Vulnerability curves from conifer sapwood sections exposed over solutions with known water potentials.

ABSTRACTConcurrent measurements of heat pulse velocity and ultrasound acoustic emission were performed on the trunks of adult Quercus petraea plants under different water stress conditions. On the trunk section of the plants the wood density was measured non‐destructively using a mobile computer tomograph which measures the attenuation of a collimated beam of radiation which traverses the trunk. By scanning the trunk in different directions, it is possible to map wood density in the section. As wood density is strictly correlated with water content, this method allows evaluation of the water content in the trunk section and the water conditions in the different parts of the section. The computer‐tomograph technique is non‐invasive and is not influenced by climatic fluctuations. A close agreement was found between wood density and heat pulse velocity; the relationship between these two parameters and ultrasound acoustic emission is discussed. Trunks of sessile oak appear well suited as water storage reservoirs playing an important role in tree survival during extended periods of low soil water availability, especially in the context of global climatic change. Here the computer‐tomograph methodology is described and suggestions arc made for further research development.

The vulnerability of the xylem to cavitation is an important trait in plant drought resistance and has been quantified by several methods. We present a modified method for the simultaneous measurement of cavitations, recorded as ultrasound acoustic emissions (UAEs), and the water potential, measured with a thermocouple psychrometer, in small samples of conifer wood. Analyzing the amplitude of the individual signals showed that a first phase, during which the mean amplitude increased, was followed by a second phase with distinctly lower signal amplitudes. We provide a method to separate the two groups of signals and show that for many samples plausible vulnerability curves require rejecting late low-energy UAEs. These very likely do not result from cavitations. This method was used to analyze the differences between juvenile wood, and early and late mature wood in Picea abies (L.) Karst. Juvenile earlywood was more resistant to cavitation than mature earlywood or latewood, which we relate to the tracheid anatomy of the samples.

The aim of this study was to assess the hydraulic vulnerability of Norway spruce (Picea abies) trunkwood by extraction of selected features of acoustic emissions (AEs) detected during dehydration of standard size samples. The hydraulic method was used as the reference method to assess the hydraulic vulnerability of trunkwood of different cambial ages. Vulnerability curves were constructed by plotting the percentage loss of conductivity vs an overpressure of compressed air. Differences in hydraulic vulnerability were very pronounced between juvenile and mature wood samples; therefore, useful AE features, such as peak amplitude, duration and relative energy, could be filtered out. The AE rates of signals clustered by amplitude and duration ranges and the AE energies differed greatly between juvenile and mature wood at identical relative water losses. Vulnerability curves could be constructed by relating the cumulated amount of relative AE energy to the relative loss of water and to xylem tension. AE testing in combination with feature extraction offers a readily automated and easy to use alternative to the hydraulic method.

The vulnerability of xylem vessels to water stress-induced cavitation was studied by measuring hydraulic conductivity and ultrasound acoustic emissions (AEs) in Fagus sylvatica L. f. purpurea (Ait.) Schneid. and Populus balsamifera L. The occurrence of xylem embolism in summer was investigated in relation to leaf water potential and stomatal conductance. Populus was extremely vulnerable to cavitation, losing functional vessels due to embolism at water potentials lower than −0.7 MPa. Fagus experienced embolism when water potential fell below −1.9 MPa. Midday water potentials often approached these threshold values. When evaporative demand increased rapidly on sunny days, water loss became limited by low stomatal conductance. Thus water potentials fell only slightly below the threshold values inducing cavitation. Despite the differences in vulnerability, both species tolerated a similar embolism rate of about 10% in the summer. There was no embolism reversal during prolonged periods of rain. AEs were predictive of loss in hydraulic conductivity, indicating that AEs were mainly confined to vessels. Finally, vessel length distribution, vessel diameter (tangential axis), vessel density, and vessel wall thickness had been determined for both species investigated. Populus had longer and wider vessels than Fagus, whereas vessel wall thickness was similar in both species.

Drought vulnerability of trees and other woody plants is much debated in the context of climate change, which creates a high interest in understanding plant water relations.The role and functioning of internal water storage is crucial, but still insufficiently understood.Drought vulnerability is typically assessed by considering loss in conductivity in function of decreasing xylem water potential, in a so-called 'vulnerability curve'.The xylem water potential at which a certain percentage of conductivity is lost (usually 50%) gives an indication of the vulnerability to cavitation.In a 'desorption curve', we can examine the release of water from internal storage tissues with decreasing water potential.Both curves are very valuable, but rely on a sequence of manual measurements (xylem water potential, hydraulic conductivity and water content) and are time-consuming.Therefore, we propose a new type of vulnerability curve that is based on continuous measurements of diameter shrinkage and ultrasonic acoustic emissions (UAE).We monitored weight loss, xylem diameter shrinkage and UAE and measured xylem water potential during the dehydration of excised branches of Vitis vinifera L. 'Johanniter'.The vulnerability curves could be interpreted in terms of water loss in elastic and inelastic tissues.The proposed method can be a tool to assess hydraulic capacitance and conductivity of the xylem.

ABSTRACTConcurrent measurements of sap velocity (heat pulse) and ultrasound acoustic emission were performed on the trunks of mature Turkey oak (Quercus cerris) and sessile oak (Quercus petraea) trees. Plant water status was assessed by measuring leaf water potential, leaf conductance and transpiration. Wood density was estimated non‐destructively on the trunk section of the plants by mobile computer tomography, which measures the attenuation of a collimated beam of radiation traversing the trunk in several directions, as the device rotates around the tree. Absorption is proportional to the density of the wood. As wood density is strictly correlated to water content, this non‐invasive method allows the water content in the trunk section to be evaluated as well as mapped. Leaf water potential declined each morning until a minimum was reached at midday and recovered in the afternoon, lagging behind changes in transpiration rate. Good correspondence was found between the patterns of sap velocity and cavitation rate. A close correlation was demonstrated between wood density, water content and sap velocity. Sap now was always higher in Turkey oak than in sessile oak. Trunk signatures by computer tomography appeared to differentiate the two oak species, with the Turkey oak stem clearly more hydrated than the sessile oak; water storage reservoirs could play an important role in tree survival during extended periods of low soil water availability and in the relative distribution of tree species, especially in the context of global climate change. Late‐wood conducting elements of oak species seem to play a significant role in water transport. The mobile computer tomograph was confirmed as a peerless tool for investigating stem water relations. Diurnal variations in the measured parameters under natural drought conditions and the differences between the two oak species are discussed.

ABSTRACTA comparative study on stomatal control under water deficit was conducted on grapevines of the cultivars Grenache, of Mediterranean origin, and Syrah of mesic origin, grown near Montpellier, France and Geisenheim, Germany. Syrah maintained similar maximum stomatal conductance (gmax) and maximum leaf photosynthesis (Amax) values than Grenache at lower predawn leaf water potentials, Ψleaf, throughout the season. The Ψleaf of Syrah decreased strongly during the day and was lower in stressed than in watered plants, showing anisohydric stomatal behaviour. In contrast, Grenache showed isohydric stomatal behaviour in which Ψleaf did not drop significantly below the minimum Ψleaf of watered plants. When g was plotted versus leaf specific hydraulic conductance, Kl, incorporating leaf transpiration rate and whole‐plant water potential gradients, previous differences between varieties disappeared both on a seasonal and diurnal scale. This suggested that isohydric and anisohydric behaviour could be regulated by hydraulic conductance. Pressure‐flow measurements on excised organs from plants not previously stressed revealed that Grenache had a two‐ to three‐fold larger hydraulic conductance per unit path length (Kh) and a four‐ to six‐fold larger leaf area specific conductivity (LSC) in leaf petioles than Syrah. Differences between internodes were only apparent for LSC and were much smaller. Cavitation detected as ultrasound acoustic emissions on air‐dried shoots showed higher rates for Grenache than Syrah during the early phases of the dry‐down. It is hypothesized that the differences in water‐conducting capacity of stems and especially petioles may be at the origin of the near‐isohydric and anisohydric behaviour of g.

Pressurized air (3.5 MPa) produced massive embolization and loss of hydraulic conductivity in detached willow twigs (Salix viminalis L.) enclosed in a pressure collar. A state of tension in the xylem water column was not a necessary precondition for this embolization. Ultrasound acoustic emissions on the distal side and close to the collar were recorded only when air-saturated water was leaving the collar, whether moved by transpiration or by an overpressure applied to a reservoir at the base of the twig. Water released from embolized xylem elements increased the water potential of the leaves. The results show that air entry into xylem elements inside the pressure collar is the main mechanism responsible for the loss of hydraulic conductivity with the pressure collar technique, and that ultrasound signals originating within the collar are not sensed on the outside.

Xylem hydraulic properties are of great significance for plant growth and performance under drought. The ability of plants to avoid drought-induced cavitation and loss of hydraulic conductivity (K) can be characterized with vulnerability curves (VC). A VC describes the sigmoidal relationship between percentage loss of K (PLC) and xylem water potential (ψxyl). The ψ at 50% loss of conductance indicates a commonly used threshold for detrimental embolism (P50). The slope (b) represents cavitation resistance. The standard hydraulic method to determine VC's requires the measurement of water flow rate (WFR) per pressure gradient through stem segments, either by measuring outflow from the stem gravimetrically or inflow using a flow meter. In a comparative study using both measurements of inflow and outflow in asparagus stems, we found considerable disparities in the resulting shapes of VCs (P50 and b). We hypothesized that water uptake of stem tissue occurs during the pressure-driven water transit, particularly at low water potential and that differences in the initial K might result from measurements of inflow or outflow. To determine whether water uptake of stem tissue occurs during K measurements of asparagus plants, we tested for effects of ψxyl on the initial inflow and outflow K at different pressure gradients and investigated if passive water uptake can be estimated by extrapolation from the linear regression between WFR and pressure gradient based on K at two pressures. Initial K differed significantly between inlet and outlet measurements at low ψxyl, whereas maximum K did not, providing evidence of water uptake during transit through droughted stems. The resulting parameters P50 and b, and thus the shape of VCs, differed as well. The extrapolation resulted in the first estimate of passive water uptake, leading to a convergence of the VC at inlet and at outlet. We conclude that differences between in- and outflow may play a major role in K measurements.

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.

ABSTRACTWhile drying, detached leaves produced ultrasound acoustic emissions (UAE) comparable to emissions from stem and twig wood. Experiments on Ilex aquifolium L. showed that the main source of these signals was cavitation in the veins, to which conduits and fibres probably both contributed. Regions of the leaf blade with abundant mesophyll and only small veins emitted few signals. More signals were counted on the adaxial side of the midrib than on the abaxial one and on the proximal third than on the distal one, in accordance with the anatomical structure. Sound attenuation was pronounced. Eight species were compared with respect to cavitation behaviour, field water relations and pressure–volume curves, and the data showed differences in cumulative number of events and resistance of leaves to cavitation. Data were generally in good agreement with anatomical structure and habitat preferences. The number of signals per conduit counted on cross‐sections was in some leaves much higher than unity, which suggests short xylem elements or an acoustic activity of cells other than conduits. There was no correlation between cavitation threshold or cumulative number of signals and the degree of sclerophylly; unexpectedly, there was a correlation between the cumulative number of signals at a water potential of ‐1.3 MPa and the bulk modulus of elasticity.

ABSTRACTRachides of Juglans regia L. (Juglandaceae) and one‐year‐old twigs of Evonymus latifolia (L.) Mill. (Celastraceae) were cooled in air to −25 °C, with an ultrasound detector attached to the xylem where peripheral tissues had been peeled off. Ultrasound acoustic emissions started between −4·5 and −14·3 °C, as measured with a thermocouple inserted into the xylem. The number of emissions was significantly lower from saturated plant parts than from those frozen at field water potentials. Bench‐drying of saturated samples produced significantly less signals than the freezing protocols. These findings are in accordance with the hypothesis that freezing of xylem under tension induces cavitation events. They corroborate earlier work which tried to provide a logical explanation for the seemingly paradoxical cryo‐scanning electron microscope observations of changing vessel contents during a daycourse in the field.

The hydraulic performance of woody species during drought is currently of high interest in the context of climate change. It is known that woody species have the capacity to mitigate water shortage by using internally stored water. Elastic shrinkage of living cells and also water release during cavitation contribute to the so-called 'hydraulic capacitance' (C) of the plant, which adds water to the transpiration stream and buffers fluctuations in water potential. Although sap-conducting conduits may ultimately serve as a water pool, cavitation will hamper the conduction of sap. Both hydraulic conductivity and C are thus inextricably linked and the interaction between both should be studied to better understand hydraulic functioning of woody species during drought. However, measurements of C are scarce and no distinction is usually made between C from elastic storage and C supplied by cavitation. In this paper, we propose a new method to assess both the decrease in hydraulic conductivity and the change in C during bench dehydration of a whole-branch segment using continuous measurements of acoustic emissions, radial diameter shrinkage and gravimetrical water loss. With this method we could establish proper vulnerability curves for grapevine (Vitis vinifera L. 'Johanniter') and quantify C during dehydration. Our results showed that loss in hydraulic conductivity during the cavitation phase was accompanied by 22-92% gain in hydraulic capacitance; therefore, a certain degree of cavitation may be tolerated in grapevine during periods of drought stress.

Reassimilation of internal CO2 via woody tissue photosynthesis has a substantial effect on tree carbon income and wood production. However, little is known about its role in xylem vulnerability to cavitation and its implications in drought-driven tree mortality. Young trees of Populus nigra were subjected to light exclusion at the branch and stem levels. After 40d, measurements of xylem water potential, diameter variation and acoustic emission (AE) were performed in detached branches to obtain acoustic vulnerability curves to cavitation following bench-top dehydration. Acoustic vulnerability curves and derived AE50 values (i.e. water potential at which 50% of cavitation-related acoustic emissions occur) differed significantly between light-excluded and control branches (AE50,light-excluded =-1.00±0.13MPa; AE50,control =-1.45±0.09MPa; P=0.007) denoting higher vulnerability to cavitation in light-excluded trees. Woody tissue photosynthesis represents an alternative and immediate source of nonstructural carbohydrates (NSC) that confers lower xylem vulnerability to cavitation via sugar-mediated mechanisms. Embolism repair and xylem structural changes could not explain this observation as the amount of cumulative AE and basic wood density did not differ between treatments. We suggest that woody tissue assimilates might play a role in the synthesis of xylem surfactants for nanobubble stabilization under tension.

The hydraulic properties of Pinus pinea, Pinus halepensis and Tetraclinis articulata were studied in a coastal dune area from Eastern Spain. The measured variables include vulnerability to xylem embolism (vulnerability curves), hydraulic conductivity and carbon isotopic discrimination in leaves. Leaf water potentials were also monitored in the three studied populations during an extremely dry period. Our results showed that roots had always wider vessels and higher hydraulic conductivity than branches. Roots were also more vulnerable to xylem embolism and operated closer to their hydraulic limit (i.e., with narrower safety margins). Although it was not quantified, extensive root mortality was observed in the two pines during the study period, in agreement with the high values of xylem embolism (> 75%) predicted from vulnerability curves and the water potentials measured in the field. T. articulata was much more resistant to embolism than P. pinea and P. halepensis. Since T. articulata experienced also lower water potentials, safety margins from hydraulic failure were only slightly wider in this species than in the pines. Combining species and tissues, high resistance to xylem embolism was associated with low hydraulic conductivity and with high wood density. Both relationships imply a cost of having a resistant xylem. The study outlined very different water-use strategies for T. articulata and the pines. Whereas T. articulata had a conservative strategy that relied on the low vulnerability of its conducting system to drought-induced xylem embolism, the two pines showed regulatory mechanisms at different levels (i.e., embolism, root demography) that constrained the absorption of water when it became scarce.