Water and Mineral Absorption and Transport + Food Translocation and Storage
Water Potential: Definition and Components
Water potential (ψ) measures potential energy in water and is the driving force for water movement between systems. It is the difference in potential energy between a given water sample and pure water at atmospheric pressure and ambient temperature.
Pure water has a reference potential designated as zero: Water potentials in plant parts are expressed relative to this reference.
Water potential is denoted by the Greek letter psi (ψ) and is measured in megapascals (MPa).
System concept: ψsystem refers to the water potential of any particular aqueous system (soil water, root water, stem water, leaf water, or atmospheric water).
The total water potential is composed of four components: where:
= solute (osmotic) potential
= pressure (turgor) potential
= gravity potential
= matric potential
Water moves from regions of higher ψ to lower ψ to re-establish equilibrium; movement continues until the difference (ΔΨ) between the two systems is zero:
For whole-plant water movement (soil → root → stem → leaf → atmosphere) during transpiration: \Psi{soil} > \Psi{root} > \Psi{stem} > \Psi{leaf} > \Psi_{atmosphere}.
Although the total Ψsystem determines water movement, plants can manipulate the components (especially Ψs) to control water flow.
Components of Water Potential
Solute potential (Ψs), also called osmotic potential, is negative in plant cells and zero in distilled water. Typical values for cell cytoplasm are approximately Solutes lower Ψw by binding water via hydrogen bonds, reducing the energy available to do work.
Hydrophilic solutes dissolve in water because water can form hydrogen bonds; hydrophobic substances (e.g., oil) do not dissolve and thus do not contribute to Ψs.
Because Ψs is one of the Ψsystem components, a decrease in Ψs lowers the total water potential (Ψtotal) and can drive water movement by osmosis.
The cytoplasm of plant cells is more negative than pure water due to solute content, causing water to move from soil into root cells via osmosis.
Plants can metabolically manipulate Ψs (and thus Ψtotal) by adding or removing solutes, giving them control over water potential:
Pressure potential (Ψp), also called turgor potential, can be positive or negative depending on compression or tension. Positive Ψp increases Ψtotal; negative Ψp decreases Ψtotal.
Typical Ψp values in well-watered cells are about In well-hydrated tissues, Ψp can reach up to around (≈ 210 psi; note: 1.5 MPa ≈ 217.5 psi by standard conversion, the text provides ~210 psi).
Ψp is reflected in plant processes such as wilting and turgor restoration: water loss via transpiration lowers Ψp toward 0 at wilting; root uptake restores Ψp when water is available.
Stomatal opening affects Ψp indirectly: opening stomata allows transpiration, which lowers leaf Ψp and Ψtotal, creating a gradient that drives water movement from the petiole (xylem) into the leaf.
Gravity potential (Ψg) is always negative or zero in plants with height. It reduces Ψtotal by acting downward; it becomes more influential with plant height. It cannot be directly manipulated by the plant. A typical gradient per unit height is about , so taller trees require a larger driving force.
Matric potential (Ψm) is always negative to zero and reflects water binding to a matrix (e.g., cell walls). It is very large (negative) in dry tissues (e.g., seeds) or dry soils and approaches zero as tissues hydrate.
Ψm arises from water binding to hydrophilic matrix components (soluble solutes vs. insoluble cell-wall matrix). In plants, Ψm is associated with the hydrophilic cellulose matrix in cell walls.
Ψm cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.
Movement of Water and Minerals in the Xylem
Water moves from higher total water potential to lower total water potential: ↑Ψtotal → ↓Ψtotal to re-establish equilibrium, i.e., movement from areas of higher Gibbs free energy to lower Gibbs free energy → ΔΨ approaches zero.
Transpiration is the main driver of water movement in the xylem and is the loss of water from the plant via evaporation at the leaf surface. It creates negative pressure (tension) around at the leaf surface, varying with vapor pressure deficit (VPD).
The tension pulls water up from the roots through the xylem; at night, when stomata close, water is held in the stem/leaf by adhesion to xylem cell walls and cohesion among water molecules, enabling continuity of the water column.
Cohesion–tension theory of sap ascent:
Evaporation from mesophyll cells generates a negative water potential gradient.
Water is pulled upward through the xylem due to cohesion between water molecules and adhesion to xylem walls.
The leaf interior contains large intercellular air spaces for gas exchange (O2 for respiration and CO2 for photosynthesis).
On the surface of mesophyll cells, water forms a thin film on primary cell-wall cellulose; evaporation into leaf air spaces decreases the surface film, increasing tension on surface water and pulling more water up the xylem.
Xylem structure specific adaptations to cope with tension:
Vessel rings help maintain tubular shape under pressure.
Small perforations between vessel elements limit the formation and spread of gas bubbles (cavitation).
Cavitation can form embolisms that disrupt continuous water flow; embolisms are more likely in tall trees due to greater tension requirements.
The taller the plant, the greater the tension required to lift water; embolisms can plug xylem and impede transport in large trees.
Which of the following statements is false? (Study question from the text)
a. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the xylem. Transpiration draws water from the leaf.
b. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the phloem. Transpiration draws water from the leaf.
c. Water potential decreases from the roots to the top of the plant.
d. Water enters the plants through root hairs and exits through stoma. 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. However, transpiration is tightly controlled.
Regulation of Transpiration
The atmosphere surrounding the leaf drives transpiration but also causes substantial water loss; up to about 90% of the water taken up by roots can be lost to transpiration.
Leaves are coated with a waxy cuticle that reduces water loss; regulation is primarily achieved by opening/closing stomata on the leaf surface.
Stomata are surrounded by guard cells that respond to environmental cues such as light intensity and quality, leaf water status, and CO2 concentrations.
Stomata must be opened for CO2 to diffuse into the leaf for photosynthesis and respiration, but opening also leads to water vapor loss.
Plants have evolved adaptations to reduce transpiration in response to local environmental conditions:
Xerophytes (desert plants) and epiphytes (plants growing on others) typically have a thicker waxy cuticle.
They may also have sunken stomata and trichomes (hair-like epidermal structures) that hinder air flow and reduce transpiration.
Some species develop multiple epidermal layers or other morphological features.
Hydrophytes (aquatic plants) have their own adaptations to thrive in water-saturated environments.
The balance between efficient photosynthesis and water conservation drives the evolution of stomatal regulation and leaf adaptations.
Photosynthates Transport in the Phloem (Translocation)
Plants store energy as polymers (e.g., starch) in seeds and bulbs; these are converted metabolically to sucrose for transport once green tissues are active.
Photosynthates produced in leaves are mainly in the form of sucrose and are transported to sinks via the phloem in a process called translocation.
Sources are tissues that produce photosynthates (e.g., mature leaves); sinks are tissues that consume or store photosynthates (e.g., roots, developing seeds, young leaves, tubers).
Pathways of flow in the phloem differ from the xylem: phloem transport is bidirectional and depends on source-to-sink demand, whereas xylem transport is unidirectional (soil → leaf → atmosphere).
The direction and pattern of flow change during development and season:
Early in development, photosynthates are directed toward roots.
During vegetative growth, delivery is split toward shoots/leaves.
During reproductive development, flow is directed toward seeds and fruits; they may also be directed toward storage organs such as tubers.
Loading of photosynthates into the phloem:
Photosynthates are loaded from mesophyll cells into phloem sieve-tube elements (STEs) starting from the source.
Cytoplasmic channels called plasmodesmata connect mesophyll cells to phloem cells, allowing movement to STEs.
Sucrose is actively transported against its concentration gradient into phloem cells via the electrochemical potential of the proton gradient, coupled to uptake by a sucrose-H+ symporter using ATP.
Structure of phloem:
Phloem sieve-tube elements (STEs) have reduced cytoplasmic contents and are connected by sieve plates with pores that allow bulk flow of phloem sap.
Companion cells are associated with STEs and provide metabolic energy and regulatory support for STEs.
Lateral sieve areas connect sieve-tube elements to companion cells.
Phloem sap composition:
Phloem sap is an aqueous solution that can contain up to about 30% sugar, minerals, amino acids, and plant growth regulators.
The high sugar concentration lowers Ψs, which lowers Ψtotal and causes water to move osmotically from the adjacent xylem into the phloem, raising hydrostatic pressure in the phloem (pressure-flow mechanism).
Movement of phloem sap:
The resultant positive pressure (hydrostatic pressure) drives bulk flow from source to sink (Figure 30.37 in the text).
In sinks, sucrose concentration is lower because sucrose is being consumed for growth or converted to starch or other polymers (e.g., cellulose).
Unloading at the sink occurs by diffusion or active transport from regions of higher concentration to lower concentration.
Water diffuses from the phloem to sink cells and is then transpired or recycled via the xylem back into the phloem sap, maintaining the cycle.
Exchange with xylem: loading into phloem reduces Ψs and Ψtotal in the phloem, which draws water from the xylem into the phloem; unloading at the sink reduces the local solute concentration and water returns to the xylem or is transpired.
Summary: Phloem transport uses a pressure-flow mechanism driven by osmotic gradients created by solute loading, with companion cells providing energy and sieve-tube elements forming a continuous conduit for bidirectional flow to meet developmental needs.
Phloem Structure and Functional Details
Sieve-tube elements (STEs) are the conducting cells of the phloem with sieve plates at their ends that have pores; these plates enable the bulk flow of phloem sap along the tube.
Companion cells accompany STEs and are metabolically active; they produce energy and help maintain the function of STEs, including loading/unloading processes.
Plasmodesmata connect MESOPHYLL CELLS to phloem cells, enabling loading of photosynthates into the phloem.
Phloem sap can be rich in sugars and various organic and inorganic solutes, and its movement is driven by pressure generated by osmotic differences (not by ATP directly).
Plant Sensory Systems and Responses to Light (30.6)
Plants detect and respond to environmental factors such as light, gravity, temperature, and mechanical touch.
Receptors sense environmental factors and relay information to effector systems via chemical messengers to elicit responses.
Photomorphogenesis: growth and development of plants in response to light; helps optimize light capture and space usage.
Photoperiodism: ability to track time (day length) using light cues; influences flowering and seasonal development.
Phototropism: directional growth toward or away from light; enables plants to optimize light capture.
Photoreceptors consist of a protein covalently bonded to a light-absorbing pigment called a chromophore; together they form a chromoprotein.
Connections to foundational principles:
Light sensing integrates with hormonal signaling and growth responses.
Light quality and intensity modulate stomatal behavior, photosynthetic rates, and resource allocation.
Gravity sensing (gravitropism) coordinates growth direction with plant architecture.
Mechanical cues from touch can trigger rapid movement or growth adjustments (thigmotropism, thigmonasty, thigmogenesis).
Photomorphogenesis, Photoperiodism, and Phototropism: Key Concepts
Photomorphogenesis: light-driven development patterns that optimize light capture and spatial arrangement of tissues.
Photoperiodism: timing of developmental transitions (e.g., flowering) based on day length.
Phototropism: growth toward light, mediated by asymmetric distribution of growth hormones (typically auxins) in response to light gradients; directs shoot and root growth.
Photoreceptors and chromoproteins integrate light signals to regulate gene expression and physiological responses.
Practical and Real-World Relevance
Water potential components and their regulation underpin plant hydration, drought tolerance, and irrigation strategies.
The cohesion–tension theory explains how water can be transported to great heights (e.g., coastal redwoods) using only physical forces, without metabolic energy input.
Understanding xylem cavitation and embolism helps explain why tall trees face hydraulic limits and why drought/heat can limit growth or cause dieback.
Phloem transport is essential for distributing sugars produced by photosynthesis; source-sink dynamics guide where growth and storage occur (e.g., roots vs seeds vs tubers).
Adaptations such as thicker cuticles, sunken stomata, and trichomes in xerophytes illustrate how plants balance photosynthesis and water loss in arid environments.
Light sensing governs seasonal timing of growth and reproduction, helping crops, horticultural species, and wild plants optimize performance and yield.