Spring 2000 Volume 9, Number 1

WATER AND CHEMICAL

MOVEMENT BENEATH THE BARK

by Dr. Williarn R. Chaney, Purdue University

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. They must also understand the structure and organization of the cells involved in three transport and the physiological processes that are responsible for uptake and movement of applied substances. Arborists who are well versed in basic tree biology will understand the rationale for their tree care recommendations, and will have a better chance to gain the confidence of their customers.

Fertilizers, pesticides, and plant growth regulators that arborists use can be applied via the leaves, roots, or trunk. Spraying sub-stances onto leaves of young trees is easy. But the problem of drift, and the cost of 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 transport-ed 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 its ability to be absorbed by the water moving in the transpiration stream. Regardless of the application method, the cells of the xylem and phloem provide the conduits through which substances are transported.

Movement in Phloem

If the dead outer bark of a tree is peeled away, the living phloem tissue is the first revealed. Phloem provides the pathway along which sugars produced in photosynthesis - as well as hormones and some organic pesticides - are transported. Movement both up and down trees occurs through this tissue, providing a means for distribution of food, hormones, and other chemical substances. Phloem cells are linked together by their living protoplasm, and extend from new shoots to fine roots. Conduction in the phloem results in a positive pressure in the cells. The best evidence of this is aphid feeding.

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 living cells. Chemicals with this characteristic are called phloem mobile, but are not restricted to movement in the phloem. They may also 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 Munch 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 (a sink such as a root or fruit), the decline in sugar concentration causes water to move out of the phloem cells. Because water is moving in at a source and out of a sink, there is a mass flow of water and substances in the phloem.

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 the treelike 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 is shed with the dead bark - the old xylem is retained, and may be used to conduct water for a few years. It eventually just provides structural support as the tree grows larger. At least 90 percent or more of a tree's trunk is xylem or wood.

Xylem Cells

Xylem consists of only four kinds of cells, but their size, shape, arrangement, and proportions are such that wood of each tree species is unique. A good wood anatomist can identify trees simply by inspecting the wood.

The most primitive type of water-conducting cell is the tracheid. This narrow, tapered cell is usually only 1/25 inch long. It has small pits in the side-walls and closed on both ends. These cells are oriented vertically and joined together via pit pairs. Water and substances dissolve in them, and move upward in the cells for a short distance. They must then 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. They still have pits in the sidewalks 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. In some species, they extend 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 provide structural strength. Parenchyma are cubical-shaped cells that remain 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 trunk wound 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.

Sapwood and Heartwood

Sapwood is the physiologically active part of the xylem. This is the tissue through which water and dissolved substances 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.

Heartwood does not conduct water, and even the parenchyma cells are dead. They may have died from lack of oxygen, being cutoff by the accumulating rings of sapwood. Or, it also is possible that the parenchyma cells are killed by toxic tannins and phenois that trees deposit in them. The heartwood is particularly decay-resistant because of the accumulation of these compounds. They deter decay organisms and account for its darker color.

Xylem Structure

There are three principle 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. Tracheids produced by the vascular cambium in the first part of a growing season - the so-called early-wood - have larger diameters and thinner sidewalls than tracheids of the latewood produced later in the growing season. Since water follows the path of least resistance, it's 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 evolutionary 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, ring porous wood contains vessels that are distinctly larger in diameter in the earlywood. A third separation, semi-ring porous is also sometimes used, but it is rather variable. Semi-ring porous woods have a gradual change in pore size across the ring.

Xylem Anatomy and Transport

Water movement varies with tree anatomy. 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, vessels of three to four annual growth rings of the outer sapwood conduct water and dissolved substances. In ring porous species such as the oaks, hickories, elms, ashes, hackberry, block locust, and mulberry, only large diameter vessels in the earlywood of the current growth increment are used to conduct water. This very narrow bond of sapwood just beneath the bark is responsible for 99 percent of the upward conduction of water and dissolved substances.

An understanding of the wood anatomy of trees and the pat-tern of conduction in the xylem is necessary before injecting materials into tree trunks. For trees with nonporous or diffuse porous wood, materials injected into the most recent three to four 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.

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.

Environmental Influences on Transpiration

Because water movement is related to transpiration, environ-mental 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 substance uptake injected into the trunk should be relatively rapid because transpiration is high. However, when the temperature is cool or the relative humidity high, the rate of uptake from the soil will be slow. Transpiration and 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 uptake and distribution of applied chemicals will be slower during drought periods.

Beneath the bark, a fascinating arrangement of living and dead cells 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 uptake and movement of water and applied chemicals can only enhance the professionalism of arborists.

Home