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Department of Biology, University of Utah, Salt Lake City, Utah 84112
Received for publication September 18, 1998. Accepted for publication February 23, 1999.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: cavitation; cohesion theory; compensating pressure theory; embolism; water transport
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), the American Journal of Botany recently featured the theory as a "Special Invited Paper" (Canny, 1998a
). At the heart of his theory is that pressures below approximately -0.3 MPa do not exist in xylem because of cavitation. These modestly negative pressures are maintained by compression of the water column by surrounding turgid tissue. This compensating tissue pressure not only minimizes the negative pressures necessary for transport, but quickly refills any cavitated and embolized conduits by squeezing water from the cells to fill up the empty conduits. The result is a "self-sustaining, ultra-stable pathway" (Canny, 1998a
). To develop a new water transport theory and to have it be accommodated so enthusiastically, there must be significant problems with our understanding of water transport in plants as represented in the cohesion theory of the textbooks. Of course, what has dogged the cohesion theory from the outset is its prediction of substantial negative pressures. These pressures are required to extract water from dry or saline substrates, move it against the frictional resistance of soil and plant, and against gravity. Water pressures down to -10 MPa or lower have been claimed in the xylem of plants. These pressures are far below the vapor pressure of water, and therefore highly metastable. Disbelief of these low negative pressures is the motivation for alternative explanations.
The deciding point in the cohesion vs. compensation debate is therefore the evidence for the existence of substantial negative pressures in xylem. We will not repeat extensive discussions of the evidence for the cohesion theory (Tyree, 1997
), and critiques of xylem pressure probe experiments of U. Zimmermann and colleagues (Zimmermann et al., 1994
) that claim to show a negative pressure limit of approximately -0.4 MPa in the xylem (Steudle, 1995
; Sperry et al., 1996
; Tyree, 1997
). We will not indulge in exposing internal flaws in Canny's theory, although this is a useful exercise (see Comstock, this issue). What we will do is focus on the validity of what we feel is the most direct test of the negative pressure limit in intact xylem conduits of plant xylem.
This test is a natural extension of Brigg's classic experiment using centrifugal force to demonstrate cavitation pressures below -25 MPa in water-filled capillary tubes (Briggs, 1950
). The test consists of replacing the capillary tube with a stem segment (Holbrook, Burns, and Field, 1995
; Pockman, Sperry, and O'Leary, 1995
). The results show that the negative pressure generated in this manner matches pressure chamber measurements and that negative pressures below -3 MPa can exist without cavitation. The cavitation pressure was inferred from "vulnerability curves" that show the relationship between negative xylem pressure and the loss of conductivity (Fig. 1). It is assumed that the cause of this loss of conductivity is the gas blockage of xylem conduits ("embolism") that follows their cavitation (Pockman, Sperry, and O'Leary, 1995
).
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)! Canny interprets vulnerability curves as exposing the limits not to negative pressure, but to the compensating tissue pressure derived from the turgor pressure of the living cells in the tissue. The compensating pressure is seen as refilling the vessels that actually cavitate at approximately -0.3 MPa, creating the illusion that cavitation has not occurred. Loss of conductivity is only seen when the cells have been exposed to sufficient stress to eliminate turgor pressure and, thus, their refilling activity. Canny's critique of the other methods we have employed for studying the cavitation process (Sperry and Saliendra, 1994
) suggests that they, too, were in error.
Is Canny correct about the cavitation pressure in plants? To answer this question, we conducted simple experiments that tested the most fundamental predictions of the compensating pressure theory. Some of the experiments were suggested by Canny himself as crucial to settling the question (Canny, 1998a
).
If Canny is right, vulnerability curves of killed or otherwise turgor-reduced stems will show a loss of conductivity near approximately -0.3 MPa because the turgor-powered refilling process would be eliminated (Fig. 1, dashed curve). The loss of conductivity in these curves would reflect the true cavitation pressure of the xylem. Living control stems would show a loss of conductivity at some lower negative pressure indicating the lower limit to the refilling activity of the living cells (Fig. 1, solid curve).
We also tested the relative refilling capability of control vs. killed stems where gas blockage was induced without stressing the tissue and thus eliminating its putative refilling capability. If Canny's interpretation of the vulnerability curve is correct, embolized conduits in living tissue must refill within the 5-min period between the induction of cavitation in the centrifuge and the subsequent conductivity measurement. The dead stems should refill much more slowly, if at all.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Experimental treatments
Three 142-mm long segments (3.5–6.0 mm diameter) were cut from the stems of each seedling under water. Each segment was subjected to a different treatment. A total of seven seedlings were used, giving a sample size of seven stems per treatment. Control stems were untreated. Killed stems were heated in a microwave oven (Panasonic NN-5369A, Matsuhita Electric Corp. of America, Franklin Park, Illinois) at medium power. They were heated in three 2-min intervals with 1-min breaks to promote even heat distribution. Temperatures reached at least 75°C inside the stems based on thermocouple measurements made after removal from the oven. The heat treatment was sufficient to eliminate respiration as measured by a gas exchange system (Fig. 2A). The gas exchange system has been described earlier (Saliendra, Sperry, and Comstock, 1995
) and differed only in that a smaller chamber (volume 72 cm3) was used to accommodate the stems. Respiration rates of killed stems remained zero after 3 and 6 h of soaking stems in water (Fig. 2A). We also made hand sections of the xylem and compared the appearance of the cytoplasm with control stems. In "blind" tests, we were able to distinguish the heated stems from controls based on the disorganization of the cytoplasm.
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). Stems were perfused for a minimum of 3 h at a head of 
5 KPa. This was sufficient to lower the water potential of the xylem tissue (measured with psychrometers) to within a few tenths of a megapascal of the solution water potential (Fig. 2B, solid line). During perfusion, stems were also immersed in a mannitol solution of the same strength. Microscopic observations of bark and leaf cells in the mannitol solution indicated that it did not cause obvious plasmolysis.
Vulnerability curves
After being treated, stems were used for vulnerability curve measurements. Vulnerability curves show the percentage loss of hydraulic conductivity caused by centrifugally induced negative pressure. Prior to the first conductivity measurement, the stems were flushed for at least 15 min with purified water with an 50 kPa hydraulic pressure head to refill any embolized vessels already present. Killed stems were flushed after the heat treatment to remove all gas bubbles that might have been caused by localized boiling during heating. Mannitol stems were flushed with purified water prior to being perfused for 3 h with mannitol as described above.
The hydraulic conductivity of flushed stems was calculated from the flow rate of measuring solution through the stem divided by the pressure gradient driving the flow. Pressure was induced by an hydraulic head of 5 kPa, and the flow rate was measured gravimetrically. To improve the accuracy of the measurement, the flow under zero hydraulic head was measured before and after the pressure-induced flow for each stem. Small zero-pressure flow rates can exist because of osmotic or capillary water uptake by tissue and leaks in the system. The zero-pressure flow rate was averaged and subtracted from the flow under pressure to obtain the net flow rate caused by the pressure. Net flow rate was used to calculate the conductivity. The measuring solution was filtered (0.2-µm retention), purified water for killed and control stems and a 10% w/w mannitol solution made from the same water for the mannitol stems. During the conductivity measurement, stems were covered with either water (control, killed) or mannitol solution (mannitol) to eliminate evaporation from the stems.
After the conductivity measurement, stems were spun for 4 min in a custom-built rotor designed to fit a Sorvall model RC5C centrifuge (Kendro Laboratory Products, Newton, Connecticut). In this way we could create precisely known negative pressures in the xylem water column (Pockman, Sperry, and O'Leary, 1995
; Alder et al., 1997
). After spinning, conductivity was measured and expressed as a percentage loss of the initial value. The stems were successively spun to increasingly lower pressures and remeasured until conductivity dropped > 95% from its initial value.
Refilling experiments
In a second experiment the refilling capability of six control and six killed stems was compared. Stems were flushed for 15 min with filtered and purified water at 50 kPa hydraulic pressure to refill embolized vessels. The conductivity was measured, stems were submerged in water, and air was injected at 0.1 MPa into the upstream end (with respect to the flow measurement) of the stems for 10 min. The low air pressure, short duration of injection, and submersion of the stems in water were to minimize any dehydration of the living cells in control stems that might impair any refilling capability derived from their turgor pressure. The conductivity was measured immediately after the air injection and again after 30 min. Between the conductivity measurements no pressure head was applied, but stems were submerged in water to encourage refilling.
Statistics
Data were analyzed with SPSS 8.0 for Windows (SPSS Inc., Chicago, Illinois). We compared vulnerability curves across treatments using a one-way ANOVA (0.05 significance level) for each test pressure. In addition, the mean cavitation pressure was calculated from each vulnerability curve by converting it to a noncumulative distribution that showed the loss of conductivity associated with each pressure decrement. The mean of the noncumulative distribution was calculated using the midpoint of the pressure decrement.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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).
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We conclude that vulnerability curves do reflect the true range of cavitation pressures in plants. The cavitation pressure is highly variable depending on habitat, species, and organ. Woody perennial species in deserts are extremely resistant to cavitation, with 100% loss of conductivity in stems occurring as low as -13 MPa (Kolb and Davis, 1994
; Sperry, 1995
). Plants growing in wetter habitats can be completely cavitated by -1.5 MPa (Tyree et al., 1991
). Roots, in particular, appear to be exceptionally vulnerable to cavitation with 50% cavitation at pressures as high as -0.5 MPa (Sperry et al., 1998
).
There is no necessary conflict between the available data on cavitation resistance drawn from vulnerability curves and the recent reports of very vulnerable xylem in corn roots and sunflower petioles using a combination of flash-freezing and microscopic observation (Canny, 1997
; McCully, Huang, and Ling, 1998
). The freezing experiments indicate considerable cavitation at pressures of approximately -0.3 MPa, and they have been used as support for the compensation theory (Canny, 1998a
). While it is desirable to make a direct comparison between the two methods, at face value the collective data simply indicate that in some species and organs, some of the xylem can cavitate at relatively high negative pressure. There is no evidence from flash-freezing experiments that all of the xylem is cavitated when it should not be according to the cohesion theory.
Observations of frozen xylem have suggested refilling of the more vulnerable conduits in sunflower and corn within a few hours of their cavitation (Canny, 1997
; McCully, Huang, and Ling, 1998
). Refilling is not precluded by the cohesion theory and has been documented on several occasions in a wide variety of species (for example, Hacke and Sauter, 1996
; Sperry et al., 1994
). However, the theory does set lower limits on the xylem pressure allowing the dissolution of gas and bubble collapse, assuming hydraulic contact between the water in the refilling conduit and the surrounding transpiration stream. Under the best of circumstances (a steady stream of gas-free xylem sap running past an air bubble, or a bubble of pure water vapor), refilling could occur for xylem pressures a few kilopascals below the vapor pressure of water (approximately -0.12 MPa relative to atmospheric (Yang and Tyree, 1992
).
Refilling under lower pressures has been reported not only for the frozen xylem studies in corn and sunflower (McCully, Huang, and Ling, 1998
), but for conifers (Edwards et al., 1994
) and dicot trees (Salleo et al., 1996
). These interesting findings need more confirmation and study, but do not by themselves require a new theory of water transport. There are a number of possible mechanisms by which refilling could occur in the presence of low negative pressures. These mechanisms could involve the action of living cells, including some of the reverse-osmosis processes mentioned by Canny (1998a)
and Milburn (1996)
. Also required would be a means of isolating the refilling conduit from the water pressure of the surrounding xylem.
To be able to publish in 1998 a paper the purpose of which is to show that plants do generate negative pressures as required by the cohesion theory is an indication of how stubborn the taboo against negative pressure is in some circles. This disbelief persists in spite of over a century of experiment and theory demonstrating the tensile strength of water and the generation of large negative pressures in simple laboratory systems (citations in Pickard, 1981
). A branch of physical chemistry is devoted to the study of metastable liquids, including water (Trevena, 1975
; Debenedetti, 1996
). If Berthelot could generate a negative pressure of -5 MPa in the laboratory glassware of the 1800s (Dixon, 1914
), it does not seem unusual that hundreds of millions of years of natural selection could accomplish the same feat in the xylem conduits of plants.
In contrast, the requirements of compensating pressure theory are quite a bit more remarkable and implausible (see Comstock, this issue). According to the theory, once water enters the xylem of the roots, it moves down a gradient in pressure just as in the cohesion theory (Canny, 1998a
). The total pressure drop required to overcome gravity and friction in the root-to-leaf xylem path can exceed 1.0 MPa based on transpiration rate and values of xylem conductivity measured with techniques similar to those in this paper (Tyree et al., 1991
, their fig. 7). Although Canny disputed these pressure estimates on the grounds that conductivity measurements are influenced by tissue pressure (Canny, 1998a
), our results have shown that conductivity measurements are not influenced by tissue pressure. So, for the compensating pressure mechanism to keep pressures above -0.3 MPa throughout the continuous apoplastic flow path of the xylem (and it typically is continuous; Zimmermann, 1983
) while faced with a 1.0 MPa total pressure drop, the xylem pressure at the upstream end would need to be at least +0.7 MPa. Such pressures would cause bleeding from cuts in the xylem, yet this is not seen under transpirational conditions. Furthermore, the squeezing of the xylem conduits required to keep the xylem pressure above -0.3 MPa would only increase the resistance to flow and the pressure drop, leading to a vicious cycle wherein the conduits must be squeezed out of existence. All this assumes that the lignified conduits could be squeezed in the first place by the expansion of turgid parenchyma tissue, when it is known that the bulk of the parenchyma tissue shrinks considerably under dehydrative stress, while the woody lignified tissue tends to maintain its dimensions (e.g., Brough, Jones, and Grace, 1986
). In any event, the experiments of Strasburger (1891)
showed over a century ago that living tissue was not necessary to sustain the movement of water in the xylem, and our data suggest that nothing has changed since that time.
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Briggs, L. J. 1950 Limiting negative pressure of water. Journal of Applied Physics 21: 721–722.
Brough, D. H., H. G. Jones, and J. Grace. 1986 Diurnal changes in water content of the stems of apple trees as influenced by irrigation. Plant, Cell and Environment 9: 1–7.
Canny, M. J. 1997 Vessel contents during transpiration: embolisms and refilling. American Journal of Botany 84: 1223–1230.[Abstract]
———. 1998a Applications of the compensating pressure theory of water transport. American Journal of Botany 85: 897–909.[Abstract]
———. 1998b Transporting water in plants. American Scientist 86: 152–159.[CrossRef]
Debenedetti, P. G. 1996 Metastable liquids. Princeton University Press, Princeton, NY.
Dixon, H. H. 1914 Transpiration and the ascent of sap. Macmillan, London.
Edwards, W. R. N., P. G. Jarvis, J. Grace, and J. B. Moncrieff. 1994 Reversing cavitation in tracheids of Pinus sylvestris L. under negative water potentials. Plant, Cell and Environment 17: 389–397.[CrossRef]
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Saliendra, N. Z., J. S. Sperry, and J. P. Comstock. 1995 Influence of leaf water status on stomatal response to humidity, hydraulic conductance, and soil drought in Betula occidentalis. Planta 196: 357–366.
Salleo, S., M. A. Lo Gullo, D. De Paoli, and M. Zippo. 1996 Xylem recovery from cavitation-induced embolism in young plants of Laurus nobilis: a possible mechanism. New Phytologist 132: 47–56.[CrossRef][Web of Science]
Sperry, J. S. 1995 Limitations on stem water transport and their consequences. In B. L. Gartner [ed.], Plant stems: physiology and functional morphology, 105–124. Academic Press, San Diego, CA.
———, F. R. Adler, G. S. Campbell, and J. P. Comstock. 1998 Limitation of plant water use by rhizosphere and xylem conductance: results from a model. Plant, Cell and Environment 21: 347–359.[CrossRef]
———, K. L. Nichols, J. E. M. Sullivan, and S. E. Eastlack. 1994 Xylem embolism in ring-porous, diffuse-porous, and coniferous trees of northern Utah and interior Alaska. Ecology 75: 1736–1752.[CrossRef][Web of Science]
———, and N. Z. Saliendra. 1994 Intra- and inter-plant variation in xylem cavitation in Betula occidentalis. Plant Cell and Environment 17: 1233–1241.
———, ———, W. T. Pockman, H. Cockhard, P. Cruiziat, S. D. Davis, F. W. Ewers, and M. T. Tyree. 1996 New evidence for large negative xylem pressures and their measurement by the pressure chamber method. Plant, Cell and Environment 19: 427–436.[CrossRef]
Steudle, E. 1993 Pressure probe techniques: basic principles and application to studies of water and solute relations at the cell, tissue and organ level. In J. A. C. Smith and H. Griffiths [eds.], Water deficits: plant responses from cell to community, 5–36. Bios Scientific Publishers Ltd., Oxford.
———. 1995 Trees under tension. Nature 378: 663–664.[CrossRef]
Strasburger, E. 1891 Ueber den Bau und den Verrichtungen der Leitungsbahnen in den Pflanzen. Histologische Beitraege. Gustav Fischer, Jena.
Trevena, D. H. 1975 The liquid phase. Wykeham Publications Ltd., London.
Tyree, M. T. 1997 The cohesion-tension theory of sap ascent: current controversies. Journal of Experimental Botany 48: 1753–1765.
———, D. A. Snyderman, T. R. Wilmot, and J. L. Machado. 1991 Water relations and hydraulic architecture of a tropical tree (Schefflera morototoni). Plant Physiology 96: 1105–1113.
Yang, S., and M. T. Tyree. 1992 A theoretical model of hydraulic conductivity recovery from embolism with comparison to experimental data on Acer saccharum. Plant, Cell and Environment 15: 633–643.
Zimmermann, M. H. 1983 Xylem structure and the ascent of sap. Springer-Verlag, Berlin.
Zimmermann, U., F. C. Meinzer, R. Benkert, J. J. Zhu, H. Schneider, G. Goldstein, E. Kuchenbrod, and A. Haase. 1994 Xylem water transport: is the available evidence consistent with the cohesion theory? Plant, Cell and Environment 17: 1169–1181.[CrossRef]
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