2 8.2 Roots to Shoots

In previous chapters on nutrient absorption, we have discussed how water uptake occurs in a plant’s roots. The movement of water is a similarly complex and harmonious process. Water enters a plant through its roots, travels through the shoots, and evaporates through the leaves via transpiration. These processes are reliant on different potentials, cohesion, and adhesion.

Figure  30.34   The cohesion–tension theory of sap ascent is shown. Evaporation from the mesophyll cells produces a negative water potential gradient that causes water to move upwards from the roots through the xylem.

Water Potential

Root uptake of water begins in the root hairs, where maximized surface area allows water to enter the plant. Water enters by osmosis,  moving from the soil (where water potential is higher) into root cells (where water potential is lower due to higher solute concentration). Root uptake of water relies on the difference in water potential to drive the movement of water through the plant. (Link to active membrane transport section)

Plants are similar to hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree (Figure 30.31a). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure 30.31b). Plants achieve this because of water potential.

Figure  30.31   With heights nearing 116 meters, (a) coastal redwoods (Sequoia sempervirens) are the tallest trees in the world. Plant roots can easily generate enough force to (b) buckle and break concrete sidewalks, much to the dismay of homeowners and city maintenance departments. (credit a: modification of work by Bernt Rostad; credit b: modification of work by Pedestrians Educating Drivers on Safety, Inc.)

Water potential is a measure of the potential energy in water. In practical terms, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψwpure H2O) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water everywhere else in a plant are expressed relative to Ψwpure H2O.

The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. “System” can refer to the water potential of the soil water (Ψsoil), root water (Ψroot), stem water (Ψstem), leaf water (Ψleaf) or the water in the atmosphere (Ψatmosphere): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. By manipulating the individual components (especially Ψs), a plant can control water movement.

Solute Potential

Solute potential (Ψs), also called osmotic potential, is related to the solute concentration (in molarity). The solute potential is negative in a plant cell and zero in distilled water. TSolute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Because of this difference in water potential water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential.

Figure  30.32   In this example with a semipermeable membrane between two aqueous systems, water will move from a region of higher to lower water potential until equilibrium is reached. Water moves in response to the difference in water potential between two systems (the left and right sides of the tube).

Pressure Potential

Pressure potential (Ψp), also called turgor potential, may be positive or negative (Figure 30.32). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive Ψp (compression) increases Ψtotal, and a negative Ψp (tension) decreases Ψtotal. Positive pressure inside cells is contained by the cell wall, producing turgor pressure. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered (Figure 30.33). Water is lost from the leaves via transpiration (approaching Ψp = 0 MPa at the wilting point) and restored by uptake via the roots.

A plant can manipulate Ψp (turgor pressure) via its ability to manipulate Ψs (osmotic pressure?) and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψs will decline, Ψtotal will decline, the difference between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and Ψp will increase. Ψp is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψp and Ψtotal of the leaf and increasing Ψ between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.

Figure  30.33   When (a) total water potential (Ψtotal) is lower outside the cells than inside, water moves out of the cells and the plant wilts. When (b) the total water potential is higher outside the plant cells than inside, water moves into the cells, resulting in turgor pressure (Ψp) and keeping the plant erect. (credit: modification of work by Victor M. Vicente Selvas)

Cohesion and Adhesion

Cohesion and adhesion are essential for their role in the transportation of water within the plant body. Cohesion allows water molecules to stick to one another. This property enables the formation of a continuous water column within the xylem vessels. This water column hydrates the plant, and delivers nutrients dissolved within the water to the appropriate location for plant usage. Adhesion moves water molecules up the xylem by sticking them to the xylem’s wall. The net effect of cohesion and adhesion is pulling water from the roots to the leaves during transpiration.

Both of these properties rely on hydrogen bonds for movement. You can review the properties of water here (LINK) to further this connection on your own!

Figure  30.37   Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements. This reduces the water potential, which causes water to enter the phloem from the xylem. The resulting positive pressure forces the sucrose-water mixture down toward the roots, where sucrose is unloaded. Transpiration causes water to return to the leaves through the xylem vessels.

Transpiration

Transpiration—the loss of water vapor to the atmosphere through stomata—is a passive process, meaning that metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled.

The role of cohesion and adhesion in assisting the plant does not begin and end with moving material from the root to the leaf. It also facilitates the generation of negative pressure.

At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other

When transpiration occurs, negative pressure is created on the leaf’s surface. When water evaporates from the leaf’s surface through its stomata, the leaving water pulls on existing water in the leaf due to cohesion. The process creates a level of tension, or negative pressure, in the xylem. The suction force draws more water upwards from the roots to the leaf.