Ultrasonic Acoustic Emissions Research Articles

Xylem embolism and bubble formation during freezing suggest complex dynamics of pressure in Betula pendula stems.

Freeze-thaw-induced embolism is a key limiting factor for perennial plants in frost-exposed environments. Gas bubbles are formed during freezing and expand during thawing resulting in xylem embolism. However, when water freezes, its volume increases by 9%, generating local pressures, which can affect the formation of bubbles. To characterize local dynamic of pressure-tension and physical state of the sap during freeze-thaw cycles, we simultaneously used ultrasonic acoustic emissions analysis and synchrotron-based high resolution computed tomography on the diffuse-porous species Betula pendula. Visualization of individual air-filled vessels was performed to measure freeze-thaw induced embolism after successive freeze-thaw cycles down to -10°C or -20°C during the leafy and the leafless periods. We also measured the distribution of gas bubbles in frozen xylem of Betula pendula, and made additional continuous monitoring of embolism spreading during one freeze-thaw cycle using a dedicated cooling system that allowed X-ray scanning during freezing and thawing. Experiments confirmed that ultrasonic emissions occurred after the onset of ice formation, together with bubble formation, whereas the development of embolism took place after thawing in all cases. The pictures of frozen tissues indicated that the positive pressure induced by the volumetric increase of ice can provoke inward flow from the cell wall toward the lumen of the vessels. We found no evidence that wider vessels within a tissue were more prone to embolism although the occurrence of gas bubbles in larger conduits would make them prone to earlier embolism. These results highlight the need to monitor local pressure as well as ice and air distribution during xylem freezing to understand the mechanism leading to frost-induced embolism.

Propagating ice front induces gas bursts and ultrasonic acoustic emissions from freezing xylem.

Ice formation and propagation in the xylem of plants is a complex process. During freezing of xylem sap, gases dissolved in liquid sap are forced out of the ice lattice due to their low solubility in ice, and supersaturation of xylem sap as well as low water potential (Ѱ) are induced at the ice-liquid interface. Supersaturation of gases near the ice front may lead to bubble formation and potentially to cavitation and/or to burst of gases driven out from the branch. In this study, we investigated the origin and dynamics of freezing-related gas bursts and ultrasonic acoustic emissions (AEs), which are suggested to indicate cavitation. Picea abies (L.) H. Karst. and Salix caprea L. branch segments were exposed to frost cycles in a temperature test chamber, and CO2 efflux (indicating gas bursts) and AEs were recorded. On freezing, two-thirds of the observed gas bursts originated from the xylem and only one-third from the bark. Simultaneously with gas bursts, AEs were detected. Branch Ѱ affected both gas bursts and AEs, with high gas burst in saturated and dry samples but relevant AEs only in the latter. Repeated frost cycles led to decreasing gas burst volumes and AE activity. Experiments revealed that the expanding ice front in freezing xylem was responsible for observed gas bursts and AEs, and that branch Ѱ influenced both processes. Results also indicated that gas bursts and cavitation are independently induced by ice formation, though both may be relevant for bubble dynamics during freezing.

Experimental Study of Damage Development in Salt Rock under Uniaxial Stress Using Ultrasonic Velocity and Acoustic Emissions

Ultrasonic waves and acoustic emissions (AE) are important technologies to reveal rock damage. However, few studies have simultaneously monitored both types of data in the same experiment because of limitations on the experimental apparatus. In this study, an integrated ultrasonic wave and AE testing device was developed to investigate the deformation characteristics and damage development of salt rock. Fracture experiments under uniaxial compression were carried out on three samples from the Jintan Salt Mine, Jiangsu Province, China. The deformation process can be divided into five stages. In the compression fissure and linear deformation stages, P- and S-waves rose slightly to stability, and acoustic emission activity was weak (0.04% and 2.66%, respectively). Subsequently, S-wave velocity slowly declined and AE events become more active, with about 13.8% of the total in the stabilized-growth cracks stage. When the salt rock entered the accelerated-growth cracks stage, AE events increased to 75.27%; with features of an earthquake swarm, the velocities of P- and S-waves began to fall significantly. After the peak stress, salt rock produced only a small number of AE events. The beginning stress of rock damage and dilatancy were about 42–50% and 62–67% of the uniaxial compressive strength, respectively. The ultrasonic wave velocity ratio, Ib-value, and r-value effectively predicted rock failure, but the r-value was superior owing to its sensitivity and ease of measurement.

A Comparison of Petiole Hydraulics and Aquaporin Expression in an Anisohydric and Isohydric Cultivar of Grapevine in Response to Water-Stress Induced Cavitation.

We report physiological, anatomical and molecular differences in two economically important grapevine (Vitis vinifera L.) cultivars cv. Grenache (near-isohydric) and Chardonnay (anisohydric) in their response to water-stress induced cavitation. The aim of the study was to compare organ vulnerability (petiole and stem) to cavitation by measuring ultrasonic acoustic emissions (UAE) and percent loss of conductance of potted grapevines subject to the onset of water-stress. Leaf (ψL) and stem water potential (ψS), stomatal conductance (gs), transpiration (E), petiole hydraulics (KPet), and xylem diameter were also measured. Chardonnay displayed hydraulic segmentation based on UAE, with cavitation occurring at a less negative ψL in the petiole than in the stem. Vulnerability segmentation was not observed in Grenache, with both petioles and stems equally vulnerable to cavitation. Leaf water potential that induced 50% of maximum UAE was significantly different between petioles and stems in Chardonnay (ψ50Petiole = -1.14 and ψ50Stem = -2.24 MPa) but not in Grenache (ψ50Petiole = -0.73 and ψ50Stem = -0.78 MPa). Grenache stems appeared more susceptible to water-stress induced cavitation than Chardonnay stems. Grenache displayed (on average) a higher KPet likely due to the presence of larger xylem vessels. A close relationship between petiole hydraulic properties and vine water status was observed in Chardonnay but not in Grenache. Transcriptional analysis of aquaporins in the petioles and leaves (VvPIP1;1, VvPIP2;1, VvPIP2;2 VvPIP2;3, VvTIP1;1, and VvTIP2;1) showed differential regulation diurnally and in response to water-stress. VvPIP2;1 showed strong diurnal regulation in the petioles and leaves of both cultivars with expression highest predawn. Expression of VvPIP2;1 and VvPIP2;2 responded to ψL and ψS in both cultivars indicating the expression of these two genes are closely linked to vine water status. Expression of several aquaporin genes correlated with gas exchange measurements, however, these genes differed between cultivars. In summary, the data shows two contrasting responses in petiole hydraulics and aquaporin expression between the near-isohydric cultivar, Grenache and anisohydric cultivar, Chardonnay.

Cavitation and water fluxes driven by ice water potential in Juglans regia during freeze-thaw cycles.

Freeze-thaw cycles induce major hydraulic changes due to liquid-to-ice transition within tree stems. The very low water potential at the ice-liquid interface is crucial as it may cause lysis of living cells as well as water fluxes and embolism in sap conduits, which impacts whole tree-water relations. We investigated water fluxes induced by ice formation during freeze-thaw cycles in Juglans regia L. stems using four non-invasive and complementary approaches: a microdendrometer, magnetic resonance imaging, X-ray microtomography, and ultrasonic acoustic emissions analysis. When the temperature dropped, ice nucleation occurred, probably in the cambium or pith areas, inducing high water potential gradients within the stem. The water was therefore redistributed within the stem toward the ice front. We could thus observe dehydration of the bark's living cells leading to drastic shrinkage of this tissue, as well as high tension within wood conduits reaching the cavitation threshold in sap vessels. Ultrasonic emissions, which were strictly emitted only during freezing, indicated cavitation events (i.e. bubble formation) following ice formation in the xylem sap. However, embolism formation (i.e. bubble expansion) in stems was observed only on thawing via X-ray microtomography for the first time on the same sample. Ultrasonic emissions were detected during freezing and were not directly related to embolism formation. These results provide new insights into the complex process and dynamics of water movements and ice formation during freeze-thaw cycles in tree stems.

Characteristics of ultrasonic acoustic emissions from walnut branches during freeze–thaw-induced embolism formation

Ultrasonic acoustic emission (UAE) methods have been applied for the detection of freeze-thaw-induced embolism formation in water conduits of tree species. Until now, however, the exact source(s) of UAE has not been identified especially in angiosperm species, in which xylem tissues are composed of diverse types of cells. In this study, UAE was recorded from excised branches of walnut (Juglans regia cv. Franquette) during freeze-thaw cycles, and attempts were made to characterize UAEs generated by cavitation events leading to embolism formation according to their properties. During freeze-thaw cycles, a large number of UAEs were generated from the sample segments. However, the cumulative numbers of total UAE during freeze-thawing were not correlated with the percentage loss of hydraulic conductivity after thawing, suggesting that the sources of UAE were not only cavitation leading to embolism formation in vessels. Among the UAEs, cumulative numbers of UAEs with absolute energy >10.0 fJ strongly correlated with the increase in percentage loss of hydraulic conductivity. The high absolute energy of the UAEs might reflect the formation of large bubbles in the large lumen of vessels. Therefore, UAEs generated by cavitation events in vessels during freeze-thawing might be distinguished from other signals according to their magnitudes of absolute energy. On the other hand, the freezing of xylem parenchyma cells was followed by a certain number of UAEs. These results indicate the possibility that UAE methods can be applied to the detection of both freeze-thaw-induced embolism and supercooling breakdown in parenchyma cells in xylem.

Relating Xylem Cavitation to Gas Exchange in Cotton

Acoustic emissions (AEs) from xylem cavitation events are characteristic of transpiration processes. Though a body of work exists describing AEs and limited stem hydraulic conductivity under water stress, there is limited information about the effects of AEs on stomatal aperture and limitation on carbon assimilation. The objective of this work was to relate AEs to drought stress in cotton. Cotton was grown in mini-lysimeters in the greenhouse and instrumented with a portable photosynthesis system and ultrasonic transducers connected to a digital signal-processing unit. Whole plant transpiration, leaf level gas exchange and ultrasonic AEs were measured. Xylem cavitation events temporally associated with the onset of drought stress. The results are consistent with stomatal closure in response to reduced hydraulic conductance from xylem cavitation events. Clear direct empirical evidence of a reduction in carbon assimilation associated with xylem cavitation resulting from water stress is presented.

Study on Plant Water Stress Based on Ultrasonic Acoustic Emissions

The soil water deficit and strong transpiration can give rise to the phenomenon of plant water stress. Because of the water stress produces the fracture of the water column in conduits, and the fracture is the reflection of energy release which can be detected by the ultrasonic acoustic emissions (UAEs) technology. In order to avoid background noise interference, the UAEs detecting frequency is between 100K Hz and 1 MHz. The PCI-2 data acquisition (DAQ) card and R15 sensors are used to improve the precision of UAEs detection. When the water stress and dehydration gets heavier, the UAEs get higher. Use the tomato plant data with the empirical deduction under the modern greenhouse conditions, the relationships among UAEs, transpiration and UAEs signal strength is described by curve. The UAEs signals occur generally from 5:00~7:00 am, and stop after 20:00 at night. In the daytime, when the plant body water storage is few, and transpiration is strong, the UAEs occur two peaks, called the “Twin Peaks Area” (TPA). In the different conditions of soil water content status and environmental factors, the TPA occur time will be advance or lag, which is generally in the range of 8:00~15:00. An acoustic emission event maybe produces several UAEs counts, while the UAEs counts have a correspondence with the UAEs signal strength. It is better to use UAEs technique to diagnose the plant water status and carry out automatic and precise irrigation for the plant and to improve the effect of Water-saving irrigation.

Study on Irrigation Model Based on Plant Ultrasonic Acoustic Emissions

Produced by cohesion between water molecules or adhesion to the conduit wall, the plant cavitation is accompanied by the vessel walls vibration which is the indicator of the water stress. The ultrasonic acoustic emissions (UAEs) are used to detect the plant cavitation events which are the transmission of vibration. The UAEs could be detected in ultrasonic frequencies between 100K Hz and 1 MHz. The PCI-2 data acquisition (DAQ) equipment and R15 sensors are used to improve the precision of cavitation detection. When the water stress and dehydration gets heavier, the UAEs rate gets higher. Use the tomato practice data with the empirical deduction under the modern greenhouse conditions, the UAEs and Transpiration Index (UAETI) and the UAEs Irrigation Index (UAEII) irrigation model are draw from the relationship between transpiration and UAEs. UAETI can be used to analysis the transpiration, and UAEII can be used to analysis the irrigation time. It is better to use UAEs technique to carry out automatic and precise irrigation for the plant and to improve the effect of Water-saving irrigation.

NEW TYPE OF VULNERABILITY CURVE GIVES INSIGHT IN THE HYDRAULIC CAPACITANCE AND CONDUCTIVITY OF THE XYLEM

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.

Green's function of the ultrasound path of the acoustic emission noise monitoring system in the plates

The mathematical model of the ultrasonic acoustic emissions monitoring channel is presented. The main characteristics of Lamb wave excitation, propagation and detection processes are considered in the model. Green’s function is obtained for the ultrasonic channel and may be utilized in further acoustic emissions research. It is established that the efficiency of detection of high-frequency signal components can be improved by minimizing the area of the mechanical contact between the transducer and the surface of a tested object. It is shown that the shape and the size of the domain of coherent acoustic emissions determine the modal structure of the ultrasonic field within the plate and thus make a strong impact on the spectral characteristics of the transducer’s response.

Natural and laser-induced cavitation in corn stems: On the mechanisms of acoustic emissions

Water in plant xylem is often superheated, and therefore in a meta-stable state. Under certain conditions, it may suddenly turn from the liquid to the vapor state. This cavitation process produces acoustic emissions. We report the measurement of ultrasonic acoustic emissions (UAE) produced by natural and induced cavitation in corn stems. We induced cavitation and UAE in vivo, in well controlled and reproducible experiments, by irradiating the bare stem of the plants with a continuous-wave laser beam. By tracing the source of UAE, we were able to detect absorption and frequency filtering of the UAE propagating through the stem. This technique allows the unique possibility of studying localized embolism of plant conduits, and thus to test hypotheses on the hydraulic architecture of plants. Based on our results, we postulate that the source of UAE is a transient "cavity oscillation"' triggered by the disruptive effect of cavitation inception.Received: 14 March 2011, Accepted: 23 April 2012; Edited by: V. Lakshminarayanan; Reviewed by: C. Negreira, Laboratorio de Acústica Ultrasonora, Universidad de la República, Uruguay; DOI: http://dx.doi.org/10.4279/PIP.040003Cite as: E. Fernández, R. J. Fernández, G. M. Bilmes, Papers in Physics 4, 040003 (2012)

Evidence for xylem embolism as a primary factor in dehydration‐induced declines in leaf hydraulic conductance

Hydraulic conductance of leaves (K(leaf)) typically decreases with increasing water stress and recent studies have proposed different mechanisms responsible for decreasing K(leaf) . We measured K(leaf) concurrently with ultrasonic acoustic emissions (UAEs) in dehydrating leaves of several species to determine whether declining K(leaf) was associated with xylem embolism. In addition, we performed experiments in which the surface tension of water in the leaf xylem was reduced by using a surfactant solution. Finally, we compared the hydraulic vulnerability of entire leaves with the leaf lamina in three species. Leaf hydraulic vulnerability based on rehydration kinetics and UAE was very similar, except in Quercus garryana. However, water potentials corresponding to the initial decline in K(leaf) and the onset of UAE in Q. garryana were similar. In all species tested, reducing the surface tension of water caused K(leaf) to decline at less negative water potentials compared with leaves supplied with water. Microscopy revealed that as the fraction of embolized xylem increased, K(leaf) declined sharply in Q. garryana. Measurements on leaf discs revealed that reductions in lamina hydraulic conductance with dehydration were not as great as those observed in intact leaves, suggesting that embolism was the primary mechanism for reductions in K(leaf) during dehydration.

Study on Plant Water Stress Based on Ultrasonic Acoustic Emissions

Study on Plant Water Stress Based on Ultrasonic Acoustic Emissions

Study on Irrigation Model based on Plant Ultrasonic Acoustic Emissions

Study on Irrigation Model based on Plant Ultrasonic Acoustic Emissions

Freeze–thaw‐induced embolism in Pinus contorta: centrifuge experiments validate the ‘thaw‐expansion hypothesis’ but conflict with ultrasonic emission data

*The 'thaw-expansion hypothesis' postulates that xylem embolism is caused by the formation of gas bubbles on freezing and their expansion on thawing. We evaluated the hypothesis using centrifuge experiments and ultrasonic emission monitoring in Pinus contorta. *Stem samples were exposed to freeze-thaw cycles at varying xylem pressure (P) in a centrifuge before the percentage loss of hydraulic conductivity (PLC) was measured. Ultrasonic acoustic emissions were registered on samples exposed to freeze-thaw cycles in a temperature chamber. *Freeze-thaw exposure of samples spun at -3 MPa induced a PLC of 32% (one frost cycle) and 50% (two cycles). An increase in P to -0.5 MPa during freezing had no PLC effect, whereas increased P during thaw lowered PLC to 7%. Ultrasonic acoustic emissions were observed during freezing and thawing at -3 MPa, but not in air-dried or water-saturated samples. A decrease in minimum temperature caused additional ultrasonic acoustic emissions, but had no effect on PLC. *The centrifuge experiments indicate that the 'thaw-expansion hypothesis' correctly describes the embolization process. Possible explanations for the increase in PLC on repeated frost cycles and for the ultrasonic acoustic emissions observed during freezing and with decreasing ice temperature are discussed.

Ultrasonic emissions from conifer xylem exposed to repeated freezing

Ultrasonic emission measurements enable the analysis of xylem cavitation induced by drought and freeze–thaw events. Several studies have indicated that ultrasonic acoustic emissions (UAE) in conifers occur upon freezing and not upon thawing, although classical theory has postulated gas bubble formation during freezing and cavitation during thawing. We analyzed the pattern and quality of freeze–thaw-induced UAE in seven conifers ( Abies alba, Larix decidua, Juniperus communis, Picea abies, Pinus cembra, Pinus mugo, Pinus sylvestris). Axes samples dehydrated to different water potentials were exposed to repeated frost cycles. The number, amplitude and energy of UAE signals were registered and related to water potential, temperature course and wood characteristics (wood density, tracheid diameter). For P. abies, ultrasonic emission analysis was also performed on bark samples, xylem samples without bark, as well as on stems of young potted trees. In all conifers, UAE were registered in water-stressed samples but not in saturated or dehydrated samples. No signals were emitted by the bark of P. abies. Ultrasonic activity occurred only upon freezing, and identical patterns were observed in axes samples and stems of potted P. abies trees. A weak positive relationship between tracheid diameter and UAE energy was observed, indicating wide tracheids to emit signals with higher energy. The classical bubble formation hypothesis cannot sufficiently explain the occurrence of UAE during freezing and upon repeated temperature cycles, as demonstrated in this study. We suggest that the low water potential of ice induces air-seeding near the ice-water interface, and consequently, causes UAE.

Low‐decibel ultrasonic acoustic emissions are temperature‐induced and probably have no biotic origin

Low‐decibel ultrasonic acoustic emissions are temperature‐induced and probably have no biotic origin

Ultrasonic acoustic emissions in drought‐stressed trees – more than signals from cavitation?

Ultrasonic acoustic emission (UAE) in trees is often related to collapsing water columns in the flow path as a result of tensions that are too strong (cavitation). However, in a decibel (dB) range below that associated with cavitation, a close relationship was found between UAE intensities and stem radius changes. UAE was continuously recorded on the stems of mature field-grown trees of Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens) at a dry inner-Alpine site in Switzerland over two seasons. The averaged 20-Hz records were related to microclimatic conditions in air and soil, sap-flow rates and stem-radius fluctuations detrended for growth (Delta W). Within a low-dB range (27 +/- 1 dB), UAE regularly increased and decreased in a diurnal rhythm in parallel with DeltaW on cloudy days and at night. These low-dB emissions were interrupted by UAE abruptly switching between the low-dB range and a high-dB range (36 +/- 1 dB) on clear, sunny days, corresponding to the widely supported interpretation of UAE as sound from cavitations. It is hypothesized that the low-dB signals in drought-stressed trees are caused by respiration and/or cambial growth as these physiological activities are tissue water-content dependent and have been shown to produce courses of CO(2) efflux similar to our courses of Delta W and low-dB UAE.

Seasonal ultrasonic acoustic emissions of Quercus ilex L. trees in a Mediterranean forest

Ultrasonic acoustic emissions were measured in Quercus ilex trees of a Mediterranean forest in Catalonia (NE Spain) each season from summer of 2004 to autumn of 2005. Acoustic emissions were maximum during hot and dry summer periods. Acoustic emissions started below 17% soil moisture, 0.85 RWC, and 2.5 MPa leaf water potential. They were negatively correlated with soil mois- ture and leaf water potential. The relationship between acoustic emissions and leaf water potential was the stron- gest, indicating that xylem tension is the most important factor inducing both cavitation (acoustic emissions) and a decrease in leaf water potential. Future increase of xylem cavitation derived from climate change may result in growth and survival limitations for this species in the drier southern limits of its current distribution.