Biology subjects

Horak, H.

Publications and source records attributed to Horak, H..

8 recordsLinked to original sources

bioRxiv2026

Higher stomatal density and smaller stomatal size associate with higher grain yield in winter wheat

In wheat (Triticum aestivum L.), flag leaf is crucial in determining grain-filling rate and final grain yield. Stomata, microscopic pores on plant surfaces, balance CO2 uptake and evaporative water loss; their numbers and aperture sizes depend on environmental factors. Previous studies have reported associations between stomatal traits and grain yield; mostly under controlled conditions. Our study aimed to investigate flag leaf stomatal anatomical and physiological traits, and their relationships with winter wheat grain yield under field conditions. We measured stomatal density and size, and stomatal conductance on adaxial and abaxial flag leaf surfaces in a three-year field study with twenty-five winter wheat varieties. Stomatal density ratio remained constant despite different stomatal densities occurring in different years, indicating coordinated adjustments of stomatal numbers between leaf surfaces in response to environmental conditions. Stomatal conductance ratios varied widely, indicating leaf-surface specific stomatal responses to environmental conditions. Grain yield was positively associated with stomatal density and negatively with stomatal size. Flag leaf removal reduced grain weight per spike and did not affect grain numbers per spike, unlike in spring wheat. Our field study provides valuable insights into the relationships between winter wheat stomatal traits and grain yield that can be used in breeding.

plant biology↗

bioRxiv2025

Low relative air humidity leads to smaller, denser stomata and higher stomatal ratios in Arabidopsis

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.

plant biology↗

bioRxiv2025

Higher adaxial stomatal density is associated with lower grain yield in spring wheat

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.

plant biology↗

bioRxiv2024

Stomatal patterning is differently regulated in adaxial and abaxial epidermis in Arabidopsis

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 thaliana, 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 addressed the role of leaf number in determination of stomatal density and stomatal ratio, and studied adaxial and abaxial stomatal patterns in mutants deficient in known abaxial stomatal development regulators. We found that stomatal density in some genetic backgrounds varies between different fully expanded leaves and 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.

plant biology↗

bioRxiv2023

Low relative air humidity and increased stomatal density independently hamper growth in young Arabidopsis

Stomatal 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 with the cost of producing stomata; future experiments need to test if high stomatal densities might pay off in later life stages. Significance statementThis study delves into the relationship between stomatal density, sensitivity, and environment in Arabidopsis. These findings not only enhance our comprehension of plant responses to humidity but also lay the groundwork for future studies aimed at optimising plant adaptability to varying environmental conditions.

plant biology↗

bioRxiv2023

NPR1 protects young leaves from systemic salicylic acid-induced damage during bacterial infection

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 Fv/Fm, 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 Fv/Fm 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.

plant biology↗

bioRxiv2023

Species-specific stomatal ABA responses in juvenile ferns grown from spores

Adjustable stomatal pores in leaves control the balance between CO2 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.

plant biology↗

bioRxiv2021

Concentration matters: different stomatal CO2-responses at sub-ambient and above-ambient CO2 levels

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. It is unknown whether stomatal responses to sub-ambient and above-ambient CO2 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 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 SLAC1 more strongly affected the speed of above-ambient CO2-induced stomatal closure. Elevated VPD caused lower stomatal conductance in all and faster CO2-responsiveness in some studied genotypes and CO2-transitions. Our results highlight the importance of experimental set-ups in interpreting stomatal CO2- responsiveness, as stomatal movements under different CO2 concentration ranges are controlled by distinct mechanisms. Sometimes elevated CO2 and VPD responses also interact. Hence, multi-factor treatments are needed to understand plant behavior under future climate conditions.

plant biology↗