Triggers of tree mortality under drought.

Severe droughts have caused widespread tree mortality across many forest biomes with profound effects on the function of ecosystems and carbon balance. Climate change is expected to intensify regional-scale droughts, focusing attention on the physiological basis of drought-induced tree mortality. Recent work has shown that catastrophic failure of the plant hydraulic system is a principal mechanism involved in extensive crown death and tree mortality during drought, but the multi-dimensional response of trees to desiccation is complex. Here we focus on the current understanding of tree hydraulic performance under drought, the identification of physiological thresholds that precipitate mortality and the mechanisms of recovery after drought. Building on this, we discuss the potential application of hydraulic thresholds to process-based models that predict mortality.

Cross-sectional study of the prevalence of hyperextension of the first metatarsophalangeal joint and its relationship to onycholysis in women with hallux valgus.

BACKGROUND: Hallux Valgus (HV) deformity is associated with misalignment in the sagittal plane that affects the first toe. However, the repercussions of the first toe hyperextension in HV have been scarcely considered. The purpose of this study was to provide evidence of the association between first-toe hyperextension and the risk of first toenail onycholysis in HV. METHODS: A total of 248 HV from 129 females were included. The extension of 1st MTP joint was measured while the patient was in the neutral position of the hallux using a two-branch goniometer. The classification of the HV severity stage was determined by the Manchester visual scale, and the height of the first toe in the standing position was measured using a digital meter. An interview and clinical examination were performed to collect information on the presence of onycholysis of the first toe. RESULTS: Of the 248 HV studied, 100 (40.3%) had onycholysis. A neutral extension > 30 degrees was noted in 110 (44.3%) HV. The incidence of onycholysis was higher in HV type C than in type B (p = 0.044). The probability of suffering onycholysis in the right foot was 2.3 times greater when the neutral position was higher than 30 degrees (OR = 2.3; p = 0.004). However, this was not observed in the left foot (p = 0.171). Onycholysis was more frequent in HV with more than 2 cm height of the first toe (p < 0.001). For both feet, the probability of suffering onycholysis was greater for each unit increase in hallux height (right foot OR = 9.0402, p = 0.005; left foot OR = 7.6633, p = 0.010). CONCLUSIONS: The incidence of onycholysis appears to be significantly associated with HV showing more than 30º extension, and more than 2 cm height of the first toe. Height and hyperextension of the first toe together with first toenail pathology should be mandatory in the evaluation of HV.

Rhizobia induce SYMRK endocytosis in Phaseolus vulgaris root hair cells.

MAIN CONCLUSION: PvSYMRK-EGFP undergoes constitutive and rhizobia-induced endocytosis, which rely on the phosphorylation status of T589, the endocytic YXXØ motif and the kinase activity of the receptor. Legume-rhizobia nodulation is a complex developmental process. It initiates when the rhizobia-produced Nod factors are perceived by specific LysM receptors present in the root hair apical membrane. Consequently, SYMRK (Symbiosis Receptor-like Kinase) becomes active in the root hair and triggers an extensive signaling network essential for the infection process and nodule organogenesis. Despite its relevant functions, the underlying cellular mechanisms involved in SYMRK signaling activity remain poorly characterized. In this study, we demonstrated that PvSYMRK-EGFP undergoes constitutive and rhizobia-induced endocytosis. We found that in uninoculated roots, PvSYMRK-EGFP is mainly associated with the plasma membrane, although intracellular puncta labelled with PvSymRK-EGFP were also observed in root hair and nonhair-epidermal cells. Inoculation with Rhizobium etli producing Nod factors induces in the root hair a redistribution of PvSYMRK-EGFP from the plasma membrane to intracellular puncta. In accordance, deletion of the endocytic motif YXXØ (YKTL) and treatment with the endocytosis inhibitors ikarugamycin (IKA) and tyrphostin A23 (TyrA23), as well as brefeldin A (BFA), drastically reduced the density of intracellular PvSYMRK-EGFP puncta. A similar effect was observed in the phosphorylation-deficient (T589A) and kinase-dead (K618E) mutants of PvSYMRK-EGFP, implying these structural features are positive regulators of PvSYMRK-EGFP endocytosis. Our findings lead us to postulate that rhizobia-induced endocytosis of SYMRK modulates the duration and amplitude of the SYMRK-dependent signaling pathway.

Mechanisms of xylem hydraulic recovery after drought in Eucalyptus saligna.

The mechanisms by which woody plants recover xylem hydraulic capacity after drought stress are not well understood, particularly with regard to the role of embolism refilling. We evaluated the recovery of xylem hydraulic capacity in young Eucalyptus saligna plants exposed to cycles of drought stress and rewatering. Plants were exposed to moderate and severe drought stress treatments, with recovery monitored at time intervals from 24 h to 6 months after rewatering. The percentage loss of xylem vessels due to embolism (PLV) was quantified at each time point using microcomputed tomography with stem water potential (Ψx ) and canopy transpiration (Ec ) measured before scans. Plants exposed to severe drought stress suffered high levels of embolism (47.38% ± 10.97% PLV) and almost complete canopy loss. No evidence of embolism refilling was observed at 24 h, 1 week, or 3 weeks after rewatering despite rapid recovery in Ψx . Recovery of hydraulic capacity was achieved over a 6-month period by growth of new xylem tissue, with canopy leaf area and Ec recovering over the same period. These findings indicate that E. saligna recovers slowly from severe drought stress, with potential for embolism to persist in the xylem for many months after rainfall events.

Leaf water potential measurements using the pressure chamber: Synthetic testing of assumptions towards best practices for precision and accuracy.

Leaf water potential (ψleaf ), typically measured using the pressure chamber, is the most important metric of plant water status, providing high theoretical value and information content for multiple applications in quantifying critical physiological processes including drought responses. Pressure chamber measurements of ψleaf (ψleafPC ) are most typical, yet, the practical complexity of the technique and of the underlying theory has led to ambiguous understanding of the conditions to optimize measurements. Consequently, specific techniques and precautions diversified across the global research community, raising questions of reliability and repeatability. Here, we surveyed specific methods of ψleafPC from multiple laboratories, and synthesized experiments testing common assumptions and practices in ψleafPC for diverse species: (i) the need for equilibration of previously transpiring leaves; (ii) leaf storage before measurement; (iii) the equilibration of ψleaf for leaves on bagged branches of a range of dehydration; (iv) the equilibration of ψleaf across the lamina for bagged leaves, and the accuracy of measuring leaves with artificially 'elongated petioles'; (v) the need in ψleaf measurements for bagging leaves and high humidity within the chamber; (vi) the need to avoid liquid water on leaf surfaces; (vii) the use of 'pulse' pressurization versus gradual pressurization; and (viii) variation among experimenters in ψleafPC determination. Based on our findings we provide a best practice protocol to maximise accuracy, and provide recommendations for ongoing species-specific tests of important assumptions in future studies.

Visualization of the Crossroads between a Nascent Infection Thread and the First Cell Division Event in Phaseolus vulgaris Nodulation.

The development of a symbiotic nitrogen-fixing nodule in legumes involves infection and organogenesis. Infection begins when rhizobia enter a root hair through an inward structure, the infection thread (IT), which guides the bacteria towards the cortical tissue. Concurrently, organogenesis takes place by inducing cortical cell division (CCD) at the infection site. Genetic analysis showed that both events are well-coordinated; however, the dynamics connecting them remain to be elucidated. To visualize the crossroads between IT and CCD, we benefited from the fact that, in Phaseolus vulgaris nodulation, where the first division occurs in subepidermal cortical cells located underneath the infection site, we traced a Rhizobium etli strain expressing DsRed, the plant cytokinesis marker YFP-PvKNOLLE, a nuclear stain and cell wall auto-fluorescence. We found that the IT exits the root hair to penetrate an underlying subepidermal cortical (S-E) cell when it is concluding cytokinesis.

Coordination of stem and leaf traits define different strategies to regulate water loss and tolerance ranges to aridity.

Adaptation to drought involves complex interactions of traits that vary within and among species. To date, few data are available to quantify within-species variation in functional traits and they are rarely integrated into mechanistic models to improve predictions of species response to climate change. We quantified intraspecific variation in functional traits of two Hakea species growing along an aridity gradient in southeastern Australia. Measured traits were later used to parameterise the model SurEau to simulate a transplantation experiment to identify the limits of drought tolerance. Embolism resistance varied between species but not across populations. Instead, populations adjusted to drier conditions via contrasting sets of trait trade-offs that facilitated homeostasis of plant water status. The species from relatively mesic climate, Hakea dactyloides, relied on tight stomatal control whereas the species from xeric climate, Hakea leucoptera dramatically increased Huber value and leaf mass per area, while leaf area index (LAI) and epidermal conductance (gmin ) decreased. With trait variability, SurEau predicts the plasticity of LAI and gmin buffers the impact of increasing aridity on population persistence. Knowledge of within-species variability in multiple drought tolerance traits will be crucial to accurately predict species distributional limits.

Living on the edge: A continental-scale assessment of forest vulnerability to drought.

Globally, forests are facing an increasing risk of mass tree mortality events associated with extreme droughts and higher temperatures. Hydraulic dysfunction is considered a key mechanism of drought-triggered dieback. By leveraging the climate breadth of the Australian landscape and a national network of research sites (Terrestrial Ecosystem Research Network), we conducted a continental-scale study of physiological and hydraulic traits of 33 native tree species from contrasting environments to disentangle the complexities of plant response to drought across communities. We found strong relationships between key plant hydraulic traits and site aridity. Leaf turgor loss point and xylem embolism resistance were correlated with minimum water potential experienced by each species. Across the data set, there was a strong coordination between hydraulic traits, including those linked to hydraulic safety, stomatal regulation and the cost of carbon investment into woody tissue. These results illustrate that aridity has acted as a strong selective pressure, shaping hydraulic traits of tree species across the Australian landscape. Hydraulic safety margins were constrained across sites, with species from wetter sites tending to have smaller safety margin compared with species at drier sites, suggesting trees are operating close to their hydraulic thresholds and forest biomes across the spectrum may be susceptible to shifts in climate that result in the intensification of drought.

Rhizobium etli CE3-DsRed pMP604: a useful biological tool to study initial infection steps in Phaseolus vulgaris nodulation.

MAIN CONCLUSION: Rhizobium etli CE3-DsRed pMP604 drives infection 12-24 h earlier than R. etli CE3-DsRed and it is an excellent tool in live-cell imaging studies of IT developement in P. vulgaris roots. The study of the cellular dynamics of nodulation has frequently been limited by the difficulty of performing live-cell imaging in nodule primordia and legume roots, which are constituted by multiple cell layers, such is the case of Phaseolus vulgaris. Seeking conditions to reduce the time it takes for rhizobia to infect P. vulgaris root, we decided to explore the nodulation properties of Rhizobium etli CE3 pMP604, a strain that constitutively produces Nod factors through a flavonoids-independent transcriptional activation which is often used to purify Nod factors. Even though the strain infects 12-24 h earlier than the parental R. etli CE3 strain, infection thread (IT) formation, nodule organogenesis processes and N2-fixation activity are similar for both strains. Additionally, we have confirmed that R. etli CE3-DsRed pMP604 is an excellent tool to trace IT development in P. vulgaris roots.

Correlated evolution of morphology, gas exchange, growth rates and hydraulics as a response to precipitation and temperature regimes in oaks (Quercus).

It is hypothesised that tree distributions in Europe are largely limited by their ability to cope with the summer drought imposed by the Mediterranean climate in the southern areas and by their competitive potential in central regions with more mesic conditions. We investigated the extent to which leaf and plant morphology, gas exchange, leaf and stem hydraulics and growth rates have evolved in a coordinated way in oaks (Quercus) as a result of adaptation to contrasting environmental conditions in this region. We implemented an experiment in which seedlings of 12 European/North African oaks were grown under two watering treatments, a well-watered treatment and a drought treatment in which plants were subjected to three cycles of drought. Consistent with our hypothesis, species from drier summers had traits conferring more tolerance to drought such as small sclerophyllous leaves and lower percent loss of hydraulic conductivity. However, these species did not have lower growth rates as expected by a trade-off with drought tolerance. Overall, our results revealed that climate is an important driver of functional strategies in oaks and that traits have evolved along two coordinated functional axes to adapt to different precipitation and temperature regimes.

Visual and hydraulic techniques produce similar estimates of cavitation resistance in woody species.

Hydraulic failure of the plant vascular system is a principal cause of forest die-off under drought. Accurate quantification of this process is essential to our understanding of the physiological mechanisms underpinning plant mortality. Imaging techniques increasingly are applied to estimate xylem cavitation resistance. These techniques allow for in situ measurement of embolism formation in real time, although the benefits and trade-offs associated with different techniques have not been evaluated in detail. Here we compare two imaging methods, microcomputed tomography (microCT) and optical vulnerability (OV), to standard hydraulic methods for measurement of cavitation resistance in seven woody species representing a diversity of major phylogenetic and xylem anatomical groups. Across the seven species, there was strong agreement between cavitation resistance values (P50 ) estimated from visualization techniques (microCT and OV) and between visual techniques and hydraulic techniques. The results indicate that visual techniques provide accurate estimates of cavitation resistance and the degree to which xylem hydraulic function is impacted by embolism. Results are discussed in the context of trade-offs associated with each technique and possible causes of discrepancy between estimates of cavitation resistance provided by visual and hydraulic techniques.

Non-invasive imaging reveals convergence in root and stem vulnerability to cavitation across five tree species.

Root vulnerability to cavitation is challenging to measure and under-represented in current datasets. This gap limits the precision of models used to predict plant responses to drought because roots comprise the critical interface between plant and soil. In this study, we measured vulnerability to drought-induced cavitation in woody roots and stems of five tree species (Acacia aneura, Cedrus deodara, Eucalyptus crebra, Eucalytus saligna, and Quercus palustris) with a wide range of xylem anatomies. X-ray microtomography was used to visualize the accumulation of xylem embolism in stems and roots of intact plants that were naturally dehydrated to varying levels of water stress. Vulnerability to cavitation, defined as the water potential causing a 50% loss of hydraulic function (P50), varied broadly among the species (-4.51 MPa to -11.93 MPa in stems and -3.13 MPa to -9.64 MPa in roots). The P50 of roots and stems was significantly related across species, with species that had more vulnerable stems also having more vulnerable roots. While there was strong convergence in root and stem vulnerability to cavitation, the P50 of roots was significantly higher than the P50 of stems in three species. However, the difference in root and stem vulnerability for these species was small; between 1% and 31% of stem P50. Thus, while some differences existed between organs, roots were not dramatically more vulnerable to embolism than stems, and the differences observed were less than those reported in previous studies. Further study is required to evaluate the vulnerability across root orders and to extend these conclusions to a greater number of species and xylem functional types.

The temperature optima for tree seedling photosynthesis and growth depend on water inputs.

Understanding how tree growth is affected by rising temperature is a key to predicting the fate of forests in future warmer climates. Increasing temperature has direct effects on plant physiology, but there are also indirect effects of increased water limitation because evaporative demand increases with temperature in many systems. In this study, we experimentally resolved the direct and indirect effects of temperature on the response of growth and photosynthesis of the widely distributed species Eucalyptus tereticornis. We grew E. tereticornis in an array of six growth temperatures from 18 to 35.5°C, spanning the climatic distribution of the species, with two watering treatments: (a) water inputs increasing with temperature to match plant demand at all temperatures (Wincr ), isolating the direct effect of temperature; and (b) water inputs constant for all temperatures, matching demand for coolest grown plants (Wconst ), such that water limitation increased with growth temperature. We found that constant water inputs resulted in a reduction of temperature optima for both photosynthesis and growth by ~3°C compared to increasing water inputs. Water limitation particularly reduced the total amount of leaf area displayed at Topt and intermediate growth temperatures. The reduction in photosynthesis could be attributed to lower leaf water potential and consequent stomatal closure. The reduction in growth was a result of decreased photosynthesis, reduced total leaf area display and a reduction in specific leaf area. Water availability had no effect on the response of stem and root respiration to warming, but we observed lower leaf respiration rates under constant water inputs compared to increasing water inputs at higher growth temperatures. Overall, this study demonstrates that the indirect effect of increasing water limitation strongly modifies the potential response of tree growth to rising global temperatures.

On the minimum leaf conductance: its role in models of plant water use, and ecological and environmental controls.

Contents Summary 693 I. Introduction 693 II. Comparison of various definitions and measurement techniques of minimum conductance 694 III. Cuticular conductance 695 IV. Contribution of stomata 696 V. Environmental and ecological variation in minimum conductance 696 VI. Use of minimum conductance in models 698 VII. Conclusions 703 Acknowledgements 703 References 703 SUMMARY: When the rate of photosynthesis is greatly diminished, such as during severe drought, extreme temperature or low light, it seems advantageous for plants to close stomata and completely halt water loss. However, water loss continues through the cuticle and incompletely closed stomata, together constituting the leaf minimum conductance (gmin ). In this review, we critically evaluate the sources of variation in gmin , quantitatively compare various methods for its estimation, and illustrate the role of gmin in models of leaf gas exchange. A literature compilation of gmin as measured by the weight loss of detached leaves is presented, which shows much variation in this trait, which is not clearly related to species groups, climate of origin or leaf type. Much evidence points to the idea that gmin is highly responsive to the growing conditions of the plant, including soil water availability, temperature and air humidity - as we further demonstrate with two case studies. We pay special attention to the role of the minimum conductance in the Ball-Berry model of stomatal conductance, and caution against the usual regression-based method for its estimation. The synthesis presented here provides guidelines for the use of gmin in ecosystem models, and points to clear research gaps for this drought tolerance trait.

An Improved Version of the Classical Banister Model to Predict Changes in Physical Condition.

In this paper, we formulate and provide the solutions to two new models to predict changes in physical condition by using the information of the training load of an individual. The first model is based on a functional differential equation, and the second one on an integral differential equation. Both models are an extension to the classical Banister model and allow to overcome its main drawback: the variations in physical condition are influenced by the training loads of the previous days and not only of the same day. Finally, it is illustrated how the first model works with a real example of the training process of a cyclist.

[Clinical and epidemiological characteristics of patients with paracoccidioidomycosis diagnosed in a hospital of Resistencia, Chaco]. / Características clínicas y epidemiológicas de pacientes con paracoccidioidomicosis diagnosticados en un hospital de Resistencia, Chaco.

In order to describe the clinical and epidemiological characteristics of paracoccidioidomycosis, a descriptive study of all the cases diagnosed by the Clinical Microbiology Service at Dr. Julio C. Perrando hospital in the city of Resistencia (Chaco Province, Argentina) was conducted. Between 2011 and 2014, 46 cases were detected. In the period 2013-2014, an almost 4-fold increase in the incidence rate was detected. The chronic form of the disease was predominant with an average age of 53 years. Serological tests in 39 out of 46 patients were performed. In 15 of 39 patients, serological tests were the only diagnostic tool while in 4 patients with a microbiological diagnosis serological tests were non-reactive. In patients from endemic areas with non-specific infectious syndrome it is important to include paracoccidioidomycosis in the differential diagnosis and to apply all available diagnostic tools to reach a timely diagnosis and to reduce the long-term sequelae and their socio-economic impact.

Trees tolerate an extreme heatwave via sustained transpirational cooling and increased leaf thermal tolerance.

Heatwaves are likely to increase in frequency and intensity with climate change, which may impair tree function and forest C uptake. However, we have little information regarding the impact of extreme heatwaves on the physiological performance of large trees in the field. Here, we grew Eucalyptus parramattensis trees for 1 year with experimental warming (+3°C) in a field setting, until they were greater than 6 m tall. We withheld irrigation for 1 month to dry the surface soils and then implemented an extreme heatwave treatment of 4 consecutive days with air temperatures exceeding 43°C, while monitoring whole-canopy exchange of CO2 and H2 O, leaf temperatures, leaf thermal tolerance, and leaf and branch hydraulic status. The heatwave reduced midday canopy photosynthesis to near zero but transpiration persisted, maintaining canopy cooling. A standard photosynthetic model was unable to capture the observed decoupling between photosynthesis and transpiration at high temperatures, suggesting that climate models may underestimate a moderating feedback of vegetation on heatwave intensity. The heatwave also triggered a rapid increase in leaf thermal tolerance, such that leaf temperatures observed during the heatwave were maintained within the thermal limits of leaf function. All responses were equivalent for trees with a prior history of ambient and warmed (+3°C) temperatures, indicating that climate warming conferred no added tolerance of heatwaves expected in the future. This coordinated physiological response utilizing latent cooling and adjustment of thermal thresholds has implications for tree tolerance of future climate extremes as well as model predictions of future heatwave intensity at landscape and global scales.

Localized conformational changes trigger the pH-induced fibrillogenesis of an amyloidogenic λ light chain protein.

Solvent conditions modulate the expression of the amyloidogenic potential of proteins. In this work the effect of pH on the fibrillogenic behavior and the conformational properties of 6aJL2, a model protein of the highly amyloidogenic variable light chain λ6a gene segment, was examined. Ordered aggregates showing the ultrastructural and spectroscopic properties observed in amyloid fibrils were formed in the 2.0-8.0 pH range. At pH <3.0 a drastic decrease in lag time and an increase in fibril formation rate were found. In the 4.0-8.0 pH range there was no spectroscopic evidence for significant conformational changes in the native state. Likewise, heat capacity measurements showed no evidence for residual structure in the unfolded state. However, at pH <3.0 stability is severely decreased and the protein suffers conformational changes as detected by circular dichroism, tryptophan and ANS fluorescence, as well as by NMR spectroscopy. Molecular dynamics simulations indicate that acid-induced conformational changes involve the exposure of the loop connecting strands E and F. These results are compatible with pH-induced changes in the NMR spectra. Overall, the results indicate that the mechanism involved in the acid-induced increase in the fibrillogenic potential of 6aJL2 is profoundly different to that observed in κ light chains, and is promoted by localized conformational changes in a region of the protein that was previously not known to be involved in acid-induced light chain fibril formation. The identification of this region opens the potential for the design of specific inhibitors.

Visualization of xylem embolism by X-ray microtomography: a direct test against hydraulic measurements.

X-ray microtomography (microCT) is becoming a valuable noninvasive tool for advancing our understanding of plant-water relations. Laboratory-based microCT systems are becoming more affordable and provide better access than synchrotron facilities. However, some systems come at the cost of comparably lower signal quality and spatial resolution than synchrotron facilities. In this study, we evaluated laboratory-based X-ray microCT imaging as a tool to nondestructively analyse hydraulic vulnerability to drought-induced embolism in a woody plant species. We analysed the vulnerability to drought-induced embolism of benchtop-dehydrated Eucalyptus camaldulensis plants using microCT and hydraulic flow measurements on the same sample material, allowing us to directly compare the two methods. Additionally, we developed a quantitative procedure to improve microCT image analysis at limited resolution and accurately measure vessel lumens. Hydraulic measurements matched closely with microCT imaging of the current-year growth ring, with similar hydraulic conductivity and loss of conductivity due to xylem embolism. Optimized thresholding of vessel lumens during image analysis, based on a physiologically meaningful parameter (theoretical conductivity), allowed us to overcome common potential constraints of some lab-based systems. Our results indicate that estimates of vulnerability to embolism provided by microCT visualization agree well with those obtained from hydraulic measurements on the same sample material.

Drought-induced shoot dieback starts with massive root xylem embolism and variable depletion of nonstructural carbohydrates in seedlings of two tree species.

Combining hydraulic- and carbon-related measurements helps to understand drought-induced plant mortality. Here, we investigated the role that plant respiration (R) plays in determining carbon budgets under drought. We measured the hydraulic conductivity of stems and roots, and gas exchange and nonstructural carbohydrate (NSC) concentrations of leaves, stems and roots of seedlings of two resprouting species exposed to drought or well-watered conditions: Ulmus minor (riparian tree) and Quercus ilex (dryland tree). With increasing water stress (occurring more rapidly in larger U. minor), declines in leaf, stem and root R were less pronounced than that in leaf net photosynthetic CO2 uptake (Pn ). Daytime whole-plant carbon gain was negative below -4 and -6 MPa midday xylem water potential in U. minor and Q. ilex, respectively. Relative to controls, seedlings exhibiting shoot dieback suffered c. 80% loss of hydraulic conductivity in both species, and reductions in NSC concentrations in U. minor. Higher drought-induced depletion of NSC reserves in U. minor was related to higher plant R, faster stomatal closure, and premature leaf-shedding. Differences in drought resistance relied on the ability to maintain hydraulic conductivity during drought, rather than tolerating conductivity loss. Root hydraulic failure elicited shoot dieback and precluded resprouting without root NSC reserves being apparently limiting for R.