism characteristically obtain much of their atmospheric carbon dioxide at night and restrict daytime opening. Carbon acquired at night is stored in organic acids and released internally during the day. This timing can reduce the water cost of carbon acquisition, although CAM expression includes several variants and changes with environmental conditions. (pmc.ncbi.nlm.nih.gov)
Development and patterning
Stomatal formation involves a regulated sequence of cell-fate changes and cell divisions. In the model plant Arabidopsis thaliana, asymmetric divisions produce precursors that eventually become guard mother cells. Each guard mother cell then divides symmetrically to form a guard-cell pair. The transcription factors SPEECHLESS, MUTE, and FAMA regulate successive developmental transitions. (pmc.ncbi.nlm.nih.gov)
Intercellular signaling coordinates stomatal placement. EPIDERMAL PATTERNING FACTOR peptides help enforce spacing, typically leaving at least one intervening epidermal cell between stomata in Arabidopsis. Stomatal density describes the number per unit surface area, whereas stomatal index expresses stomatal number relative to the total number of stomata and other epidermal cells. These measurements distinguish aspects of developmental allocation from changes caused by epidermal expansion. (pmc.ncbi.nlm.nih.gov)
Evolution and research
Stomata occur in vascular plants and in the sporophytes of many mosses and hornworts, but are absent from living liverworts. Fossil examples are known from the approximately 410-million-year-old Rhynie chert. Conserved developmental regulators across distant plant lineages provide evidence for an ancient origin, although the evolution of stomatal movement and the functions of early stomata remain subjects of investigation. Stomata in bryophyte reproductive structures cannot simply be assumed to perform the same water-regulating role as those in vascular-plant leaves. (pmc.ncbi.nlm.nih.gov)
Stomatal traits are investigated in crop research because they influence water consumption and responses to drought and elevated carbon dioxide. Experiments with rice engineered to have reduced stomatal density demonstrated lower water use and improved drought tolerance under the tested conditions. Such results are conditional: the effects of changing stomatal traits depend on growth environment, photosynthetic demand, and the degree of anatomical alteration. (eprints.whiterose.ac.uk)