Tree Physiology

O_LIPit membranes play a crucial role in water transport between neighbouring xylem conduits by providing hydraulic safety and flow resistance. While effects of pit membranes thickness on embolism resistance and temporal changes in the ultrastructure of pit membranes have been documented between sapwood and heartwood, these changes are largely unknown within conduits of the current-year. C_LIO_LIWe studied interconduit pit membranes of branches of eight angiosperm species by sampling wood from a temperate forest over four consecutive seasons, focusing on the latest growth ring. We quantified the pit membrane thickness and greyscale intensity (as a proxy for electron density) using transmission electron microscopy and image analysis. C_LIO_LIOur observations showed considerable interspecific variation in changes to pit membrane thickness and electron density. Several species exhibited the thinnest pit membranes and highest electron density at the end of a growing season, while others showed minimal variation over time. Intra-tree variation showed that changes in pit membrane shrinkage and electron density were associated with conduit diameter: pit membranes in wide conduits showed larger modification over time than narrow ones. C_LIO_LIOur results indicate seasonal changes in the structure and chemistry of angiosperm pit membranes, even within the latest growth ring. While their shrinkage might increase resistance to flow, the occurrence of darker pit membranes indicates coating and penetration by polar lipids, affecting the behaviour of gas-liquid interfaces. We speculate that the degree of modification that pit membranes undergo is mechanistically driven by the sap flow rate, and possibly conduit dimensions. C_LI

The sequence of leaf physiological thresholds underlying plant responses to water deficit is thought to be functionally coordinated; yet, to what extent this coordination is maintained across genotypes and environments remains poorly documented at the intraspecific level. We characterized the sequence of stomatal closure, turgor loss and xylem embolism in the leaves of two genotypes of the riparian species Populus nigra (DRA-038 vs. PG-31) subjected to control, additional nitrogen or additional potassium treatments. Under control conditions, embolism measurements using the optical vulnerability method showed that DRA-038 was more vulnerable than PG-31, in agreement with measurements performed on stems with the reference Cavitron method. Stomatal closure consistently preceded xylem embolism, while bulk leaf turgor loss was typically observed once xylem embolism had already reached 50%. Hydraulic thresholds responded to treatments in a genotype-dependent manner, the intrinsically more vulnerable genotype DRA-038 being typically more plastic. However, despite variations across genotypes and treatments, the trait sequence remained tightly coordinated such that stomatal safety margins (SSMs) remained virtually null. These findings support a strong mechanistic integration of leaf hydraulic thresholds in poplar across genetic units and varying environments, questioning whether to favour intrinsic tolerance or plastic capacities in breeding future drought-tolerant genotypes.

Bacterial canker remains a major constraint affecting apricot production in South-East of France. It is primarily caused by Pseudomonas syringae, a Gram-negative bacterium, many strains of which exhibit ice nucleation activity. By promoting ice formation at relatively high subzero temperatures, ice nucleation-active bacteria may facilitate tissue disruption and pathogen entry. Concurrently, climate-driven shifts toward warmer winter-spring periods have advanced flowering phenology, increasing exposure to late frost events. Despite breeders having developed less susceptible varieties to bacterial canker and early flowering varieties, the link between these traits and frost sensitivity, an emerging risk in this location, remains unresolved. Here, we have evaluated the links between canker susceptibility and frost sensitivity using three cultivar pairs contrasting in disease response and flowering time. Ice nucleation temperature was measured in excised buds under controlled conditions throughout the frost-risk period, alongside field-based diameter variation monitoring over two years. Disease susceptibility (P < 0.001), phenology (P = 0.003), varieties (P < 0.001), locations (P < 0.001), and sampling date (P < 0.001) significantly affected nucleation temperature, whereas epiphytic bacterial abundance and xylem vessel diameter did not. Trees froze at higher temperatures in situ than in laboratory assays (1 to -2{degrees}C versus -3 to 4{degrees}C, respectively), indicating strong environmental modulation of freezing processes beyond Psy-like bacterial activity, which is reflected in contrasting disease susceptibilities (P < 0.001) and precocities (P < 0.001). These results shed light on the complexity of the freezing process in trees under natural conditions. We discuss the potential roles of microclimatic conditions and alternative ice nucleation sources beyond Psy-like bacteria in driving these physiological processes.

Xylem and bark properties influence tree growth and drought resistance, yet their functional coordination and their environmental drivers remain unclear. We assessed xylem-bark coordination in branches of eight temperate woody Rosaceae species spanning different ecological preferences. We quantified xylem hydraulic efficiency and safety alongside bark traits governing permeability, hygroscopic water exchange, water storage, and anatomy, and evaluated phylogenetic signal and climatic associations. Bark water vapor conductance (Gbark) increased with maximum xylem hydraulic conductivity (Kh) and with xylem water potential at 50% loss of conductivity (P50), indicating species with more efficient but more embolism-vulnerable xylem developed more permeable bark. Species with higher Gbark showed reduced hygroscopic absorption time, consistent with faster rehydration from atmospheric water vapor. Both Gbark and P50 were phylogenetically conserved and covaried with climatic factors, namely air temperature, vapor pressure deficit (VPD), and isothermality. Species from warmer, high-VPD climates with greater diurnal temperature variability combined higher bark permeability with more vulnerable xylem, implying a shift from embolism avoidance to embolism tolerance strategies. Overall, xylem and bark hydraulics in Rosaceae evolved in concert along diurnal and annual gradients of evaporative demand, showing that drought resistance in woody angiosperms cannot be understood without considering bark traits alongside xylem function. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=104 SRC="FIGDIR/small/730605v1_ufig1.gif" ALT="Figure 1"> View larger version (41K): org.highwire.dtl.DTLVardef@14872c2org.highwire.dtl.DTLVardef@1662c8corg.highwire.dtl.DTLVardef@f6b4aaorg.highwire.dtl.DTLVardef@cf1aef_HPS_FORMAT_FIGEXP M_FIG C_FIG Caption: This study shows that xylem and bark in Rosaceae species form an integrated functional system in which xylem hydraulic safety, efficiency, and bark permeability are jointly tuned along diurnal and annual gradients of air temperature and evaporative demand.

Trees are routinely pruned to mitigate the risk of wind damage, but there are few studies examining changes in wind loads after pruning, especially for large conifers. In this study, ten Colorado spruces (Picea pungens) were monitored before and after a series of pruning treatments. Trees were pruned to raise or thin crowns over a range of severities between 0% and 40%. Wind-induced bending moments were measured using two calibrated displacement probes installed orthogonally on the lower stem of each tree. Using a hierarchical Bayesian model, the relationship between maximum wind speeds and bending moments was quantified, consistent with theoretical and empirical expectations, as a non-linear power law. Random intercepts for model coefficients were used to account for individual variability in aerodynamic behavior among experimental trees, and predictions were made using the median response marginalized over the observed trees. The modeled relationship between wind speeds and bending moments was physically reasonable and like existing measurements with scaling exponents below two. Despite considerable variation among experimental trees, the aerodynamic behavior of trees, as indicated by model coefficients, was not clearly altered by pruning treatments, and, correspondingly, model predictions of bending moments over the range of observed wind speeds remained similar for all pruning treatments. Ultimately, the study yielded weak evidence for a change in bending moments following conventional pruning treatments for Colorado spruce, and the practical value of pruning to mitigate risk appeared limited for the studied conditions. Highlights- Wind loads were monitored on large Colorado spruce after crown raising and thinning - A hierarchical Bayesian model quantified wind speed and bending moment power laws - Negligible change in bending moments was found for all pruning types and severities - Conventional pruning methods may not mitigate risk for Colorado spruce

O_LIDuring the extreme drought period of 2018-2020, secondary pest attacks were observed in some declining beech trees (Fagus sylvatica). The main species identified are Agrilus viridis and Taphrorychus bicolor, which are known to attack beech trees more frequently after periods of intense drought. The aim of this study is to investigate if and how these two cambium-feeding insects in interaction with drought contribute to beech decline. We assessed the modification of the carbon reserves stock and carbon allocation to growth and fruiting in declining beech trees. C_LIO_LIPast radial growth, non-structural carbohydrates (NSC) concentrations in several organs and the quantity of beechnuts were measured in 38 beech trees with contrasting levels of biotic attack in 2020. Total amount of carbon reserves was calculated on the basis of NSC concentrations, and the amount of carbon allocated to growth and fruiting at the tree level was assessed using allometric relationships. C_LIO_LIBeech trees most severely attacked by secondary pests showed lower radial growth for last ten years than trees that have not been attacked. NSC concentrations and the amount of carbon reserve in the coarse roots and stems of trees were also lower in beech trees with intense biotic symptoms compared to unaffected trees. However, the amount of carbon allocated to growth and fruiting during the 2020 growing season did not differ significantly among biotic attack classes. C_LIO_LISynthesis: Regardless of biotic attack intensity, declining beech trees exhibited abnormally low carbon reserves. Trees most severely affected by insects in 2020 appeared to be the most vulnerable, as evidenced by reduced radial growth over the past decade. Future experiments are needed to explore further whether insects impacted directly carbon reserves, or whether low carbon reserves in trees is a vulnerability factor promoting insects attacks. Finally, declining beech trees allocated a similar amount of carbon to fruiting as reported in healthy stands, suggesting that fruiting is a prioritised carbon sink, even under severe drought and biotic attack. C_LI

BackgroundThe mechanism underlying drought-induced embolism in angiosperm xylem has been attributed to air-seeding. This concept describes the bulk flow of gas from embolised to neighbouring conduits through the penetration of gas-liquid menisci across pores in interconduit pit membranes. While there is compelling evidence for the spatial propagation of embolism, air-seeding rests on various simplifying assumptions. Among others, air-seeding presumes that xylem sap has physical properties comparable to pure water, that pit membranes can be approximated as structures with simple pores, and that embolism occurs whenever a gas-liquid interface crosses a pit membrane. ScopeRecent experimental and theoretical work demonstrates that the biophysical conditions and processes governing gas-liquid interactions at interconduit pit membranes are fundamentally more dynamic and complex than assumed by air-seeding. These phenomena include: (1) gas movement through constriction pore networks, (2) insoluble, polar lipids at conduit surfaces and interfaces, (3) dynamic surface tension of xylem sap that depends on the local packing density of interfacial lipids, (4) bubble snap-off dynamics within pit membranes, (5) surfactant-stabilized nanobubbles in sap that is oversaturated with dissolved gas, and (6) electrostatic interactions between charged interfaces. Importantly, embolism propagation involves bubble generation and embolism formation as distinct, temporarily and spatially separated processes. Embolism formation occurs when nanobubbles become unstable, whereas nanobubbles below critical stability thresholds can remain stable in sap-filled conduits. ConclusionsTogether, these findings reconfirm that pit membranes function as safety valves enabling water transport according to the cohesion-tension theory, and provide mechanistic insights into embolism propagation. They address the question why plants do not suffer constant embolism formation despite negative xylem pressures. We conclude that a revised framework explicitly accounting for the 3D structure of pit membranes, and multiphase, dynamic processes operating within them are required to explain the biophysics underlying water transport and embolism resistance in angiosperm xylem.

We tested whether seasonal plasticity in photosystem II (PSII) heat tolerance aligns with leaf phenology in tropical trees, comparing evergreen and deciduous species across the wettopostwet transition in a seasonally dry tropical forest of the central Western Ghats, India. This transition, preceding droughtinduced senescence, represents the key window to assess true unstressed thermal plasticity. PSII thermal traits: damage onset (T5), damage midpoint (T50), and temperature between damage onset to full loss (T95-T5), decline width (DW) were quantified in 27 cooccurring species during the wet (27.5{square}{degrees}C) and postwet (31.6{square}{degrees}C) periods. Contrary to phenologybased predictions, PSII plasticity was not structured by leaf habit or successional status. Both T5 (+1.7{square}{degrees}C) and T50 (+0.9{square}{degrees}C) increased significantly across seasons, but responses were speciesspecific, with evergreen and deciduous trees acclimating similarly. The preventionversusforbearance tradeoff (T5 - DW relationship) remained conserved, though leaf habits diverged under postwet conditions. Thermal safety margins based on T50 were large, but T5 revealed vulnerable latesuccessional evergreens (Saraca asoca, Ficus spp.) and Careya arborea. These results show that PSII thermotolerance regulation operates largely independently of droughtavoidance phenology, indicating species identity and not leaf habit drives plasticity in PSII thermal response in seasonally dry tropical forests.

The article introduces a new Forest Stress Index (ISF) based on a plant hydraulic modelling approach rather than classical climatic drought indices. Unlike other index like scPDSI or SPEI, ISF is grounded in xylem embolism dynamics simulated with the mechanistic SurEau model. The goal is to better link climatic anomalies to tree physiological functioning and mortality risk. ISF is defined using a locally adapted ideotype characterized by an optimal P50 value under a reference hydraulic functioning threshold. Simulations are performed across Europe and France using multiple climate datasets. The index is robust to model parameterization choices and assumptions about plant functional traits. Results show strong spatial and temporal consistency and significant correlations with SPEI and scPDSI. However, ISF more strongly highlights extreme drought years and exhibits a more skewed distribution. Future projections under SSP5-8.5 indicate a widespread increase in hydraulic stress with strong regional contrasts. Overall, ISF provides a mechanistic and complementary drought indicator more directly linked to forest mortality processes.

O_LIEuropean beech (Fagus sylvatica L.) is a widely distributed, ecologically and economically important deciduous tree species in European forests, but is increasingly threatened by drought stress. Volatile organic compounds (VOCs) are ubiquitous plant metabolites that may serve as non-invasive biomarkers of drought stress, yet they have rarely been studied in European beech. C_LIO_LIIn this study, we examined VOC responses of European beech to experimental drought across diverse genetic backgrounds in a common garden. The 72 four-year-old beech saplings represented three genetic clusters, seven provenances (geographic seed sources), and 12 maternal seed families. Half of the saplings were assigned to the drought treatment and received no water for 14 days, while the remaining saplings served as controls and were watered as required. VOC profiles, quantified as peak heights of mass spectral features, were measured for all individuals during pre-drought, drought, and rewatering periods. C_LIO_LIWe found that pre-drought VOC profiles, in particular monoterpenes, varied significantly among genetic backgrounds. Experimental drought significantly altered VOC profiles, characterized by increased green leaf volatiles and decreased monoterpenes, oxidized terpenoid derivatives, and other fatty acid derivatives. Reductions in monoterpenes persisted after rewatering, indicating a drought legacy effect. Drought responses were largely conserved across genetic backgrounds, with significant seed family-specific responses detected for only three VOC features. C_LIO_LIOur findings suggest that VOC profiles are genetically structured yet highly plastic under drought and highlight their potential as non-invasive biomarkers for monitoring drought stress in European beech under climate change. C_LI

Temperate forests increasingly face extreme air temperature, but plant physiological responses, particularly alterations in carbon allocation or protection via volatile organic compound (VOC) emissions, remain poorly understood. We pulse-labelled well-watered saplings of Fagus sylvatica and Pseudotsuga menziesii in a controlled heat stress experiment with 13CO2 to quantify heat-induced shifts in CO2, VOC and C pool exchange, specifically analyzing compound-specific {delta}13C of terpenoids, water-soluble organic matter (WSOM) and dark respiration. Under heat stress, up to 50% of fresh assimilates were directed to maintenance respiration and 1-2% to VOC emissions, while net assimilation and water use efficiency decreased by 50-75% in both species. Heat directly affected metabolic processes and reduced turnover rates of fresh assimilates in F. sylvatica, but accelerated them in P. menziesii. Strong 13C labelling of some compounds, particularly acyclic ones, suggested increased de novo synthesis of specific terpenoids for heat stress protection. By tracing the fate of recently assimilated 13CO2 we demonstrate that heat stress reduces net carbon uptake and water use efficiency, disrupts turnover of C pools and increases carbon loss via respiration and de novo synthesis of specific VOCs, potentially diminishing net carbon uptake of forests under future heat extremes.

Rising winter temperatures threaten deciduous fruit tree productivity by depleting carbohydrate reserves during dormancy. This study investigated Stem Photosynthetic Capacity (SPC), a rare adaptive trait from wild Prunus arabica, as a mechanism to enhance almond carbon economy. Using extreme segregating groups from the F1 population (P. dulcis X P. arabica), we evaluated physiological performance through high-resolution lysimetric and multi-year orchard monitoring. High-SPC [SPC(+)] genotypes maintained significantly greater stem CO2 assimilation and transpiration during leafless periods compared to low-SPC [SPC(-)] progenies. Over five successive seasons, SPC(+) trees exhibited a 33.3% increase in trunk secondary growth and reached 10% bloom approximately 8 days earlier. Most importantly, the SPC(+) group achieved a 4.6-fold increase in mean kernel yield when compared to SPC(-) group. These findings demonstrate that SPC provides a flexible, supplementary winter carbon source that directly supports both vegetative and reproductive development. Integrating SPC into commercial almond breeding programs may offer a valuable strategy to improve climate resilience and help sustain yields under warming conditions. HighlightIntegrating stem photosynthesis into commercial almond hybrids provides a winter carbon source that advances blooming, expands trunk growth by [~]33%, and increases kernel yields by more than 4.5-fold.

O_LIThe heat tolerance of tree species has recently received increased attention, as it is critical for the response of forest ecosystems to climate change. Most studies rely on laboratory experiments with detached leaves, where the thermostability of photosystem II (PS II) is analyzed using chlorophyll fluorescence analysis. The temperature at which the maximum quantum yield (Fv/Fm) of PS II is reduced by 50% (T50) is often used as key measure of heat tolerance despite of weak mechanistic corroboration and inconsistent definitions. We propose a new metric consisting of a critical temperature (TIP) and a corresponding fluorescence value (FIP). C_LIO_LIWe analyzed the effect of incubation time and temperature on different versions of T50. C_LIO_LIT50 was strongly dependent on the duration of heat exposure, decreasing exponentially with incubation time. T50 and TIP values derived from short-term treatments of different incubation times <1 h are not comparable, but stabilize after longer intervals of heat exposure. C_LIO_LIT50 of Fv/Fm specified without considering incubation time is a meaningless metric. Specifications derived from short-term heat treatments should be avoided, because the result is highly influenced by the experimental setup. Different calculation methods for T50 influence the result and elucidate different aspects of the heat response. C_LI

Internal decay in living trees is an important component of carbon and nutrient cycling as well as species and structural diversity maintenance in forest ecosystems. We used sonic and electrical resistance tomography to evaluate and compare the prevalence and severity of stem decay in 57 living trees among four common species (Acer rubrum L., Nyssa sylvatica Marsh., Quercus rubra L., and Tsuga canadensis (L.) Carriere)) with overlapping and non-overlapping distributions across wetland and upland habitat types at the Harvard Forest in Petersham, MA, USA. Independent of tree size, site identity best explained variation in the prevalence of decay across trees sampled, whereas species identity best explained the severity of decay. We categorized trees as having no decay, incipient decay, active decay, or cavities based on combined sonic and electrical resistance metrics, the latter generated by a custom image analysis application. About 31% of wetland trees exhibited incipient decay (compared to 11% in the upland), whereas about 32% of upland trees exhibited active decay (compared to 10% in the wetland). Our study highlights a new quantitative framework for decay categorization through normalized principal component analysis (PCA) and decay analysis software that complements dual tomographic methodology for future investigations of ecological drivers of decay presence and susceptibility.

Direct heat damage has been considered secondary as a cause of climate change-induced tree mortality and productivity declines in forests compared with climate change effects on tree water relations. However, evidence from temperate and tropical forests is accumulating that direct heat damage in the photosystem II (PS II) that is independent of water relations is also a realistic scenario under climate change. We analyzed PS II heat tolerance in Larix sibirica, which represents a dominant boreal tree species in Siberia and northern Central Asia in cold environments with subzero or near-zero mean annual temperatures, but nevertheless warm summers. Thermal imaging was applied to relate heat thresholds found in the laboratory to canopy temperatures in forests on north-facing mountain slopes, which are the main habitat of L. sibirica. L. sibirica showed slight decreases of the maximum quantum yield of PS II (Fv/Fm) at 35{degrees}C and 40{degrees}C after up to 4 h, but strong reductions at [&ge;]45{degrees}C and minor increases in Fv/Fm in late summer, which could be interpretation as heat acclimation. Canopy temperatures in the study year did not reach the thresholds for serious PS II heat damage. However, L. sibirica was more strongly sensitive to heat than temperate conifers. This first combined study of heat tolerance and canopy temperatures from boreal forests points to the possibility of low heat tolerance of boreal tree species, but such conclusion would require the study of more tree species.

Dendrometer-derived tree water deficit (TWD) contains physiologically rich information and is increasingly used to monitor tree drought stress, yet process-based forest models rarely include a directly comparable representation of TWD dynamics. Existing hydraulic models can represent internal water storage in detail, but their parameter demands limit broader application. Here, we introduce the Tree Water Imbalance and Storage Tracker (TWIST), a parsimonious and physiologically interpretable framework that derives volume-based TWD dynamics. The module is driven by transpiration and relative soil water content and uses three empirical parameters to control transpiration-driven internal water depletion, deficit refilling, and additional soil-water uptake limitation. It also derives relative tree water content (RWCtree) from the simulated deficit and an estimate of the available internal water pool. We tested TWIST by coupling it to the process-based ecosystem model LandscapeDNDC. Parameters were optimized for 2018 and evaluated independently for 2019-2024 against normalized dendrometer-derived TWD at a Czech beech site. Simulated TWD trajectories broadly agreed with observed daily and seasonal dynamics, while RWCtree translated them into a physiologically interpretable proxy for internal dehydration. TWIST demonstrated capability to reproduce key TWD drought-response patterns, including diurnal depletion-replenishment cycles, reduced nocturnal rehydration with declining soil moisture, and progressive deficit accumulation. By representing TWD and RWCtree as diagnostic model outputs, TWIST makes dendrometer-derived drought-stress information more directly usable in forest models. It thereby provides a practical basis for linking tree-level drought-stress signals with stand-level simulations and, potentially, remotely sensed indicators of canopy water status.

Global climate change has increased the frequency and intensity of drought events, making it urgent to understand how native species respond to water deficit (WD). In biodiverse environments such as tropical forests, simple methods are needed to study multiple species simultaneously. This can help predict how natural environments will respond to climate change and guide the strategic selection of drought-resistant species for reforestation. This study aimed to: (1) adapt an existing simple and inexpensive method to apply a controlled WD on tree seedlings from tropical species commonly produced in nurseries for restoration projects, suitable for greenhouse experiments; and (2) evaluate the effectiveness of this method in generating ecophysiological responses to WD that allow the estimation of species' drought resistance. Ten native tree species from the Semideciduous Seasonal Forest (SSF), a phytophysiognomy of the Atlantic Forest, were selected. An existing method was adapted to implement capillary irrigation, in which the bases of the seedling tubes were placed in floral foam blocks positioned inside 15 L plastic containers filled with water. A gradual and severe WD was applied to five seedlings of each species by removing all water from the containers, leaving only the water retained in the saturated floral foam available for plant uptake. The remaining seedlings were maintained well-watered (containers full and foam saturated) as the control group. Stomatal conductance (gs) was measured daily for all seedlings until they reached 50% or less of their initial gs (igs); at this point, stem water potential ({Psi}w) was measured. Both gs and {Psi}w differed significantly among treatments and species (p < 0.01). Ficus guaranitica and Heliocarpus popayanensis were the only species that did not show significant {Psi}w differences between treatments, indicating higher drought resistance. In contrast, Campomanesia xanthocarpa and Eugenia uniflora had the lowest {Psi}w values under WD, suggesting lower drought resistance. The remaining species were distributed along a gradient of responses to WD. Additionally, no correlation was found between {Psi}w and gs at 50% igs in the WD group (rho = 0.16, p = 0.26). The method proved effective in inducing controlled WD and generating measurable ecophysiological responses, offering a useful tool for screening native species for drought resistance.

Photorespiration significantly influences terrestrial carbon fluxes, yet empirical measurements of its variability across tree species and temperature conditions remain limited, constraining predictions of vegetation and climate models. We quantified apparent photorespiratory CO2 loss (Lapp) and its temperature response for seven temperate broadleaf tree species in northern Europe, using in situ O2-shift measurements in Uppsala, Sweden during peak summer. Apparent loss was derived as the difference between net CO2 assimilation under ambient (Anet) and O2-free conditions at three leaf temperatures (25, 30, and 35 {degrees}C), spanning typical and heat-wave scenarios. Apparent photorespiratory CO2 loss showed pronounced interspecific variation and increased with temperature, while net photosynthesis remained relatively stable. The ratio of apparent loss to net photosynthesis ({phi} = Lapp/Anet) rose sharply with temperature, reaching species-mean values up to 0.94 at 35 {degrees}C, indicating that photorespiration can represent nearly the entirety of net carbon gain under heat stress even when leaves remain net CO2 sinks. Suppression of photorespiration under N2 and associated changes in leaf temperature systematically reallocated photosynthetic electron transport: the fraction of ambient electron transport rate (ETR) allocated to net CO2 assimilation declined with temperature, whereas the complementary fraction allocated to apparent photorespiratory loss and other O2-dependent sinks increased, with ETR-based apparent loss and its proportional expression rising steeply across the 25-35 {degrees}C range. Together, these in situ flux and partitioning measurements reveal high variability and strong temperature sensitivity in apparent photorespiration among temperate trees. Compared to crop-based parameterisations, the {phi} values we report for temperate trees are substantially higher and more temperature-dependent, providing species-specific constraints that can improve Farquhar-von Caemmerer-Berry-type vegetation model representations of photorespiration in forest ecosystems.

Tropospheric ozone (O) is a major atmospheric pollutant that affects plant carbon metabolism, redox homeostasis, and secondary metabolism, including the biosynthesis and emission of biogenic volatile organic compounds (BVOCs). However, the contribution of BVOCs to O3 tolerance, particularly in tropical woody species, remains poorly understood. Here, we investigated whether acute O exposure (cumulative AOT40 of 3497.82 ppb h) induces alterations in photosynthetic performance, redox homeostasis, and BVOC partitioning in Eugenia uniflora. We evaluated gas exchange, photosynthetic pigments, ascorbate and glutathione pools, emitted BVOCs, modeled intercellular BVOC concentrations, and the relative carbon cost associated with BVOC emissions. O exposure significantly increased net CO2 assimilation without affecting stomatal conductance, transpiration, leaf water status, or chlorophyll concentrations, indicating maintenance of photosynthetic performance. Carotenoid concentrations and total glutathione decreased, whereas glutathione redox status was maintained. O induced marked compound-specific changes in BVOC composition and partitioning. Several monoterpenes appeared exclusively under O exposure, {gamma}-elemene emission increased significantly, and the relative distribution of individual BVOCs between the modeled intercellular and emitted pools was altered. These findings show that the response of E. uniflora to acute O exposure was characterized by interplay among carbon assimilation, glutathione redox regulation, and BVOC partitioning rather than by increased total volatile emission. Enhanced carbon assimilation occurred without additional carbon loss through BVOC release, while changes in the modeled intercellular pool indicate that part of the volatile response remained within the leaf. Our findings highlight BVOC partitioning as an important dimension of the plant response to oxidative stress and demonstrate that emission measurements alone may not fully capture the fate and potential physiological role of volatile carbon under O exposure. O_FIG O_LINKSMALLFIG WIDTH=178 HEIGHT=200 SRC="FIGDIR/small/743946v1_ufig1.gif" ALT="Figure 1"> View larger version (52K): org.highwire.dtl.DTLVardef@653af1org.highwire.dtl.DTLVardef@ca5forg.highwire.dtl.DTLVardef@1e641bforg.highwire.dtl.DTLVardef@1e68fae_HPS_FORMAT_FIGEXP M_FIG C_FIG BVOC Partitioning Contributes to Oxidative Stress Defence Under Acute O Exposure

Chronic infection by vascular pathogens is conventionally expected to severely constrain host biomass accumulation, yet empirical evidence from long-lived woody plants remains inconsistent. We investigated the long-term impacts of Xylella fastidiosa colonization on the radial growth and climate sensitivity of adult Mediterranean almond trees (Prunus dulcis), aiming to resolve how persistent vascular infections modulate tree performance and resilience under a changing climate. We coupled high-resolution dendrochronological analysis with a novel, ring-resolved molecular reconstruction of individual infection histories across 706 annual rings. This hindcasting approach allowed for the retrospective identification of precise colonization dates, bacterial loads (Ct values), and pathogen subspecies (subsp. fastidiosa vs. subsp. multiplex). Growth-climate relationships were modelled using standardized ring-width indices (RWI) against a crop-weighted water deficit index (CWDi). Intra-host colonization followed a steep radial gradient, with active bacterial abundance concentrated in newly formed, functional outer xylem. Surprisingly, chronic infection did not trigger a sustained reduction in baseline annual ring width. Instead, pathogens fundamentally reshaped climate-growth sensitivity. Hosts infected by subsp. fastidiosa maintained high plastic growth tracking during wet years, whereas this capacity was significantly attenuated in those harbouring subsp. multiplex. Despite the absence of a chronic signal in trunk radial growth, vascular impairment was tightly associated with severe retrograde canopy dieback. Our findings indicate that chronic infection by X. fastidiosa can act as a latent biotic stressor, altering host physiological sensitivity to environmental fluctuations without directly suppressing baseline stem growth. This pattern is consistent with the marked temporal decoupling between spring cambial activity and late-summer bacterial proliferation, together with progressive vascular dysfunction leading to severe canopy dieback. These results suggest that current abiotic-centred frameworks of drought-induced decline may underestimate the contribution of cryptic vascular pathogens to vegetation mortality under intensifying climate change.