Chapter 4 – Water Balance of Plants
Water in the Soil
- In sandy soils, the spaces between particles are so large that water tends to drain from them and
remain only on the particle surfaces and at interstices between particles. In clay soils, the channels are
small enough that water is retained against the forces due to gravity. The moisture holding capacity of
soil is called field capacity.
- Field capacity is water content of soil after it is saturated with water and excess has been drained
away. Clay soils or soils with high humus content have large field capacity. Sand particles are larger and
have less surface area per gram. Clay particles are smaller and have more surface area per gram.
Negative Hydrostatic pressure in soil water lowers soil water potential
- The osmotic potential (Ψ ) of soil water is usually neglible because solute concentrations are low
i.e. -0.02MPa. For soils with substantial concentrations of salt Ψ can be significant i.e. -0.2 MPa or
lower. Hydrostatic pressure (Ψ ) for pet soils is close to 0. As soil dries, Ψ can bpcome negative.
- As water content of soil decreases, water recedes into interstices between soil particles forming air-
water surfaces whose curvature represents balance between tendency to minimize surface area of
air-water interface and attraction of water for soil particles. Water under curved surfaces develop
negative pressure according to Ψ = -2Tpr where T is surface tension of water (7.28x10 ) and r is -8
radius of curvature.
- As water dries, it is removed from the largest spaces making Ψ negativp from increasing curvature
of air-water surfaces in smaller diameter pores.
Water moves through soil by bulk flow
- Water flows from regions of high soil water content where water filled spaces are large, to regions of
lower soil water content where smaller size of water filled spaces is associated with more curved air
- As plants absorb water from soil, they deplete the soil of water near the roots. This reduces Ψ in p
water near root surface and causes pressure gradient with respect to neighbouring regions of soil
that have higher Ψ vapues.
- Rate of water flow depends on: size of pressure gradient and hydraulic conductivity of soil.
Hydraulic conductivity is measure of ease with which water moves through soil. Sandy soils have
large spaces so they have large hydraulic conductivity and clay has low hydraulic conductivity.
- As water content decreases, hydraulic conductivity decreases because replacement of water in soil
by air. In dry soils, water potential (Ψ w may fall below permanent wilting point. Water potential is
so low that plant cannot regain turgor pressure even if all water loss through transpiration ceases.
So Ψ w= Ψ . s
Water Absorption by Roots
- Root hairs increase surface area of the root allowing for absorption of more water and ions. Older
parts of root have outer protective layer called exodermis that is hydrophobic (impermeable to water).
This allows new roots further down and area near root tip to absorb water and create a larger gradient.
- Intimate contact between soil and root is easily ruptured if soil is disturbed so newly transplanted
seedlings need to be protected from water loss for the first few days so new root growth into soil can
establish soil root contact.
Water moves in root via apoplast, symplast and transmembrane pathways
- Apoplast is continuous system of cell walls, intercellular air spaces and lumen of cells that have lost
cytoplasm (xylem conduits and fibers). Water moves through cell walls and water filled extracellular
spaces (without crossing any membranes) as it travels across root cortex.
- Symplast consists of entire network of cell cytoplasm interconnected by plasmodesmata. Water
travels across root cortex by passing from one cell to next via plasmodesmata. Since water
movement in apoplast and symplast don’t have to cross semipermeable membranes, driving force is
gradient in hydrostatic pressure.
- Transmembrane pathway water enters cell on one side and exits on other side. Water crosses at
least two membranes for each cell in its path (plasma membrane on entering and exiting). Transport
across tonoplast may be involved. Presence of semipermeable membranes means driving force is
total water potential gradient. - Water movement through apoplast pathway is obstructed by Casparian strip, radial cell walls in
the endodermis that is impregnated with waxlike, hydrophobic suberin. Suberin forms a barrier to
water and solute movement. Endodermis is suberized in nongrowing part of root, far up from the
root tip. Forces water to pass through plasma membrane.
- Requirement that water move symplastically across endodermis explains by permeability of roots to
water depends strongly on Aquaporins. Down regulating aquaporin genes reduces hydraulic
conductivity of roots causing wilting or producing larger root systems.
- Water uptake decreases in low temp or anaerobic conditions or respiratory inhibitors. Submerged
roots run out of oxygen which is provided by diffusion through air spaces in soil. Permeability of
Aquaporins can be regulated in response to intracellular pH. Decreased rates of respiration lead to
increases in pH altering conductance of Aquaporins.
Solute accumulation in xylem can generate root pressure
- Roots absorb ions and concentrate them in xylem. Buildup of solutes in xylem leads to decreases in
osmotic potential Ψ asd water potential Ψ . Lower Ψ providws driving force for water absorption,
leading to + hydrostatic pressure in xylem.
- Root pressure likely occurs when soil water potential is high and transpiration rates low. High
transpiration leads to rapid water loss to the atmosphere and root pressure never develops. Plants
that develop root pressure frequently produce liquid droplets on edges of leaves, a process called
- + xylem pressure leads to exudation of xylem sap through special pores called hydathodes.
Guttation most noticeable when transpiration is low and humidity is high i.e. night.
Water Transport through Xylem
Xylem consists of two types of tracheary elements
- Vessel elements are found in angiosperms and some gymnosperms. Tracheids are found in both
angiosperms and gymnosperms. Both involve production of secondary cell wall and death leading to
no membranes or organelles. Only lignified cell walls remains forming hollow tubes for water flow
with little resistance.
- Tracheids elongated spindle shaped cells arranged in overlapping vertical files. Water flows
between tracheids through pits where secondary cell wall is absent and primary wall is thin and
porous. Pits on one tracheid are located opposite pits on adjoining tracheid forming pit pairs. Pit
pairs are low resistance pathways. Porous layer between pit pairs consist of two primary walls and
middle lamella called pit membrane.
- Pit membranes in tracheids of conifers have a torus which acts like a valve to close pit by lodging
itself in circular or oval wall thickenings bordering these pits. Prevents gas bubbles from spreading
into neighbouring tracheids. Pit membranes in all other plants lack a torus. Because water filled
pores in these pit membranes are small they also block movement of gas bubbles (emboli).
- Vessel elements are shorter and wider than tracheids and have perforated end walls that form
perforation plate. Have pits on lateral walls. Perforated end walls allow vessel members to be
stacked end to end to form large conduit called a vessel. Because of open end walls, vessels
provide efficient pathway for water movement. Vessel members at extreme ends of vessel lack
perforations in end walls and communicate with neighboring vessels via pit pairs.
What pressure difference is needed to lift water 100 meters to a treetop?
- A pressure difference of about 3MPa is needed from the base to the top branches is needed to carry
water up the tallest trees.
Cohesion tension Theory explains water transport in xylem
- Water at the top of a tree develops large tension (negative hydrostatic pressure) and this tension
pulls water through xylem. This cohesion tension theory of sap ascent requires cohesive properties
of water to sustain large tensions in xylem water columns.
- The negative pressure that causes water to move up through the xylem develops at surface of cell
walls in leaf. Water adheres to