Table of Contents
Overview
Water and dissolved mineral ions taken up by the roots must travel upward through the stem to reach leaves and other organs. This transport mainly occurs in the xylem and is driven by a combination of physical forces and plant physiological processes. In this chapter, we focus on how water and ions move through the stem, which tissues are involved, and what forces contribute to long‑distance transport.
Pathways of Transport in the Stem
Xylem as the Main Transport Tissue
In the stem, the xylem forms continuous, mostly vertical pathways from the root to the leaves. It consists mainly of:
- Tracheids (in all vascular plants)
Long, narrow cells with tapered ends and thick, lignified cell walls. They are dead at maturity and overlap like a bundle of pipes with pits (thin wall areas) that allow lateral water movement between cells. - Vessel elements (especially in angiosperms)
Shorter, wider cells stacked end to end. Their end walls are highly perforated or almost absent, forming continuous tubes called vessels. These offer lower resistance to flow but are more vulnerable to damage (e.g., embolisms).
These dead, hollow conduits act like capillaries: water and ions move through their lumens and across pits and perforation plates.
Arrangement of Xylem in the Stem
The position and organization of xylem vary:
- Herbaceous dicots: Xylem occurs in discrete vascular bundles, with xylem typically on the inner side and phloem on the outer side.
- Woody dicots and gymnosperms: Xylem develops into wood (secondary xylem), arranged in annual growth rings.
- Monocots: Numerous scattered vascular bundles containing xylem.
This arrangement affects the mechanical stability of the stem but not the fundamental principle that xylem provides long‑distance pathways for water and ions.
Apoplastic and Symplastic Routes in the Stem
Although long‑distance movement in the stem is through xylem conduits (a largely apoplastic route—the cell wall and intercellular space continuum), water can also exchange with living stem tissues:
- Apoplastic flow: Through cell walls and xylem lumens, without crossing cell membranes; dominant for bulk flow in the stem.
- Symplastic flow: From cell to cell via plasmodesmata through cytoplasm; important for local redistribution, buffering, and temporary storage.
- Transmembrane pathway: Repeated crossing of plasma membranes, combining features of both routes.
Living xylem parenchyma cells can take up and release water and ions, influencing local concentrations and buffering fluctuations.
Driving Forces of Upward Water Transport
Long‑distance ascent of xylem sap in the stem is mainly a passive process, driven by physical forces established by the leaves and environment, not by a “pump” in the stem.
Root Pressure vs. Transpiration Pull
Two major forces can contribute to upward water movement:
- Root pressure
- Generated by active ion uptake in roots, which lowers the water potential in xylem and drives water in osmotically.
- Can cause guttation in leaves at night or under high humidity.
- Usually modest and rarely sufficient alone to lift water to great heights.
- Transpiration pull (cohesion–tension mechanism)
- Evaporation of water from leaf surfaces (transpiration) creates a water potential gradient from leaves down through the stem to the roots.
- Water is pulled upward along this gradient as a continuous column in xylem.
- This is the dominant mechanism in most conditions, especially in tall plants and trees.
In the stem, these forces manifest as negative pressure (tension) in the xylem sap during active transpiration.
Water Potential Gradient
Water moves from regions of higher water potential ($\Psi$) to lower water potential. Along the soil–plant–atmosphere continuum:
$$
\Psi_{\text{soil}} \;>\; \Psi_{\text{root}} \;>\; \Psi_{\text{stem}} \;>\; \Psi_{\text{leaf}} \;>\; \Psi_{\text{air}}
$$
Within the stem, this gradient is maintained by:
- Loss of water at the leaves (transpiration)
- Capillary properties of the xylem conduits
- Cohesive and adhesive forces (see below)
Physical Principles: Cohesion–Tension Theory
The cohesion–tension theory explains how water can be transported upward through the stem as a continuous column under tension.
Cohesion and Adhesion
- Cohesion: Attraction between water molecules via hydrogen bonds. It gives water a high tensile strength, allowing columns of water to withstand significant negative pressure without breaking.
- Adhesion: Attraction between water molecules and xylem wall surfaces (cellulose, lignin). Helps water “stick” to walls and counteract gravity.
Together, cohesion and adhesion enable water columns to be pulled upward from the leaves, through the stem, down to the roots.
Tension in the Xylem
Transpiration from leaves generates tension in the leaf xylem:
- Water evaporates from cell walls in leaf air spaces.
- This draws water from cells adjacent to the air spaces.
- Tension is transmitted back through the water column in the xylem.
Because water columns in the xylem are continuous, this tension is transmitted down the stem like a rope under pull.
Capillarity and Conduit Diameter
In narrow tubes, capillary rise depends on tube radius $r$:
$$
h \propto \frac{1}{r}
$$
Where $h$ is the height of capillary rise. The small diameters of tracheids and vessels help with:
- Adhesion to walls
- Stabilization of water columns under tension
However, capillary rise alone is insufficient to account for water transport in tall trees; it supports the cohesion–tension mechanism rather than replacing it.
Flow Resistance and Flow Rate in the Stem
Water flow through xylem can be considered in terms of flow resistance and driving pressure (tension).
Hagen–Poiseuille Relationship (Qualitative)
Flow through a cylindrical conduit is described by the Hagen–Poiseuille equation:
$$
Q \propto \frac{r^{4} \cdot \Delta P}{\eta \cdot L}
$$
Where:
- $Q$ = volume flow rate
- $r$ = radius of the conduit
- $\Delta P$ = pressure difference (here, tension gradient)
- $\eta$ = viscosity of the fluid
- $L$ = length of the conduit
Consequences for the stem:
- Small increases in vessel diameter greatly increase potential flow ($r^4$ relationship).
- Narrower tracheids provide lower risk of cavitation but higher resistance.
- Wider vessels offer high conductivity but are more vulnerable to embolisms.
Longitudinal vs. Lateral Transport
- Longitudinal flow (along the stem): Mainly through vessel and tracheid lumens; primary pathway for long‑distance transport.
- Lateral flow (across the stem):
- Through pits between adjacent xylem conduits.
- Via rays (radial parenchyma) that move water and solutes between xylem and phloem, and between inner and outer stem regions.
This lateral component is vital for distribution to different parts of the stem and for maintaining uniform hydration.
Transport of Mineral Ions in the Stem
Water in the xylem is not pure; it is xylem sap containing dissolved ions and some organic substances.
Composition of Xylem Sap
Typical solutes include:
- Cations: $K^+$, $Ca^{2+}$, $Mg^{2+}$, $Na^+$ (often regulated), $Fe^{2+}/Fe^{3+}$ (chelated)
- Anions: $NO_3^-$, $H_2PO_4^-/HPO_4^{2-}$, $SO_4^{2-}$, $Cl^-$
- Trace elements and occasionally organic molecules (e.g., amino acids, organic acids, hormones like cytokinins, ABA).
Passive Transport with Bulk Flow
Once ions have been loaded into the xylem (largely at the root level):
- They are carried passively with the bulk water flow driven by transpiration.
- No further energy input is needed along the stem for their upward movement.
Ion concentrations can change slightly along the stem due to unloading into stem tissues or refilling.
Ion Exchange with Stem Tissues
Living cells in and around the xylem influence ionic composition:
- Xylem parenchyma can:
- Take up ions from the sap (storage, detoxification).
- Release ions back into the sap (redistribution, buffering).
- Ray parenchyma moves ions radially:
- From xylem to phloem (e.g., for redistribution to roots or growing tissues).
- From phloem to xylem (e.g., during remobilization from storage organs).
Ion channels, carriers, and pumps in parenchyma membranes regulate these exchanges, but the main long‑distance movement in the stem remains passive with the water stream.
Stability and Vulnerability of the Water Column
The water column under tension in the xylem is stable under normal conditions but can be disrupted.
Cavitation and Embolism
- Cavitation: Formation of vapor bubbles when tension exceeds water’s tensile strength or when gas is introduced (e.g., through freeze–thaw cycles, mechanical damage, drought).
- Embolism: A persistent air bubble that fills part of a conduit and blocks water flow.
Consequences:
- Reduced hydraulic conductivity in affected xylem segments.
- Potential loss of supply to parts of the crown in severe cases.
Structural and Functional Protection Mechanisms
Plants possess several strategies:
- Narrow conduits and intervessel pits:
- Small pit pores and specialized pit membranes limit spread of air bubbles.
- Tracheids, being narrow and overlapping, localize embolisms more than wide vessels.
- Redundant pathways:
- Many parallel conduits mean that blockage of some does not necessarily interrupt overall transport.
- Refilling mechanisms (in some species):
- Living xylem parenchyma may help dissolve or isolate embolisms, especially when transpiration is low.
- Root pressure can help re‑fill embolized conduits in certain plants and seasons.
Seasonal and Environmental Influences
- Winter freezing: Ice formation can exclude gases, leading to embolism upon thawing.
- Drought: Low soil water potential and high transpiration demand increase xylem tension and cavitation risk.
- Species differences: Drought‑tolerant species often have narrower vessels and more cavitation‑resistant xylem.
These factors determine how reliably water and ions can be transported through the stem under changing environmental conditions.
Functional Significance of Stem Transport
The efficiency and reliability of water and ion transport in the stem affect:
- Leaf function:
- Adequate water supply for photosynthesis and transpiration cooling.
- Delivery of mineral nutrients required for enzyme function and chlorophyll synthesis.
- Growth and development:
- Supply to developing buds, flowers, and fruits.
- Support of cambial activity in woody stems (formation of new xylem and phloem).
- Whole‑plant water balance:
- Coordination with root uptake and leaf transpiration.
- Response to environmental stress (drought, salinity, temperature extremes).
The stem thus serves not only as a mechanical support but also as a dynamic hydraulic link between the soil and the atmosphere, enabling plants to maintain metabolism and growth in a terrestrial environment.