BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. PLANT TRANSPORT

13.4. Water Ascent in the Xylem

The xylem of flowering plants contains Two Types of Water-conducting structures: tracheids and vessels. In Section 6.2.1, we discussed what these structures look like under a Light Microscope, as well as in scanning electron micrographs (Fig. 6.12). The Structure of secondary xylem (wood) is examined in Chapter 22. The xylem and phloem together form the vascular tissue of higher, or vascular, plants. This tissue consists of so-called vascular bundles, the structure and distribution of which in the stems of primary-structure dicotyledonous plants are shown in Fig. 13.16.

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Fig. 13.16. A. Anatomy of a young stem of a typical dicotyledonous plant — annual sunflower (Helianthus annuus). B. Transverse section of a sunflower stem under a light microscope at low magnification. C. Transverse section of a vascular bundle in a sunflower stem at high magnification. D. Cytology/practical/54.html">Longitudinal section of a sunflower stem under a microscope.

13.15. What is the overall three-dimensional shape in the stem of a dicot of the following Tissues: a) epidermis; b) xylem; c) pericycle; d) pith?

That water ascends specifically through the xylem is easily demonstrated by immersing the cut end of a SHOOT in a dilute aqueous solution of a dye, such as eosin. The colored liquid spreads upward through the stem, filling the network of Veins permeating the leaves. If thin sections are then cut and examined under a light microscope, the dye is found to be localized within the xylem.

More dramatic proof of water ascent via the xylem comes from "ringing" (girdling) experiments. Such experiments were conducted long before radioactive isotopes became available to easily trace The pathway of substances in living organisms. In one variation of the experiment, a narrow ring of bark—including the phloem—is removed from a woody stem. For quite some time afterward, the shoots above the cut ring continue to grow normally: consequently, such ringing does not affect the upward transport of water through the stem. However, if a flap of bark is lifted and the underlying segment of wood (i.e., xylem) is excised, the plant wilts rapidly. Thus, water moves from the soil into the shoots precisely through this conducting tissue.

Any theory explaining water transport in the xylem must account for the following observations:

1. The anatomical elements of the xylem are narrow, dead tubes whose diameter ranges from 0.01 mm in "summer" wood to 0.2 mm in "spring" wood.

2. Large quantities of water move through the xylem at a relatively high velocity: up to 8 m h^-1 in tall trees, and about 1 m h^-1 in other plants.

3. Lifting water to the top of a tall tree through such tubes requires a pressure of approximately 4000 kPa. The tallest trees—redwoods in California and eucalypts in Australia—reach heights of over 100 m. Water is able to rise in narrow, wettable tubes due to its high surface tension (a phenomenon known as capillarity); however, relying solely on these forces, water cannot rise higher than about 3 m, even in the finest xylem vessels.

A satisfactory explanation for these facts is provided by the cohesion-tension theory. According to this theory, the upward Movement of water from the roots is driven by its evaporation from leaf Cells. As discussed in Section 13.3, evaporation lowers The water potential of the mesophyll cells adjacent to the xylem, and water enters these cells from the xylem sap, which has a higher water potential; in doing so, it passes through the moist Cell walls at the vein endings, as shown in Fig. 13.8.

Xylem vessels are filled with a continuous water Column; as water exits the vessels, tension is generated within this column. This tension is transmitted down the stem all the way to the ROOT due to the cohesion of water molecules. These molecules tend to "cling" to one another because they are polar and attracted by electrical forces, and are further held together by Hydrogen Bonds (Section 3.1.2). In addition, they are attracted to the walls of the xylem vessels, a phenomenon known as adhesion. The strong cohesion of water molecules means that the water column is difficult to break—it has a high tensile strength. The tensile stress in the xylem cells generates a force capable of pulling the entire water column upward via bulk flow. Water enters the xylem from neighboring root cells at the base. Crucially, the walls of the xylem elements are rigid and do not collapse under the negative pressure inside, unlike what happens when drinking a beverage through a soft straw. This structural rigidity is provided by Lignin. Evidence that the liquid inside the xylem vessels is under strong tension (stretched) is provided by diurnal fluctuations in the diameter of tree trunks, measured with an instrument called a dendrograph. The minimum diameter is recorded during the day, when Transpiration rates are highest. The tension in the water column slightly pulls the vessel walls inward (due to adhesion), and the combination of these microscopic compressions results in the overall trunk "shrinkage" detected by the device.

Estimates of the tensile strength of the xylem sap column have ranged from 3000 to 30,000 kPa, with lower values obtained in more recent studies. Water potentials of approximately -4000 kPa have been recorded in leaves, and the tensile strength of the xylem sap column is presumably sufficient to withstand the resulting tension. Of course, it is possible that the water column may occasionally break, particularly in vessels of large diameter.

Critics of this theory emphasize that any disruption in the continuity of the sap column should immediately halt the entire flow, as the vessel would fill with air and vapor (a phenomenon known as cavitation). Cavitation can be triggered by severe mechanical Shock, stem bending, or water deficit. It is well known that over the course of the summer, the water content in a tree trunk gradually decreases, and the wood becomes filled with air. This is exploited by loggers because such trees are easier to float. However, the breakage of the water column in some vessels has little effect on the overall rate of bulk flow. This is likely because water can flow laterally into parallel vessels or bypass the air blockage by moving through adjacent parenchyma cells and cell walls. Furthermore, calculations indicate that to maintain the observed flow rates, it is quite sufficient for only a small fraction of xylem elements to be functional at any given time. In some trees and shrubs, water moves exclusively through the younger, outer wood known as sapwood. In oak and ash, for example, the conducting function is performed primarily by the current year's vessels, while the rest of the sapwood acts as a water reserve. New xylem vessels are formed throughout the growing season, but chiefly at its beginning, when The rate of water flow is at its maximum.

The second force driving Water movement through the xylem is root pressure. This can be detected and measured when the shoot system is cut off, and the remaining stump and roots continue to exude sap from the xylem vessels for some time. This process is inhibited by respiratory poisons such as cyanide, and ceases under oxygen deprivation or low temperatures. The operation of this mechanism is apparently driven by the active secretion of salts and other water-soluble substances into the xylem sap. As a result, its water potential drops, and water enters the xylem from adjacent root cells by osmosis.

This mechanism generates a hydrostatic pressure of about 100–200 kPa (reaching 800 kPa in exceptional cases); while generally insufficient on its own to drive water to the top of tall plants, it undoubtedly contributes to maintaining xylem flow in many species. In slowly transpiring herbaceous plants, this pressure is entirely sufficient to cause guttation—the exudation of water from the plant surface¹ in liquid form rather than as vapor. All conditions that inhibit transpiration, such as low light intensity and high humidity, promote guttation. It is common in many rainforest species and frequently observed at the leaf tips of grass seedlings.

13.16. List The properties of the xylem that enable its ability to transport water and dissolved substances over long distances.

¹ This occurs through specialized Stomata known as hydathodes. — Trans.