Question

Explain the 3 hypotheses for the flow of water through a stem

Answer 1

There are three hypotheses that explain the movement of water up a plant against gravity. These hypotheses are not mutually exclusive, and each contribute to movement of water in a plant, but only one can explain the height of tall trees:

1.            Root pressure pushes water up

2.            Capillary action draws water up within the xylem

3.            Cohesion-tension pulls water up the xylem

Root pressure relies on the positive pressure that forms in the roots as water moves into the roots from the soil. Water moves into the roots from the soil by osmosis, due to the low solute potential in the roots (lower ?s in roots than in soil). This intake of water in the roots increases ?p in the root xylem, driving water up. In extreme circumstances, root pressure results in guttation or secretion of water droplets from stomata in the leaves. However, root pressure can only move water against gravity by a few meters, so it is not strong enough to move water up the height of a tall tree.

Capillary action or capillarity is the tendency of a liquid to move up against gravity when confined within a narrow tube (capillary). Capillarity occurs due to three properties of water:

1.            Surface tension, which occurs because hydrogen bonding between water molecules is stronger at the air-water interface than among molecules within the water.

2.            Adhesion, which is molecular attraction between “unlike” molecules. In the case of xylem, adhesion occurs between water molecules and the molecules of the xylem cell walls.

3.            Cohesion, which is molecular attraction between “like” molecules. In water, cohesion occurs due to hydrogen bonding between water molecules.

On its own, capillarity can work well within a vertical stem for up to approximately 1 meter, so it is not strong enough to move water up a tall tree.

The cohesion-tension hypothesis is the most widely-accepted model for movement of water in vascular plants. Cohesion-tension essentially combines the process of capillary action with transpiration, or the evaporation of water from the plant stomata. Transpiration is ultimately the main driver of water movement in xylem. The cohesion-tension model works like this:

1.            Transpiration (evaporation) occurs because stomata are open to allow gas exchange for photosynthesis. As transpiration occurs, it deepens the meniscus of water in the leaf, creating negative pressure (also called tension or suction).

2.            The tension created by transpiration “pulls” water in the plant xylem, drawing the water upward in much the same way that you draw water upward when you suck on a straw.

3.            Cohesion (water sticking to each other) causes more water molecules to fill the gap in the xylem as the top-most water is pulled toward the stomata.

Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells, thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.