JXB Advance Access originally published online on April 28, 2003
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Journal of Experimental Botany, Vol. 54, No. 387, pp. 1597-1605,
June 1, 2003
© 2003 Oxford University Press
Branch junctions and the flow of water through xylem in Douglas-fir and ponderosa pine stems
Received 22 November 2002; Accepted 10 March 2003
1 Department of Biological Sciences, University of Nevada, Las Vegas, NV 89154-4004, USA
2 Western Ecology Division, NHEERL, ORD, US Environmental Protection Agency, Corvallis, OR 97333, USA
3 To whom correspondence should be addressed. Fax: +1 702 895 3956. E-mail: schulte{at}ccmail.nevada.edu
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Abstract |
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Water flowing through the xylem from the roots to the leaves of most plants must pass through junctions where branches have developed from the main stem. These junctions have been studied as both flow constrictions and components of a hydraulic segmentation mechanism to protect the main axes of the plant. The hydraulic nature of the branch junction also affects the degree to which branches interact and can respond to changes in flow to other branches. The junctions from shoots of two conifer species were studied, with particular emphasis on the coupling between the downstream branches. Flow was observed qualitatively by forcing stain through the junctions and the resulting patterns showed that flow into a branch was confined to just part of the subtending xylem until a considerable distance below the junction. Junctions were studied quantitatively by measuring flow rates in a branch before and after flow was stopped in an adjacent branch and by measuring the hydraulic resistance of the components of the junction. Following flow stoppage in the adjacent branch, flow into the remaining branch increased, but considerably less than predicted based on a simple resistance analogue for the branch junction that assumes the two branches are fully coupled. The branches downstream from a junction, therefore, appear to be limited in their interconnectedness and hence in their ability to interact.
Key words: Hydraulic conductance, modelling, resistance, tracheids, water flow.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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The water flow pathway from the roots to the leaves of most plants passes through a number of structural divisions or branch junctions, particularly in the case of large woody plants. From the hydraulic perspective alone, branch junctions are important components in flow and have been studied extensively as possible flow constrictions or as segmentation points in the overall hydraulic architecture of trees, but not as they relate to how branches may interact hydraulically within a crown.
The view of junctions as constrictions considers the location of resistances along the hydraulic pathway within the plant and how these junctions might influence flow and the drop in water potential between the soil and the leaf. Tyree and Ewers (1991) speculated that the effect of a constriction at a junction might be to favour one pathway over another. Constriction at a junction could occur if the xylem conduits are narrower within the junction than in the upstream or downstream stems. From studies of primary xylem development, the nodal region at the base of leaves in Populus deltoides has been described as a constriction zone based on the observed reduction in number and width of vessels (Isebrands and Larson, 1977; Larson and Isebrands, 1978). In first-year twigs of Quercus, Lo Gullo et al. (1995) found that junctions contain a greater number of vessel endings and narrower vessels than do internodes. Ellison et al. (1993) found junction constrictions due to a reduction in vessel diameter, but the occurrence of such constrictions depended on the growth conditions of their plants. Tyree and Ewers (1991) argue, however, that, at least for some species, these constrictions may not be an important component of the total resistance of the pathway. Tyree and Alexander (1993) found that the effect of a constriction at a branch junction would be similar in terms of resistance to adding between 0.6 and 7.5 cm in height to trees that typically grow to 10–20 m heights. Thus, considerable variation appears to exist among plants in the extent of constriction within the junction and it has been speculated that a large degree of constriction may correspond with species having lower order branching patterns (Tyree and Ewers, 1991).
An additional role for junctions is presented in the hydraulic segmentation concept (Zimmermann, 1978, 1983), whereby structural features of the stem system lead to the preferential occurrence of cavitations in distal organs rather than main stems. Zimmermann (1983) speculated that hydraulic constrictions at leaf and branch junctions could be an architectural feature that confines cavitations to small expendable distal organs. As discussed by Tyree and Ewers (1991), this segmentation may arise in two forms; either a vulnerability segmentation, whereby distal organs have greater cavitation vulnerability, or a hydraulic segmentation whereby constrictions would lead to lower water potentials in distal organs and hence their preferential loss in comparison to the main stem. Tyree et al. (1993) argue that hydraulic segmentation alone without a vulnerability segmentation is not likely to be of significance for most plants. The role of branch junctions as a feature which enhances segmentation could also depend on the vulnerability to cavitation within the junction, but the present authors are unaware of any studies that have investigated this directly. For branch junctions in roots of desert succulents, the hydraulic resistance to flow through the junction may increase during soil drying due to embolisms in vessels within the junction (North et al., 1992). As the authors discuss, this manner of segmentation may be important in preventing water loss from roots back into a dry soil.
Another possible significance of branch junctions becomes apparent when one considers the nature of flow through the junction. The simplest electrical circuit analogue model of a two-way junction (one main stem and two branches) would consist of three resistors, all connected together at a point representing the centre of the junction. Although this model may have some usefulness in that it allows flow to ‘divide’ between the two branches in some manner dependent upon the relative magnitude of their resistances, it also implies that the two branches are intimately coupled and interactive.
Interactions between branches at the whole-tree scale have been the subject of numerous studies with somewhat variable results. In these studies, stomatal conductance is often measured as the indicator of a leaf response to changing water availability. In some cases, the gradual removal of foliage from a tree crown appears to induce stomatal opening in the remaining foliage, suggesting that, at the crown level, branches do interact and can respond to flow changes in other branches (Troeng and Langstrom, 1991; Pataki et al., 1998). However other studies have found only limited change in stomatal conductance on the remaining branches when large parts of the canopy are removed (Syvertsen, 1994; Whitehead et al., 1996; Hubbard et al., 1999). Preliminary studies of branch junctions by the present authors (Brooks et al., 2002) found limited response by sunlit foliage to shading of competing foliage on the same branch and subsequent flow studies implied that junctions may not behave according to the three-resistor model described above.
The purpose of the present study was to examine in greater detail the degree of interaction between branches for two species of coniferous trees, Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb) Franco) and ponderosa pine (Pinus ponderosa Dougl. ex Laws.). In the light of previous work, the question was: can branches respond to changes in flow to adjacent branches? In other words, can a reduction in water loss from a branch potentially lead to an improvement in the water status of the neighbouring branch? Branch junctions were perceived in the extreme as either being fully-coupled as in the three-resistor model above (Fig. 1A) or as uncoupled (Fig. 1B). In the latter case, xylem conduits would be poorly interconnected in the lateral direction or, in flow terms, radial conducting ability would be extremely low compared with axial conducting ability. For the uncoupled model, developmentally speaking, portions of the xylem of the main stem would be committed to one branch or the other and changes in the flow through one branch would not affect flow in the other branch. For the fully-coupled model, the flow into a branch would respond to changes in flow in the adjacent branch.
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Plant materials
Branch junctions for the two species studied vary with respect to the number of branches arising from a common stem and so for simplicity, junctions with only two branches (Y-shaped) were chosen for study. Branches of ponderosa pine were collected from trees (3–5 m height, 10–15 years old) at the Metolius River young ponderosa pine Ameriflux research site in the central Oregon Cascade Range near Black Butte, Oregon (44° 26' N, 121° 34' W, 1190 m elevation). Douglas-fir branches were collected from trees (15–20 m height, 24 years old) at the Wind River Experimental Forest of the Gifford Pinchot National Forest within the southern Washington Cascade Range (45° 49' N, 121° 59' W, 560 m elevation). At the time of collection, branches were wrapped with moist paper and sealed in plastic bags for processing that same day.
Staining patterns in branch junctions
A qualitative approach to the hydraulic nature of the branch junction was to observe the patterns that became visible following the flow of stain through the junction. Excised branch junctions were cut under water and sealed into a system of tubing. The tubing at the base of the main stem was filled with a 0.5% safranin solution and the cut surface of one branch was sealed to prevent flow. A partial vacuum (20– 30 kPa) was applied to the tubing at the other branch and flow was allowed to proceed until stain was clearly observed flowing profusely downstream from the branch (approximately 45 min of stain flow). The main stem section was then dissected into 1 cm segments from the base of the junction and photographed on the transverse face (cross-section) of each segment. The individual segments were sliced longitudinally and oblique photographs obtained so as to show both the transverse and radial-longitudinal faces.
Flow and conductance measurements
For the quantitative approach, branch junctions were excised and recut under water prior to attaching to a pressure–flow system for the measurement of flow rate under an applied pressure. Tissues external to the xylem were removed because of the presence of large resin ducts which could release materials that would obstruct water flow through the xylem tracheids. The basal main-stem section of the excised junction was attached with tubing to a source of partial vacuum and a pressure meter. Pipettes were attached to the two branches for measurement of flow rate by timing the passage of a meniscus through the pipette. The tubing connections to the stems were sealed with a clear acrylic copolymer (Wet ‘n Wild nail polish). Pressure was applied (20–25 kPa) and flow rate was measured into the two branches. One branch (component b, Fig. 1) was excised and the cut surface was sealed. Flow rate was again measured into the remaining branch (component a). Then the junction was cut apart and segments removed from components a, b, and c. These segments were sealed into a similar pressure–flow system for the measurement of hydraulic conductance per unit length (K, m4 MPa–1 s –1; Schulte et al., 1987, 1989). Lengths and diameters of all segments as well as the components remaining as part of the junction were obtained with calipers. An estimate of the total flow resistance of each component of the junction was calculated from the inverse conductance per unit length multiplied by the total length of that component of the junction. Solutions for flow studies were made from degassed water and 20 mM in KCl (Zimmerman, 1978). Flow through the intact junctions occurred in the reverse of normal flow direction (unlike in the staining experiment above), but preliminary studies showed that the conductance of stem segments for both species was the same when subjected to flow in either direction (data not shown). Statistical analyses involving linear regression and species comparisons utilized the ProStat software from Poly Software International, Inc.
Branch analysis
For calculating the flow increase expected for the fully-coupled model, an electrical circuit analogue for the branch junction was developed (Fig. 2 ). The predicted flow increase depends on how much the pressure drop across Ra changes when branch b is removed. First, the flow (Q; m3 s–1) through branch a may be described by way of an Ohm’s Law analogy:
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where 
Pa and P are the pressure drops across Ra before and after, respectively, the removal of branch b. The pressure drops across branch a before and after removing branch b are:
where Rab and Rabc are combined resistances calculated as:
Then the theoretical flow increase after removing branch b is calculated as:
Substituting equation (2) into equation (1) and subsequently into equation (4) gives the predicted, theoretical change in flow as a function of the various resistances:
The presence of flow constrictions within branch junctions could be assessed using data collected for the previous analysis. Measurements on segments from components a, b, and c gave an estimate of resistance per unit length for each component. Assuming this resistance is constant along each branch and the main stem right to the centre of the junction (i.e. no constriction is present), the predicted flow through the entire junction (both branches combined) was calculated for the pressure applied. This predicted flow was then compared to the measured flow (again, both branches combined). If the junction is not constricted, then the predicted and measured flows should match. A lower measured flow relative to the predicted flow would suggest that there is a hydraulic constriction in the junction, i.e. the measured resistance per unit length of each branch and the main stem cannot be projected right to the centre of the junction and there must be an increased resistance per unit length within the junction (a constriction).
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Results |
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Qualitative analysis of junctions: staining patterns
The patterns observed following staining, when flow was induced in only one branch, were similar within species and so results from only one sample per species are presented. For Douglas-fir branch junctions, the pith, primary xylem and perhaps also the oldest secondary xylem growth ring did not conduct any stain, and may have acted as barriers to lateral flow across the main stem (Fig. 3). Thus, flow in the main stem and into the branch appeared to use only the xylem on that side of the main stem (top panel, Fig. 3). Several centimetres below the junction, however, stain begins to appear on the opposite side of the main stem. It is apparent from the images that stain is not crossing the centre of the stem, but gradually appearing tangentially around the growth rings.
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A similar pattern was observed for ponderosa pine (Fig. 4) as in Douglas-fir. Stain does not appear to cross the pith and regions of older xylem. Near the junction (top panel, Fig. 4), stain was confined to the same side of the main stem as the conducting branch. Several centimetres below the junction, stain eventually appears on the opposite side of the main stem. The staining patterns apparent on the transverse faces for ponderosa pine appear qualitatively similar to Douglas-fir in the gradual appearance of stain tangentially around each growth ring.
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It is important to note that the observed patterns originated from stain applied at the base of the main stem and forced up towards the junction, and not from the branch down. It should also be noted that the staining patterns shown (Figs 3, 4) were obtained from junctions where flow was allowed to proceed for approximately 45 min. If the junction was stained only long enough for stain to appear at the branch cut surface (a few minutes) lateral staining was not apparent except for the first millimetre above the cut surface in contact with stain. Once flow was halted, little or no additional movement of the stain was apparent, at least over the time frame required for taking photographs (1–2 h).
Quantitative analysis of junctions
Following removal of the adjacent branch, an increase of flow into the remaining branch was always observed for both Douglas-fir (Fig. 5) and ponderosa pine (Fig. 6) branch junctions. In Douglas-fir, measured flow increased by 9–33% for the seven junctions studied. However, the increase predicted from equation (5) for these junctions ranged from 25% to 122% and was always considerably greater than the actual flow increase as measured. Considering each junction individually, the measured flow increase averaged about one-quarter of the predicted flow increase. For ponderosa pine, measured flow increased by 14–24% for the seven junctions studied. However as with Douglas-fir, the predicted flow increases were always greater than measured, ranging from 22% to 82%. The average measured flow increase for ponderosa pine was about one-third of that predicted.
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Considerable variation was apparent between individual branch junctions, although the measured flow increase into the remaining branch after removing the adjacent branch was always less than that predicted as noted above. The predicted flow increase does appear to be related to the measured increase (Fig. 7). In addition, the magnitude of the flow increase for junctions affected by the relative size of the two branches can be compared (Fig. 8). A larger flow increase was observed when the branch that was removed was much larger than the remaining branch compared with junctions where the branches were similar in size.
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In assessing the presence of constrictions within the branch junctions when flow is occurring through both branches, measured flow through the entire branch junction was compared to a prediction based on the assumption that the hydraulic resistance per unit length of each segment (the two branches and the main stem) beyond the junction was the same as the resistances within the junction. Measured flows for Douglas-fir were similar to predicted flows and so these junctions do not appear to contain a constriction or region of higher resistance within the junction (Fig. 9). However, for ponderosa pine, measured flows averaged 79% of predicted (significantly different than Douglas-fir at the 5% level), suggesting that ponderosa pine junctions are constricted. Within a species, the measured to predicted flow ratio did not appear to depend on the size of the branch junction for the range of junctions studied.
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Discussion |
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The qualitative study of staining patterns following flow directed into one branch of a junction indicates that a considerable degree of isolation exists between the branches supplied through a junction. The staining pattern gradually spread around the main stem tangentially through individual growth rings suggesting that, at some distance from the junction (several centimetres for the junctions studied), a branch can begin to utilize the entire main stem xylem if flow is not occurring in the adjacent branch. The observed patterns must arise from rapid flow on the side of the main stem nearest the branch where the partial vacuum was applied and progressively slower axial flow with distance around the stem tangentially and with distance from the junction. Therefore, the staining patterns suggest that the entire stem can participate in flow into the active branch, but this participation develops gradually with distance as xylem sap crosses the stem laterally along several centimetres of stem length basal to the junction.
The quantitative study of flow responses and the analysis of the electrical circuit analogue provide further support for the partial isolation of the branches supplied by a branch junction. If the junction was truly fully-coupled and reasonably represented by the simple three-resistor model, then the flow into a branch following the stoppage of flow into the adjacent branch should have increased 3–4-fold over what was actually observed. On the other hand, the branches are not completely isolated because there was always at least some response in flow to the remaining branch.
Considerable variation was apparent among the individual junctions in terms of measured responses to adjacent branch removal. The predicted and actual responses were correlated, suggesting that the fully-coupled flow model based on segment resistances does account for some of the observed variation. It appears that another component in the degree of interaction between branches involves the relative size of the two branches. A small branch will show a greater response to flow changes in the larger adjacent branch as compared with a junction where the branches are balanced in size. This seems intuitively reasonable; stopping the flow in the larger of the branches in an unbalanced junction might cause a greater change in the xylem pressure driving flow through the junction than stopping flow in the smaller branch. An alternate perspective would be that if junctions are somewhat isolating, the xylem of the main stem is proportionally committed to branches depending on their size. Therefore, if flows stops in the larger branch, a greater proportion of xylem capacity in the main stem becomes available to the remaining branch.
The staining patterns and flow studies both indicate that the xylem within the stem clearly has lower flow resistance in the axial than in the lateral directions. Although rays are present in the xylem and may provided for some lateral flow, it is also possible to consider an anatomical basis for this anisotropy from the nature of flow through tracheids. Tracheids are imperforate cells and xylem sap flowing from cell-to-cell depends on the presence of pits in the lateral cell walls and overlap between tracheids. For most conifers, pits are located exclusively on the radial walls of the cells, except perhaps for a few rows of the last cells produced late in the growing season (Panshin and de Zeeuw, 1980). Assuming the cells are roughly 100-fold longer than wide, the axial flow of water from the centre of one cell to the centre of an adjacent cell would also result in a slight movement in the lateral–tangential direction (around the growth ring). Alternately, an axial flow of, say, 1 mm distance might involve flow through only one set of pits whereas flow over the same distance in a lateral direction would necessitate crossing 50 sets of pits (assuming a tracheid diameter of 20 µm). Therefore, a consideration of xylem anatomy for conifers suggests a basis for differences in axial and lateral conducting ability and, hence, at least some degree of isolation between branches. Such hypothetical considerations based on pits occurring only on radial walls are supported by the staining patterns observed in the present study which show that lateral flow in the stem occurs more readily around the xylem growth ring (tangentially) than across growth rings.
A number of studies of flow through xylem at the whole-organism level have produced evidence for isolation between xylem elements around the stem. In work with very old (
1000 years), stunted eastern white cedar, Larson et al. (1993) demonstrated a close association (temporal and spatial) between root and shoot mortality events during the history of these trees and suggest that the stem must be sectored into functionally independent areas in order to isolate parts of the tree while maintaining intimate contact between particular roots and portions of the crown. Dye ascent experiments with white cedar provided further support for sectoring (Larson et al., 1994). Bristlecone pine trees show similar segmented mortality and survival where a single branch and a root connected by a strip of xylem and phloem may be all the living tissue on an ancient tree (Lanner, 1984). Therefore, hydraulic isolation within the xylem may be an important adaptation for the development and survival of trees by preventing stresses localized in one part of the root system from being dispersed throughout the crown. The sectorialized transport process has also been observed in grape vines (Shani et al., 1993), through dye ascent experiments whereby dye supplied to an individual root was not dispersed throughout the above-ground stem. On the other hand, previous studies of dye ascent in a variety of tree species have shown that lateral flow occurs readily for many species, with the possible exception of eastern hemlock showing a sectored pattern (Kozlowski and Winget, 1963). MacKay and Weatherly (1973) found that the stems of Acer pseudoplatanus and Gossypium hirsutum are capable of supporting flow even when overlapping cuts are made to the stem xylem, presumably because of lateral flow. For honey mesquite (Ainsley et al., 1991) and Pacific silver fir (Teskey et al., 1985), responses of the tree canopy to root severing experiments suggest that the branches are responding more as an integrated whole than as sectored components. Therefore, it appears likely that the degree of isolation between portions of the xylem is quite species-dependent. These studies of branch junctions also support the presence of some degree of lateral flow. However, for the two conifers studied here, the observed degree of isolation between the branches downstream from a junction appears likely to limit the ability of flow in a branch to respond to changes in flow in adjacent branches that share a common junction. Such isolation at junctions could partially explain why stomatal conductance responses to leaf area removal do not occur until whole tree transpiration is reduced (Pepin et al., 2002; Whitehead et al., 1996), and why experiments at the branch level do not show responses similar to those at the whole-crown level (Brooks et al., 2002; Hubbard et al., 1999).
Although not the primary purpose of the present study, these data allowed the degree of constriction present in these branch junctions to be analysed. Ponderosa pine junctions appeared to have a significant increase in resistance within the junction, relative to the resistances of the main stem and each branch outside of the junction, roughly similar constrictions as described for Acer saccharum by Tyree and Ewers (1991). With the present data, it is not possible to quantify a resistance per unit length for this constricted region without specifying an appropriate ‘length’ with which to define the junction, however, an analysis by Tyree and Alexander (1993) for three angiosperm species suggested that small hydraulic constrictions in the junction would not contribute very much to the overall flow resistance in stems. Previous studies (Zimmermann, 1983) have suggested that constrictions in junctions may be an important component in branch segmentation, whereby branches will be preferentially sacrificed to preserve the main stem. More recently, it appears that differences in cavitation vulnerability may be more important than hydraulic constrictions alone as a segmentation mechanism (Tyree and Ewers, 1991; Tyree et al., 1993).
The results of this study indicate that a fully-coupled, three-resistor model of a branch junction is far from adequate in describing the interactions between branches. The observed degree of isolation is intermediate between fully-coupled and uncoupled models. In terms of electrical circuit analogues, perhaps a more reasonable model would have the main stem constructed as a ladder-like network of resistors with the axial components representing axial flow through tracheids and the lateral components corresponding to a combination of radial flow across the stem and tangential flow around the growth rings. In further work, it would be useful to quantify the lateral flow resistances and develop an understanding of the anatomical basis for these hydraulic components.
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Acknowledgements |
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This work was supported by the Department of Biological Sciences, University of Nevada, Las Vegas and the US Environmental Protection Agency. This manuscript has been subjected to the Environmental Protection Agency’s peer and administrative review, and it has been approved for publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. Thanks to JC Domec and C McFarlane for reviewing earlier versions of the manuscript.
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