Plants that grow on land (terrestrial plants) find the materials they require for life in two different locations. The soil is the source of water and minerals to be used for a variety of functions, while the atmosphere provides carbon dioxide for photosynthesis. The root system takes up water and minerals from the soil, while the shoot system, consisting of leaves and stems, carries out photosynthesis. As larger plants evolved, the roots and shoots became increasingly distant from each other, and long-distance transport systems (xylem and phloem) became necessary for survival. Clearly, one of the most important functions of the root system is the absorption of water. How does the root absorb water? Once inside the plant, how do water and dissolved minerals move from the root to the shoot? What happens to the water once it is delivered to the leaves by the xylem? To answer these questions, it is useful to discuss the end of the transport process first, since it is there that the driving force is found.
Nearly 99 percent of the water taken up by plants is lost to the atmosphere through small pores located mainly on the lower surface of leaves. Estimates of water loss by a single corn plant exceed 200 liters (53 gallons) over a growing season. This loss of water by the shoots of plants is called transpiration. Transpiration provides the driving force for the movement of water up the plant from the roots to the leaves. This movement ultimately results in further uptake of water from the soil.
Of course, the water taken up by the plant serves functions within the plant as well. Water is the environment in which life and its reactions occur. Water, and the materials dissolved or suspended in it, make up the cytoplasm of cells and the interior of cellular compartments. It is the uptake of water that drives the growth of plant cells. Water enters into many reactions or chemical changes in cells, including the reactions that capture light energy during photosynthesis.
The Causes of Transpiration.
The small pores through which shoots lose water to the atmosphere are called stomata. These pores, which allow carbon dioxide into the plant from the surrounding air, are actually spaces between the cells that make up the "skin," or epidermis, of the shoot. Stomata can be open or closed, depending on the action of a pair of cells, called guard cells, surrounding the pore.
Stomata open in response to the plant's requirement for carbon dioxide in photosynthesis. The carbon dioxide cannot move directly into the shoot cells because the outside of the shoot is covered with an impenetrable waxy coating, the cuticle . This coating prevents the plant from drying out but also prevents the movement of carbon dioxide into the leaves. The presence of the stomata allows for sufficient carbon dioxide to reach the leaf cells mainly responsible for photosynthesis.
Not only does carbon dioxide move into the plant when the stomata are open, but water in the form of water vapor also moves out of the plant, driven by the difference in water concentration between the plant and the atmosphere. The opening and closing of the stomatal pores maximize the up-take of carbon dioxide and minimize the loss of water by the shoot. Thus, two of the major signals that cause the stomata to open are the absorption of light and the low concentration of carbon dioxide in the leaves. As a result of these signals, most plants open their stomata during the day, when light energy is available for photosynthesis, and close them at night. Water stress, that is, a water deficit sufficient to prevent normal functioning, can override these signals and cause the pores to close in order to prevent excess water loss. For a plant, taking up carbon dioxide for growth is secondary when further water loss might threaten survival.
While transpiration is often described as a necessary evil, it does serve to cool leaves under conditions of high light absorption. The evaporation of water from leaves thus serves the same purpose for plants as sweating does for humans. Transpiration also speeds up the flow of water and dissolved minerals from the roots to the shoots. In the absence of transpiration, however, other mechanisms would cool the leaves, and water would continue to flow, though at a much lower rate, as it was used in the leaves.
The Nature of Transpiration.
Transpiration is an example of diffusion, the net movement of a substance from a region of high concentration to a region of lower concentration. Diffusion accounts for transpiration because the air inside the plant is very moist, while the atmosphere surrounding the plant almost always contains less water vapor than the inside of the plant. The relative humidity of the air inside a leaf usually ranges from 98 to 100 percent, while the atmosphere rarely approaches such high values. (Relative humidity [RH] is the amount of water in the air compared to the maximum amount that could be held at that temperature.) These differences in relative humidity reflect differences in water vapor concentration, the driving force for diffusion. For example, assume that both the leaf and the atmosphere are at the same temperature of 20°C (68°F) and that the atmosphere is at a relative humidity of 50 percent. Then the air inside the leaf holds 10.9 grams of water per cubic meter, and the atmosphere holds only 5.5 grams of water per cubic meter. If the leaf is warmer than the atmosphere, a common occurrence, the difference in water vapor concentration will be even higher.
The Transpiration-Cohesion-Tension Mechanism for the Transport of Water in the Xylem
The problem of how water moves upward in plants from roots to shoots is most extreme in the tallest trees, where distances to be traveled are the greatest. Some of the tallest trees are at least 120 meters (394 feet) tall. If a hypothesis or model can explain water movement in these tallest plants, then the model can also explain it in smaller examples. Much of the research on the mechanism of water transport has been performed on relatively tall trees.
Since around the 1960s one mechanism has been the most widely accepted explanation for how water moves in the xylem. This mechanism is intimately connected to the process of transpiration described above.
Xylem Transport Cells.
The xylem has a number of kinds of cells and so is called a complex tissue. The xylem vessels are the cells that actually transport water and dissolved minerals from the root. Two types of xylem vessels exist: vessel members and tracheids.
While vessel members and tracheids differ in a number of respects, they share one prominent structural feature: both are dead when transporting water. For xylem vessels, the production of strong secondary walls and the death of the cell are key characteristics that must be taken into account by any proposed mechanism. Also significant is the fact that cell walls are permeable to the flow of water, though they do offer some resistance to the flow.
Inadequate Explanations for the Movement of Water from Roots to Shoots.
Ideas to explain the uptake and transport of water are of two basic types. Either water can be pushed (pumped) from the bottom of the plant, or it can be pulled to the top. Early experimenters attempted to explain water movement in terms of pumps, which were thought to be located either in the roots or all along the path of water movement. Pumping water requires energy, and only the living cells in the plant expend energy. So, in 1893 a German researcher named Eduard Strasburger tested the hypothesis that living cells in the plant push water up the stem. He cut twenty-meter-tall trees off at the base and placed the cut stumps in buckets of poisons that would kill any living cells that were contacted. The trees continued to transport water despite the death of the living cells in the trunks. This experiment demonstrated that living stem cells are not required for transport of water through the stem. Further, the experiment showed that the roots are not necessary for the transport of water.
One "pull" model for water transport in the xylem is capillarity, the rise of some liquids in small tubes that are made of materials to which the liquid is attracted. However, capillarity can only pull water to a height of less than half a meter (ten and a half feet) in tubes the size of xylem vessels. This mechanism is clearly inadequate to explain water transport in tall trees.
The Transpiration-Cohesion-Tension Mechanism
An alternative pull model, the transpiration-cohesion-tension mechanism, is accepted as the best explanation thus far for water movement. The hypothesis has several components, as the name implies. Transpiration, the loss of water by the above-ground or aerial parts of the plant, has already been described. Cohesion refers to an attraction between molecules that are alike, in this case, water molecules. When water molecules interact, they are attracted to each other, the partial negative charge on the oxygen of one molecule being attracted to the partial positively charged hydrogen of another. These attractions, called hydrogen bonds, are quite important in the transpiration-cohesion mechanism.
Another important but less familiar idea is tension, or negative pressure. When a substance is compressed, a positive pressure higher than atmospheric pressure is generated. When a substance is being pulled from both ends, rather than compressed, a negative pressure, or tension, results that is lower than atmospheric pressure. One way to visualize a tension is to think about liquid in a syringe, which has the needle end sealed. When the plunger is pulled toward the back of the barrel, the liquid is pulled and is under a tension.
The transpiration-cohesion-tension mechanism can be explained as a series of sequential steps:
- The first step is the diffusion of water from the leaf through the stomatal pores. This is transpiration, and it provides the force driving the transport of water.
- As water exits the leaf, more water evaporates from the cell walls of the leaf into the extensive air spaces within the leaf.
- As water evaporates from the cell walls, this loss pulls more liquid water from the xylem of the leaf. This flow occurs because the water in the tiny pores and crevices in the cell walls is continuous with the water in the xylem vessels and because the continuous water molecules are held together by the high cohesion of water.
- Cohesion also accounts for the flow of the column of water from the xylem of the root to the xylem of the leaves, as water is lost through the stomata.
- As water flows upward in the root, water influx from the soil fills the void.
The flow of water and dissolved minerals in the xylem is an example of bulk flow, the movement of a solution as a whole rather than as individual molecules. A more common example of bulk flow is the streaming of water with its dissolved minerals in the pipes of a house. One difference between the house and xylem examples is that the pressures in plumbing are positive pressures, and water is pushed out the faucet. The pressures in the xylem are negative pressures, and water is pulled up the plant. In both cases transport occurs from a region of higher pressure to lower pressure. The transport cells in the xylem are also unlike the living cells of the plant, which have positive pressures on the order of five to ten times atmospheric pressure.
Evidence for the Transpiration-Cohesion-Tension Mechanism.
According to the transpiration-cohesion-tension mechanism, the process of water transport in the xylem is purely physical, with no energy input from living cells of the plant. This is consistent with the structure of the xylem vessels, which are nonliving. The cell walls of the xylem vessels are strong secondary walls, which can withstand the very negative pressures in the xylem stream. In order to pull water to the top of the tallest trees, the tensions at the top must be around negative thirty atmospheres.
Experimental evidence supports the transpiration-cohesion-tension mechanism as well. If the pressures in the xylem were positive and higher than atmospheric pressure, cutting through a xylem vessel would open the cut end to atmospheric pressure, and water would exude from the cut end. Since the pressures in the xylem are negative and lower than atmospheric, air is actually sucked into the cut end when the xylem vessels are cut, and water retreats into the xylem vessels. (Recutting cut flowers under water is good advice, since this refills the xylem vessels with water and keeps the flowers fresh longer.)
The retreat into the xylem vessels allows the tensions in the xylem to be measured by a device called a pressure bomb or pressure chamber. A leaf or twig is cut from the plant, and the blade is sealed into a strong metal chamber with the leaf stalk (petiole) or stem extending into the air. The chamber is pressurized until the contents of the xylem are forced back to their original position at the cut surface. The positive pressure required to restore the water to its original position is equal to, but opposite in sign, to the tension that existed in the xylem before it was cut. Tensions measured in the xylem higher up a tree are more negative than lower in the tree, in keeping with the proposed mechanism.
Questions About the Mechanism.
The main controversy surrounding transpiration-cohesion-tension mechanism questions whether water columns in the xylem can withstand the tensions required for transport. In a large pipe a column of water can withstand very little tension before the column breaks and an air bubble forms. Bubble formation (cavitation) stops the flow of water in a xylem vessel in much the same way that vapor lock stops the flow of gasoline in the fuel line of a car. However, the tension that a liquid can sustain increases as the diameter of the vessel decreases, and xylem vessels are very small (twenty to several hundred micrometers in trees, for example), and as a result the tension they can sustain without cavitation is very high.
Henry Dixon, an early researcher important in developing the cohesion theory, actually devised the pressure chamber in 1914, but made his chambers out of glass, with explosive results. Per Scholander first successfully used the pressure chamber in 1965 to measure the xylem tensions in tall Douglas-fir trees. He obtained his twig samples with the help of a sharpshooter who shot them down with a rifle.
Even so, bubbles do form in the xylem under various conditions such as drought, when very negative tensions are required to transport water. Using sensitive microphones, researchers can detect individual cavitation events as clicks that occur when the water column breaks. Although bubbles do form, water can move around a blocked xylem vessel, and plants can restore the continuity of the water columns in various ways. Researchers continue to investigate and question the cohesion mechanism, particularly new methods to measure tensions in the xylem.
Uptake of Water by Roots and the Movement of Water Into the Xylem
The mechanism for water uptake into roots differs from that for transpiration (diffusion) and for the flow of sap in the xylem (bulk flow). In order for water and dissolved minerals from the soil to enter the xylem of the root, cell membranes must be crossed. Osmosis is the movement of water across membranes that allow water to move freely but that control the movement of dissolved substances, such as mineral ions.
Why must water cross living cells as it moves across the root? Why can't water simply flow around the cells in the cell walls and enter the root xylem directly? The primary roots of plants generally consist of three concentric rings of tissue. On the outside is the epidermis, a single layer of cells usually lacking a thick waxy coating, suiting the root well for its role in water absorption. Next toward the center are layers of parenchyma cells that make up the cortex of the root. At the center of the root is the vascular tissue, including the xylem. The innermost layer of the cortex, the endodermis or "inner skin," has a special feature that prevents water movement through the cell walls of this layer. The walls that are perpendicular to the surface of the root contain waxy deposits called Casparian strips. The Casparian strips are impermeable to water and dissolved substances like mineral ions and thus block transport in the cell walls at the endodermis. Both water and minerals must enter the cytoplasm of the endodermal cells by crossing cell membranes. The water and minerals then cross into the vascular tissue by moving through living cells and cytoplasmic connections between them, finally exiting into the xylem by crossing yet another membrane.
The role of the Casparian strips is probably related more to the movement of mineral ions into the plant than to the movement of water. Mineral ions must cross membranes by means of special proteins in the membrane. Sometimes these proteins concentrate ions and other substances inside the cells by using cellular energy. By forcing mineral ions to cross membranes, the Casparian strips allow the plant to control which minerals will enter and to accumulate these ions to levels higher than in the soil water.
see also Anatomy of Plants; Cells, Specialized Types; Leaves; Photosynthesis, Carbon Fixation and; Photosynthesis, Light Reactions and; Roots; Stems; Vascular Tissues.
Susan A. Dunford
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