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Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824-1312 USA
Received for publication November 5, 1999. Accepted for publication May 2, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: embolism • frost depth • phenology • Rhus • roots • Sassafras • seasonal conductivity • stems • tree • xylem vessels
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Sperry and Sullivan, 1992
; Tognetti and Borghetti, 1994
; Magnani and Borghetti, 1995
; Hacke and Sauter, 1996
). However, seasonal studies of xylem embolism are lacking for roots of woody plants that experience severe winter freezing conditions.
Freezing-induced embolism occurs in stems when the xylem sap freezes and air bubbles are forced out of solution. Upon thaw, and especially when the stem xylem sap is under negative pressure (tension), the air bubbles expand, forming embolisms that block water transport. The larger the xylem conduit, the more vulnerable it is to cavitation during freeze-thaw events (Ewers, 1985
; LoGullo and Salleo, 1993
; Sperry et al., 1994
; Sperry, 1995
; Davis, Sperry, and Hacke, 1999
). In addition, with wider conduits it is more difficult to reverse embolism once it has occurred. By Henry's law, the critical pressure (Pc), that is, the minimum pressure required to dissolve an embolism, given sufficient time, is equal to -4
/D, where is surface tension and D is vessel diameter in µm (Tyree and Yang, 1992
). At 25°C, Pc = -284 kPa µm/D. Therefore, embolism reversal can occur at lower (more negative) pressures in narrow vessels than in wide vessels, for instance, at -5.7 kPa for 50-µm diameter vessels vs. at -0.6 kPa for 500-µm vessels.
In general, stems of temperate woody plants are able to overcome the potentially debilitating effects of freezing-induced embolisms. The two most well-known mechanisms for embolism recovery are replacement of embolized xylem by new xylem production and refilling of embolized xylem by positive root pressure. In temperate ring-porous trees the wide earlywood vessels of stems generally remain conductive for only one growing season. Such trees can tolerate the loss of function in the previous year's growth rings because of the production of new, wide diameter, earlywood vessels in the early spring, prior to leaf maturation (Zimmerman, 1983
; Ellmore and Ewers, 1986
; Cochard and Tyree, 1990
; Sperry et al., 1994
; Hacke and Sauter, 1996
). Xylem refilling by positive root pressure has been reported for many temperate deciduous diffuse-porous trees and for temperate species of Vitis (Scholander, Love, and Kanwisher, 1955
; Sperry et al., 1987, 1994
; Sperry, Donnelly, and Tyree, 1988a
; Sperry, 1993
), but not for temperate ring-porous trees. Root pressures, which are able to dissolve embolisms by raising the pressure within the xylem conduits to atmospheric pressures or above, originate from water absorption into the roots. Positive root pressures have been detected for many plants when transpiration levels are minimal such as before dawn, during rainstorms, and in the spring before bud burst (Sperry et al., 1987
; Cochard, Ewers and Tyree, 1994
; Ewers, Cochard and Tyree, 1997
; Fisher et al., 1997
).
Some studies suggest that roots of diffuse-porous trees are potentially more vulnerable to drought-induced embolism than stems. This is based upon vulnerability curves of roots vs. stems in Betula occidentalis (Sperry and Saliendra, 1994
) and Acer grandidentatum (Alder, Sperry, and Pockman, 1996
). Because tree roots tend to have wider vessel diameters than stems (Zimmermann and Potter, 1982
; Gasson, 1985
; Gartner, 1995
; Pate, Jeschke, and Aylward, 1995
; Ewers et al., 1997
), roots could also be more vulnerable to freezing-induced embolism.
The present study investigated the seasonal patterns of xylem conductivity and embolism in the roots vs. stems of two clonal tree species, Sassafras albidum and Rhus typhina. Preliminary results suggested that in roots of Sassafras albidum, the vessels remained conductive for many years, vs. just one year in stems (Bosela and Ewers, unpublished data). As such, our hypothesis was that the roots, but not the stems, would show a seasonal reversal of winter freezing-induced embolism.
Both species in the study are ring-porous, temperate, deciduous trees that spread clonally from root buds that arise from horizontally growing parent roots. The clonal growth habit of the two species allowed for genetically identical samples to be taken monthly from clones throughout 1996 and 1997. Frost gauges were used to determine whether the soil water surrounding the horizontal parent roots froze. In addition to monitoring the seasonal change in conductivity and embolism, various phenological events were recorded including bud burst, cessation of leaf growth, cessation of twig elongation, peak flowering, appearance of fruit (Rhus only), leaf color change, and leaf drop. The mechanism for possible recovery from winter embolism was also investigated using bubble manometers to measure root pressure. In addition, crystal violet dye was used to mark the conducting xylem elements in roots and stems sampled throughout the study period, to see which elements were more prone to embolism and to see whether embolism was related to vessel diameter.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Each stem arising from the horizontal root system was considered a ramet. The smallest sampled Sassafras and Rhus clones had 119 and 161 ramets, respectively. Four clones of each species were monitored in 1996–1997 and eight in 1997–1998. In 1996, 32 Sassafras and 34 Rhus ramets were sampled, vs. 52 ramets of each species in 1997. Different clones of a species were separated from each other by more than 160 m in each case, so each clone was assumed to be a separate genet.
Hydraulic conductance
Measurements were made as described by Sperry, Donnelly, and Tyree (1988b)
and Tyree and Ewers (1991)
. A single ramet from each of the clones was sampled per month throughout the monitoring period. The horizontal root systems of the sampled ramets were excavated to (25 cm both proximally and distally. The depth of the root from the soil level was recorded in each case. Roots were cut under water in the field and ramets were transported to the laboratory in water. Stem, "junction," and root segments were then cut under water to 
14 cm and the flow rate per unit pressure gradient (Kh) and percentage embolism were measured using a Sperry Apparatus (Sperry, Donnelly, and Tyree, 1988b
). The junction segments were actually the basal segments of the stem, immediately above the point of attachment to the root. For Sassafras, in most cases the 14-cm junction segments had been entirely subterranean. In Rhus, the basal portion of the junction segment was always subterranean in the intact plant, but the upper portion had usually been above soil level. The cut ends of all segments were recut with a razor blade and connected to sections of vinyl tubing. A solution of 10 mol/m3 citric acid (pH = 3) was used for conductivity measurements in order to discourage microbial growth. The solution was run through a 0.2-µm mesh Gelman filter. A pressure head of 2.5 kPa was used for measuring the conductivity in the native state (Kh initial), and a pressure of 173 kPa was used to flush out any embolism. Volumetric flow rate was determined with a 0.1-mL pipette and a stopwatch. This process was repeated until a maximum value (Kh max) was achieved.
To determine which were the conductive xylem elements, segments were perfused for 15 min with 0.5% crystal violet to mark conductive vessels. Dyes were used both on control segments, to represent the native or initial state, and on segments with the embolisms removed (final).
Specific conductivity (Ks), which is Kh divided by the xylem area, was calculated from the entire xylem diameter (not just sapwood), with the pith area subtracted. We used total xylem area since the sapwood area of the stems was often very narrow and difficult to measure accurately.
Anatomical study
To determine the number of conductive xylem growth rings, all segments were cut 3 cm from the perfusion port on a band saw and recut with a fresh razor. The smooth end was then examined under a dissecting microscope so that the number of xylem growth rings and the number of active rings could be determined. A growth ring was not counted as conductive unless it had more than 20 conductive vessel elements in transverse view.
To determine diameters of conductive vessel elements, all segments from July, September, October, and November of 1997 were cut transversely with a sliding microtome set at a thickness of 40 µm. Sections were rapidly moved through an ethanol-xylene dehydration series and mounted in permount. Vessel lumen areas were measured using a light microscope interfaced with a high-resolution CCD video camera and multi-scan analog monitor (model VE 1000 CCD, Dage-MTI, Inc., Michigan City, Indiana, USA), and image analysis was achieved using NIH Image 1.5. All the vessels in a randomly selected pie-shaped wedge were sampled for each slide, with N > 100 vessels per slide. In some of the smaller roots all the vessels in the transverse section were measured. Vessel diameters were calculated from lumen areas, and vessel diameters <25 µm were excluded in order to avoid including fibers within the measurements.
Macerations were done to determine vessel diameter for comparison with the image analysis technique, as described by Hargrave et al. (1994)
. Vessel members were sampled randomly in the fashion described by Ewers and Fisher (1989)
. Vessel lumen diameters were measured at vessel element mid-points with an optical micrometer, with 100 vessels sampled for roots, junctions, and stems of each species.
Phenology
One sample ramet from each of the 16 clones was monitored throughout the 1997 growing season for recording phenological events. Five marked buds per tagged ramet were monitored for bud burst beginning in early May 1997. Twenty-two marked leaves in Sassafras and 16 in Rhus were measured weekly throughout the growing season in 1997, to generate the mean date of cessation of leaf growth. The same method was used to generate the mean date of cessation of twig elongation. Eight twigs were tagged and monitored per species. Peak flowering was described as mean date at which >50% of the individual flowers on an inflorescence were open. Appearance of fruit was noted only in Rhus. Mean date of fruit visibility was the date by which all five monitored female ramets in a clone had visible fruit. Mean date of leaf color change was calculated by averaging the dates that colored leaves appeared in each clone. Leaf drop was defined as the mean date by which all leaves on monitored ramets had fallen. Data was recorded every week through September and then every other week through leaf drop.
Root pressures
The possibility of positive root pressure was investigated using bubble manometers (Fisher et al., 1997
). For both species in 1996 and 1997, xylem sap pressures were measured on both junctions and horizontal roots in the early spring following soil thaw but before bud burst. Measurements for Sassafras began the first week and continued through the third week of May. For Rhus, measurements were taken in the first week of May and June. Root pressures were measured on at least one individual per clone for both species in 1996 and 1997. Manometers were attached in the evening and checked the following morning before dawn. The root or junction was then recut with a fresh razor and the vinyl tubing reattached. The same root or junction was checked for three consecutive days after which a new root or junction was selected.
Frost depth
The depth of soil frost was monitored for the winters of 1996–1997 and 1997–1998 with frost gauges (Rickard and Brown, 1972
). Five sites were monitored each winter, two of which remained the same for both years. Each site had five gauges that were placed throughout the clone. Gauges were inserted 1.0 m into the soil. Freezing depth was measured every other week from the first frost through the last frost and on any date when the air temperature was colder than had previously been recorded that season.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Percentage embolism
For both Rhus and Sassafras, stem segments and junctions were fully embolized during the periods from November through May. In contrast, the sampled roots were always conductive, although it should be noted that plants were not sampled in mid-winter when the soil was frozen (Fig. 1). Also for both species, following the spring thaw, stems and junctions remained nearly 100% embolized until after bud burst. The reduction in percentage embolism in stems and junctions was concomitant with the maturation of the new earlywood vessels, which occurred for Rhus by July in 1996 (by June in 1997), and for Sassafras by June in 1996 (by July in 1997). Even during the summer, the percentage embolism was fairly high in stems and junctions of both species in both years, ranging from 60 to 90%. Stems of both species were 100% embolized by October, which, notably, was before the first freeze (Fig. 1).
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75% in April to a low of 40% in November. In contrast, in Sassafras the root embolism increased from a low of 25% in April, up to 40% in the summer and fall of 1996, and up to 70% in the summer and fall of 1997 (Fig. 1).
Specific conductivity (Ks)
For Rhus, the mean segment diameters (±1 SE) for sampled roots, junctions, and stems were, respectively, 7.9 ± 0.5, 11.9 ± 0.2, and 9.7 ± 0.2 mm. For Sassafras the corresponding dimensions were, respectively, 5.5 ± 0.2, 10.0 ± 1.0, and 9.3 ± 1.0 mm.
In both years, for both species, Ks max values were always much greater in the roots than in the stems (Fig. 2). However, in Rhus, unlike in Sassafras, in November of each year, about one-half of the stems and junctions had a Ks max value of zero. The zero values were associated with abundant tyloses in the vessels of the current year's xylem. By April and May, all of the stems and junctions of Rhus had Ks max values near zero. By then all the vessels were plugged in a manner that could not be reversed by the high-pressure perfusion technique.
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Sassafras stems and junctions reached their peak Ks max values in July of 1997. These were means ±1 SE of 11.0 ± 7.6 kg·s-1·MPa-1·m-1 for stems and 2.5 ± 0.4 kg·s-1·MPa-1·m-1 for junctions. The lower values of Ks max in the junction segments were associated with greater heartwood formation in the lower stem (note that our ks values were based on the entire xylem area, not just the sapwood). Unlike in Rhus, stems and junctions of Sassafras always had Ks max values above zero, even when Ks initial values were at zero in the fall and spring. As in Rhus, the Ks max values for the roots changed very little seasonally and fluctuated around an overall mean in 1997 of 27.3 ± 2.7 kg·s-1·MPa-1·m-1. That was 3.6 times greater than the peak values for stems, and 10.9 times greater than for junctions.
Anatomical study
Percentage conductive lumen area
In the stems of both species, based on crystal violet dyes, the percentage conductive lumen area prior to embolism removal (initial or native state) declined from about 32% in July down to 0% in October/November (Fig. 3). In contrast, for roots of both species, there was no significant change in the percentage conductive lumen area from July through October/November, with values fluctuating around an overall mean of 42%. The junctions, not shown in Fig. 3, displayed a pattern very similar to the stems.
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For Rhus, stems (initial) had no conductive rings until June in 1997 (Fig. 4). For June, July, and August, stems (initial) had 1.0 ± 0.0 conductive rings. This value dropped back down to zero by October. In the roots (initial), the number of conductive rings did not show a seasonal pattern, with about four or five growth rings conductive at every sample date.
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In Sassafras, as in Rhus, the number of conductive xylem growth rings in stems (initial) prior to embolism removal varied seasonally (Fig. 4). From April to August of 1997, the number of conductive xylem growth rings increased from zero to 2.1 ± 0.9 rings. In August, most stems had only one conductive growth ring but one stem had seven out of seven growth rings conductive. The number of conductive growth rings decreased to zero again by October. Roots (initial) fluctuated around an overall mean of 5.1 ± 0.3 rings, and did not demonstrate a clear seasonal pattern.
Unlike in Rhus, following embolism removal, stems (final) of Sassafras had a significantly greater number of conductive rings than native stems (initial) (paired t test, P < 0.000005). The number of conductive xylem growth rings in stems (final) fluctuated around an overall mean of 3.6 ± 0.2 rings throughout the monitoring period but were always above zero. The average ratio of conductive rings to total rings was 0.9 ± 0.03, which indicated that most embolisms could be removed in this species by the high-pressure flow treatment. Junctions (data not shown) followed the same seasonal trend as stems. As with stems, in roots of this species removal of embolisms also resulted in a significantly greater number of conductive xylem growth rings (paired t test, P < 0.01).
Vessel diameters
In both species, for both stems and roots, there was no clear seasonal pattern during the period from July through September regarding diameters of conductive vessels as seen in the initial (native) state vs. final (embolisms removed) state. Therefore the vessel diameter results were pooled for that time period (Table 1). For roots of both species and for stems of Rhus, there was no difference in the mean diameter of stained vessels in the native versus final state. However, in stems of Sassafras, mean diameter of stained vessels was significantly lower in the initial state than in the final state, indicating that the wider vessels were more likely to be embolized. For stems of both species, in October and November none of the vessels were stained in the initial state, indicating that by October all vessels were embolized, regardless of vessel diameter.
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Root pressures
Bubble manometers were attached to both species in early spring of 1996 and 1997 before bud burst or significant leaf expansion had occurred. Manometers were attached to both the horizontally spreading roots near the junction (N = 12 Rhus and 3 Sassafras) and to stems cut off just above the root-stem junction (N = 11 Sassafras), with each predawn measurement repeated for 3 d. No positive pressures were detected in either species in either year.
Frost depth
The depth of soil freezing was variable between sites and between years. The deepest freezing at a site in the winter of 1996/1997 was 315 vs. 242 mm in the winter of 1997/1998. For the winter of 1996/1997, the mean maximum depth of frost at all sites was 204 vs. 149 mm in 1997/1998.
The roots of Rhus that were sampled in 1997 ranged in depth from 30 to 110 mm below the soil surface. This was shallower in each case than the depth of frost recorded by the 30 frost gauges in the previous winter, which ranged from 125 to 315 mm. Therefore all the sampled roots of Rhus were in soil that froze at least once in the previous winter (Table 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The relationship between ring-porous wood and leaf phenology has been widely recognized. The present study is consistent with previous studies that have indicated that Sassafras and Rhus leaf out later than diffuse-porous trees at the same location (McGee, 1986
; Lechowicz, 1995
). Presumably, temperate ring-porous trees, which have large diameter earlywood vessels, are much more vulnerable to freezing-induced embolism than diffuse-porous trees. In ring-porous trees the stem mostly depends upon the current year's xylem for conductivity. Fore example, in Ulmus americana 92% of the conductivity were attributed to the outer growth ring (Ellmore and Ewers, 1986
). Apparently there has been natural selection for ring-porous trees to delay maturation of their leaves and their new xylem until danger of frost is past, thus allowing the mature leaves to have a reliable water supply (Zimmerman, 1983
; Lechowicz, 1984
; Wang, Ives, and Lechowicz, 1992
).
In previous studies on temperate ring-porous trees, the wide earlywood vessels remained conductive for only one growing season, while the narrow latewood vessels remained conductive for several years (Zimmermann, 1983
; Ellmore and Ewers, 1986
; Cochard and Tyree, 1990
; Ewers and Cruiziat, 1991
). In those species the wide, but not the narrow vessels in the wood, were vulnerable to freezing-induced embolism. What may be peculiar to stems of Sassafras and Rhus is the fact that, in the fall, there was "preemptive" 100% winter embolism, affecting both wide and narrow vessels, before the first freezing temperatures. Perhaps the xylem embolism in the stems was due to successive degradation of vessel pit membranes, such as has been reported for the diffuse-porous tree Populus tremuloides (Sperry, Perry, and Sullivan, 1991
). By that model, degradation of the pit membrane would allow for greater vulnerability to drought-induced embolism, as allowed for by the air-seeding hypothesis (Zimmerman, 1983
).
Could it be to the plants' advantage to create preemptive embolism in stems? Populus tremuloides, Rhus, and Sassafras are all examples of trees that sprout from horizontal parent roots (Bosela and Ewers, 1997). Perhaps the preemptive embolism is related to the clonal habit. Preemptive embolism of stems, but not roots, would tend to enhance the hydraulic advantage of the parent roots over sprouts, allowing perhaps for aggressive clonal spread.
In terms of embolism, seasonal conductivity, and the number of conductive growth rings, the Sassafras and Rhus "junctions" behaved more like stems than roots. Although the junction segments were partly or completely below ground in the intact plants, the junction segments were, anatomically, the basalmost part of the stem. Thus the functional properties of the junction segments appeared to be related more to organ anatomy than to position relative to the soil level.
Percentage embolism values for Sassafras and Rhus were high even during the summer and they were never at 0%. The stems and roots used were several years old, and the high pressure perfusion treatment, to determine the Ks max value, may have refilled vessels that had long since become nonconductive. Based on results for Quercus (Cochard and Tyree, 1990
), if current-year stems were used in our study, the embolism values probably would have been much lower.
Embolism in the stems of Sassafras could be reversed in the early spring and late fall when stems were 100% embolized, suggesting the vessels remain free of blockage by gums, resins, or tyloses for several years even though they were nonconductive. In contrast, by November, conductance in the stems of Rhus could not be restored following high pressure perfusion, suggesting permanent blockage had occurred following embolism. In stems of black locust, Robinia pseudoacacia, tylose growth blocked the earlywood vessels of the outer growth ring by December (von Reichenbach, 1846
; Fujita et al., 1978
; Zimmermann, 1979
). In stems of Quercus rubra and Q. alba, the earlywood vessels became embolized with the first freeze in the fall, but tyloses did not completely block the embolized vessels until the next growing season (Cochard and Tyree, 1990
).
Values for Ks reported in this study were calculated using total xylem cross-sectional area, minus the pith, rather than sapwood area. The Ks values are thus lower than if they had been calculated using sapwood area. This is especially true for the stems, where much of the total xylem area was nonconductive.
The pattern of higher Ks values for roots vs. stems of Sassafras and Rhus is consistent with patterns seen in many trees and shrubs (Gartner, 1995
; Pate, Jeschke, and Aylward, 1995
). This can be attributed, in part, to the roots having many more conductive growth rings than the stems. In the case of Sassafras, but not Rhus, the roots had wider vessels than the stems, which would further add to Ks values for roots.
The soil water surrounding the sampled roots of Rhus, and about one-half of the sampled roots of Sassafras, froze during the previous winter. Whether the root xylem sap froze is unknown. By our data, the roots never became highly embolized during the 1996 and 1997 monitoring periods, and so our original hypothesis, that the roots would show seasonal reversal of freezing-induced embolism, was not supported. This was despite the fact that the average root vessels were wide enough, much greater than the critical 44 µm diameter of the Davis, Sperry, and Hacke (1999)
model, that they should have become completely embolized with a freeze/thaw cycle at modest negative xylem pressures. However, the difficulty of obtaining "natural" samples from the frozen soil prevented sampling throughout the winter. It may be that the winter xylem pressures in the roots were not negative enough to induce cavitation following freeze/thaw cycles. It is also possible that the roots became fully embolized in winter, but the embolisms were dissolved before our first measurements in April. If freeze/thaw embolism of the root xylem did occur, then roots apparently reversed embolism by a mechanism other than positive root pressures, since we could not detect positive root pressures in these species.
To conclude, for Sassafras and Rhus, the seasonality of embolism is quite different for woody roots vs. woody stems, but the extent to which this is due more to differences in embolism formation or embolism reversal is unclear. For stems of Laurus nobilis, Salleo et al. (1996)
reported reversal of embolism with xylem pressures below -1 MPa. However, the mechanism for embolism reversal at such low water potentials is arguable (Holbrook and Zwieniecki, 1999
; Tyree et al., 1999
). Within roots of plants, probably no new paradigm is needed to explain embolism reversal. When xylem pressures are at atmospheric pressure or only very slightly negative, given sufficient time, embolisms will dissolve according to Henry's Law (Tyree and Yang, 1992
; Ewers, Cochard, and Tyree, 1997
). Given the maximum vessel diameters in Rhus and Sassafras of 112 and 160 µm, respectively, embolism reversal could occur in the roots of these species at xylem pressures above -2.5 and -1.8 kPa. In Michigan, such conditions might occur whenever the plants are leafless and the soil is saturated with water, as is often the case during the time period from late fall through early spring.
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FOOTNOTES |
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2 Author for correspondence (ewers{at}msu.edu
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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