The functional significance of the stomatal size to density relationship: Interaction with atmospheric [CO2] and role in plant physiological behaviour

Roots of domestication: unveiling the dynamics of domestication through trait-based analysis of olive trees in northern Morocco

The domestication of crops, a transformative milestone in human history, has largely contributed to reshaping agricultural practices and plant characteristics. This study investigates the functional responses along the wild-cultivated continuum in olive trees in northern Morocco, focusing on leaf functional traits to elucidate domestication effects. We compared wild olive (Olea europaea subsp. europaea var. sylvestris) with traditional cultivated varieties (O. e. subsp. e. var. europaea). Our results reveal clear distinctions in leaf traits, including leaf area, specific leaf area and leaf dry matter content, indicating divergent resource-use strategies. Cultivated varieties displayed traits associated with thicker, denser leaves and higher stomatal density, suggesting adaptations to stress conditions such as water scarcity. Principal component analysis highlighted a leaf economic spectrum, which differentiated wild and cultivated forms and supported the functional trade-off between resource acquisition and conservation. Intraspecific trait variability was substantial, driven by both genetic factors and phenotypic plasticity in response to local environmental gradients. These findings underscore the significant impact of domestication on olive trees, providing insights into the adaptive mechanisms underlying crop resilience in traditional agroecosystems. Our research emphasizes the importance of conserving these traditional olive varieties, not only for their historical and cultural significance but also for the deep understanding they offer regarding the evolving relationship between humans and the plant world.This article is part of the theme issue 'Unravelling domestication: multi-disciplinary perspectives on human and non-human relationships in the past, present and future'.

Synergistic governance of urban heat islands, energy consumption, carbon emissions, and air pollution in China: Evidence from a Spatial Durbin Model

Advancing the synergistic governance of urban heat island (UHI), energy consumption (EC), CO2 emissions and air pollution (collectively referred to as heat-energy-carbon-pollution, HECP) is essential for China's green transition. This study examined five representative indicators of HECP-namely, urban heat island intensity (UHII), EC, CO2 emissions, PM2.5 and O3 concentrations-using panel data from 269 prefecture-level cities in China from 2005 to 2020. The Spatial Durbin Model (SDM) was employed to identify the key drivers of HECP governance at both national and regional levels. Results indicated significant spatiotemporal disparities in UHII, EC, CO2 emissions, concentrations of PM2.5 and O3, exhibiting a north-south gradient and pronounced intensities in eastern China. At the national level, energy structure (ES), population density (POP), and normalized difference vegetation index (NDVI) were identified as crucial determinants of HECP synergistic governance. Regional variations were also observed, particularly in eastern China, where industrial structure (IS) and POP played significant roles. Moreover, the spillover effects of these drivers were stronger than their direct effects, highlighting the necessity of interregional cooperation for effective HECP synergistic governance. Our findings underscore the need for a green energy transition, strategic population management, and expanded vegetation coverage. Furthermore, the study highlights the critical role of cross-regional collaboration in improving governance efficiency, offering empirical support for China's integrated environmental and climate policies.

Trait plasticity and adaptive strategies of vascular epiphytes to a large-scale experimental reduction of fog immersion in a tropical montane cloud forest

PREMISE

Tropical montane cloud forests (TMCF) are characterized by frequent fog immersion and host a rich epiphyte community. Epiphytes rely on atmospheric inputs of water, making them susceptible to reductions in fog immersion, which are predicted with climate change.

METHODS

We experimentally reduced the fog in a Peruvian TMCF to examine the ability of eight abundant species of vascular epiphytes in the families Orchidaceae, Bromeliaceae, Ericaceae, Dryopteridaceae, Piperaceae and Clusiaceae to respond to reduced fog immersion via plasticity in morphological and physiological traits.

KEY RESULTS

We found that across all species combined, fog reduction led to a decrease in stomatal length (SL) and foliar water uptake (FWU) capacity. Disterigma sp. (Ericaceae), an epiphytic shrub, reduced leaf thickness (LT) with fog reduction, likely a result of reduced water storage. Comparing across species, we found significant differences in traits related to drought tolerance, including the turgor loss point (TLP), relative water content at TLP (RWCTLP) and osmotic potential at full saturation (πo) indicating that two studied fern species in the Elaphoglossum genus (Dryopteridaceae) may tolerate low water potentials.

CONCLUSION

Our results revealed that some vascular epiphyte species can adjust certain morphological and physiological traits to acclimate to reduced fog immersion. Additionally, our findings support differences in ecological strategies across epiphyte functional groups to either maximize water storage in specialized tissue or to increase drought tolerance. These results give early indications of the likely vulnerability of some epiphyte groups to projected shifts in fog immersion across TCMFs globally.

The combined effect of decreased stomatal density and aperture increases water use efficiency in maize

Stomata play a crucial role in balancing carbon dioxide uptake and water vapor loss, thereby regulating plant water use efficiency (WUE). Enhancing WUE is important for sustainable agriculture and food security, particularly for crops such as maize (Zea mays L.), as climate change and growing global food demand exacerbate limitations on water availability. Genetic factors controlling stomatal density and levels of the plant hormone abscisic acid (ABA) in leaves, which affect stomatal aperture, are key determinants of stomatal conductance (gs) and intrinsic WUE (iWUE). In this study, we demonstrate that stomatal density and stomatal aperture have a combined effect on gs and iWUE in maize. Using near-isogenic lines (NILs) and CRISPR/Cas9 mutants, we show that combining reduced stomatal density and reduced stomatal aperture can improve iWUE without compromising photosynthesis. This effect is pronounced at both, optimal and high temperatures. These findings highlight the potential of targeting multiple stomatal traits through genetic stacking to enhance WUE, offering a promising strategy for crop adaptation to water-limited environments.

Impacts of nitrogen fertilization and planting date on the physiology and yield of purple sweet potato at the extreme Northern edge of cultivation

Global warming causes plant stress and reduces crop productivity. Cultivation of the warmer region crop sweet potato (Ipomoea batatas (L.) Lam) in Northern regions can be an opportunity to benefit from climate warming, but there is little information of how growing season length interacts with agricultural practices such as nitrogen (N) fertilization. We studied the photosynthetic characteristics, biomass accumulation, carbon (C) and N contents of plant organs of the cultivar 'Purple Bud' in relation to the planting date (the 2nd of May, 10th of May, 20th of May, 30th of May and 10th of June) and N fertilization (kg ha-1; N0, N50, N100 and N150). Nitrogen content of leaves (NL) and tubers (NT) increased with N application dose and was moderately affected by planting time. Despite the fertilization-dependent increase of leaf N content, photosynthesis rate (A) was unaffected or somewhat reduced by N fertilization. This reflected reductions in stomatal conductance (gs) and ratio of intercellular CO2 to ambient CO2 (Ci/Ca), suggesting that enhanced N availability and concomitant increase in whole plant area resulted in reduced plant water availability. The highest values of leaf C/N ratio, tuber to root mass ratio and dry weight content of roots (DWR) were found in N0 plants and the ones planted on the 10th of May and 20th of May. Our results collectively demonstrate that the growth and productivity of sweet potato is strongly dependent on the length of the growing season, and can be further constrained by utilization efficiency of N. We conclude that future research should focus on optimum sweet potato cultivation technologies at Northern latitudes.

Heat stress reduces stomatal numbers in Ginkgo biloba: Implications for the stomatal method of palaeo-atmospheric [CO2] reconstruction during episodes of global warming

The stomata of fossil plants are commonly used as proxies to reconstruct palaeo-atmospheric carbon dioxide concentrations (palaeo-[CO2]). Stomatal reconstruction of palaeo-[CO2] during global greenhouse periods or episodes of global warming, are particularly important to our understanding of the role of CO2 as a climate system driver. However, the efficacy of the 'stomatal method' for palaeo-[CO2] reconstruction depends upon the strength of the inverse relationship between stomatal number and the [CO2] in which the leaf developed. However, the impact of heat stress on stomatal initiation and development are largely unknown. Ginkgo biloba, a living fossil species, seedlings were grown in controlled environment chambers under 20/25 °C and 30/35 °C night/day temperature regimes. Heat stress in the 30/35 °C treatment impaired photosynthetic function, decreased stomatal conductance (Gs), and reduced stomatal index (SI), indicative of lower stomatal initiation. Modelled theoretical Gs did not correlate with observed measured Gs, undermining the utility of palaeo-[CO2] reconstructions based on stomatal diffusion modelling. The lower stomatal initiation of G. biloba leaves from the higher temperature resulted in greater estimates of [CO2] based on SI values using the nearest living equivalent and SI-[CO2] transfer function approaches. Heat stress may diminish the effectiveness of the stomatal method in reconstructing palaeo-[CO2] during intervals of global warming marked by floral turnover in Earth history.

Synthetic alleles to study MUTE-dependent molecular transitions in stomatal development

Stomatal abundance sets plants' potential for gas exchange, impacting photosynthesis and transpiration and, thus, plant survival and growth. Stomata originate from cell lineages initiated by asymmetric divisions of protodermal cells, producing meristemoids that develop into guard cell pairs. The transcription factors SPEECHLESS, MUTE, and FAMA are essential for stomatal lineage development, sequentially driving cell division and differentiation events. Their absence produces stomataless epidermis, hindering analysis of their roles during lineage development. MUTE drives the transition from proliferating meristemoids to guard mother cells, committed to stomatal fate. We aim to explore the molecular mechanisms underlying MUTE activity, using partial loss-of-function alleles predicted to impair DNA-binding and to potentially alter MUTE transcriptional activity. We engineered mutant allele coding sequences, generated Arabidopsis lines carrying them and analyzed their epidermal and transcriptional phenotypes using microscopy and RNA-seq. Synthetic alleles driven by the MUTE promoter rescued the stomata less phenotype of the seedling-lethal mute-3 mutant, enabling stomata differentiation and resulting in viable, fertile plants. Further examination of the developmental consequences of MUTE partial loss-of-function revealed arrested lineages, reduced stomatal abundance and altered stomatal spacing. Transcriptomic analysis of very young cotyledons from complemented lines indicated that only some MUTE targets require an intact MUTE bHLH domain. Comparison with existing lineage cell-specific transcriptional profiles showed that lineage development in the mutant lines was delayed compared to the wild-type but followed similar gene networks. These synthetic alleles provide new insight into MUTE ability to accurately and timely specify stomata formation.

Molecular Regulation of Photosynthetic Carbon Assimilation in Oat Leaves Under Drought Stress

Common oat (Avena sativa L.) is one of the important minor grain crops in China, and drought stress severely affects its yield and quality. To investigate the drought resistance characteristics of oat seedlings, this study used Baiyan 2, an oat cultivar at the three-leaf stage, as the experimental material. Drought stress was simulated using polyethylene glycol (PEG) to treat the seedlings. The photosynthetic parameters and physicochemical indices of the treatment groups at 6 h and 12 h were measured and compared with the control group at 0 h. The results showed that drought stress did not significantly change chlorophyll content, but it significantly reduced net photosynthetic rate and other photosynthetic parameters while significantly increasing proline content. Transcriptome analysis was conducted using seedlings from both the control and treatment groups, comparing the two treatment groups with the control group using Tbtool software (v2.136). This analysis identified 344 differentially expressed genes. Enrichment analysis of these differentially expressed genes revealed significant enrichment in physiological pathways such as photosynthesis and ion transport. Ten differentially expressed genes related to the physiological process of photosynthetic carbon assimilation were identified, all of which were downregulated. Additionally, seven differentially expressed genes were related to ion transport. Through gene co-expression analysis combined with promoter region structure analysis, 11 transcription factors (from MYB, AP2/ERF, C2C2-dof) were found to regulate the expression of 10 genes related to photosynthetic carbon assimilation. Additionally, five transcription factors regulate the expression of two malate transporter protein-related genes (from LOB, zf-HD, C2C2-Dof, etc.), five transcription factors regulate the expression of two metal ion transporter protein-related genes (from MYB, zf-HD, C2C2-Dof), five transcription factors regulate the expression of two chloride channel protein-related genes (from MYB, bZIP, AP2/ERF), and two transcription factors regulate the expression of one Annexin-related gene (from NAC, MYB). This study provides a theoretical foundation for further research on the molecular regulation of guard cells and offers a molecular basis for enhancing drought resistance in oats.

Linking stomatal size and density to water use efficiency and leaf carbon isotope ratio in juvenile and mature trees

Water-use efficiency (WUE) is affected by multiple leaf traits, including stomatal morphology. However, the impact of stomatal morphology on WUE across different ontogenetic stages of tree species is not well-documented. Here, we investigated the relationship between stomatal morphology, intrinsic water-use efficiency (iWUE) and leaf carbon isotope ratio (δ13C). We sampled 190 individuals, including juvenile and mature trees belonging to 18 temperate broadleaved tree species and 9 genera. We measured guard cell length (GCL), stomatal density (SD), specific leaf area (SLA), iWUE and bulk leaf δ13C as a proxy for long-term WUE. Leaf δ13C correlated positively with iWUE across species in both juvenile and mature trees, while GCL showed a negative and SD a positive effect on iWUE and leaf δ13C. Within species, however, only GCL was significantly associated with iWUE and leaf δ13C. SLA had a minor negative influence on iWUE and leaf δ13C, but this effect was inconsistent between juvenile and mature trees. We conclude that GCL and SD can be considered functional morphological traits related to the iWUE and leaf δ13C of trees, highlighting their potential for rapid phenotyping approaches in ecological studies.

Leaf functional trait evolution and its putative climatic drivers in African Coffea species

BACKGROUND AND AIMS

Leaf traits are known to be strong predictors of plant performance and can be expected to (co)vary along environmental gradients. We investigated the variation, integration, environmental relationships and evolutionary history of leaf functional traits in the genus Coffea, typically a rainforest understorey shrub, across Africa. A better understanding of the adaptive processes involved in leaf trait evolution can inform the use and conservation of coffee genetic resources in a changing climate.

METHODS

We used phylogenetic comparative methods to investigate the evolution of six leaf traits measured from herbarium specimens of 58 African Coffea species. We added environmental data and data on maximum plant height for each species to test trait-environment correlations in various (sub)clades, and we compared continuous trait evolution models to identify variables driving trait diversification.

KEY RESULTS

Substantial leaf trait variation was detected across the genus Coffea in Africa, which was mostly interspecific. Of these traits, stomatal size and stomatal density exhibited a clear trade-off. We observed low densities of large stomata in early-branching lineages and higher densities of smaller stomata in more recent taxa, which we hypothesize to be related to declining CO2 levels since the mid-Miocene. Brownian motion evolution was rejected in favor of white noise or Ornstein-Uhlenbeck models for all traits, implying these traits are adaptively significant rather than driven by pure drift. The evolution of leaf area was likely driven by precipitation, with smaller leaves in drier climates across the genus.

CONCLUSIONS

Generally, Coffea leaf traits appear to be evolutionarily labile and governed by stabilizing selection, though evolutionary patterns and correlations differ depending on the traits and clades considered. Our study highlights the importance of a phylogenetic perspective when studying trait relationships across related taxa, as well as the consideration of various taxonomic ranges.

Inside-out: Synergising leaf biochemical traits with stomatal-regulated water fluxes to enhance transpiration modelling during abiotic stress

As the global climate continues to change, plants will increasingly experience abiotic stress(es). Stomata on leaf surfaces are the gatekeepers to plant interiors, regulating gaseous exchanges that are crucial for both photosynthesis and outward water release. To optimise future crop productivity, accurate modelling of how stomata govern plant-environment interactions will be crucial. Here, we synergise optical and thermal imaging data to improve modelled transpiration estimates during water and/or nutrient stress (where leaf N is reduced). By utilising hyperspectral data and partial least squares regression analysis of six plant traits and fluxes in wheat (Triticum aestivum), we develop a new spectral vegetation index; the Combined Nitrogen and Drought Index (CNDI), which can be used to detect both water stress and/or nitrogen deficiency. Upon full stomatal closure during drought, CNDI shows a strong relationship with leaf water content (r2 = 0.70), with confounding changes in leaf biochemistry. By incorporating CNDI transformed with a sigmoid function into thermal-based transpiration modelling, we have increased the accuracy of modelling water fluxes during abiotic stress. These findings demonstrate the potential of using combined optical and thermal remote sensing-based modelling approaches to dynamically model water fluxes to improve both agricultural water usage and yields.

Peroxidase gene TaPrx109-B1 enhances wheat tolerance to water deficit via modulating stomatal density

Increasing the tolerance of crops to water deficit is crucial for the improvement of crop production in water-restricted regions. Here, a wheat peroxidase gene (TaPrx109-B1) belonging to the class III peroxidase gene family was identified and its function in water deficit tolerance was revealed. We demonstrated that overexpression of TaPrx109-B1 reduced leaf H2O2 level and stomatal density, increased leaf relative water content, water use efficiency, and tolerance to water deficit. The expression of TaEPF1 and TaEPF2, two key negative regulators of stomatal development, were significantly upregulated in TaPrx109-B1 overexpression lines. Furthermore, exogenous H2O2 downregulated the expression of TaEPF1 and TaEPF2 and increased stomatal density, while exogenous application of diphenyleneiodonium chloride, a potent NADPH oxidase inhibitor that repressed the synthesis of H2O2, upregulated the expression of TaEPF1 and TaEPF2, decreased stomatal density, and enhanced wheat tolerance to water deficit. These findings suggest that TaPrx109-B1 influences leaf stomatal density by modulation of H2O2 level and the expression of TaEPF1 and TaEPF2. The results of the field trial showed that overexpressing TaPrx109-B1 increased grain number per spike, which reduced the yield loss caused by water deficiency. Therefore, TaPrx109-B1 has great potential in breeding wheat varieties with improved water deficit tolerance.

Decoding stomatal characteristics regulating water use efficiency at leaf and plant scales in rice genotypes

MAIN CONCLUSION

Stomatal traits in rice genotypes affect water use efficiency. Low-frequency small-size stomata correlate with whole plant efficiency, while low-frequency large-size stomata show intrinsic efficiency and responsiveness to vapour pressure deficit. Leaf surface and the patterning of the epidermal layer play a vital role in determining plant growth. While the surface helps in determining radiation interception, epidermal pattern of stomatal factors strongly regulate gas exchange and water use efficiency (WUE). This study focuses on identifying distinct stomatal traits among rice genotypes to comprehend their influence on WUE. Stomatal frequency ranged from 353 to 687 per mm2 and the size varied between 128.31 and 339.01 μm2 among 150 rice germplasm with significant variability in abaxial and adaxial surfaces. The cumulative water transpired and WUE determined at the outdoor phenomics platform, over the entire crop growth period as well as during specific hours of a 24 h-day did not correlate with stomatal frequency nor size. However, genotypes with low-frequency and large-size stomata recorded higher intrinsic water use efficiency (67.04 μmol CO2 mol-1 H2O) and showed a quicker response to varying vapour pressure deficit that diurnally ranged between 0.03 and 2.17 kPa. The study demonstrated the role of stomatal factors in determining physiological subcomponents of WUE both at single leaf and whole plant levels. Differential expression patterns of stomatal regulatory genes among the contrasting groups explained variations in the epidermal patterning. Increased expression of ERECTA, TMM and YODA genes appear to contribute to decreased stomatal frequency in low stomatal frequency genotypes. These findings underscore the significance of stomatal traits in breeding programs and strongly support the importance of these genes that govern variability in stomatal architecture in future crop improvement programs.

Characterization of carbon fluxes, stock and nutrients in the sacred forest groves and invasive vegetation stands within the human dominated landscapes of a tropical semi-arid region

In the semi-arid plains of Southern India, outside the protected area network, sacred groves forests and the barren lands invaded by Prosopis juliflora are reckoned to be the major greenery, but have homogenous and heterogeneous vegetation respectively. This study attempted to compare 50 Sacred Groves Stands (SGS) and 50 monodominant Prosopis juliflora Stands (PJS) for the functional diversity, evenness, floral diversity, carbon stock and dynamics, carbon-fixing traits, dendrochronology of trees, soil nutrient profiles, and soil erosion. Quadrat sample survey was adopted to record stand density, species richness, abundance, basal area and leaf area index; composite soil samples were collected at depths 0-30 cm for nutrient profiling (N, P, K, and OC). Photosynthesis rate (µmole co2 m2/sec), air temperature (°c), leaf intracellular co2 concentration (ppm), ambient photosynthetic active radiation (µmole m2/sec), transpiration rate (m. mole H2O m2/sec) were determined for the 51 tree species existed in SGS and PJS using Plant Photosynthesis system. Structural Equation Model (SEM) was applied to derive the carbon sequestering potential and photosynthetic efficiency of eight dominant tree species using vital input parameters, including eco-physiological, morphological, and biochemical characterization. The Revised Universal Soil Loss Equation (RUSLE) model, in conjunction with ArcGIS Pro and ArcGIS 10.3, was adopted to map soil loss. Carbon source/sink determinations inferred through Net Ecosystem Productivity (NEP) assessments showed that mature SGS potentially acted as a carbon sink (0.06 ± 0.01 g C/m2/day), while matured PJS acted as a carbon source (-0.34 ± 0.12 g C/m2/day). Soil erosion rates were significantly greater (29.5 ± 13.4 ton/ha/year) in SGS compared to PJS (7.52 ± 2.55 ton/ha/year). Of the eight selected tree species, SEM revealed that trees belonging to the family Fabaceae [Wrightia tinctoria (estimated coefficient: 1.28, p = 0.02) > Prosopis juliflora (1.22, p = 0.01) > Acacia nilotica (1.21, p = 0.03) > Albizia lebbeck (0.97, p = 0.01)] showed comparatively high carbon sequestering ability.

Comparing Methodologies for Stomatal Analyses in the Context of Elevated Modern CO2

Leaf stomata facilitate the exchange of water and CO2 during photosynthetic gas exchange. The shape, size, and density of leaf pores have not been constant over geologic time, and each morphological trait has potentially been impacted by changing environmental and climatic conditions, especially by changes in the concentration of atmospheric carbon dioxide. As such, stomatal parameters have been used in simple regressions to reconstruct ancient carbon dioxide, as well as incorporated into more complex gas-exchange models that also leverage plant carbon isotope ecology. Most of these proxy relationships are measured on chemically cleared leaves, although newer techniques such as creating stomatal impressions are being increasingly employed. Additionally, many of the proxy relationships use angiosperms with broad leaves, which have been increasingly abundant in the last 130 million years but are absent from the fossil record before this. We focus on the methodology to define stomatal parameters for paleo-CO2 studies using two separate methodologies (one corrosive, one non-destructive) to prepare leaves on both scale- and broad-leaves collected from herbaria with known global atmospheric CO2 levels. We find that the corrosive and non-corrosive methodologies give similar values for stomatal density, but that measurements of stomatal sizes, particularly guard cell width (GCW), for the two methodologies are not comparable. Using those measurements to reconstruct CO2 via the gas exchange model, we found that reconstructed CO2 based on stomatal impressions (due to inaccurate measurements in GCW) far exceeded measured CO2 for modern plants. This bias was observed in both coniferous (scale-shaped) and angiosperm (broad) leaves. Thus, we advise that applications of gas exchange models use cleared leaves rather than impressions.