This Editorial introduces the Virtual Issue ‘Stomata’ that includes the following papers: Apigo et al . (2026), Bernardo et al . (2026), Brench et al . (2026), Busby et al . (2026), Caine et al . (2019, 2026), Chen et al. (2026), Ding et al . (2026), Drake et al . (2019), Erberich et al . (2026), Fan et al . (2025), Grenzi et al . (2026), Gustavsson et al . (2026), Hõrak (2026), Huang et al . (2026), Kou et al . (2026), Li et al . (2026), Liu et al . (2025), Nguyen et al . (2026), Raven (2002), Roelfsema & Hedrich (2002), Samantara et al. (2025), Sicangco et al . (2026), Silva‐Alvim et al . (2026), Sinha et al . (2022), Song et al . (2014), Sun et al . (2025, 2026), Tan et al . (2026), Wilson et al . (2025), Woning et al . (2026), Yan et al . (2026), Yang et al . (2026), Zavala‐Paez et al . (2026), Zhang et al . (2025). Access the Virtual Issue at www.newphytologist.com/virtualissues .

In land plants, stomatal pores on leaf surfaces developed to control gas exchange between leaf and surrounding air, but also to enable nutrient uptake and leaf cooling. Traits such as stomatal density (SD), guard cell size, stomatal distribution between the upper (adaxial) and lower (abaxial) leaf surfaces (stomatal ratio), and stomatal aperture width exhibit notable variation across different genotypes and environments. These traits influence leaf photosynthesis, water loss, growth, productivity, and pathogen susceptibility. Here, we studied different stomatal traits of spring wheat flag leaves and their relationship with grain yield in field experiments during 2022-2023. Significant genotypic variation among adaxial and abaxial SDs and stomatal ratios was detected, whereas stomatal conductance was mostly affected by annual differences in weather. A strong negative relationship between adaxial stomatal density and grain yield was detected under all conditions, when abiotic factors (water stress or nutrient limitation) resulted in yield losses, whereas under favourable conditions, there was no significant relationship between adaxial stomatal density and grain yield. The effects of leaf surface-specific traits on yield are often overlooked in physiological and breeding experiments. Our results indicate that higher-than-optimal adaxial SD values may result in wheat yield losses under stresses imposed by future climate conditions. ### Competing Interest Statement The authors have declared no competing interest.

Stomatal pores mediate leaf CO2 uptake and water loss. The number, size, and aperture of stomata determine plant gas-exchange potential and affect productivity and stress tolerance. To date, stomatal research has mostly focused on abaxial stomata (stomata on the lower leaf surface) to understand the mechanisms and regulation of stomatal development and stomatal opening and closure. Many plants are amphistomatous and produce stomata also on the adaxial (upper) leaf surface. Relatively, little is known of how adaxial stomata function, how they develop, how different stomatal distributions between leaf surfaces are achieved and how they affect plant physiology. Here, I discuss current knowledge on the function and making of adaxial stomata, and address the role and regulation of stomatal distribution between adaxial and abaxial leaf surfaces in amphistomatous plants. Evidence in the literature points toward independent adaxial and abaxial stomatal function and development, and indicates that stomatal distribution in leaves is an important functional trait.

Atmospheric dryness is increasing, bringing about decreases in plant productivity. Stomatal pores mediate plant gas-exchange with the environment, balancing CO2 uptake with water loss. Stomatal anatomical and physiological traits respond to changes in environment, potentially affecting plant growth and yield under future environments. Designing stomatal patterns to suit future climate conditions has the potential to improve plant water use efficiency or productivity. By combining mutations in signaling pathways that control stomatal development and apertures, we designed plants that combine high stomatal densities with more open stomata and show that respective mutations independently affect stomatal conductance and density. Analyses of adaxial and abaxial stomatal conductances showed that in Arabidopsis, adaxial stomata are responsible for a significant proportion of leaf gas-exchange. Adaxial and abaxial stomatal physiology were largely similarly affected by mutations in stomatal regulation pathways but adaxial stomata tended to be relatively more closed than abaxial stomata. We show that growth under low relative air humidity leads to higher stomatal densities and smaller stomata. Stomatal development in the adaxial and abaxial leaf surface responded differently to dry air conditions: adaxial stomatal index increased, whereas abaxial stomatal index decreased. Stomatal ratio increased under dry air conditions, leading to a higher degree of amphistomaty. Plant growth was independently suppressed by dry air and high stomatal density and index. Our results suggest that acclimation to decreasing air humidity leads to stomatal anatomical adjustments that help to maximize plant gas-exchange potential under conditions where water supply may be limited and sporadic. ### Competing Interest Statement The authors have declared no competing interest.

Climate change is projected to increase environmental humidity in northern latitudes, yet its effects on tree hydraulics remain underexplored. We investigated how elevated air relative humidity and increased soil moisture influence water relations, gas exchange, and aquaporin (AQP) expression in silver birch (Betula pendula) at the free air humidity manipulation experiment. We applied air humidification and soil irrigation treatments and measured leaf hydraulic conductance, gas exchange parameters, and AQP transcript levels in leaves to assess physiological and molecular responses. Both treatments significantly decreased leaf hydraulic efficiency, that is the capacity of leaves to transfer water in liquid phase. However, AQP expression responded divergently: air humidification downregulated most AQP transcripts, whereas soil irrigation upregulated them. Despite these changes, gas exchange remained stable, but stomatal sensitivity to vapour pressure deficit declined under both treatments. Our findings suggest distinct regulatory mechanisms of AQPs in response to atmospheric vs edaphic moisture, while resulting in convergent physiological outcomes. The reduced stomatal sensitivity and decoupling between hydraulic performance and photosynthetic activity under nonstress conditions indicates a shift in water-use strategies and highlights the pronounced hydraulic plasticity of B. pendula. These results have implications for predicting tree and forest resilience under future climate scenarios, where increasing humidity may compromise hydraulic safety and water-use regulation during extreme weather events.

Abstract Plants provide humankind with a habitable environment, food, and oxygen. Due to changing climate and increasing global population, there is pressure to increase agricultural production while limiting negative impacts on natural ecosystems. Understanding plant physiology and plant‐environment interactions is needed to further sustainable agricultural practices and maintain a green and diverse environment. Early detection of signals characteristic to plant stress can help to design interventions to preserve yield or diversity. Therefore, technology development is important to monitor plant health by measuring a variety of parameters related to the microclimate around plants and plant physiology. This review focuses on state‐of‐the‐art wearable sensors developed for plants.

Stomatal pores in leaves mediate CO2 uptake into the plant and water loss via transpiration. Most plants are hypostomatous with stomata present only in the lower leaf surface (abaxial epidermis). Many herbs, including the model plant Arabidopsis, have substantial numbers of stomata also on the upper (adaxial) leaf surface. Studies of stomatal development have mostly focused on abaxial stomata and very little is known of adaxial stomatal formation. We analysed the role of leaf number in determining stomatal density and stomatal ratio, and studied adaxial and abaxial stomatal patterns in Arabidopsis mutants deficient in known abaxial stomatal development regulators. We found that stomatal density in some genetic backgrounds varies between different fully expanded leaves, and thus we recommend using defined leaves for analyses of stomatal patterning. Our results indicate that stomatal development is at least partly independently regulated in adaxial and abaxial epidermis, as (i) plants deficient in ABA biosynthesis and perception have increased stomatal ratios, (ii) the epf1epf2, tmm, and sdd1 mutants have reduced stomatal ratios, (iii) erl2 mutants have increased adaxial but not abaxial stomatal index, and (iv) stomatal precursors preferentially occur in abaxial epidermis. Further studies of adaxial stomata can reveal new insights into stomatal form and function.

SUMMARYStomatal pores in plant leaves mediate CO2 uptake for photosynthesis and water loss via transpiration. Altered stomatal density can affect plant photosynthetic capacity, water use efficiency, and growth, potentially providing either benefits or drawbacks depending on the environment. Here we explore, at different air humidity regimes, gas exchange, stomatal anatomy, and growth of Arabidopsis lines designed to combine increased stomatal density (epf1, epf2) with high stomatal sensitivity (ht1‐2, cyp707a1/a3). We show that the stomatal density and sensitivity traits combine as expected: higher stomatal density increases stomatal conductance, whereas the effect is smaller in the high stomatal sensitivity mutant backgrounds than in the epf1epf2 double mutant. Growth under low air humidity increases plant stomatal ratio with relatively more stomata allocated to the adaxial epidermis. Low relative air humidity and high stomatal density both independently impair plant growth. Higher evaporative demand did not punish increased stomatal density, nor did inherently low stomatal conductance provide any protection against low relative humidity. We propose that the detrimental effects of high stomatal density on plant growth at a young age are related to the cost of producing stomata; future experiments need to test if high stomatal densities might pay off in later life stages.

Stomatal pores that control plant CO2 uptake and water loss affect global carbon and water cycles. In the era of increasing atmospheric CO2 levels and vapor pressure deficit (VPD), it is essential to understand how these stimuli affect stomatal behavior. Whether stomatal responses to sub-ambient and above-ambient CO2 levels are governed by the same regulators and depend on VPD remains unknown. We studied stomatal conductance responses in Arabidopsis (Arabidopsis thaliana) stomatal signaling mutants under conditions where CO2 levels were either increased from sub-ambient to ambient (400 ppm) or from ambient to above-ambient levels under normal or elevated VPD. We found that guard cell signaling components involved in CO2-induced stomatal closure have different roles in the sub-ambient and above-ambient CO2 levels. The CO2-specific regulators prominently affected sub-ambient CO2 responses, whereas the lack of guard cell slow-type anion channel SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1) more strongly affected the speed of above-ambient CO2-induced stomatal closure. Elevated VPD caused lower stomatal conductance in all studied genotypes and CO2 transitions, as well as faster CO2-responsiveness in some studied genotypes and CO2 transitions. Our results highlight the importance of experimental setups in interpreting stomatal CO2-responsiveness, as stomatal movements under different CO2 concentration ranges are controlled by distinct mechanisms. Elevated CO2 and VPD responses may also interact. Hence, multi-factor treatments are needed to understand how plants integrate different environmental signals and translate them into stomatal responses.

Adjustable stomatal pores in leaves control the balance between CO 2 entry for photosynthesis and water loss via transpiration. The drought and low humidity-induced phytohormone abscisic acid (ABA) is the major regulator of active stomatal closure responses in angiosperms. Whether the ABA signalling pathway for stomatal closure functions similarly in older land plant groups, such as lycophytes and ferns, is still unclear: some studies find no stomatal ABA response in ferns, others find that ABA response is present or triggered by specific environmental conditions. Here we analysed steady-state gas-exchange, stomatal density and stomatal response to exogenously applied ABA in nine fern species grown from spores under controlled growth conditions. We find that ABA responses in ferns are species-specific: stomata in four out of nine species closed in response to ABA. The ABA-sensitive species mostly had slow responses of low magnitude, suggesting reduced ABA-sensitivity of ABA signalling pathway in ferns. Species with larger stomatal conductance tended to close stomata in response to ABA, whereas a relatively strong response of ~35% was also found in Cyrtomium falcatum , a fern with low stomatal conductance. Our results show that ferns constitute a diverse group with varying degree of stomatal ABA-sensitivity. Further characterisation of ABA signalling pathway components in diverse fern species is needed to understand the genetic basis for the variable ABA-sensitivity in ferns.

The phytohormone salicylic acid (SA) is an important molecular signal that mediates pathogen defence mechanisms, including triggering Arabidopsis immune responses to the hemi-biotroph Pseudomonas syringae pv. tomato ( Pst ). SA induces the expression of a myriad of defence genes via its receptor and transcriptional regulator NONEXPRESSER OF PR GENES 1 (NPR1). Here, we used chlorophyll fluorescence imaging of F v /F m , to detect damage to photosystem II before Pst -induced disease symptoms were visible. We observed that the pathogen only induced damage, and subsequent cell death, in mature leaves while developing leaves in the center of the rosette appeared to be protected. However, in the npr1-1 mutant, Pst -infected mature leaves were able to systemically transmit a signal that caused damage to the photosynthetic machinery in uninfected young leaves. Reductions in F v /F m could also be induced systemically in developing npr1-1 leaves by high levels of SA in mature leaves, and rescued by SA biosynthesis deficiency in npr1-1sid2-2 mutants. Together, these results indicate that, in addition to its well-known role as a positive regulator of SA responses, NPR1 also acts to suppress SA-dependent immune responses and thereby protects developing leaves from autoimmune damage.

Methyl jasmonate (MeJA) induces various defence responses in seed plants, but for early plant lineages, information on the potential of jasmonates to elicit stress signalling and trigger physiological modifications is limited. The spikemoss Selaginella martensii was exposed to a range of MeJA concentrations (0, 10, 25, and 50 mM), and biogenic volatile organic compound (BVOC) emissions, photosynthetic rate (A), and stomatal conductance (gs) were continuously measured. In addition, changes in phytohormone concentrations and gene expression were studied. Enhancement of methanol, lipoxygenase pathway volatiles and linalool emissions, and reductions in A and gs, were MeJA dose-dependent. Before MeJA treatment, the concentration of 12-oxo-phytodienoic acid (OPDA) was 7-fold higher than jasmonic acid (JA). MeJA treatment rapidly increased OPDA and JA concentrations (within 30 min), with the latter more responsive. Some genes involved in BVOC biosynthesis and OPDA-specific response were up-regulated at 30 min after MeJA spraying, whereas those in the JA signalling pathway were not affected. Although JA was synthesized in S. martensii, OPDA was prioritized as a signalling molecule upon MeJA application. MeJA inhibited primary and enhanced secondary metabolism; we propose that fast-emitted linalool could serve as a marker of elicitation of stress-induced metabolism in lycophytes.

The shoot apical meristem (SAM) and the root apical meristem (RAM) contain stem cells that give rise to plant organs above and below the ground. Maintenance of meristems is essential for the development and growth and hence regu-lated by signal transduction systems that contain multiple components that function redundantly. The CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-related (CLE) peptides are major regulators of meristem homeostasis that are perceived by leucine-rich repeat receptor-like protein kinases (LRR-RLKs) together with their co-receptors in the plasma membrane. However, the intracellular signal transduction events that follow CLE peptide perception are largely unknown. peptides 25 45 receptor The , Wenping Wang, and the receptor-like kinases (RLCKs) PBS1-like and 36 (PBL34/35/36) act as intracellular signaling components downstream of CLE peptide sensing by the plasma membrane receptor complexes in

Stomatal pores that control plant CO 2 uptake and water loss affect global carbon and water cycles. In the era of increasing atmospheric CO 2 levels and vapor pressure deficit (VPD), it is essential to understand how these stimuli affect stomatal behavior. It is unknown whether stomatal responses to sub-ambient and above-ambient CO 2 levels are governed by the same regulators and whether these responses depend on VPD. We studied stomatal conductance responses in Arabidopsis stomatal signaling mutants under conditions where CO 2 levels were either increased from sub-ambient to ambient (400 ppm) or from ambient to above-ambient levels under normal or elevated VPD. We found that guard cell signaling components involved in CO 2 -induced stomatal closure have different roles in the sub-ambient and above-ambient CO 2 levels. The CO 2 -specific regulators prominently affected sub-ambient CO 2 responses, whereas the lack of guard cell slow-type anion channel SLAC1 more strongly affected the speed of above-ambient CO 2 -induced stomatal closure. Elevated VPD caused lower stomatal conductance in all and faster CO 2 -responsiveness in some studied genotypes and CO 2 -transitions. Our results highlight the importance of experimental set-ups in interpreting stomatal CO 2 - responsiveness, as stomatal movements under different CO 2 concentration ranges are controlled by distinct mechanisms. Sometimes elevated CO 2 and VPD responses also interact. Hence, multi-factor treatments are needed to understand plant behavior under future climate conditions.

Stomatal densities, aperture openness and their responsiveness to environmental change determine plant water loss and regulate entry of pathogens. Stomatal responsiveness is usually assessed on restricted areas of leaves or isolated epidermal peels floated in solution. Analyzing these responses in the whole plant context could give valuable additional information, for example on the role of mesophyll in stomatal responses. We analyzed stomatal responses to the phytohormone abscisic acid (ABA) and pathogenic elicitors in intact plants by dynamic measurement of leaf temperature. We tested whether ABA-induced stomatal closure in wheat requires external nitrate and whether bacterial elicitor-induced stomatal closure can be detected by dynamic thermal imaging in intact Arabidopsis. We found that wheat was hypersensitive to all applied treatments, as even mock-treated leaves showed a strong increase in leaf temperature. Nevertheless, ABA activated stomatal closure in wheat independent of exogenous nitrate. Pathogenic elicitors triggered a fast and transient increase in leaf temperature in intact Arabidopsis, indicating short-term stomatal closure. The data suggest that the dynamics of pathogen-induced stomatal closure is different in whole plants compared to epidermal peels, where elicitor-induced stomatal closure persists longer. We propose that dynamic thermal imaging could be applied to address the effect of pathogenic elicitors on stomatal behavior in whole plants to complement detached sample assays and gain a better understanding of stomatal immunity.