We were interested whether the species specific differences, from e. Based on the allometric relations derived from the measurements, we derived scaling relations to estimate how whole tree xylem sapwood and phloem volume, nitrogen content and hydraulic conductance change as a function of tree size and how these properties are axially distributed within a tree.
We also set out to estimate how much nitrogen was allocated to the xylem and phloem in comparison to the leaves. The power function form was used since it is generally used to describe allometric variation in biological properties e. See Table 1 for the list of symbols used in the manuscript.
We reason that the actual amount of conducting sapwood, and thus xylem hydraulic conductance, must be in between these two extreme scenarios, and exploring the space between these two extremes describes how sensitive the scaling predictions are to changes in the radial profile of xylem hydraulic conductance.
In addition, we assumed two extreme scenarios for xylem heartwood nitrogen content, where A heartwood nitrogen concentration was the same as in sapwood and B where heartwood nitrogen content was zero.
Again, the actual amount of nitrogen in the xylem must lie in between these two extreme scenarios. We made an assumption that the cross-sectional area of the xylem sapwood is conserved at branching junctions.
This assumption is from the pipe model theory formulated originally by Shinozaki et al. The pipe model assumption has been shown to hold reasonably well for the tree species used in our measurements Kaufmann and Troendle, ; Ilomaki et al. Note that the original pipe model assumption, presented e.
We further assumed leaf area to be proportional to xylem cross-sectional sapwood area. This assumption is also from the Pipe model theory Shinozaki et al. Note that scaling of phloem and leaf properties are affected by the assumptions made about sapwood turnover to heartwood.
This behavior stems from the pipe model assumption. The amount of heartwood affects the number of furcations [i. The more heartwood there is, the higher the furcation number n is at any given height, and the more phloem tissue there is.
We used the allometric relations in Equation 1 to scale whole tree xylem and phloem volume, conductance and nitrogen amount with tree height for two of the measured species one gymnosperm and one angiosperm ; pine and aspen.
Trees varied between 6. The trees were harvested between May and October In addition, two additional pine trees were harvested in September for some additional nitrogen content measurements. All harvested trees were healthy. The cardinal points were marked to the sample trees before felling. Stem diameters were measured from various relative locations within the stem 1. Bark thickness measurement were made from tree trunk at breast height and at 6 m height.
We numbered all living branches from the base of the crown to the top of the tree and measured their heights from the tree base and marked their compass direction, length, and base diameters. Branch diameters were measured beyond noticeable basal swelling and the distance from tree trunk and apex were measured.
The crown was divided into segments of height and compass directions and a total of 10—15 sample branches were selected from each tree from different heights and sides of the tree so that the branch size distribution was evenly represented in the sample. Only healthy appearing and non-damaged branches entered the sample. We measured the length of sample branches from the cut surface to branch apex and measured over and under bark diameters from the base of the branch.
Subsequently we divided each branch into segments. The first segment was from the base to the first fork and the following segments were between subsequent forking points. Thickness of the bark was calculated as the difference between under bark and over bark diameter divided by two. Altogether, we measured branch or stem diameters with and without bark between 0.
For measurements of the dimensions of the living bark we cut 85 stem disks from different heights of each tree 2. The cut surfaces were sanded and scanned, and the thicknesses of the periderm and living bark were measured using a self-made image analyzer program. We used 62 samples from stem and branch disks 3. The bark was removed from xylem. The periderm of bark was then removed, and the rest of the bark was termed as living bark, which consisted of the primary and secondary phloem, and vascular cambium.
Allometric equations were fitted to the measured data using non-linear regression between sample diameter and xylem and bark properties with Sigmaplot Sigmaplot for Windows version There are different estimates in the literature for the scaling of phloem conduit radius with tree height. This is an intermediate scaling between the scaling exponent of 0.
We assumed that a constant fraction of the xylem and phloem cross-sectional area was conducting lumen volume the rest being conduit walls, parenchyma, etc. This is in agreement with a formulation described e. Simultaneously, the maximum sapwood depth r sw , max was varied between 2 and cm. Table 2. The model calculates xylem and phloem pressure and sugar concentrations and their within tree axial gradients in steady state.
Pressure differences drive xylem and phloem transport, i. Phloem sap viscosity was made to be sugar concentration dependent. Transpiration rate, phloem loading made equal to photosynthesis rate and unloading rates and soil water potential were given as boundary conditions, and xylem and phloem hydraulic conductance and tree height were given as structural parameters. The position of phloem unloading could be varied in the transport model so that we ran simulations where phloem unloading was made to occur either evenly along the phloem transport pathway or exclusively in the roots.
We did three simulations with the model. We used a 10 m pine with a maximum sapwood depth of 2 cm as an example, and took the axial distribution of xylem and phloem conductivity from the equations shown in the Appendix A3 and demonstrated in Figure 7.
In other words, the total amount of phloem tissue was preserved, but was distributed unevenly as a function of axial position. For this, we used the scaling relations for whole tree xylem and phloem hydraulic conductance as a function of tree height derived in Appendixes A1 and A2 and demonstrated in Figure 4 and Table 4 for the case of pine. In this simulation, leaf gas exchange rates i. Photosynthesis rate was made to be proportional to transpiration rate. Note that this resulted in different initial values for phloem conductance between the cases where no heartwood was assumed and the case where the maximum sapwood depth was set at 2 cm.
We also varied the initial value of phloem conductive and distribution of phloem unloading to see their effects on the results. The equations obtained for the scaling of xylem and phloem properties as a function of tree height L starting from a distance L 0 from leaf apex are as follows.
The derivation of the equations is presented in the Appendix A1. These equations apply only to the case without heartwood. The numerical equations for the whole tree scaling relations including sapwood to heartwood turnover are shown in Appendix A2. The values of 0. Xylem and phloem properties were given constant values at branches than smaller than this. The values for L 0 were chosen large enough so that we had measurements from branches of corresponding diameter.
The cross-sectional area of the whole bark A b , i. The scaling exponent ranged from 1. The scaling exponents were rather close to each other across the species. When testing the difference, the logarithmic transformation changed the exponents somewhat 1. Instead, the bark was divided into outer and inner bark, and the latter represents the functional phloem tissue.
Figure 1. For inter-species comparison of inner bark thickness there was sufficient data for aspen and pine. Their exponents were not significantly different from each other in the ln-transferred data.
Figure 2. Measured inner bark i. Nitrogen content increased clearly with decreasing stem diameter in both the living bark and the whole bark, but remained fairly constant for the xylem Figure 3. While all species seemed to follow similar pattern for the living bark, there seemed to be a level difference for the whole bark so that there was the most nitrogen in the aspen bark and least in pine bark for the same diameter.
Figure 3. The data measurement points in living bark and xylem were from 8 trees 4 species. Table 3. Whole tree scaling relation predictions were made for two example species: pine and aspen.
The allometric relations used in the scaling of whole tree xylem and phloem volume, nitrogen content and hydraulic conductance are presented in Table 3. Figure 4 shows the scaling relations for phloem and leaf properties in relation to the xylem properties, and Figure 5 shows the absolute values for xylem, phloem and leaf properties.
Aspen had a larger amount of phloem and higher phloem to xylem ratio in relation to pine. Leaves were the largest sink of nitrogen in small trees, but xylem and phloem exceeded the leaves as a nitrogen sink with increases in tree height Figures 4C,D , 5E,F. The total nitrogen content of the phloem was smaller than that of the xylem in pine and large aspen trees. The total nitrogen content of the phloem exceeded the xylem nitrogen content in small aspen trees Figures 4C,D , 5E,F.
Assumptions on heartwood proportions and nitrogen content of the heartwood caused the relative nitrogen contents between the tissues to vary strongly.
However, when there was no heartwood, or the nitrogen content of heartwood was assumed to be same as that of the sapwood, then the role of the phloem as a nitrogen sink decreased in relation to xylem with increases in tree size. Table 4 present the absolute values for scaling of tree xylem and phloem volume, nitrogen content, conductance, and leaf area-specific conductance as a function of tree size. Note that scaling is not strictly allometric [see Equation 2 and Appendix A2], although very close to it, for each case.
Figure 4. The predictions for the whole tree phloem volume in relation to xylem sapwood volume A , phloem hydraulic conductance in relation xylem hydraulic conductance B , total phloem and leaf nitrogen content in relation to xylem hydraulic content for the scenarios in which the heartwood has the same nitrogen content as the sapwood and for the case of no heartwood C , and phloem and leaf nitrogen content in relation to xylem hydraulic content for the case where the heartwood has the same nitrogen content as the sapwood D.
In B the same area-specific conductivity was assumed for xylem and phloem. Figure 5. The predictions for the absolute values for whole tree volume of xylem and phloem A,B , hydraulic conductance of xylem and phloem C,D , nitrogen content of xylem, phloem and leaves E,F , and hydraulic conductance of xylem and phloem per leaf area G,H as a function of tree height. Table 4. The results for scaling of tree properties as a function of tree height L. Figure 6 shows the minimum and maximum xylem and phloem volume, nitrogen content and conductance in relation to a 10 m tree obtained from the sensitivity analysis done with parameter combinations.
The general trends within remained unchanged, although the xylem, phloem and leaf properties overlapped with each other. Xylem and leaf properties seemed to be more sensitive to parameter combination than those of phloem. Figure 6. The minimum and maximum xylem sapwood and phloem volume A , nitrogen content B and conductance C in relation to a 10 m tree obtained from the sensitivity analysis done with parameter combinations.
Also total leaf nitrogen content is shown in B. Within a 10 m tree taken as an example here phloem cross-section and volume was distributed very much toward the apex, whereas xylem sapwood cross-section was evenly distributed axially, following from our pipe model assumption Figure 7A.
Water transport via symplastic and apoplastic routes. The cross section of a dicot root has an X-shaped structure at its center. The X is made up of many xylem cells. Phloem cells fill the space between the X. A ring of cells called the pericycle surrounds the xylem and phloem. The outer edge of the pericycle is called the endodermis.
A thick layer of cortex tissue surrounds the pericycle. The cortex is enclosed in a layer of cells called the epidermis. The monocot root is similar to a dicot root, but the center of the root is filled with pith. The phloem cells form a ring around the pith. Round clusters of xylem cells are embedded in the phloem, symmetrically arranged around the central pith. The outer pericycle, endodermis, cortex and epidermis are the same in the dicot root. There are three hypotheses that explain the movement of water up a plant against gravity.
These hypotheses are not mutually exclusive, and each contribute to movement of water in a plant, but only one can explain the height of tall trees:. Root pressure relies on positive pressure that forms in the roots as water moves into the roots from the soil. In extreme circumstances, root pressure results in guttation , or secretion of water droplets from stomata in the leaves. However, root pressure can only move water against gravity by a few meters, so it is not strong enough to move water up the height of a tall tree.
Capillary action or capillarity is the tendency of a liquid to move up against gravity when confined within a narrow tube capillary. Capillarity occurs due to three properties of water:. On its own, capillarity can work well within a vertical stem for up to approximately 1 meter, so it is not strong enough to move water up a tall tree. This video provides an overview of the important properties of water that facilitate this movement:.
The c ohesion-tension hypothesis is the most widely-accepted model for movement of water in vascular plants. Cohesion-tension essentially combines the process of capillary action with transpiration , or the evaporation of water from the plant stomata. Phloem transports sucrose and amino acids up and down the plant. This is called translocation. In general, this happens between where these substances are made the sources and where they are used or stored the sinks.
This means, for example, that sucrose is transported:. The cells that make up the phloem are adapted to their function:. Plant transport tissues - xylem and phloem Xylem The xylem transports water and minerals from the roots up the plant stem and into the leaves. Vessels: Lose their end walls so the xylem forms a continuous, hollow tube. Become strengthened by a chemical called lignin. The cells are no longer alive. Lignin gives strength and support to the plant.
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