Plant, Cell & Environment

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.

O_LIThe future climate will be characterized by an increase in frequency and duration of drought and warming that exacerbates atmospheric evaporative demand. How trees acclimate to long-term soil moisture changes and whether these long-term changes alter trees sensitivity to short-term (day to months) variations of vapor pressure deficit (VPD) and soil moisture is largely unknown. C_LIO_LILeaf gas exchange measurements were performed within a long-term (17 years) irrigation experiment in a Scots pine-dominated forest in one of Switzerlands driest areas on trees in naturally dry (control), irrigated, and irrigation-stop (after 11 years of irrigation) conditions. C_LIO_LISeventeen years of irrigation increased photosynthesis (A) and stomatal conductance (gs) and reduced the gs sensitivity to increasing VPD but not to soil drying. Following irrigation-stop, gas exchange did not decrease immediately, but after three years, had decreased significantly in irrigation-stop trees. Vcmax and Jmax recovered after five years. C_LIO_LIThese results suggest that long-term release of soil drought reduces the sensitivity to atmospheric evaporative demand and that atmospheric constraints may play an increasingly important role in combination with soil drought. In addition, they suggest that structural adjustments lead to an attenuation of initially strong leaf-level acclimation to strong multiple-year drought. C_LI

O_LIRising atmospheric CO2 levels are predicted to influence forest health directly and indirectly, yet the long-term effects of elevated CO2 (eCO2) on mature trees in natural ecosystems remain poorly understood. Understanding how eCO2 affects susceptibility to biotic stress and alters leaf metabolism is critical for predicting forest responses to climate change. C_LIO_LIWe examined the effects of eCO2 (+150 ppm) on 180-year-old Quercus robur at the Birmingham Institute of Forest Research (BIFoR) Free Air CO2 Enrichment (FACE) facility. From 2016 (pre-treatment) to 2024 (year 8 of enrichment), we monitored natural powdery mildew infection and insect herbivory, alongside targeted and untargeted metabolomic profiling of leaf material collected across the growing season. C_LIO_LIWhile seasonal patterns and an overall decline in PM and herbivory were observed, no consistent differences in biotic stress incidence emerged due to eCO2. Metabolomic data revealed subtle but widespread shifts, especially in amino acid, CoenzymeA, and redox pathways. C_LIO_LIThese results suggest that although eCO2 drives extensive metabolic changes, it does not alter biotic stress resistance in mature oaks. Instead, eCO2 appears to promote physiological plasticity that may shape future responses to combined environmental stressors. These insights offer a valuable reference point for interpreting long-term ecosystem dynamics. C_LI

Understory plant species take on different functional strategies, whereby some exploit periods of available light in springtime before the canopy closes, and others also benefit from sunlight later in autumn when the canopy opens again. These strategies involve understory species coordinating phenological events to pre-empt canopy leaf out and to extend their growing season beyond canopy leaf senescence, meanwhile accumulating photo-protective pigments which mitigate periods of high-light exposure. Canopy closure brings shade to the understory, but also causes drastic changes in light quality. Whilst many experiments manipulating spectral quality have revealed understory plant responses to the changing R:FR ratio in shade, effect of the blue and UV regions have been examined very little. We installed filters attenuating short wavelength regions of the solar spectrum in a forest understory in southern Finland, creating the following treatments: a transparent control filter, and filters attenuating UV radiation < 350 nm, all UV radiation, and both UV and blue light. In eight understory species, representing different plant functional types, we repeatedly assessed leaf optical properties to obtain epidermal flavonol and anthocyanin contents from leaf emergence in spring to leaf senescence in autumn, during both 2017 and 2018. Flavonols responded more to seasonal changes in light quality in relatively light-demanding species than in shade-tolerant and wintergreen species; and were particularly responsive to blue light. However, anthocyanins were largely unaffected by our filter treatments, suggesting that other cues such as cold temperatures govern their seasonal variation. UV radiation only accelerated leaf senescence in Acer platanoides seedlings, but blue light accelerated leaf senescence in all species measured apart from Quercus robur. In summary, seasonal changes in understory solar radiation in the blue and UV regions affected leaf pigments and leaf phenology; particularly for more light-demanding species. An increase in canopy duration under climate change will extend the period of shade in the understory, with consequences for the spectral cues available to understory plants. The resultant reduction in blue and UV radiation in shade, could delay leaf senescence in the understory even further.

In the near future, climate change will cause enhanced frequency and/or severity in terrestrial ecosystems, including tropical forests. Drought responses by tropical trees may affect their carbon use, including production of volatile organic compounds (VOCs), with unpredictable implications for carbon cycling and atmospheric chemistry. It remains unclear how metabolic adjustments by mature tropical trees in response to drought will affect their carbon fluxes associated with daytime CO2 production and VOC emission. To address this gap, we used position-specific 13C-pyruvate labeling to investigate leaf CO2 and VOC fluxes from four tropical species before and during a controlled drought in the enclosed rainforest of Biosphere 2. Overall, plants that were more sensitive to drought had greater reductions in daytime CO2 production. Although daytime CO2 production was always dominated by non-mitochondrial processes, the relative contribution of CO2 from the tricarboxylic acid cycle tended to increase under drought. A notable exception was the legume tree Clitoria fairchildiana, which had less anabolic CO2 production than the other species even under pre-drought conditions, perhaps due to more efficient refixation of CO2 and anaplerotic use for amino acid synthesis. C. fairchildiana was also the only species to allocate detectable amounts of 13C label to VOCs, and was a major source of VOCs in the Biosphere 2 forest. In C. fairchildiana leaves, our data indicate that intermediates from the mevalonic acid pathway are used to produce the volatile monoterpene trans-{beta}-ocimene, but not isoprene. This apparent crosstalk between the mevalonic acid and methylerythritol phosphate pathways for monoterpene synthesis declined with drought. Finally, although trans-{beta}-ocimene emissions increased under drought, it was increasingly sourced from stored intermediates and not de novo synthesis. Unique metabolic responses of legumes may play a disproportionate role in the overall changes in daytime CO2 and VOC fluxes in tropical forests experiencing drought.

Leaf gas exchange is the key driver of forest carbon uptake and directly determines forest carbon sink activity. Additionally, plants release a variety of biogenic volatile organic compounds (VOCs) acting as stress signals of trees. However, continuous hourly resolved measurements of leaf gas exchange and VOC emissions in tall tree canopies are challenging and remain scarce. To this end, we developed a sophisticated in-situ leaf gas exchange measurement system with 24 cuvettes deployed on mature Fagus sylvatica (n=3) and Pseudotsuga menziesii (n=3) individuals in a mixed temperate forest. We additionally measured sap flux density (Js), radial growth and tree water deficit (TWD) to gain a holistic picture of seasonal leaf and stem water and carbon flux dynamics during the summer of 2024. During midsummer, we found a gradual reduction of stomatal conductance (gs) and VOC emissions of sun, but not shade branchlets of P. menziesii in response to moderate atmospheric and edaphic drying. Decreased gs led to a downregulation of transpiration (E), Js, and carbon isotope discrimination accompanied by an increase in TWD and intrinsic water used efficiency. Leaf gas exchange of shade branchlets remained unaffected due to microclimatic buffering effects. Contrarily, sun leaves of F. sylvatica, profited from sunny midsummer conditions and increased leaf gas exchange, whereas shade leaves benefitted from more diffuse light during early summer exhibiting similar carbon assimilation, transpiration and VOC emissions as sun leaves. For both species we found a clear time lag of four to five hours between maximum leaf and stem water fluxes and a delay of up to 20 hours for the recovery of TWD, highlighting the role of stem water reserves. Pronounced seasonal and diurnal differences of leaf gas exchange, stem water fluxes and VOC emissions showed, that continuous data are essential to better understand variability of ecosystem flux dynamics.

Plants perceive the presence and defense status of their neighbors through light and volatile cues, but how plants integrate both stimuli is poorly understood. We investigated if and how low Red to Far red light (R:FR) ratios, indicative of shading or canopy closure, affect maize (Zea mays) responses to herbivore-induced plant volatiles (HIPVs), including the green leaf volatile (Z)-3-hexenyl acetate. We modulated light signaling and perception by using FR supplementation and a phyB1phyB2 mutant, and we determined volatile release as a response readout. To gain mechanistic insights, we examined expression of volatile biosynthesis genes, hormone accumulation, and photosynthesis. Exposure to a full blend of HIPVs or (Z)-3-hexenyl acetate induced maize volatile release. Short-term FR supplementation increased this response. In contrast, prolonged FR supplementation or constitutive phytochrome B inactivation in phyB1phyB2 plants showed the opposite response. Short-term FR supplementation enhanced photosynthesis and stomatal conductance and (Z)-3-hexenyl acetate-induced JA-Ile levels. We conclude that a FR-enriched light environment can prompt maize plants to respond more strongly to HIPVs emitted by neighbors, which might be explained by changes in photosynthetic processes and phytochrome B signaling. Our findings reveal interactive responses to light and volatile cues with potentially important consequences for plant-plant and plant-herbivore interactions.

Wood formation is a crucial process for carbon sequestration, yet how variations in carbon supply affect wood formation and carbon dynamics in trees more generally remains poorly understood. To better understand the role of carbon supply in wood formation, we restricted phloem transport using girdling and compression around the stem of mature white pines and monitored the effects on local wood formation and stem CO2 efflux, as well as nonstructural carbon concentrations in needles, stems, and roots. Growth and stem CO2 efflux varied with location relative to treatment (i.e., above or below on the stem). We observed up to a two-fold difference in the number of tracheids formed above versus below the manipulations over the remaining growing season. In contrast, the treatments did not affect mean cell size noticeably and mean cell-wall area decreased only slightly below them. Surprisingly, nonstructural carbon pools and concentrations in the xylem, needles, and roots remained largely unchanged, although starch reserves declined and increased marginally below and above the girdle, respectively. Our results suggest that phloem transport strongly affects cell proliferation and respiration in the cambial zone of mature white pine, but has little impact on nonstructural carbon concentrations. These findings contribute to our understanding of how wood formation is controlled. HighlightRestrictions in phloem transport designed to affect carbon supply, lead to changes in wood formation and stem respiration of mature white pines without substantially changing local nonstructural carbon concentrations.

O_LIPlant leaves that are exposed to herbivore induced plant volatiles (HIPVs) respond by increasing their defenses. Whether this phenomenon also occurs in the roots is unknown. C_LIO_LIUsing maize (Zea mays), whose leaves respond strongly to leaf HIPVs, we measured the impact of root HIPVs, emanating from plants infested by the banded cucumber beetle (Diabrotica balteata), on constitutive and herbivore-induced levels of root soluble sugars, starch, total soluble proteins, free amino acids, volatile and non-volatile secondary metabolites, defense gene expression, growth and root herbivore resistance of neighboring plants. C_LIO_LIHIPV exposure did not alter constitutive or induced levels of any of the measured root traits. Furthermore, HIPV exposure did not reduce the performance and survival of banded cucumber beetle larvae on maize or teosinte. Cross-exposure experiments revealed that maize roots, in contrast to maize leaves, neither emit nor respond strongly to defense-regulating HIPVs. C_LIO_LITogether, these results demonstrate that volatile-mediated defense regulation is restricted to the leaves of maize and teosinte, a finding which is in line with the lower diffusibility of volatiles in the soil and the availability of other, potentially more efficient information conduits below ground. C_LI

O_LIPhotosynthetic assimilation (Anet) and stomatal conductance (gs) are usually strongly coupled, but this relationship is decreased or even lost at high temperatures (Tair). The contributions of environmental drivers (Tair, vapour pressure deficit (VPD), and soil moisture) in interaction with the physiological mechanisms behind this process are still unclear. C_LIO_LIWe exposed saplings of three temperate and tropical species to rising Tair (20 to 40{degrees}C) at low (1.2 to 1.9 kPa) and increasing VPD (1.1 to 5.6 kPa), and at stable Tair (35{degrees}C) to increasing VPD (1.4 to 4.3 kPa) under well-watered or chronic soil drought conditions ([&le;]10 %). Anet, gs, and transpiration (E) in the light and the dark and leaf thermoregulation were tracked throughout the experiment. C_LIO_LIWhen VPD remained low, gs continued to increase while Anet decreased at Tair > 35{degrees}C, leading to stomatal decoupling. In contrast, under rising VPD, trees maintained the coupling between Anet and gs at high Tair. C_LIO_LIWhile a decoupling of Anet and gs only occurred when VPD was low, Anet and E decoupled under both VPD regimes at high Tair. C_LIO_LIOur results indicate that, since gs and VPD collectively drive E, stomatal decoupling is needed to increase E when VPD is not sufficiently high. C_LI

Grasslands, like many temperate ecosystems, are threatened by heatwaves which have been shown to be more frequent. Plant diversity can buffer yield loss due to heatwaves, but this positive effect may be modified by nutrient availability. Soil sulfur (S) has declined over the last decades and although it is crucial for coping with abiotic stresses, its role in grassland responses to heatwaves remains poorly documented. We aimed to determine how S nutrition modulates the responses of grasslands to heatwaves. Four monocultures and two mixtures were grown under two S levels and exposed to four thermoprotocols: i) control, ii) two successive mild heatwaves, iii) one severe heatwave, and iv) a recurrent sequence combining mild and severe heatwaves. We measured biomasses, leaf S compounds, {delta}13C as a proxy of water-use efficiency, leaf temperature, and photosystem II efficiency. S nutrition interacted with thermoprotocols. Under a severe heatwave, standard S promoted higher shoot production and better water-use efficiency. In contrast, under recurrent events, S limitation enhanced root:shoot ratio, water-use efficiency, shoot growth, and led to lower leaf temperature. S-containing metabolites did not vary with heatwaves, suggesting no direct metabolic. Under recurrent heatwaves, S limitation could indirectly improve tolerance to severe heatwaves. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=114 SRC="FIGDIR/small/691764v1_ufig1.gif" ALT="Figure 1"> View larger version (42K): org.highwire.dtl.DTLVardef@184f87borg.highwire.dtl.DTLVardef@d55d3forg.highwire.dtl.DTLVardef@1dd1426org.highwire.dtl.DTLVardef@1560116_HPS_FORMAT_FIGEXP M_FIG C_FIG

Although chiral monoterpenes emitted by plants above- and belowground shape the chemical landscape of many ecosystems, their biosynthesis and emissions, especially in response to drought, are poorly understood. We imposed a 6-week drought on two-year old, potted saplings of Norway spruce and analysed chiral monoterpene emissions and tissue concentrations from needles and roots. Isotopically labelled pyruvate was used to compare tissue-specific contributions of de novo synthesis to chiral monoterpene concentrations. While de novo synthesis of (-)--pinene and both enantiomers of limonene was apparent in needle emissions, no label was incorporated in roots. Drought reduced chiral monoterpene emissions to 30% of control levels, but increased needle and root tissue concentrations by 150 and 230%, respectively. Aboveground monoterpene concentrations were dominated by (-)-limonene, whereas belowground concentrations mainly consisted of the (-)-enantiomers of -pinene, {beta}-pinene, {beta}-phellandrene and camphene. Chiral composition in needles shifted in response to drought but remained stable in roots. We conclude that chiral monoterpene composition is tissue-specific and likely related to tissue-specific functioning. Instead of being passively emitted from storage pools, our results suggest active control mechanisms regulating chiral monoterpene emissions under drought conditions. Our findings imply important ramifications for understanding the regulation of emissions in relation to storage pools and plant-environmental interactions. Summary StatementEmissions of chiral monoterpenes and their composition is tissue-specific and regulated independent from storage pools in Picea abies. Chiral monoterpene ratios shift aboveground in response to drought, but are not affected belowground.

O_LIProtection from excess solar radiation and access to sufficient water are important problems for terrestrial plants to solve. Desiccation tolerance (DT), defined as the ability to equilibrate to dry air and resume normal metabolic activity after rehydration, allows organisms to survive dry periods by limiting metabolic activity to periods of moisture availability. We compared separate and combined effects of chronic ultraviolet radiation (UVR) treatments (UV-A and UV-A/B) and a dehydration treatment (as a surrogate for desiccation) in the mosses Syntrichia ruralis and S. caninervis to uncover the nature of correlation between DT and UVR tolerance (UVRT). C_LIO_LIUsing a fully factorial experiment with combined transcriptomics and metabolomics, we tested for cross-talk (overlap in signaling pathways in response to different stressors but separate mechanisms of protection) in the genetic underpinnings of DT and UVRT and cross-tolerance (overlap in the mechanism of protection) these two stressors. C_LIO_LIShared transcriptomic response to the two stressors with no significant interaction between them suggested cross-talk between UVRT and DT for S. caninervis. Phenolic metabolites and transcripts were involved in the response to UVR and dehydration in both species. C_LIO_LISome candidate UVRT genes and metabolites were induced by UVR in S. ruralis, but not S. caninervis, supporting the hypothesis that S. ruralis has a more plastic, acclimatable UVR response than S. caninervis, and that these differences are predictable by their unique interaction with these stressors as poikilohydric organisms. C_LI

Cold acclimation in plants is a complex phenomenon involving numerous stress-responsive transcriptional and metabolic pathways. Existing gene expression studies have primarily addressed short-term cold acclimation responses in herbaceous plants, while few have focused on perennial evergreens, such as conifers, that survive extremely low temperatures during winter. To characterize the transcriptome changes during cold acclimation in Picea abies (L.) H. Karst (Norway spruce), we performed RNA-Sequencing analysis of needles and roots subjected to a chilling progression (5 {degrees}C) followed by 10 days at freezing temperature (-5 {degrees}C). Comparing gene expression responses of needles against Arabidopsis thaliana L. (Arabidopsis) leaves, our results showed that early transient inductions were observed in both species but the transcriptional response of Norway spruce was delayed. Our results indicate that, similar to herbaceous species, Norway spruce principally utilizes early response transcription factors (TFs) that belong to the APETALA 2/ethylene-responsive element binding factor (AP2/ERF) superfamily and NACs. However, unique to the Norway spruce response was a large group of TFs that mounted a late transcriptional response to low temperature. A predicted regulatory network analysis identified key conserved TFs, including a root-specific bHLH101 homolog and other members of the same family with a pervasive role in cold regulation, such as homologs of ICE1 and AKS3 and also homologs of the NAC (anac47 and anac28) and AP2/ERF superfamilies (DREB2 and ERF3), providing new functional insights into cold stress response strategies in Norway spruce. One sentence summaryNorway spruce shares elements of the cold regulon described in herbaceous species but has undescribed components that contribute to the cold tolerance of this evergreen coniferous species.

In agricultural crops, forests and grasslands, water deficit often occurs in the presence of cues from neighbouring vegetation. However, most studies have addressed separately the mechanisms of plant growth responses to these two aspects of the environment. Here we show that transferring Arabidopsis thaliana seedlings to agar containing polyethylene glycol (PEG) to restrict water availability reduces hypocotyl growth responses to shade without simultaneous affecting cotyledon expansion or its response to shade. Water restriction diminished the activity of the PHYTOCHROME INTERACTING FACTOR 4 (PIF4), PIF5, PIF3 and PIF3-LIKE 1 gene promoters, particularly in seedlings exposed to simulated shade. The response of PIF4 expression to PEG required the presence of its positive morning regulators CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY), which also reduced their expression in response to PEG. Water restriction diminished the nuclear abundance of PIF4 in hypocotyl cells only in the seedlings exposed to shade. In addition to the changes in PIF4 levels, post-transcriptional processes also contributed to the response to PEG. Hypocotyl growth showed significant triple interaction among water availability, shade and the presence of PIF4, PIF5 and PIF3. Collectively, these results unveil PIFs as a hub that interlinks shade and drought information to control growth.

Rising atmospheric CO2 has profound implications for crop productivity and food security. Based on studies in C3 plants, elevated CO2 (eCO2) can shape plant-pathogen interactions, although the outcomes are often variable. The question of how eCO2 influences immunity and disease development in C4 plants, such as the globally important cereal crop maize (Zea mays L.), has not been systematically examined. We challenged maize plants grown under ambient CO2 (aCO2, 420 ppm) and eCO2 (550 ppm) with bacterial, viral, fungal, and oomycete pathogens. Plants grown in eCO2 were more susceptible to sugarcane mosaic virus, suggesting compromised antiviral defenses, less susceptible to Clavibacter nebraskensis, Exserohilum turcicum, and Colletotrichum graminicola, and susceptibility to Puccinia sorghi and Pythium sylvaticum was unchanged. Reduced susceptibility to C. nebraskensis was associated with enhanced basal immune responses. These results establish a foundation for dissecting eCO2-responsive defense mechanisms, and they highlight a critical need to understand how eCO2 will impact plant responses to microbes, pests, and abiotic stresses under future conditions.

Trees in temperate and boreal latitudes synchronize their growth-dormancy cycles with seasonal environmental variations to ensure their survival over the years. Dormancy control is crucial during winter when plants cease growth and establish buds to protect their apical meristems from cold temperatures. To overcome endormancy, initiate bud break, and restore growth, plants must be exposed to a specific duration of chilling, referred to as the chilling requirement, which is species- and ecotype-dependent. In this work, we study the novel roles of two TEMPRANILLO-like genes (TEML1 and TEML2) in the annual cycle of poplar. We demonstrated that Populus TEML genes are regulated by photoperiod, cold temperatures and the circadian clock, and they play a role in the control of endodormancy. Notably, their function diverges from the role of its Arabidopsis ortholog AtTEM, which regulates FLOWERING LOCUS T (FT) transcription and the photoperiodic flowering transcription. Transcriptomic analysis of endodormant buds during winter revealed that the activation of TEML1 and TEML2 promotes endodormancy release by modulating the expression of endodormancy regulators and growth-promoting genes.

The circadian clock synchronizes plants with the rhythmic changes in their environment created by the cycle of day and night. Plant-pathogen interactions are influenced by this rhythmic cycle and a functioning circadian clock is essential for eQective resistance to pathogens. In this study we investigate the relationship between PAMP-triggered immune signalling and the circadian clock in Arabidopsis. We found that early PAMP-triggered immune responses including flg22- induced FRK1 transcript levels and the flg22-induced ROS burst are enhanced at subjective dusk rather than subjective dawn. Overexpression of clock gene TOC1 supressed these defence responses to flg22 and resulted in increased susceptibility to Pseudomonas syringae. Additionally, treatment with flg22 and elf18 altered the rhythmicity of CCA1 and TOC1. These results contribute to the currently small amount of research investigating the interactions between the circadian clock and PAMP-triggered immunity. Importantly, we establish the detrimental eQect of TOC1 overexpression on PAMP-triggered defence responses. Further investigation into how clock components regulate early immune responses is essential for improving our understanding of plant health.

Conifer forests worldwide are increasingly threatened by environmental stressors, especially bark beetle infestations. In a novel experimental field setup with controlled attacks on clonal trees, we investigated the molecular defense responses of 35-year-old Norway spruce trees to colonization attempts by the spruce bark beetle, Ips typographus. A multi-omics approach was used to study tree responses to bark beetle pioneers under natural conditions. At the site of the attacks, increased levels in jasmonic acid, together with changes in other phytohormones, induced differential gene expression of up to 1,900 genes from multiple defense-related pathways. These transcriptomic alternations were corroborated by proteomic and metabolomic shifts, which ultimately showed increased levels of compounds involved in deterring bark beetle attacks, such as phenolic aglycones, diterpene resin acids, mono- and sesquiterpenes, and lignin. Our study highlights the intricate yet rapid and effective local defense response of Norway spruce against pioneer bark beetle attacks and lays a foundation for in-depth studies of this and other conifers resistance to phloem-feeding insects.

Residual water losses after stomatal closure have recently been identified as key determinants of drought-induced hydraulic failure, particularly under heatwave conditions. However, little is known about the intraspecific variability of residual conductance (gres) and its thermal sensitivity. Here, we investigated the genetic and environmental sources of variation in gres and its associated thermal parameters (phase transition temperature T_, and temperature sensitivities Q10a and Q10b) in Abies alba Mill., together with vulnerability to xylem embolism (P50). Measurements were performed using the Drought-Box on seven French provenances grown in a common garden to assess genetic variability, and on trees growing across contrasting forest sites to quantify phenotypic plasticity. Seasonal dynamics and within-canopy microclimatic effects were also examined, and linked to needle biochemical traits. Residual conductance exhibited a marked seasonal decline, with high values in newly formed needles followed by a stabilization from late summer to the following spring, closely tracking the accumulation of cuticular waxes. In contrast, Klason lignin content showed little seasonal variation. Difference between provenances was weak for all investigated parameters, suggesting strong constraints on these safety-related traits. By contrast, gres showed significant environmental plasticity, with lower values at more climatically constrained sites, while thermal parameters and P50 remained relatively conserved. Our results identify gres as a developmentally dynamic and environmentally plastic trait in silver fir, potentially representing a key lever of acclimation to drought. Incorporating such variability into mechanistic models should improve predictions of tree vulnerability under future climates combining intensified droughts and heatwaves. Key message.Residual conductance in Abies alba is developmentally dynamic and environmentally plastic but genetically constrained, highlighting its key role in acclimation to drought and heatwave-driven hydraulic failure.