Journal of Experimental Botany, Vol. 52, No. 357, pp. 783-789,
April 15, 2001
© 2001 Oxford University Press
Original Papers |
Intra-specific variation in xylem cavitation in interior live oak (Quercus wislizenii A. DC.)
1 Department of Biology, Augustana College, Sioux Falls, SD 57197, USA
2 Department of Agronomy and Range Science, University of California, Davis, CA 95616, USA
3 Department of Land, Air, and Water Resources, University of California, Davis, CA 95616, USA
Received 12 September 2000; Accepted 5 October 2000
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Abstract |
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Xylem cavitation induced by water stress reduces plant hydraulic conductance and can indicate the habitat a species evolved in and its phylogenetic background. Species differ widely in cavitation resistance, but less is known about intra-specific variation. Cavitation resistance was assessed for field-collected adult and sapling size classes from three populations of interior live oak (Quercus wislizenii A. DC.) in California, USA. Root and stem cavitation resistance of two-year old seedlings from a greenhouse experiment was also measured. Cavitation resistance curves were determined by injecting air into the vascular system to induce cavitation and measuring the subsequent decline in hydraulic conductance. Based on the air-seeding hypothesis, the absolute value of the air pressures should be equivalent to the tensions that cause cavitation under dehydrating conditions. Conductance declined exponentially with applied pressure for both roots and stems. Comparisons between populations did not reveal significant differences despite good statistical power. The 50% loss in conductance point occurred between 1.0–1.6 MPa; conductance declined more slowly thereafter. Conductance was 21–30% of maximum at 4.0 MPa and 7–14% at 8.0 MPa. Saplings exhibited a nearly identical pattern compared with adults except at 4.0 MPa, where saplings exhibited slightly less cavitation (7%). Greenhouse seedling stems were more resistant compared with both field-collected adults and with seedling roots. The 50% loss in conductance point occurred at 0.83 and 2.6 MPa for seedling roots and stems, respectively. Seedling stems maintained conductance of 20.9% at 8.0 MPa while most roots were fully cavitated between 5.0–8.0 MPa.
Key words: Xylem cavitation, size class, population, Quercus, drought stress.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Xylem cavitation induced by water stress is common and can result in substantial seasonal losses in xylem conductance in trees (Tyree and Sperry, 1989
; Alder et al., 1996). Several mechanisms for drought-induced xylem cavitation have been proposed; currently, the mechanism with the most experimental support is the air-seeding hypothesis (Zimmerman, 1983; Sperry and Tyree, 1988; Tyree et al., 1994). Air seeding occurs when an air bubble is pulled into a water-filled conduit from an adjacent air-filled conduit through pores in inter-conduit pit membranes in the xylem conduit wall. The primary factor determining the vulnerability to cavitation therefore, is the permeability of the interconduit pit membranes (Sperry and Tyree, 1988). The size of the pores in the pit membranes determines the negative pressure at which bubbles would be aspirated and cause xylem element cavitation (Tyree et al., 1994). It is also possible to push a bubble through an intervessel pit pore by pressurizing the stem with a positive pressure equal in absolute value to the negative pressure normally required to force air through the pores in the intervessel pits (Sperry and Saliendra, 1994). This method was used to determine cavitation vulnerability curves, i.e. the relationship between the loss in hydraulic conductance due to xylem cavitation and xylem tension.
The shapes of vulnerability curves vary greatly among different taxa. It is thought that they reflect the water-stress environment under which the plants have evolved and that these curves may provide insight into a taxon's ability to tolerate water stress (Tyree and Sperry, 1988, 1989; Tyree and Ewers, 1991; Tyree et al., 1994). Although differences between species can be quite pronounced, much less is known concerning the variation in this trait within a species. The choice was made to assess intraspecific variation in Q. wislizenii, an evergreen oak species that is widely distributed in California, inhabiting the Sierra and Coastal foothills below about 1500 m. The region has a ‘Mediterranean-type’ climate with a dry season that can extend from April to November. The region also encompasses a variety of soil types and parent materials and areas can differ in the extent of summer drought (Griffin, 1973, 1988; Momen et al., 1992). These factors may result in differences in the minimum water potential experienced for the various populations and size classes of Q. wislizenii. Variation in 
pre values have been shown for different size classes and populations of Q. douglasii (Griffin, 1973; SL Matzner, unpublished results) which co-occurs with Q. wislizenii over much of its range (including the three study sites). Size class differences in pre have also been shown for Q. agrifolia, another evergreen oak which is dominant along the coast of central and southern California (Griffin, 1973). The objectives of this study were to assess intraspecific variation in vulnerability to xylem cavitation in Q. wislizenii. Comparisons were made between three populations, two size classes and between roots and shoots from a separate greenhouse experiment.
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Materials and methods |
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Field-collected plant material
Field measurements and plant collections were made from oak woodland communities at three sites in California, USA where Q. wislizenii and Q. douglasii Hook and Arn. (blue oak) are co-dominant. The three sites were the Sierra Foothill Research and Extension Center (SFS, Yuba county, 121°18'W, 39°15'N), Hopland Research and Extension Center (HFS, Mendocino county, 123°04'W, 39°00'N), and San Joaquin Experimental Range (SJER, Madera county, 119°44'W, 37°06'N). These sites differ in annual precipitation, with long-term annual rainfall averages of 720, 930 and 480 mm per year, respectively. All three sites have a Mediterranean-type climate characterized by cool wet winters and hot dry summers with little or no rain falling between April and November. The SFS site is located in the Northern Sierra Nevada foothills, the SJER site is located within the Central Sierra Nevada foothills and the HFS site is located in the southern part of the North Coast Range. Both sapling and adult size classes of Q. wislizenii were used in this study. All adults were reproductively mature and ranged in size from 3.8–65 cm diameter at breast height (DBH). The variation in DBH was due to the occurrence of multiple stemmed individuals that typically had a much smaller DBH. Sapling individuals were generally less than 2 m in height with basal diameters ranging between 1.6–3.8 cm.
Field measurements
Seasonal patterns of stomatal conductance (gs, mmol m-2 s-1) were measured at the SFS site between April and September of 1996 with a steady-state porometer (Li-Cor Inc., Lincoln, NE). Midday xylem water potentials (mid, MPa) of uncovered leaves were measured with a pressure chamber, PMS Inc., Corvalis OR. All gs and mid values were measured between 10.00 h and 15.00 h (solar time). Diurnal gs and mid measurements did not reveal large changes during that time period. Late-season gs and mid for all three sites were made between 29 June and 7 August, 1996. Soil water potentials were determined from soil samples using Peltier thermocouple psychrometers (JRD Merrill Specialty Equipment, Logan, UT) controlled by a CR7 data logger (Campbell Scientific Inc., Logan, UT), or were estimated from the relationships between the soil dielectric constant and laboratory-determined soil water potentials. Dielectric constant measurements of the soil were made using a Troxler soil moisture probe (Troxler International LTD, Research Triangle Park, NC).
Greenhouse experiments
Acorns were collected from all three sites in the fall of 1994. Germinated acorns were planted in December of 1994 in a naturally lighted greenhouse on the campus of the University of California, Davis, Yolo Co., California. Pots were made from PVC pipe, 80 in cm height, 15 cm in diameter, and were filled with 3:1 (v:v) fritted clay, sand mixture. The plants were kept well-watered using a 1/10 concentration nutrient solution (Epstein, 1972). Vulnerability curves were generated for individual plants between January and June of 1996.
Vulnerability curves
Stem segments, at least 20 cm in length and less than 1 cm in diameter, were selected. This usually included the current and previous year's growth for field collected specimens. Stem preparation followed the protocols described previously (Sperry and Saliendra, 1994); stem segments were flushed for at least 15 min at c. 0.1 MPa to refill any embolized vessels. A hydraulic head of 0.003 MPa was used to induce flow of a filtered (0.22 µm) 0.5% sodium hypochlorite perfusion solution (to prevent microbial growth) through the stem. Preliminary experiments indicated that the sodium hypochlorite solution worked as well as an HCl (pH<2.0) solution and that it caused less discoloration of the stems. Stem exudate was collected and weighed. Hydraulic conductance was defined as the mass flow rate through the stem divided by the pressure gradient across the stem segment (kg m s-1 MPa-1). A portion of each stem segment was sealed in a double-ended pressure sleeve. By increasing and holding the air pressure around the stem segment within the sleeve, air was forced into xylem elements, inducing cavitation. The amount of cavitation was quantified by measuring hydraulic conductance after applying pressures of 0, 0.5, 1.5, 4.0, and 8.0 MPa. For current year stem material, hydraulic conductance measured at 0 or 0.1 MPa of applied pressure is usually used as the maximum conductance and conductance measurements at higher pressure levels would be expressed as the percentage of that maximum. The results of this study, however, are being reported using conductance at 0.5 MPa of applied pressure as the maximum because it was necessary to use at least two years growth to get the required stem length. Old dysfunctional xylem elements may become refilled during the flushing process (JS Sperry, personal communication) and will artificially increase the maximum conductance, but may quickly cavitate, giving the appearance of greater vulnerability. Even under well-watered conditions, Q. wislizenii pre-dawn water potentials average –0.5 MPa (Momen et al., 1992) so any vessels that cavitate below 0.5 MPa of applied pressure would very likely be non-functional during most normal field conditions. In order to make the comparisons consistent, conductance at 0.5 MPa was used as the maximum for the roots as well. Using conductance at 0.5 MPa as the maximum rather than conductance at 0 MPa did not change the general shapes of the curves however, the 50% loss in conductance point occurred at pressures 0.4–0.6 MPa higher.
Statistics
Population and size class differences in vulnerability to xylem cavitation were analysed using a repeated-measures analysis of variance in the general-linear-models procedure (SAS Inst. Inc., Cary, NC). Hydraulic conductance measurements on single stems at different levels of applied pressure were the repeated factor. Level of applied pressure was the within-subject effect. Population, size class and root versus stem differences in the shapes of the curves were tested as the within-subject by between subject interaction. Values of percentage of maximum conductance were transformed using a natural logarithmic transformation to meet normality assumptions.
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Results |
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Long-term (34–54 years) average annual precipitation ranged between 480–931 mm at the three sites (Table 1
). Year to year variation was considerable as can be observed from the standard errors and minimum precipitation values. Minimum annual precipitation was similar for SFS and SJER but higher for HFS. Average precipitation in 1996 was above average for both HFS and SFS, but below average for SJER. Long-term average temperatures (July–September) were similar for SFS and SJER, but slightly lower for HFS. Long-term average maximum temperatures (July–September) were similar for HFS and SFS. HFS had significantly higher late-season gs (P<0.01) and mid (P<0.01) values compared with SFS and SJER. The soil for the top 30 cm of soil at the three sites differed considerably for September 1995. Precipitation for 1995 was 600, 479 and 339 mm for HFS, SFS and SJER respectively, which was well below average. Values of soil at HFS were –3.5 MPa by September, which was considerably more positive than soil at SJER. SJER has very uniform, sandy textured soil with a gravelly parent material below about 1 m depth. The soil at this site averaged –6.1 MPa (SE=0.2) in July and by September the soil had dried beyond the detection limits of the thermocouple psychrometers (–6 to –7 MPa). SFS exhibited the most positive soil of the three sites. It should be noted, however, that the extremely rocky and high clay content soils at this site might have affected these measurements. The rocky soil may have limited root penetration and the observed soil value may be more positive than the soil volume that was accessible to the oak roots.
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Seasonal stomatal conductance (gs) declined exponentially as soil water potentials declined for adult, sapling and seedling size classes at the SFS site (Fig. 1
). Stomatal conductance was between 250–350 mmol m-2 s-1on 18 June when soil water potentials averaged –0.5 MPa. Stomatal conductance for the three size classes was highest at this time with adults having slightly higher, and seedling slightly lower conductance than saplings. Between 18 June and 3 July, stomatal conductance declined slightly for adults and saplings, but declined much more for seedlings, reflecting their greater dependence on the more rapidly drying, shallow soil layers. On 8 August, the average maximum mid-day xylem water potential for adults was –2.27 MPa (SE=0.16, n=3) and stomatal conductance fell to between 15–30% of the June values, despite only a modest decline in average soil water potential (0–60 cm depth) to about –1.0 MPa. By 18 September, soil water potential (0–60 cm depth) had dropped to –2.8 MPa, but adult xylem water potential values dropped only slightly to –2.68 MPa (SE=0.24, n=3) and stomatal conductance was still at 10–20% of the June rates.
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Hydraulic conductance declined rapidly with increasing applied pressure for Q. wislizenii adults from three Californian populations (Fig. 2
). The 50% loss in conductance point occurred between 1.1–1.3 MPa of applied pressure. With 1.5 MPa of applied pressure, hydraulic conductance fell to between 43–48% of maximum. Conductance was maintained between 21–22% at 4.0 MPa of applied pressure and between 7–13% at 8.0 MPa. Despite slight differences in the 50% loss in conductance point and in hydraulic conductance at different levels of applied pressure, repeated measures analyses did not reveal significant differences in the shapes of the curves among adults from the three sites.
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The pattern of sapling hydraulic conductance with increasing applied pressure was very similar to the pattern found for adults (Fig. 2
). The 50% loss in conductance point occurred between 1.0–1.6 MPa for saplings. By 1.5 MPa, hydraulic conductance had declined to between 40–52% of maximum for all sites; conductance was between 27–30% at 4.0 MPa and by 8.0 MPa between 10–14%. Differences in the shapes of the curves for saplings among the three populations were not significant.
Within population comparisons between adults and saplings (Fig. 2) did not reveal significant differences in the vulnerability to xylem cavitation. Comparison of adults and saplings (Fig. 3) across all sites however, did reveal a significant pressurexsize class interaction (P<0.014) which was due to slightly higher conductance for saplings (28.5%) compared with adults (21.8%) at 4.0 MPa. The 50% loss in conductance point occurred at similar applied pressures for both saplings and adults (1.22 and 1.15 MPa, respectively). Conductance at 1.5 MPa was similar for adults (45%) and saplings (44.8%). By 8.0 MPa of applied pressure, conductance was only slightly higher for saplings (11.5%) compared to adults (10.4%).
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Roots and stems of greenhouse-grown seedlings both exhibited exponential declines in conductance with increasing applied pressure (Fig. 3
). Seedling stems were significantly more resistant than both field-collected adults (P<0.0001) and seedling roots (P<0.0001). The 50% loss in conductance point occurred at 2.6 MPa of applied pressure for stems compared with 1.4 MPa for roots. At 1.5 MPa conductance was 64.1% for stems compared with 36.9% for roots. Conductance was 44.9% and 18.1% and 29.1% and 6.1% for stems and roots, respectively, at 3.0 and 5.0 MPa. By 8.0 MPa, conductance for stems and roots was 20.0% and 1.1%, respectively.
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Discussion |
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The seasonal pattern of gs was similar for adults, saplings and seedlings. In general, gs declined exponentially, with maximum rates early in the season, a rapid decline with moderate declines in soil water potential, followed by a much slower decline with decreasing soil water potential. This pattern is similar to patterns reported for other oak species subjected to decreasing water potential (Epron et al., 1993
; Damesin and Rambal, 1995; SL Matzner, unpublished results).
As water potentials decline, most angiosperm species (including several oak species, Higgs and Wood, 1995) exhibit a sigmoidal pattern with little or no loss in conductance initially, followed by a rapid decline (Tyree and Sperry, 1989; Tyree and Ewers, 1991; Tyree et al., 1994). In contrast, Q. wislizenii stems exhibited an exponential decline in conductance with decreasing water potential (or equivalent applied pressure). This pattern is more similar to the pattern reported for roots (Sperry and Saliendra, 1994; Alder et al., 1996; Sperry and Ikeda, 1997). The 50% loss in conductance point for Q. wislizenii adults and saplings occurred between 1.0–1.6 MPa, comparable to several riparian tree species, (Tyree and Ewers, 1991; Tyree et al., 1994). Conductance between 9–13% of maximum was maintained even at 8.0 MPa of applied pressure however, indicating that this species is ultimately quite drought tolerant. Adult Q. wislizenii stems are much more resistant than would appear necessary based on mid values observed in this study (<–3.0 MPa). It should be remembered, however, that seedlings will be exposed to much more negative soil due to their more shallow root system (soil at SJER for example can be extremely dry). Seedlings of Q. douglasii which co-occurs at all three study sites can exhibit pre values of –7.0 MPa (SL Matzner, unpublished results).
Some of the differences between the shapes of the curves for Q. wislizenii compared with those reported for other species may be due to the age of the stem material used. Most measurements of vulnerability are made on current year growth. Because of the very slow growth of Q. wislizenii, this was not possible and stem segments consisted of at least two years growth. A recent study (Lo Gullo et al., 1995) compared one, two and three-year-old stem segments and found fairly dramatic increases in the vulnerability of xylem elements with age. At 2.32 MPa of applied pressure the percent loss in conductance increased from 30–68% in one-year-old stems compared with three-year-old stems. Increased vulnerability with age may be due to increased degradation of pit membranes in older xylem elements (Sperry et al., 1991).
Despite the observed differences in long-term annual rainfall averages and the differences in mid, populations of Q. wislizenii in this study did not differ in their vulnerability to xylem cavitation despite high statistical power. The small standard error bars obtained in this study reflect the power to detect within and among population differences. In fact, the response of adults and saplings at each site was surprisingly consistent. A study also compared mesic and xeric sites and did not find differences in xylem vulnerability between two populations of Fagus sylvatica (Tognetti et al., 1995). Other studies have shown more xeric populations have greater resistance to cavitation than more mesic populations. Differences were found between seedlings of two populations of Eucalyptus camaldulensis (Franks et al., 1995), while differences were also found between Pinus halepensis seedlings (Tognetti et al., 1997). Significant population differences were also found between a more mesic riparian site and a more xeric slope site in Acer grandidentum roots, although these differences were not found in stems (Alder et al., 1996).
Vulnerability curves for adults and saplings were only slightly different in this study. At least two other studies have found saplings to be more resistant to cavitation than adults. Sperry and Saliendra found size class differences in Betula occidentalis (Sperry and Saliendra, 1994), while a study by KJ Rice (unpublished results) found size class differences in saplings and adults of Eucalyptus crebra and Eucalyptus drepanophylla. Differences in vulnerability between size classes might be expected because of the greater susceptibility of small individuals as a result of their shallow and less extensive root systems. The lower gs rates for seedlings (Fig. 1) may reflect their greater susceptibility to lower minimum water potentials. Although juvenile Q. wislizenii water potentials were not measured in this study, juvenile pre have been shown to be more negative for Q. douglasii (Griffin, 1973; SL Matzner, unpublished results), which is the dominant woody species at all three of the study sites, and for another Californian evergreen oak, Q. agrifolia. The similarity in the response of gs to decreasing soil at SFS for saplings and adults, however, may indicate that saplings and adults were not being exposed to dramatically different water-stress regimes.
Stems and roots from two-year-old greenhouse-grown seedlings were also compared in this study. As stated in the Materials and methods, conductance at 0.5 MPa was used as the maximum for the roots in order to make comparisons consistent. The overall shape of the curves was similar, but the 50% of maximum conductance point for roots increased from about 0.8 MPa to 1.4 MPa. Differences between roots and stems would likely be greater than that shown in Fig. 3 (i.e. roots would be more vulnerable than the figure indicates). Roots also exhibited greater variation (as seen by the standard error bars) compared with stems. Both the roots and stems in this study exhibited an exponential decline with increasing applied pressure. As has been shown in other studies (Sperry and Saliendra, 1994; Alder et al., 1996; Hacke and Sauter, 1996; Sperry and Ikeda, 1997), roots of Q. wislizenii were significantly more vulnerable than stems. The greater vulnerability of roots may be important in limiting conductance and thereby slowing transpiration as water availability declines (Alder et al., 1996; Sperry and Ikeda, 1997). By limiting water uptake this may extend the time interval that water can be taken up. Since cavitated roots are more likely to be refilled than stems (Alder et al., 1996), root cavitation may be preferable to stem cavitation when rains come after a long summer drought.
In assessing the pattern of vulnerability in adults and saplings, it was first concluded that there was little evidence for either genetic variation in the vulnerability to xylem cavitation or plasticity in this trait for this species. The addition of the seedling data, however, indicates that seedlings grown under greenhouse conditions from acorns collected at the same study sites exhibited a more resistant pattern. It is possible that the observed differences between adults and seedlings may be the result of selection between the seedling and sapling stage, but perhaps it is more likely that they are due to plastic changes or some combination of selection and plasticity. Plastic adjustment could be due to the different growing environment, ontogenetic changes, or a combination of these factors. Recent experiments have indicated that a fritted-clay textured growth medium (similar to what was used for greenhouse-grown seedlings in this study) can induce greater resistance in stems of common beans (Phaseolus vulgaris), compared with plants grown in a loamy textured soil (SL Matzner, unpublished results). Currently, little is known concerning the amount of phenotypic plasticity that occurs in the vulnerability to xylem cavitation within species. Because vulnerability depends on the size of intervessel pits (Sperry and Tyree, 1988), changes in vulnerability should only occur with new xylem production or as old xylem elements degrade. Low phosphorus concentrations have been shown to increase xylem susceptibility as a result of decreased pit membrane diameter in Populus (Harvey and van den Driessche, 1997). Water stress has been shown to reduce xylem vessel size in Vitus vinifera L. (Lovisolo and Schubert, 1998) and this may result in less vulnerable xylem because vulnerability has been shown to be negatively correlated with vessel diameter within a species (Sperry and Saliendra, 1994; Lo Gullo et al., 1995). Sperry and Ikeda observed a tendency for up-slope roots to be less vulnerable than down-slope roots and suggest that within a root system, adjustments in cavitation resistance may be possible (Sperry and Ikeda, 1997).
The response exhibited by greenhouse seedling stems does indicate that the potential for selection and/or plasticity does exist. If plastic adjustment is possible for Q. wislizenii, it is curious that adults and saplings at the three sites exhibited such a uniform response. The lack of differences in saplings and adults and between populations may simply reflect the greater importance of cavitation resistance at the seedling stage and/or in roots.
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Acknowledgments |
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This work was supported by NSF grants DEB 91-23989 and IBN 95-20679. I would like to thank Gerry Weight in the LAWR machine shop for machining the pressure sleeve. I would also like to thank Dr John Sperry for schematics of the pressure sleeve, protocols for measuring hydraulic conductance, general advice and several personal communications.
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Notes |
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4 To whom correspondence should be addressed: Fax: +1 605 274 4718. E-mail: matzner{at}inst.augie.edu
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References |
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