30.5 Transport of Water and Solutes in Plants

Water Potential and Movement in Plants

Water Potential in Plants

Water potential ($\Psi$) measures the potential energy of water in a system compared to pure water. Water always moves from areas of higher water potential (less negative) to areas of lower water potential (more negative). Pure water at standard temperature and pressure has a water potential of zero, which is the highest possible value.

Water potential has three components:

• Pressure potential ($\Psi_p$): Physical pressure exerted on water. Turgor pressure inside a plant cell is a common example. Positive pressure raises water potential and helps maintain cell shape and rigidity.
• Solute potential ($\Psi_s$): The effect of dissolved solutes (sugars, ions) on water potential. Also called osmotic potential, this value is always negative because solutes lower the free energy of water. More solutes means a more negative solute potential.
• Gravity potential ($\Psi_g$): The effect of gravity on water at different heights. Water at the top of a tall tree has lower water potential than water at the base, simply because gravity pulls it downward.

These combine in the water potential equation:

$\Psi = \Psi_p + \Psi_s + \Psi_g$

For most problems in an intro biology course, you'll focus on pressure potential and solute potential. Gravity potential becomes significant mainly in very tall plants.

Movement of Water Through Plants

Water follows a continuous water potential gradient from the soil, through the roots, up the xylem, and out through the leaves. Each step along this path has progressively lower (more negative) water potential.

How water gets pulled upward through the xylem:

1. Transpiration (evaporation of water from leaf surfaces through stomata) removes water from the leaf, creating very negative water potential in leaf cells.
2. This creates a "pull" on the water column in the xylem, drawing water upward.
3. Cohesion (hydrogen bonds between water molecules) keeps the water column intact so it doesn't break apart under tension.
4. Adhesion (attraction between water molecules and the hydrophilic walls of xylem cells) helps counteract gravity and supports the water column.

This explanation is called the cohesion-tension theory, and it accounts for how water reaches the tops of even the tallest trees.

Transpiration is the loss of water vapor from leaves through stomata. Several environmental factors affect the transpiration rate:

• Humidity: Low humidity increases the water vapor gradient between the leaf interior and the outside air, speeding up transpiration.
• Temperature: Higher temperatures increase the kinetic energy of water molecules, promoting faster evaporation.
• Wind: Moving air removes the thin layer of humid air (boundary layer) that builds up around the leaf surface, increasing the rate of water loss.

Evapotranspiration refers to the combined water loss from plant transpiration and direct evaporation from soil and plant surfaces. It plays a major role in the water cycle and can influence local water availability.

Stomatal regulation controls the balance between gas exchange and water conservation. Guard cells on either side of each stoma change shape to open or close the pore:

• When guard cells take up water and become turgid (swollen), they bow apart and the stoma opens.
• When guard cells lose water and become flaccid, they collapse together and the stoma closes.

Factors that influence stomatal opening and closing:

• Light: Stomata generally open during the day to let in $CO_2$ for photosynthesis and close at night to conserve water.
• $CO_2$ concentration: Low $CO_2$ levels inside the leaf trigger stomata to open, allowing more $CO_2$ uptake.
• Water availability: Drought stress causes guard cells to lose turgor, closing stomata to prevent excessive water loss. The hormone abscisic acid (ABA) plays a key role in signaling this closure.

Water and Solute Transport Pathways

Water and solutes can travel short distances (root to xylem, for example) through three different routes:

• Apoplast pathway: Movement through cell walls and the spaces between cells, without ever crossing a cell membrane. This is the fastest route but is blocked at the endodermis by the Casparian strip, which forces water through cell membranes for selective filtering.
• Symplast pathway: Movement through the cytoplasm of connected cells via plasmodesmata (small channels that link the cytoplasm of adjacent cells).
• Transmembrane pathway: Movement directly across cell membranes, often facilitated by aquaporins (specialized water channel proteins that speed up osmosis).

For long-distance transport, plants rely on vascular tissue: xylem carries water and dissolved minerals upward from roots, and phloem distributes sugars and other organic compounds throughout the plant.

Transport of Sugars and Photosynthetic Products

Transport of Photosynthetic Products

Sugars produced by photosynthesis travel through the phloem, which is made up of two key cell types:

• Sieve-tube elements: Living cells that lack a nucleus, ribosomes, and most organelles. This stripped-down interior creates an open pipeline for efficient transport. Sieve plates (perforated end walls) connect adjacent sieve-tube elements.
• Companion cells: Nucleated cells connected to sieve-tube elements by plasmodesmata. They supply the energy (ATP) and proteins that sieve-tube elements need to function.

The Pressure-Flow Hypothesis explains how sugars move through the phloem. Here's how it works step by step:

1. Loading at the source: Sugars (primarily sucrose) are actively loaded into sieve-tube elements at source tissues like mature leaves. This requires ATP and creates a high solute concentration inside the phloem.
2. Water entry by osmosis: The high solute concentration lowers the water potential inside the phloem, so water moves in from the nearby xylem by osmosis. This raises the turgor pressure at the source end.
3. Bulk flow from source to sink: The high turgor pressure at the source pushes the phloem sap (sugar solution) toward sink tissues, where turgor pressure is lower. This bulk flow carries sugars, amino acids, hormones, and some minerals along with the water.
4. Unloading at the sink: At sink tissues (roots, developing fruits, growing shoot tips, storage organs like tubers), sugars are unloaded from the phloem and either used in metabolism or converted to starch for storage. As solutes leave, water potential rises, and water exits the phloem by osmosis. This maintains the pressure gradient that keeps flow going.

Sucrose is the primary transport sugar in most plants because it's a non-reducing sugar, making it more chemically stable during long-distance transport than glucose or fructose.

Source-sink relationships determine the direction and rate of phloem transport. A "source" is any organ that produces or releases more sugar than it uses (typically mature leaves). A "sink" is any organ that consumes or stores more sugar than it produces (roots, fruits, young developing leaves). These relationships are flexible: a developing leaf starts as a sink but becomes a source once it's mature enough to photosynthesize on its own.