WATER TRANSPORT
❑ Water balance in plants
❑ Water absorption
❑ Water transport through xylem,
Source: Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition).
Sinauer Associates, Inc. Publishers, Sunderland, Massachusetts USA.
Water in Soil
• Water content and the rate of water movement in soils depend on
soil type and structure.
Fig: Main driving forces for water flow from the soil through
the plant to the atmosphere:
i.
differences in water vapor concentration (Δcwv) between leaf
and air are responsible for the diffusion of water vapor from
the leaf to the air;
ii.
differences in pressure potential (Δ ψP, PP) drive the bulk
flow of water through xylem conduits; and
iii.
differences in water potential (Δ ψ, WP) are responsible for
the movement of water across the living cells in the root.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers, Sunderland, Massachusetts USA.
A negative hydrostatic pressure in soil water lowers
soil water potential
• WP of soils have three components: the osmotic
potential (ψs, OP), the pressure potential (ψP,
PP), and the gravitational potential (ψg, GP).
• OP of soil water is generally negligible, because
solute concentrations are low (–0.02 MPa)
(except in saline soils, ); In saline soils ψs can be
–0.2 MPa or lower.
• PP for wet soils, ψP is very close to zero. As soil
dries out, ψP decreases and can become
negative.
• Gravity plays an important role in drainage. The
downward movement of water is due to the fact
that GP is proportional to elevation (i.e. higher at
higher elevations, and vice versa).
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers, Sunderland, Massachusetts USA.
• Water under a curved surface develops a negative pressure that may be estimated by the following
formula:
T- surface tension of water (7.28 × 10–8 MPa m); r- radius of curvature of the
air–water interface. This is the same capillarity equation, where here the soil
particles are assumed to be fully wettable (contact angle q = 0; cos q = 1).
• As soil dries out, water is first removed from the largest spaces between soil particles and
subsequently from successively smaller spaces between and within soil particles.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers, Sunderland, Massachusetts USA.
Water moves through soil by bulk flow
• Bulk or mass flow is the concerted movement of molecules en masse, in response to a pressure
gradient. E.g. bulk flow are water moving through a garden hose or down a river.
• The movement of water through soils is predominantly by bulk flow.
• Because the pressure in soil water is due to the existence of curved air–water interfaces, water
flows from regions of higher soil water content (where the water-filled spaces are larger, PP is less
negative) to regions of lower soil water content (where the water-filled spaces are smaller with more
curved air–water interfaces and a more negative PP).
• Rate of water flow in soils depends on two factors: the size of the pressure gradient through the
soil, and the hydraulic conductivity (HC) of the soil.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers, Sunderland, Massachusetts USA.
Water Absorption by Roots
• Root hairs are filamentous outgrowths of root
epidermal cells that increase the surface area of the
root, thus providing greater capacity for absorption of
ions and water from the soil.
• Water enters the root most readily near the root tip.
• Mature regions of the root are less permeable to water
because they have developed a modified epidermal
layer that contains hydrophobic materials in its walls.
Fig. Rate of water uptake by short segments (3–5 mm) at
various positions along an intact pumpkin (Cucurbita
pepo) root (A).
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
Fig. Diagram of water uptake in which the entire
root surface is equally permeable (B) or is
impermeable in older regions due to the deposition
of suberin, a hydrophobic polymer (C). When root
surfaces are equally permeable, most of the water
enters near the top of the root system, with more
distal regions being hydraulically isolated as the
suction in the xylem is relieved due to the inflow of
water. Decreasing the permeability of older regions
of the root allows xylem tensions to extend further
into the root system, allowing water uptake from
distal regions of the root system. (A after Kramer
and Boyer 1995.)
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers, Sunderland, Massachusetts USA.
Water moves in the root via
the apoplast, symplast, and
transmembrane pathways
• Apoplast pathway: In this
pathway, water moves through cell
walls and extracellular spaces
without crossing any membranes as
it travels across the root cortex.
• Symplast pathway: In the
symplast pathway, water flows in
the cortex between the cells
through the plasmodesmata without
crossing the plasma membrane.
Figure: Pathways for water uptake by the root.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
• Transmembrane pathway: is the route by which water enters a cell on one side, exits the cell on
the other side, enters the next in the series, and so on.
• And though we can define three pathways, it is important to remember that water moves not
according to a single chosen path, but wherever the gradients of WP and hydraulic resistances
direct it. A particular water molecule moving in the symplast may cross the membrane and move in
the apoplast for a moment, and then move back into the symplast again.
• At the endodermis, water movement through the apoplast pathway is obstructed by the Casparian
strip.
• The Casparian strip is a band within the radial cell walls of the endodermis that is impregnated with
lignin, a hydrophobic polymer.
• The Casparian strip forms in the nongrowing part of the root, several mm to several cm behind the
root tip, at about the same time that the first xylem elements mature.
• The Casparian strip breaks the continuity of the apoplast pathway, forcing water and solutes to
pass through the PM in order to cross the endodermis.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
Solute accumulation in the xylem can generate “root
pressure”
• Plants sometimes exhibit a phenomenon referred to as root
pressure (positive hydrostatic pressure in root).
• When transpiration is low or absent, positive hydrostatic pressure
builds up in the xylem.
• High WP, low transpiration rates → Root pressure develops.
• High transpiration rates → water is transported through the plant
and lost to the atmosphere rapidly → so a positive pressure
resulting from ion uptake never develops in the xylem.
• Plants that develop root pressure frequently produce liquid
droplets on the edges of their leaves, a phenomenon known as
guttation.
• Positive xylem pressure causes exudation of xylem sap through
specialized pores called hydathodes that are associated with
vein endings at the leaf margin.
Guttation in a leaf from lady’s mantle
(Alchemilla vulgaris). In the early
morning, leaves secrete water droplets
through the hydathodes, located at the
margins of the leaves.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers, Sunderland, Massachusetts USA.
Water Transport through Xylem
• There are two main types of water-transporting cells in the xylem: tracheids and
vessel elements.
• Vessel elements: seen in angiosperms, Gnetales of gymnosperms, and some ferns.
• Tracheids: present in both angiosperms, gymnosperms and ferns.
• Tracheids are elongated, hollow, spindle-shaped and dead cells with highly lignified
walls and are arranged in overlapping vertical files. Water flows between tracheids by
means of the numerous pits.
• The shapes of pits and the patterns of wall pitting vary with species and organ type.
• Pits of one tracheid are located opposite pits of an adjoining tracheid, forming pit
pairs.
• Pit pairs constitute a low-resistance path for water movement between tracheids. The
water-permeable layer between pit pairs, consisting of two primary walls and a middle
lamella, is called the pit membrane.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
• Pit membranes in tracheids of conifers have a
central thickening, called torus (plural tori),
surrounded by a porous and relatively flexible
region known as the margo (Fig. C).
• The torus acts like a valve: When it is centered
in the pit cavity, the pit remains open; when it is
lodged to one side of the cavity, the pit is closed.
• The pit membranes in angiosperms and non coniferous plants (Fig. D) are homogenous in structure
(i.e. tori is absent in tracheids or vessel elements). But because the water-filled pores in the pit
membranes are very small, they prevents the spread of embolism (movement of gas bubbles) and
also imparts a significant hydraulic resistance.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
• Vessel elements tend to be shorter and wider than tracheids
and have perforated end walls that form a perforation plate at
each end of the cell.
• Vessel elements are dead cells.
• Vessels are connected to other vessels and to tracheids
through pits.
• Like tracheids, vessel elements have pits on their lateral
walls. Unlike in tracheids, the perforated end walls allow
vessel elements to be stacked end to end to form a much
longer conduit called a vessel.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
• Fig: Vessels (left) and tracheids (right) form a series of
parallel, interconnected pathways for water movement.
• Cavitation blocks water movement because of the
formation of gas-filled (embolized) conduits.
• Because xylem conduits are interconnected through
openings (“bordered pits”) in their thick secondary
walls, water can detour around the blocked vessel by
moving through adjacent conduits.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
Cohesion-tension theory
• The cohesion–tension theory explains water transport in the xylem.
• Pressure gradients needed to move water through the xylem could result from the generation of
positive pressures at the base of the plant or negative pressures at the top of the plant.
• The water at the top of a tree develops a large tension (a negative hydrostatic pressure), and this
tension pulls water through the xylem.
• This mechanism is called the cohesion–tension theory of sap ascent because it requires the cohesive
properties of water to sustain large tensions in the xylem water columns.
• The xylem tensions needed to pull water from the soil develop in leaves as a consequence of
transpiration.
• As water evaporates from the surfaces of mesophyll cells, the surface of the remaining water
withdraws farther into the interstices of the cell wall where it forms curved air–water interfaces. The
cell wall is made up of cellulose and water adheres to the cellulose microfibrils and other hydrophilic
components of the cell wall.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
✓ Because cellulose is hydrophilic
(contact angle = 0°), the force resulting
from surface tension causes a negative
pressure in the liquid phase.
✓ As more water is removed from the
wall, the radius of curvature of the air–
water interfaces decreases, the
hydrostatic pressure becomes more
negative.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
Xylem transport of water in trees faces physical challenges
1. The water under tension transmits an inward force to the walls of the xylem. If the cell walls were
weak or pliant, they would collapse under this tension. The secondary wall thickenings and
lignification of tracheids and vessels are adaptations that offset this tendency to collapse.
2. Water under such tensions is in a physically metastable state.
✓
Water is stable as a liquid (when its hydrostatic pressure (HP) > its saturated vapor pressure
(SVP)).
• Water in the xylem is described as being in a metastable state because despite the existence of a
thermodynamically lower energy state (the vapor phase) —it remains a liquid (HP< SVP so water will
be in vapour form but it remains in liquid form). This situation occurs because (1) the cohesion and
adhesion of water make the free-energy barrier for the liquid-to-vapor phase change very high, and
(2) the structure of the xylem minimizes the presence of nucleation sites (sites that lower the energy
barrier separating the liquid from the vapor phase).
• The most important nucleation sites are gas bubbles.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.
Plants minimize the consequences of xylem cavitation by several ways
1. Because the water-transporting conduits in the xylem are interconnected, one gas bubble might, in
principle, expand to fill the whole network. In practice, gas bubbles do not spread far, because an
expanding gas bubble cannot easily pass through the small pores of the pit membranes. Because
the capillaries in the xylem are interconnected, one gas bubble does not completely stop water flow.
Instead, water can detour around the embolized conduit by traveling through neighboring, waterfilled conduits. Thus, the finite length of the tracheid and vessel conduits of the xylem, while
resulting in an increased resistance to water flow, also provides a way to restrict the impact of
cavitation.
2. Gas bubbles can also be eliminated from the xylem. Some plants develop positive pressures (root
pressures) in the xylem. Such pressures shrink bubbles and cause the gases to dissolve.
3. Cavitation may be repaired even when the water in the xylem is under tension (mechanism of repair
is not yet known).
4. Finally, many plants have secondary growth in which new xylem forms each year. The production of
new xylem conduits allows plants to replace losses in water-transport capacity due to cavitation.
Taiz L, Zeiger E, Møller IM, Murphy A (2015) Plant Physiology and Development (6th edition). Sinauer Associates, Inc. Publishers,
Sunderland, Massachusetts USA.