Báo cáo lâm nghiệp: "Detection of cavitation events upon freezing and of water in stems using ultrasound technique"

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Detection of cavitation events upon freezing and thawing of water in stems using ultrasound techniques R. Valentini 2 A. Raschi G.E. Scarascia- 1 Mugnozza and C. Va zana 3 C. Vazzana 3 1 C.N.R.-I.A. T.A., 50144 Firenze, 2 Biol. Selvicolturale - Univ. della Tuscia, Viterbo, and Ist. 3 Dipt. di Agronomia e Produzioni Erbacee, Univ. di Firenze, P.le delle Cascine, i8, Firenze, Italia consequences of the above Introduction gated the cycles, without directly de- mentioned monstrating the cavitative events as they In recent years, renewed attention has were taking place. Moreover, most of the been paid to cavitative events occurring in work has been carried out on conifers, plants subjected to water stress. This has which, according to the literature, are less been a result of the creation of new tech- damaged than angiosperms. In this pre- niques based on the detection of sounds liminary text, we examine the differences ultra- reliably, (Milburn, 1973) or, more in the ultrasound emissions that exist be- sounds (Tyree and Dixon, 1983) emitted tween a coniferous (Araucaria excelsa) by the plant as a consequence of the and a broad-leafed (Eucalyptus occiden- vibration of cavitating conducting ele- talis) tree during lowering and rising of the ments. The results obtained so far show temperature. that the development of emboli is a phe- nomenon that occurs much more fre- quently than was supposed, and takes place at a different frequency in different Materials and Methods species. Indeed, several observations, summarized by Zimmermann (1983), sug- gest that cavitation should take place The experiment was carried out in the laborato- ry of the Agronomy Department of the Universi- upon the freezing and the thawing of ty of Firenze, Italy, using 3 yr old potted plants water in vessels. In fact, the existence of of E. occidentalis Endl., and 4 yr old excised cavitative events as a consequence of lateral branches of A. excelsa R. Br. All plants cycles of freezing and thawing has been had been acclimated for 2 wk in a growth cham- ber and none of them had experienced water evidenced by several authors (Scholander freezing during the previous months; stress or et al., 1955; Hammel, 1967; Sucoff, 1969), water potential, measured by pressure xylem who showed the existence of permanent chamber on all plants before the experiments, damage to the conductive system in some ranged between 0.07 and 0.11 MPa. The pots species. To date, research has investi- of Eucalyptus plants were enclosed in a thermal
Results insulated box to prevent soil freezing. The cut extremity of Auracaria branches, recut under water, was kept immersed in water and en- closed in an insulated box. Plant stems were Fig. 1, the typical behavior of ultrasound In frozen in a top loading freezer, whose top cover pulses as a function of stem temperature had been substituted with expanded poly- Before ice for- in is Eucalyptus reported. styrene material. A hole had been cut in the mation, which occurred at an average new cover in order to allow the distal part of the sample, with all leaves to be kept at room supercooling temperature of -4.5°C, only temperature, enclosed in a black plastic bag. a few isolated events were recorded. A Temperature was lowered at a rate of 5°C per considerable emission rate appeared hour. To measure the acoustic emission rate, a several minutes after ice formation and Bruel and Kjaer 8312 broad band sensor was clamped in a handpiece to the lower part of the the maximum was detected at the lowest stem, which had previously been prepared by temperature reached (-8°C). As the tem- removing a small window of bark to expose the perature was increased, ultrasound emis- xylem. This area was coated with petroleum sion gradually diminished, while upon jelly to prevent water loss and then with ultra- thawing we detected only a few counts. sound gel to improve acoustic transmission. To avoid heating effects of the transducer on the Fig. 2 presents the behavior of ultra- sensor and the sample, we used the sensor sound emissions in Araucaria subjected to intermittently, switching it on only for 2 min out of 10. The acoustic emissions were counted the same kind of test. As in Eucalyptus, using an instrument similar to the one described only a few single events were recorded by Sandford and Grace (1985), slightly modi- during supercooling, but the highest num- fied. Freezing of stem tissues was monitored ber of acoustic events were detected a with thermocouple sensors (type T) connected few minutes after ice formation or imme- to a strip chart recorder.
really due to cavitation. Pre- diately after it. Emission rates then events are vious research has showed the different decreased, coming to zero in about 1 h. similar to behavior of angiosperms, which develop the trend During thawing, was that observed in emboli as a consequence of these cycles, Eucalyptus plants. and conifers in which xylem functionality is not lost. Reasons for the different behavior patterns should be found in wood struc- ture, since the bordered pits on the tra- Discussion and Conclusions cheids can isolate the freezing sap, pre- venting the expansion of emboli to the adjacent vessels. The behavior of acoustic plants, it is generally In water-stressed emissions in Eucalyptus could be linked to single emission represents agreed that a the fact that freezing does not take place the cavitation of water in one vessel. at the same time in all the elements of Doubts exist about the counting efficiency wood but is somewhat progressive. It can of this kind of sensor, which is thought to also be explained by a dehydration pro- be quite low. In our tests, no events were cess caused by the presence of extracel- recorded using the sensor alone in the lular ice, which is responsible for an exter- freezer (i.e., the environmental noise was nal force withdrawing water from cells. low) or freezing a dehydrated piece of Since we were working at saturated stem. Moreover, the detected events atmospheric potential, appeared as groups, divided by periods of water we are unable to make reliable conjectures about silence with just a few isolated events, as the behavior of cavitation in a frozen stem they appear during water stress. There- under field conditions. fore, we concluded that the recorded
Further research is needed to Sandford A.P. & Grace J. (1985) The measure- assess ment and interpretation of ultrasound from that, together with examining the modifica- woody stems. J. Exp. Bot 36, 298-311 1 tions in water uptake and water conduc- Scholander P.F., Love W.E. & Kanwsher J.W. tivity during freezing and after thawing. (1955) The rise of sap in tall grapevines. Plant Physiol. 30, 93-104 Sucoff E. (1969) Freezing of conifer xylem and the cohesion tension theory. Physiol. Plant. 22, References 424-431 Tyree M.T. & Dixon M.A. (1983) Cavitation events in Thuja occidentalis? Ultrasonic acous- Hammel H.T. (1967) Freezing of xylem sap tic emission from the sapwood can be measur- without cavitation. Plant Physiol. 42, 55-66 ed. Plant Physiol. 72, 1094-1099 Milburn J.A. (1973) Cavitation studies on whole Zimmermann M.H. (1983) In: Xylem Structure ricinus plants by acoustic detection. Planta 112, and the Ascent of Sap. Springer-Verlag, Berlin, 333-342 pp. 143