Water And Its Movement Through The Plant

If you were to cover the soil at the base of a plant with foil, place the pot where it receives light, and then put the potted plant under a glass bell jar, you would notice moisture

160 Chapter 9

160 Chapter 9

Plant Under Glass Jar
Figure Q.Q A potted plant sealed under a bell jar. The surface of the soil has been covered with foil. Note the accumulation of moisture on the inside of the glass. The moisture came through the plant by transpiration.

accumulating on the inside of the jar within an hour or two. Because of the foil barrier, the water could not have come directly from the soil; it had to have come through the plant. More than 90% of the water entering a plant passes through and evaporates—primarily into leaf air spaces and then through the stomata into the atmosphere (see Fig. 9.10)— with usually less than 5% of the water escaping through the cuticle. This process of water vapor loss from the internal leaf atmosphere is called transpiration (Fig. 9.9).

The amount of water transpired by plants is greater than one might suspect. For example, mature corn plants each transpire about 15 liters (4 gallons) of water per week, while four-tenths of a hectare (1 acre) of corn may transpire more than 1,325,000 liters (350,000 gallons) in a 100-day growing season. A hardwood tree uses about 450 liters (120 gallons) of water while producing 0.45 kilogram (1 pound) of wood (or 1,800 liters while producing 0.45 kilogram of dry weight substance), and the 200,000 leaves of an average-sized birch tree will transpire from 750 to more than 3,785 liters (200 to 1,000 gallons) per day during the growing season. Humans recycle much of their water via the circulatory system, but if they were to have requirements similar to those of plants, each adult would have to drink well over 38 liters (10 gallons) of water per day.

Why do living plants require so much water? Water constitutes about 90% of the weight of young cells. The thousands of enzyme actions and other chemical activities of cells take place in water, and additional, although relatively negligible, amounts are used in the process of photosynthesis. The exposed surfaces of the mesophyll cells within the leaf have to be moist at all times, for it is through this film of water that the carbon dioxide molecules needed for the process of photosynthesis enter the cell from the air. Water is also needed for cell turgor, which gives rigidity to herbaceous plants.

Consider also what it must be like in the mesophyll of a flattened leaf that is fully exposed to the midsummer sun in areas where the air temperature soars to well over 38°C (100°F) in the shade. If it were not for the evaporation of water molecules from the moist surfaces, which brings about some cooling, and reradiation of energy by the leaf, the intense heat could damage the plant. Sometimes, the transpiration is so rapid that more water is lost than is taken in. The stomata may then close, preventing wilting. The relation and role of abscisic acid in excessive water loss is discussed in Chapter 11.

How does water travel through the roots from 3 to 6 meters (10 to 20 feet) or more beneath the surface and then up the trunk to the topmost leaves of a tree that is more than 90 meters (300 feet) tall? We know that interconnected tubes of xylem extend throughout the plant, from the young roots up through the stem and branches to the tiny veinlets of the leaves. We also know that the water, following a water potential gradient, gets to the start of this "plumbing system" by osmosis. Water is then raised through the columns apparently by a combination of factors, and the process has been the subject of much debate for the past 200 years (Fig. 9.10).

One of the earliest explanations for the rise of water in a living plant was given in 1682 by the English scientist Nehemiah Grew. He suggested that cells surrounding the xylem vessels and tracheids performed a pumping action that propelled the water along. This was questioned, however, when it was found that water will also rise in lengths of dead stems. Then, after Marcello Malpighi suggested it, the belief that capillary action moved the water became popular.

It is well known that the height water will rise in a narrow tube is inversely proportional to the diameter of the tube. It is also known that this rise occurs through the forces involved in the forming of a concave meniscus (curved surface) at the top of the water column (Fig. 9.11). Even though water can, indeed, rise 1 meter (3 feet) or more in a very narrow tube, air must be present above the column for the forces to work, which is not the case in a plant. In fact, any air introduced into a water column in xylem interferes with the rise of water. Also, while capillarity might produce enough force to raise water a meter or two, the diameter of the tubes is not small enough to raise it more than that.

Stern-Jansky-Bidlack: I 9. Water in Plants I Text I I © The McGraw-Hill

Introductory Plant Biology, Companies, 2003

Ninth Edition

Water in Plants 161

evaporates through stomatal pores evaporates through stomatal pores

enters root hairs

^ upward through xylem fj y

Figure Q.10 Pathway of water through a plant.

The pioneer plant physiologist Stephen Hales discovered and measured root pressure as one means by which water moves through plants. When some plants are pruned after growth has begun in the spring, water will exude from the cut ends. This is the result of root pressure, which is discussed on page 160. Some plants do not "bleed" when they are pruned, and the force exerted by root pressure has been shown generally to be less than 30 grams per square centimeter (a few pounds per square inch). This is considerably less than what is needed to raise water to the tops of tall trees. Furthermore, root pressure seems to drop to negligible amounts in the summer, when the greatest amounts of water are moving through the plant.

The Cohesion-Tension Theory

Stephen Hales also identified a pulling force due to evaporation of water from leaves and stems. This has led to the cohesion-tension theory, the most satisfactory explanation for the rise of water in plants thus far suggested. Water molecules are electrically neutral, but they are asymmetrical in shape (see Fig. 2.4). This results in the molecules having very slight positive charges at one end and very slight negative charges at the other end. Such molecules are said to be polar. When the negatively charged end of one water molecule comes close to the positively charged end of another water molecule, weak hydrogen bonds hold the molecules together.

Water Through Narrow Tube
Figure 9.11 Capillarity in narrow tubes. The smaller the diameter of the tube, the greater the rise of the fluid.

We know that water molecules adhere to capillary walls (e.g., those of xylem tracheids and vessels) and cohere to each other, creating a certain amount of tension. It is possible, for example, to fill a small glass with water, place a thin, smooth sheet of cardboard over the mouth, and invert the glass without the water spilling. This is because the adhesion of the water molecules to the cardboard and the cohe-siveness of the water molecules to one another hold the cardboard against the rim of the glass.

When water evaporates from the mesophyll cells in a leaf and diffuses out of the stomata (transpires), the cells involved develop a lower water potential than the adjacent cells. Because the adjacent cells then have a correspondingly higher water potential, replacement water moves into the first cells by osmosis. This continues across rows of mesophyll cells until a small vein is reached. Each small vein is connected to a larger vein, and the larger veins are connected to the main xylem in the stem, and that, in turn, is connected to the xylem in the roots that receive water, via osmosis, from the soil. As transpiration takes place, it creates a "pull," or tension, on water columns, drawing water from one molecule to another all the way through an entire span of xylem cells. The cohesion required to move water to the top of a tall tree is considerable, but the cohesive strength of the water columns is usually more than adequate. Any breaking of the tension through the introduction of a gas bubble results in a temporary or permanent blocking of water transport. This seldom is a problem, however, because small bubbles may be redissolved and larger gas bubbles rarely block more than a few of the numerous capillary tubes of xylem at any time the tissue is functioning.

Water molecules move partly through cell cytoplasm and partly through spaces between cells; they also move between cellulose fibers in the walls and through spaces in the centers of dead cells. Most water and solutes can travel across the epidermis and cortex via the cell walls until they reach the endodermis. There, the water and solutes are forced by Casparian strips to cross the cytoplasm of the endodermal cells on their way to the vessels or tracheids of the xylem (Fig. 9.12; see also Chapter 5).

Stern-Jansky-Bidlack: I 9. Water in Plants I Text I I © The McGraw-Hill

Introductory Plant Biology, Companies, 2003

Ninth Edition

Chapter 9

wmm III

- passage cell

— endodermis

Figure Q.12 Part of the center of a buttercup root, showing endodermal cells with Casparian strips, x600.

If rapid transpiration is occurring, the roots are likely to grow rapidly toward available water. In corn plants, for example, the main roots may grow at a rate of more than 6 centimeters (2.3 inches) a day. Solutes, as well as water, may move so rapidly during periods of rapid transpiration that there is little osmosis taking place across the endodermis. Scientists believe that at such times water may be pulled through the roots by bulk flow, which is the passive movement of a liquid from higher to lower water potential.

In summary: "columns" of water molecules move through the plant from roots to leaves, and the abundant water of a normally moist soil supplies these "columns" as the water continues to enter the root by osmosis (see Fig. 9.10); simply put, the difference between the water potentials (water "concentrations") of two areas (e.g., soil and the air around stomata) generates the force to transport water in a plant.

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Responses

  • asmara
    How much water will an acre of corn transpire during a 100day growing season, on average?
    8 years ago
  • MYLES
    Where does the water travel through in a plant?
    8 years ago
  • MARKUS
    How does water travel through the roots to the topmost leaves of a tree?
    7 years ago
  • Linda
    How does water travel from the soil to the leaves of a flowering Plant?
    7 years ago
  • Danait Haylom
    How could water come from a potted plant sealed in a beaker?
    7 years ago
  • stephan
    How is the movement of water in a flowering plants?
    2 years ago

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