As current methods for measuring xylem embolism in trees are indirect and prone to artefacts, there is an ongoing controversy over the capacity of trees to resist or recover from embolism. The debate ...will not end until we get direct visualization of the vessel content. Here, we propose desktop X‐ray microtomography (micro‐CT) as a reference direct technique to quantify xylem embolism and thus validate more widespread measurements based upon either hydraulic or acoustic methods. We used desktop micro‐CT to measure embolism levels in dehydrated or centrifuged shoots of laurel – a long‐vesseled species thought to display daily cycles of embolism formation and refilling. Our direct observations demonstrate that this Mediterranean species is highly resistant to embolism and is not vulnerable to drought‐induced embolism in a normal range of xylem tensions. We therefore recommend that embolism studies in long‐vesseled species should be validated by direct methods such as micro‐CT to clear up any misunderstandings on their physiology.
Abstract
Climate change is expected to increase the frequency and intensity of droughts and heatwaves in Europe, leading to effects on forest growth and major forest dieback events due to hydraulic ...failure caused by xylem embolism. Inter-specific variability in embolism resistance has been studied in detail, but little is known about intra-specific variability, particularly in marginal populations. We evaluated 15 European beech populations, mostly from geographically marginal sites of the species distribution range, focusing particularly on populations from the dry southern margin. We found small, but significant differences in resistance to embolism between populations, with xylem pressures causing 50% loss of hydraulic conductivity ranging from −2.84 to −3.55 MPa. Significant phenotypic clines of increasing embolism resistance with increasing temperature and aridity were observed: the southernmost beech populations growing in a warmer drier climate and with lower habitat suitability have higher resistance to embolism than those from Northern Europe growing more favourable conditions. Previous studies have shown that there is little or no difference in embolism resistance between core populations, but our findings show that marginal populations have developed ways of protecting their xylem based on either evolution or plasticity.
Winter physiology of woody plants is a key issue in temperate biomes. Here, we investigated different frost resistance mechanisms on 1-year-old branches of 11 European tree species from November ...until budburst: (i) frost hardiness of living cells (by electrolyte leakage method), (ii) winter embolism sensitivity (by percentage loss of conductivity: PLC) and (iii) phenological variation of budburst (by thermal time to budburst). These ecophysiological traits were analyzed according to the potential altitudinal limit, which is highly related to frost exposure. Seasonal frost hardiness and PLC changes are relatively different across species. Maximal PLC observed in winter (PLCMax) was the factor most closely related to potential altitudinal limit. Moreover, PLCMax was related to the mean hydraulic diameter of vessels (indicating embolism sensitivity) and to osmotic compounds (indicating ability of living cells to refill xylem conducting elements). Winter embolism formation seems to be counterbalanced by active refilling from living cells. These results enabled us to model potential altitudinal limit according to three of the physiological/anatomical parameters studied. Monitoring different frost resistance strategies brings new insights to our understanding of the altitudinal limits of trees.
•Xylem embolism resistance was estimated in 11 species with contrasted wood anatomy and vessel length.•All methods tested provided consistent results in short-vessel angiosperm species.•The Pneumatic ...method underestimated embolism resistance in conifers and 2 long-vessel angiosperms.•Air-injection and Flow-centrifuge methods consistently estimated embolism resistance intraspecific variability in a conifer.•Turgor loss point measurements were used to evaluate the reliability of vulnerability curves.
Vulnerability to drought-induced embolism is a key trait that shapes drought resistance and that could be increasingly used to design climate-smart forest management guidelines and to anticipate the outcome of climate change on populations dynamics and ecosystems functioning. A panel of methods is currently available to measure embolism resistance. This makes crucial a proper identification of which methods are the most accurate for determining this trait. However, the measurement of embolism resistance is sensitive to numerous artifacts that may lead to large errors for a given species. In addition, not all methods are easily accessible because of the cost of some large equipment and/or certain lab facilities. The emergence of the easy and low cost Pneumatic method allows to perform vulnerability curves at high throughput. However, only few studies have evaluated the reliability of this method compared to others. In this study, we proposed a comparison of five methods that allowed to assess embolism resistance in eleven tree species with contrasting xylem anatomy and vessels length (six short vessel angiosperms, two tracheid bearing conifers and three long-vessel angiosperms), covering a large part of the range of embolism resistance observed in trees. Consistent results were obtained among all the methods for short-vessel angiosperm species. In tracheid-bearing conifers, the Pneumatic method overestimated vulnerability to embolism. In long-vessel species, the Pneumatic method led to inconsistent results with accurate vulnerability to cavitation curves (VCs) for one species but led to r-shaped VCs with a underestimation of incipient embolism for the two other ones. The comparison of VC parameters with turgor loss point is proposed as an indicator of the validity of the VCs. The conditions of validity, the advantages and pitfalls of the five methods are discussed. Our results warned against the widespread usages of some methods before rigorous validation tests have been performed.
The impact of water deficit on stomatal conductance (g s ), petiole hydraulic conductance (K petiole ), and vulnerability to cavitation (PLC, percentage loss of hydraulic conductivity) in leaf ...petioles has been observed on field-grown vines (Vitis vinifera L. cv. Chasselas). Petioles were highly vulnerable to cavitation, with a 50% loss of hydraulic conductivity at a stem xylem water potential (Ψ x ) of -0.95 MPa, and up to 90% loss of conductivity at a Ψ x of -1.5 MPa. K petiole described a daily cycle, decreasing during the day as water stress and evapotranspiration increased, then rising again in the early evening up to the previous morning's K petiole levels. In water-stressed vines, PLC increased sharply during the daytime and reached maximum values (70–90%) in the middle of the afternoon. Embolism repair occurred in petioles from the end of the day through the night. Indeed, PLC decreased in darkness in water-stressed vines. PLC variation in irrigated plants showed the same tendency, but with a smaller amplitude. The Chasselas cultivar appears to develop hydraulic segmentation, in which petiole cavitation plays an important role as a 'hydraulic fuse', thereby limiting leaf transpiration and the propagation of embolism and preserving the integrity of other organs (shoots and roots) during water stress. In the present study, progressive stomatal closure responded to a decrease in K petiole and an increase in cavitation events. Almost total closure of stomata (90%) was measured when PLC in petioles reached >90%.
A technique for measuring hydraulic conductances of excised xylem segments exposed to high negative pressures is described. A centrifugal force is used to generate negative pressures (P) in the ...sample and to create a positive hydrostatic pressure difference (ΔP) between its two ends. ΔP forces water through the sample at a flow rate (F) determined optically during centrifugation. The sample hydraulic conductance k is derived from F and ΔP. The sample vulnerability curve is given by the dependence of k on P. Results for Cedrus atlantica Manetti and Laurus nobilis L. shoots are given. The technique is appropriate for the analysis of xylem refilling under negative pressure.
ABSTRACT
The hydraulic architecture of Laurus nobilis L. and Juglans regia L. leaves was studied using three different approaches: (1) hydraulic measurements of both intact leaves and of leaves ...subjected to treatments aimed at removing the extra‐vascular resistance; (2) direct measurements of the vascular pressure with a pressure probe; and (3) modelling the hydraulic architecture of leaf venation system on the basis of measurements of vein densities and conductivities. The hydraulic resistance of leaves (Rleaf) either cut, boiled or frozen–thawed was reduced by about 60 and 85% with respect to control leaves for laurel and walnut, respectively. Direct pressure drop measurements suggested that 88% of the resistance resided outside the vascular system in walnut. Model simulations were in agreement with these results provided vein hydraulic conductance was 0.12–0.28 that of the conductance predicted by Poiseuille's law. The results suggest that Rleaf is dominated by substantial extra‐vascular resistances and therefore contrast with the conclusions of recent studies dealing with the hydraulic architecture of the leaf. The present study confirms the ‘classical’ view of the hydraulic architecture of leaves as composed by a low‐resistance component (the venation) and a high‐resistance component (the mesophyll).
Three methods are in widespread use to build vulnerability curves (VCs) to cavitation. The bench drying (BD) method is considered as a reference because embolism and xylem pressure are measured on ...large branches dehydrating in the air, in conditions similar to what happens in nature. Two other methods of embolism induction have been increasingly used. While the Cavitron (CA) uses centrifugal force to induce embolism, in the air injection (AI) method embolism is induced by forcing pressurized air to enter a stem segment. Recent studies have suggested that the AI and CA methods are inappropriate in long-vesselled species because they produce a very high-threshold xylem pressure for embolism (e.g., P50) compared with what is expected from (i) their ecophysiology in the field (native embolism, water potential and stomatal response to xylem pressure) and (ii) the P50 obtained with the BD method. However, other authors have argued that the CA and AI methods may be valid because they produce VCs similar to the BD method. In order to clarify this issue, we assessed VCs with the three above-mentioned methods on the long-vesselled Quercus ilex L. We showed that the BD VC yielded threshold xylem pressure for embolism consistent with in situ measurements of native embolism, minimal water potential and stomatal conductance. We therefore concluded that the BD method provides a reliable estimate of the VC for this species. The CA method produced a very high P50 (i.e., less negative) compared with the BD method, which is consistent with an artifact related to the vessel length. The VCs obtained with the AI method were highly variable, producing P50 ranging from -2 to -8.2 MPa. This wide variability was more related to differences in base diameter among samples than to differences in the length of samples. We concluded that this method is probably subject to an artifact linked to the distribution of vessel lengths within the sample. Overall, our results indicate that the CA and the AI should be used with extreme caution on long-vesselled species. Our results also highlight that several criteria may be helpful to assess the validity of a VC.
Under drought conditions, the xylem of trees that conducts ascending sap produces ultrasonic emissions whose exact origin is not clear. We introduce a new method to record simultaneously both ...acoustic events and optical observation of the xylem conduits within slices of wood that were embedded in a transparent material setting a hydric stress. In this article, we resolved the rapid development of all cavitation bubbles and demonstrated that each ultrasound emission was linked to the nucleation of one single bubble, whose acoustic energy is an increasing function of the size of the conduit where nucleation occurred and also of the hydric stress. We modelled these observations by the fact that water columns in conduits store elastic energy and release it into acoustic waves when they are broken by cavitation bubbles. Water columns are thus elastic, and not rigid, ‘wires of water’ set under tension by hydric stresses. Cavitation bubbles are at the origin of an embolism, whose development was followed in our experiments. Such an embolism of sap circulation can result in a fatal condition for living trees. These findings provide new insights for the non-destructive monitoring of embolisms within trees, and suggest a new approach to study porous media under hydric stress.
Hydraulic failure is one of the main causes of tree mortality in conditions of severe drought. Resistance to cavitation is known to be strongly related to drought tolerance and species survival in ...conifers, but the threshold of water-stress-induced embolism leading to catastrophic xylem dysfunction in angiosperms has been little studied. We investigated the link between drought tolerance, survival and xylem cavitation resistance in five angiosperm tree species known to have contrasting desiccation resistance thresholds. We exposed seedlings in a greenhouse to severe drought to generate extreme water stress. We monitored leaf water potential, total plant water loss rate, leaf transpiration, stomatal conductance and CO2 assimilation rate during drought exposure and after rewatering (recovery phase). The time required for the recovery of 50% of the maximum value of a given ecophysiological variable after rewatering was used to determine the critical water potential corresponding to the threshold beyond which the plant failed to recover. We also investigated the relationship between this potential and stem xylem cavitation resistance, as assessed from vulnerability curves. This minimum recoverable water potential was consistent between ecophysiological variables and varied considerably between species, from -3.4 to -6.0 MPa. This minimum recoverable water potential was strongly correlated with P50 and P88, the pressures inducing 50 and 88% losses of stem hydraulic conductance, respectively. Moreover, the embolism threshold leading to irreversible drought damage was found to be close to 88%, rather than the 50% previously reported for conifers. Hydraulic failure leading to irreversible drought-induced global dysfunction in angiosperm tree species occurred at a very high level of xylem embolism, possibly reflecting the physiological characteristics of their stem water-transport system.
PDF
