| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Department of Organismic Biology, Ecology, and Evolution, University of California,Los Angeles, California 90095-1606
Received for publication November 17, 1998. Accepted for publication March 19, 1999.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Key Words: Agave deserti; • embolism • hydraulic conductance • Opuntia ficus-indica; • vessel anatomy • water relations • xylem physiology
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
; Sperry and Tyree, 1988
; Sperry et al., 1996
). According to this mechanism, when the negative hydrostatic pressure within a xylem conduit is sufficient to overcome the capillary forces at the air–water interface in a pit membrane, an air bubble can be pulled into a water-filled conduit from an adjacent air-filled conduit. This air bubble is a "seed" for vaporization of the metastable water; expansion of the air bubble results in a vapor-filled conduit that is hydraulically dysfunctional. The pressure at which an air bubble is drawn through a pore in the pit membrane is a function of the pore diameter (the capillary equation; Nobel, 1991
). Experimentally, a positive air pressure applied to the outside of the water-filled xylem can force air across a pit membrane, having the same cavitational effect as a negative hydrostatic pressure within conduits in root and stem segments (Cochard, Cruiziat, and Tyree, 1992
). Recently, centrifugal force has also been used to cause cavitation by generating negative hydrostatic pressure within the xylem of excised plant segments (Alder et al., 1997
). These two methods agree well for branches of various temperate trees (Pockman, Sperry, and O'Leary, 1995
; Alder et al., 1997
), but their application to roots is limited to a single species, Betula occidentalis (Alder et al., 1997
).
A comparison of 60 temperate, tropical, and Mediterranean trees and shrubs shows a weak positive correlation of xylem conduit diameter with vulnerability to cavitation (Tyree, Davis, and Cochard, 1994
). Within an individual species, vulnerability to cavitation can increase with xylem conduit diameter, such as for B. occidentalis (Sperry and Saliendra, 1994
) and Populus balsamifera (Hacke and Sauter, 1996
) but not for others, such as Acer grandidentatum (Alder, Sperry, and Pockman, 1996
) and Alnus glutinosa (Hacke and Sauter, 1996
). Vulnerability to cavitation can also increase with xylem tissue diameter or plant segment diameter (Cochard, 1992
; Sperry and Ikeda, 1997
), despite the prediction of the air-seeding hypothesis that vulnerability to cavitation due to water stress should depend on the diameter of the pores in pit membranes.
Although a low xylem pressure can cause xylem water to cavitate, it also provides a larger driving force for water uptake from the soil. However, as xylem pressure decreases, as generally occurs during drought, complete loss of conductance will ultimately occur. The trade-off between an increasing driving force for water uptake and increasing cavitation results in a maximum water flow rate that occurs just before the complete loss of conductance (Sperry et al., 1998
). If excessive transpiration permits xylem pressures to decrease below the xylem pressure at which the rate of water flow is maximized, the subsequent complete loss of conductance will cause failure of the hydraulic system (Jones and Sutherland, 1991
). Hydraulic models for trees (Tyree and Sperry, 1988
) predict that plants will maximize water uptake by allowing xylem pressures to approach the critical xylem pressure.
The CAM succulents Agave deserti and Opuntia ficus-indica used in this study maintain high root water potentials during extended drought (Nobel, 1988
), and water uptake by their roots occurs mainly from wet soils (Nobel and Lee, 1991
; Nobel and Cui, 1992
). Cavitation occurs in the roots of A. deserti and O. ficus-indica at relatively high xylem pressures (Ewers, North, and Nobel, 1992
; North and Nobel, 1996
), limiting water uptake from the soil to the first few weeks of drought and thereafter reducing water loss from the succulent shoots back to a drying soil (Nobel and Cui, 1992
). In the present study, the loss of root hydraulic conductance due to cavitation accompanying decreasing xylem pressure (a vulnerability curve) was quantified using both air-injection and centrifugal methods. Because of its more mesic habitat (Nobel, 1988
), roots of O. ficus-indica were hypothesized to be more vulnerable to cavitation than those of A. deserti. Correlations between xylem dimensions and vulnerability to cavitation were also examined for each species. Vulnerability curves were used to predict the extent of cavitation that the roots would experience in a drying soil and to determine relative flow rates in the xylem under varying root xylem pressures. It was hypothesized that although appreciable cavitation could occur in these roots, the extent of the cavitation would be minimized through maintenance of relatively high root xylem pressures by the nearby succulent shoot tissue.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Xylem anatomy
Transverse sections were taken at midsegment from the excavated roots and stained with 0.05% toluidine blue O. The average area of vessel lumens was determined by tracing individual vessels with a digitizing tablet (Kurta, Altek Corp, Silver Spring, Maryland) using a camera lucida attached to a light microscope. For A. deserti, all vessels in a transverse cross section were measured. For O. ficus-indica, transverse sections were divided into four 90° radial sectors and 50 randomly selected vessels were measured from each sector, after which the number of vessels in an entire root cross section was determined. Vessels were nearly circular for both species, allowing vessel diameter to be calculated readily from vessel area.
Cavitation study
Two techniques were used to induce cavitation in the root xylem: an air-injection method (Cochard, Cruiziat, and Tyree, 1994
) and a centrifugal method (Holbrook, Burns, and Field, 1995
; Pockman, Sperry, and O'Leary, 1995
). For measurements using the air-injection method, a root segment was trimmed under water to 20 cm in length and then inserted into a cylindrical pressure chamber with an opening at each end. The root was sealed within the chamber with rubber stoppers and compression fittings that allowed the ends of the root segment to protrude from both ends of the chamber (Sperry and Saliendra, 1994
). To expel any embolisms that had occurred in the soil or during transport, 100 kPa of water pressure was applied to the distal end for 20 min, after which the water pressure on the distal end was reduced to 5.0 kPa. The volumetric flow rate of water (QV, in cubic metres per second) was then measured at the proximal end by collecting and weighing the extruded water in vials filled with cotton wool. The axial hydraulic conductivity (also called a conductance per unit length) of the root segment (Kh, in metres to the fourth power per megapascal per second) was calculated from
x (in metres) is the root segment length, and P (in megapascals) is the water pressure difference that caused flow along the root axis. After determination of the initial Kh, the air pressure in the chamber was increased to 500 kPa for 10 min to allow time for the pressurized air to enter the xylem conduits, after which the chamber pressure was reduced back to zero and Kh of the root segment was measured again. This process was repeated with progressively higher air pressures (in 500-kPa increments) until <5% of the initial Kh remained. The value of QV under no pressure difference was always <0.5% of QV under P. In addition, due to the small values of P used to cause flow (5–7 kPa), Kh was approximately constant along a root during a measurement.
For the centrifugal method (Alder et al., 1997
), a root segment was trimmed under water to 27 cm in length and attached to tubing that led to an analytical balance at the proximal end and to a water pressure source at the distal end while submerged. After expelling embolisms as above, the water pressure was reduced to 5.0 kPa. To account for flow that could occur without any pressure difference across a submerged root, the measurement of Kh at a particular pressure (Eq. 1) was preceded and followed by a measurement of QV under no pressure difference, which was subtracted from the QV under a pressure difference to give the actual QV for the calculation of Kh. After the initial (maximal) Kh measurement, the root was placed in a specially designed centrifuge rotor and spun about its longitudinal axis. The root ends were contained in water-filled L-shaped reservoirs that submerged them during centrifugation. The most negative pressure (Pxylem, MPa) experienced by the root xylem (at its center) equaled
 |
is the density of water (in kilograms per cubic metre), 
is the angular velocity (in radians per second), and r (in metres) is the distance from the center of the spinning root to the surface of the water that submerged the root tip. After spinning the root for 5 min at the desired negative pressure, Kh was measured within 5 min, which avoids refilling of the vessels (Alder et al., 1997
); this was repeated for progressively higher angular velocities until <5% of the initial Kh remained.
Vulnerability curves for both the air-injection and the centrifugal methods, which represent the cumulative percentage loss of Kh (Tyree and Sperry, 1988
), were expressed relative to the hydraulic conductance at the maximal pressure that the xylem of these roots experiences in wet soil (-0.25 MPa for A. deserti and -0.29 MPa for O. ficus-indica; Nobel and Lee, 1991
) and were fit with an exponential equation. The mean cavitation pressure was determined by plotting the percentage conductance loss per unit pressure change (rather than plotting the cumulative loss of conductance, as for a vulnerability curve) and taking the mean of this distribution based on the midpoint of each pressure change. For the analysis of the vulnerability curves in the following model, the osmotic pressure was assumed to be negligible so that the root xylem pressure (Pxylem) could be replaced by the root xylem water potential (
xylem; Nobel, 1991
; Tyree, 1997
).
Data were statistically analyzed by Student's t test and are presented as means ± 1 SE.
Model
Axial water movement along root xylem can be described by the equation (ignoring radial flow)
/dx (in megapascals per metre) is the water potential gradient along the xylem. Recognizing that QV is a constant along the xylem and that Kh depends on [Kh ()], as represented by a root vulnerability curve, Eq. 3 can be integrated from x = 0 to x = x after multiplying both sides by dx (Sperry et al., 1998
) 
distal is the water potential at x = 0 and proximal is the water potential at x = x. To determine the influence of cavitation on QV, Eq. 4 was solved by holding distal constant and progressively lowering proximal until QV converged to a constant value; this yielded the critical volume flow rate (QVcrit) at that particular distal and marks the threshold that will result in complete loss of conductance if it is exceeded (Sperry et al., 1998
). Because Kh() (represented by an exponential equation) mathematically never reached zero, QVcrit was taken at 98% loss of conductance. The relation between QV and proximal at a constant distal obtained from Eq. 4 under conditions of cavitation [decreasing Kh() as decreases] was compared to results obtained in the absence of cavitation [constant and maximal Kh() as decreases].
The relationship between QVcrit and distal indicates the maximum QV that could occur as distal varies. Measurements of root xylem water potential (xylem) in relation to soil water potential (soil) for these two species (Nobel and Lee, 1991
) were used to predict QV for actual roots by replacing proximal with xylem and distal with soil in Eq. 4, substituting the empirical data for the two variables, and solving for QV, which was then compared to QVcrit for that particular distal.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
|
|
|
proximal decreased (Fig. 4). At a distal of 0.0 MPa but with cavitation, QV increased up to a maximum (QVcrit) at a proximal of -3.3 MPa for A. deserti and at -2.05 MPa for O. ficus-indica (Fig. 4A), which were 32 and 34% of QV without cavitation, respectively. The value of QVcrit decreased slightly for a distal of -0.5 MPa, resulting in a QVcrit that was 21% of maximum for A. deserti and 17% for O. ficus-indica (Fig. 4B).
|
distal decreased for both species (Fig. 5), consistent with the decreasing driving force for water uptake. Values of QVcrit were above zero at a lower distal for A. deserti than for O. ficus-indica, owing to the higher vulnerability of O. ficus-indica to cavitation. Based on measurements of soil and xylem, water uptake was predicted only above a soil of -0.50 MPa for A. deserti and -0.48 MPa for O. ficus-indica (Fig. 5). Over the range of soil that water uptake would occur, QV averaged 22% of QVcrit for A. deserti and 30% for O. ficus-indica.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
). Smaller diameter conduits may not cause greater resistance to cavitation under water stress, but a causal link between pit membrane pore size and conduit diameter could explain the frequent correlation of vulnerability to cavitation and conduit diameter (Tyree, Davis, and Cochard, 1994
). At a developmental level, xylem conduit diameter typically is related to stele diameter, as the additional water uptake requirements of a maturing plant are met by larger diameter vessels or tracheids. For A. deserti and O. ficus-indica, the overall relative increase in the hydraulic conductivity Kh with stele diameter was similar, but the relative increase in mean vessel diameter was greater for A. deserti and the relative increase in the number of vessels was greater for O. ficus-indica. These observations are consistent with the lack of secondary growth in the monocotyledonous A. deserti (Carlquist, 1975
), which produces larger vessels that permit an increased xylem flow as its roots mature. Secondary growth in the vascular tissues of the dicotyledonous O. ficus-indica produces a greater number of vessels as the demand for water uptake increases.
As hypothesized, roots of O. ficus-indica were more vulnerable to cavitation than those of A. deserti, consistent with the relative habitat preferences and tolerances of low water potentials for these two species. Although the native habitat of O. ficus-indica is unknown, this species is found extensively throughout the tropics and subtropics, in contrast to the arid native habitat of A. deserti in the northwestern Sonoran Desert (Nobel, 1988
; Hickman, 1993
). For many species, drought tolerance can be correlated with vulnerability to cavitation (Carlquist, 1975
; Tyree, Davis, and Cochard, 1994
), such as for Pinus edulis and Juniperus osteosperma (Linton, Sperry, and Williams, 1998
) and three subspecies of Artemisia tridentata (Kolb and Sperry, in press).
Within the period of positive water uptake for hydrated plants (when soil > -0.5 MPa), significant cavitation occurs in these roots, with a predicted 24% loss of conductance for A. deserti and 40% for O. ficus-indica at a soil of -0.5 MPa. At this soil, the difference in cavitational loss of conductance between the two species is almost entirely determined by root vulnerability to cavitation, as the difference in the root xylem water potential (xylem) between the two species is only 0.02 MPa. At a soil below -0.5 MPa, water loss from the root to the drying soil is energetically favored, as xylem remains close to the water potential of the succulent shoot (Nobel, 1988
). As drought proceeds, the large cladodes (succulent stem segments) of O. ficus-indica maintain higher root and shoot water potentials than the water potential of the less succulent A. deserti. For instance, at the end of 3 mo of drought, xylem is approximately -1.3 MPa for A. deserti (North and Nobel, 1998
) and -0.7 MPa for O. ficus-indica (Goldstein, Andrade, and Nobel, 1991
), resulting in similar predicted losses of conductance of 69 and 62%, respectively. Six months of drought cause root water potentials of A. deserti to decrease to -2.0 MPa (North and Nobel, 1998
), with a predicted 87% loss of conductance. Rewetting of the soil after 30 d of drought partially refills cavitated conduits in both species (North and Nobel, 1995, 1996
), providing new lateral roots a hydraulic connection to the shoot. Therefore, although significant cavitation occurs in a wet soil during water uptake, cavitation in these species may be more significant in limiting water loss back to a drying soil, which occurs when soil becomes less than xylem (Nobel and Cui, 1992
).
Predicted values of QV based on empirical data were far below QVcrit, as a consequence of the relatively high xylem of A. deserti and O. ficus-indica. From an energy point of view, roots of these species would increase the rate of water uptake from the soil if xylem were lower. In addition, a lower xylem would increase the range of soil over which water uptake could occur, effectively prolonging the period of water uptake as drought progressed. This appears to be the case for Larrea tridentata, a ubiquitous shrub of the Sonoran and Mojave deserts, which experiences significant cavitation only at xylem pressures lower than -10 MPa and maintains transpiration throughout the entire year (Pockman, 1996
). For CAM succulents, however, high root water potentials are maintained by the succulent shoots. Thus, instead of xylem that is highly resistant to cavitation to allow water extraction during extended drought, these succulents are "drought avoiders" (Levitt, 1980
), where water uptake is limited to a relatively wet soil and long periods of drought are tolerated because of water storage in the succulent tissue.
The air-injection and centrifugal methods for measuring cavitation agreed for roots of A. deserti, as well as for Betula occidentalis (Alder et al., 1997
), providing additional evidence that cavitation occurs via air-seeding and strengthening the utility of the centrifugal method for measuring cavitation in roots. Recently, the interpretation of both methods has been challenged in the presentation of a "compensating pressure theory," which has been proposed as the replacement for the cohesion-tension theory of xylem transport (Canny, 1995
). In particular, the air-injection and centrifugal methods may cause complete cavitation at much less negative pressures than previously believed and conduits may be refilled by water from xylem parenchyma via a "compensating pressure" while the measurement of Kh is in progress, so that vulnerability curves actually reflect the limit at which parenchyma can refill xylem conduits (Canny, 1998
).
This compensating pressure hypothesis proposes that the maximum available "compensating pressure" for xylem refilling equals the osmotic pressure of the xylem parenchyma, leading to the prediction that the negative pressure at which xylem vessels can no longer be refilled corresponds to the osmotic pressure of the adjacent xylem parenchyma (Canny, 1998
). This interpretation appears to fit branches of some species (Pockman, Sperry, and O'Leary, 1995
; see analysis in Canny, 1998
) that experience nearly complete loss of conductance at a threshold of negative pressure that is equal but opposite in sign to the osmotic pressures of the xylem parenchyma. In contrast to branches, many root vulnerability curves show a continual increase in loss of conductance that may become asymptotic as xylem pressures continue to decrease (Sperry and Saliendra, 1994
; Alder et al., 1997
; Sperry and Ikeda, 1997
). The root vulnerability curves for A. deserti and O. ficus-indica generally follow a rectangular hyperbolic relation, with a high rate of initial loss that gradually decreases with lower xylem pressures. These roots do not appear to have a threshold of conductance loss that corresponds to the osmotic pressure of the xylem parenchyma (mean osmotic pressure of roots for A. deserti is 0.95 MPa; North and Nobel, 1998
) and therefore do not agree with the predictions of the compensating pressure theory. The asymptotic vulnerability curves for A. deserti and O. ficus-indica may be adaptive by allowing substantial cavitation at xylem pressures immediately below the point at which water uptake ceases (soil 
-0.5 MPa), thereby controlling water loss to the soil at high soil, while maintaining low levels of conductance at low xylem pressures for quicker recovery and refilling after a long drought.
|
FOOTNOTES |
|---|
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
———, J. S. Sperry, and W. T. Pockman. 1996 Root and stem xylem embolism, stomatal conductance, and leaf turgor in Acer grandidentatum populations along a soil moisture gradient. Oecologia 105: 293–301.[CrossRef][ISI]
Canny, M. J. 1995 A new theory for the ascent of sap—cohesion supported by tissue pressure. Annals of Botany 75: 343–357.
———. 1998 Applications of the compensating pressure theory of water transport. American Journal of Botany 85: 897–909.[Abstract]
Carlquist, S. 1975 Ecological strategies of xylem evolution. University of California Press, Berkeley, CA.
Cochard, H. P. 1992 Vulnerability of several conifers to air embolism. Tree Physiology 11: 73–83.[ISI][Medline]
————, P. Cruiziat, and M. T. Tyree. 1992 Use of positive pressure to establish vulnerability curves. Plant Physiology 100: 205–209.
Ewers, F. W., G. B. North, and P. S. Nobel. 1992 Root-stem junction of a desert monocotyledon and a dicotyledon: hydraulic consequences under wet conditions and during drought. New Phytologist 121: 377–385.[CrossRef][ISI]
Goldstein, G., J. L. Andrade, and P. S. Nobel. 1991 Differences in water relations parameters for the chlorenchyma and parenchyma of Opuntia ficus-indica under wet versus dry conditions. Australian Journal of Plant Physiology 18: 95–107.[ISI]
Hickman, J. C. 1993 The Jepson manual: higher plants of California. University of California Press, Berkeley, CA.
Hacke, U., and J. T. Sauter. 1996 Drought-induced xylem dysfunction in petioles, branches, and roots of Populus balsamifera L. and Alnus glutinosa (L.) Gaertn. Plant Physiology 111: 413–417.[Abstract]
Holbrook, N., M. J. Burns, and C. B. Field. 1995 Negative xylem pressures in plants: A test of the balancing pressure technique. Science 270: 1193–1194.
Jones, H. G., and R. A. Sutherland. 1991 Stomatal control of xylem embolism. Plant, Cell and Environment 18: 189–196.[CrossRef]
Kolb, K. J., and J. S. Sperry. 1998 Differences in drought adaptation between subspecies of sagebrush (Artemisia tridentata). Ecology, in press.
Levitt, J. 1980 Responses of plants to environmental stresses, 2nd edition. Academic Press, New York, NY.
Linton, M. J., J. S. Sperry, and D. G. Williams. 1998 Limits to water transport in Juniperus osteosperma and Pinus edulis: implications for drought tolerance and regulation of transpiration. Functional Ecology 12: 906–911.[CrossRef][ISI]
Nobel, P. S. 1988 Environmental biology of agaves and cacti. Cambridge University Press, New York, NY.
———. 1991 Physicochemical and environmental plant physiology. Academic Press, San Diego, CA.
———, and M. Cui. 1992 Hydraulic conductances of the soil, the root-soil air gap, and the root: changes for desert succulents in drying soil. Journal of Experimental Botany 43: 319–326.
———, and C. H. Lee. 1991 Variations in root water potentials: Influence of environmental factors for two succulent species. Annals of Botany 67: 549–554.
North, G. B., and P. S. Nobel. 1995 Hydraulic conductivity of concentric root tissues of Agave deserti Engelm. under wet and drying conditions. New Phytologist 130: 47–57.[CrossRef][ISI]
———, and ———. 1996 Radial hydraulic conductivity of individual root tissues of Opuntia ficus-indica (L.) Miller as soil moisture varies. Annals of Botany 77: 133–142.
———, and ———. 1998 Water uptake and structural plasticity along roots of a desert succulent during prolonged drought. Plant, Cell and Environment 21: 705–713.[CrossRef]
Pockman, W. T. 1996 Xylem Cavitation in Sonoran Desert Vegetation. Ph.D. dissertation. University of Utah, Salt Lake City, UT.
———, J. S. Sperry, and J. W. O'leary. 1995 Sustained and significant negative water pressure in xylem. Nature 378: 715–716.[CrossRef][ISI]
Sperry, J. S., F. R. Alder, 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]
———, and T. Ikeda. 1997 Xylem cavitation in roots and stems of Douglas-fir and white fir. Tree Physiology 17: 275–280.[ISI][Medline]
———, 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. Cochard, 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]
———, and M. T. Tyree. 1988 Mechanism of water stress-induced xylem embolism. Plant Physiology 88: 581–587.
Tyree, M. T. 1997 The cohesion-tension theory of sap ascent: current controversies. Journal of Experimental Botany 48: 1753–1765.
———, S. D. Davis, and H. Cochard. 1994 Biophysical perspectives of xylem evolution: Is there a trade-off of hydraulic efficiency for vulnerability to dysfunction? IAWA Journal 15: 335–360.[ISI]
———, and J. S. Sperry. 1988 Do woody plants operate near the point of catastrophic xylem dysfunction caused by dynamic water stress? Plant Physiology 88: 574–580.
Zimmermann, M. H. 1983 Xylem structure and the ascent of sap. Springer-Verlag, Berlin.
This article has been cited by other articles:
|
|
 |
|
  P. H. Maseda and R. J. Fernandez Stay wet or else: three ways in which plants can adjust hydraulically to their environment J. Exp. Bot., December 1, 2006; 57(15): 3963 - 3977. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. Trifilo, A. Gasco, F. Raimondo, A. Nardini, and S. Salleo Kinetics of recovery of leaf hydraulic conductance and vein functionality from cavitation-induced embolism in sunflower J. Exp. Bot., October 1, 2003; 54(391): 2323 - 2330. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. Martre, G. B. North, and P. S. Nobel Hydraulic Conductance and Mercury-Sensitive Water Transport for Roots of Opuntia acanthocarpa in Relation to Soil Drying and Rewetting Plant Physiology, May 1, 2001; 126(1): 352 - 362. [Abstract] [Full Text]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
, N = 17 roots) and Opuntia ficus-indica (
, N = 28 roots). Data are grouped into 10-µm-diameter classes and are means ± 1 SE. Hydraulic conductance was assumed to be proportional to (vessel diameter)4, as described for cylinders by the Hagen-Poiseuille relation (Nobel, 1991
, N = 8 roots) and for O. ficus-indica are from the centrifugal force method only (
) and O. ficus-indica (
) as calculated from empirical root water potentials (