| For arborists to truly understand and be able to converse with clients
about the application of chemical substances to trees, they must not only
know how to mix and apply the proper dose, but also need to appreciate
what goes on under the bark. What is the structure and organization
of the cells involved in transport in trees and what physiological processes
are responsible for uptake and movement of applied substances?
Arborists who are well versed in basic tree biology will understand the
rationale for the recommendations they make for the care of trees, and
will have a better chance to gain the confidence of their customers.
The variety of fertilizers, pesticides, and plant growth regulators
that arborists use for the care of trees can be applied via the leaves,
roots, or trunk. Spraying substances onto leaves of young trees is
easy. But the problem of drift and cost of the pumps and hoses needed
to reach the crown reduce the appeal of this approach for large trees.
In addition, many foliar applied substances do not easily penetrate the
protective waxy layers on leaf surfaces, and hence are poorly transported
away from leaves to the rest of the tree. Uptake via roots requires
incorporation or injection of chemical substances into the soil and relies
on absorption by the root system. Placement of substances near roots,
or growth of roots into the zone of placement, are essential for good uptake
and distribution throughout a tree. Injection of chemicals directly
into the trunk of trees depends on it being absorbed by the water moving
in the transpiration stream. Regardless of the method of application,
the cells of the xylem and phloem provide the conduits through which substances
are transported.
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| Movement in Phloem | ||||
If the dead outer bark of a tree is peeled away, the living phloem
tissue is the first revealed. The phloem provides the pathway along
which sugars produced in photosynthesis as well as hormones and some organic
pesticides are transported. A unique feature of phloem is that movement
both up and down trees occurs through it, providing a means for distribution
of food, hormones, and other chemical substances to and from sites of application,
production, storage, or utilization. The cells of the phloem are
linked together by their living protoplasm and extend from the new shoots
to the fine roots. Conduction in the phloem results in a  positive
pressure in the cells. The best evidence of this is the feeding of
aphids. The aphid is a clever little insect that can delicately stick
its feeding tube into a phloem cell just under the bark. Pressure
inside the phloem cell forces more sugary solution through its body than
it can digest. The result is the so-called honeydew that drips
onto sidewalks and cars beneath infested trees.
Substances applied to leaves or absorbed from the soil by roots may be carried in the phloem if they have the molecular structure necessary to penetrate the membranes of the living cells. Chemicals with this characteristic are called phloem mobile, but are not restricted to movement in the phloem. They also may be carried with water moving in the xylem. Uptake into phloem cells via trunk injection is unlikely because the positive pressure in them initially forces sap to flow out of the cells when they are damaged by the injection procedure. In addition, phloem cells quickly respond to wounding, forming plugs of gelatinous material that seal damaged cells. The most widely accepted explanation for translocation of substances
in the phloem is the Münch pressure flow hypothesis proposed in 1930.
A high concentration of sugars or any organic substance loaded inside cells
of the phloem at a source, such as a leaf where sugars are produced, creates
a diffusion gradient that draws water into the cells. The resulting
pressure causes a flow to occur. If the dissolved sugars or
other chemicals carried along with the sugars are removed from the phloem
at another place in the tree for use (a sink such as a root or fruit),
the decline in concentration of sugar causes water to move out of the phloem
cells. Because water is moving in at a source and out at a sink,
there is a mass flow of water and substances in the phloem.
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| Vascular Cambium | ||||
| Just beneath the phloem is the vascular cambium, a group of cells with the ability to divide. This lateral meristem surrounds the roots, trunk, branches, and shoots, extending throughout a tree like a glove. It is this meristem that each year produces a new layer of phloem toward the outside of the tree and a new layer of xylem, or wood, toward the inside. Unlike the phloem, which does not accumulate but instead is shed with the dead bark, the old xylem is retained, may be used to conduct water for a few years, and eventually just provides structural support as the tree grows larger. At least 90% or more of the trunk of a tree is xylem or wood. | ||||
| Xylem Cells | ||||
| Xylem consists of only four kinds of cells, but their size, shape,
arrangements, and proportions are such that the wood of each tree species
is unique. A good wood anatomist can identify trees just by inspecting
the wood.
The most primitive type of water conducting cell is the tracheid, a narrow tapered cell, usually only 1/25 of an inch long, with small pits in the sidewalls and closed on both ends. These cells are oriented vertically and joined together via pit pairs. Water and substances dissolved in it, move upward in the cells for a short distance before they must move through the pit pairs into an adjacent tracheid. Upward conduction follows a tortuous and inefficient pathway in xylem dominated by tracheids. Vessels are an evolutionary advancement for transporting water. They are two to four times wider in diameter than tracheids, frequently large enough to be visible to the naked eye. The vessel cells are normally shorter than tracheids and still have pits in the sidewalls, but most importantly, they have large pores in the end walls. These barrel-shaped cells are arranged end to end in long open stacks for conducting water. The tubes may extend uninterrupted for several feet up trees, and in some species the entire length of the tree, creating an efficient system for moving water and dissolved substances. When mature and functional for water transport, the tracheids and vessels are dead, hollow cells. The two remaining kinds of cells found in xylem are fibers and parenchyma. Fibers, usually shorter and narrower than tracheids, have very thick sidewalls containing few pit pairs and closed ends. They don't conduct water, but instead function in the xylem to provide structural strength. Parenchyma are cubical-shaped cells that remains alive in the xylem for several years. These cells are scattered in the xylem and form the vascular rays, which provide a pathway for lateral movement across the xylem. Parenchyma cells also are the storage sites for carbohydrates needed for growth and to maintain tree vigor. Because living parenchyma cells retain the ability to divide, they allow
trees to respond to wounds and are the origin of new roots and shoots on
stem cuttings. The callus that forms around the edges of a wound
on the trunk or a pruned branch arises from the parenchyma cells in the
xylem. As long as the parenchyma cells are alive, that part of the
xylem is considered part of the sapwood. When the parenchyma cells
die, the wood becomes heartwood.
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| Sapwood and Heartwood | ||||
| Sapwood is the physiologically active part of the xylem. This is the tissue through which water and substances dissolved in it move from the roots to the shoots. Here too starch is stored in living parenchyma cells. The sapwood can often, but not always, be distinguished by its lighter color. The heartwood does not conduct water and even the parenchyma cells are dead. They may have died from lack of oxygen, being cut off by the accumulating rings of sapwood. Or it also is possible that the parenchyma cells are killed by toxic tannins and phenols that trees deposit in them. The heartwood is particularly decay-resistant because of the accumulation of these compounds, which deter decay organisms and account for its darker color too. | ||||
| Xylem Structure | ||||
| There are three principal xylem anatomies; nonporous, diffuse porous,
and ring porous. In nonporous xylem of trees like pines, spruces,
firs, and other gymnosperms, only one cell type, the tracheid, is involved
in upward conduction. The tracheids produced by the vascular cambium
in the first part of a growing season, the so-called earlywood, have larger
diameters and thinner sidewalls than the tracheids of the latewood produced
later in the growing season. Since water follows the path of least
resistance, it is not surprising that the earlywood tracheids provide the
main pathway for upward movement of root absorbed and stem injected substances.
Up to three or four annual growth rings of xylem may be active in water
transport in trees with this kind of anatomy.
The evolutionarily advanced wood of hardwood trees contains vessels
as well as tracheids. The arrangement of the vessel tubes, or pores, when
viewed in a cross-section provides for the classification of wood as either
diffuse or ring porous. In diffuse porous wood, the vessels of similar
diameter are uniformly scattered throughout the earlywood and latewood
of each annual ring. In contrast, in ring porous wood the vessels
are distinctly larger in diameter in the earlywood. A third
separation, semi-ring porous also is sometimes used, but it is rather variable.
Semi-ring porous woods have a gradual change in pore size across the ring.
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| Xylem Anatomy and Transport | ||||
| The pathway of water movement varies with the type of anatomy found
in trees. It is not surprising that vessels, with their large diameters
and open end walls provide less resistance to water movement than tracheids
with only tiny pits in the sidewalls. In diffuse porous species the
vessels of 3 to 4 annual growth rings of the outer sapwood conduct water
and dissolved substances. In ring porous species such as the oaks,
hickories, elms, ashes, hackberry, black locust, and mulberry, only the
large diameter vessels in the earlywood of the current growth increment
are used to conduct water. This very narrow band of sapwood just
beneath the bark is responsible for 99% of the upward conduction of water
and dissolved substances.
Hence an understanding of the wood anatomy of trees and the pattern
of conduction in the xylem is essential to achieve the best results when
injecting materials into the trunks of trees. For trees with nonporous
or diffuse porous wood, materials injected into the most recent 3 to 4
annual growth rings will likely be carried in the transpiration stream
into the tree crown. For trees with ring porous wood, however, injected
materials must be placed just beneath the bark into the current annual
growth ring. Injection into older wood will result in very
poor upward movement of the chemical applied.
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| Transpiration | ||||
| The principal reason for water flowing upward through the dead, hollow
xylem cells is transpiration, or the evaporation of water from the leaves.
Continuous columns of water extend from the cells of the leaves through
the xylem of the branches and trunk into the roots. Water molecules
have tremendous cohesive forces that allow them to hold together under
the negative pressures that develop in the xylem when water evaporates
from the stomatal pores in leaves. A tension or negative pressure
of one atmosphere, or 0.1 MPa, is adequate to pull water about 30 feet.
Hence, the tension in the xylem at the top of a 60 foot tall tree must
be at least two atmospheres to overcome the pull of gravity.
The rate of water movement in xylem is surprisingly rapid, but does
vary markedly with the type of anatomy. Maximum rates of water flow
in trees are reported to vary between 3.3 to 6.5 feet per hour in nonporous
conifers, 3.3 to 19.7 feet per hour in diffuse porous trees, and 13 to
131 feet per hour in ring-porous trees.
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| Environmental Influences on Transpiration | ||||
| Because water movement is related to transpiration, environmental factors
such as soil moisture, air temperature, and relative humidity affect the
rate of movement. On hot days with low relative humidity, the rate
of uptake of substances injected into the trunk should be relatively rapid
because transpiration is high. However, when the temperature is cool
or the relative humidity high, conditions that slow down transpiration,
the rate of uptake from the soil or during trunk injection will be slow.
The rate of transpiration and the weather are so closely related that even
scattered clouds that temporarily block the sun can noticeably reduce the
ease with which materials are injected into a tree trunk. It
has even been reported that water flow can be quite variable around the
trunk, with sections below well-lit portions of the crown having much higher
flow rates than shaded portions. Of course, available soil
moisture is a major factor influencing transpiration too. Transpiration
rates and hence uptake and distribution of applied chemicals will be slower
during drought periods.
Beneath the bark of trees is a fascinating arrangement of living and dead cells that provide pathways for transporting water and chemicals. Movement in the xylem is due to physical forces that essentially pull water and dissolved substances up trees through the dead hollow cells. In the phloem, physiological processes move chemicals across membranes of living cells, producing pressure gradients that push material throughout the tree. An awareness of the anatomy and processes involved in the uptake and movement of water and applied chemicals can only enhance the professionalism of arborists. |
