Plant Physiology
Water balance of plants
Water in the soil
The water content and the rate of water
movement in soils depend to a large
extent on soil type and soil structure.
Sand
•Desert
Silt
•Under water bodies
(canals)
Clay
•Traditional houses
Water in the soil consists of 3 parts:
1- Gravitational water: water filled in the big spaces/interstices of soil particles and
is readily drained from them by gravitation.
• Gravitational water is found in the macropores. It moves rapidly out of well
drained soil and is not considered to be available to plants.
• It can cause upland plants to wilt and die because gravitational water occupies
air space, which is necessary to supply oxygen to the roots.
• Drains out of the soil in 2-3 days
2- Bound water: water tightly adhered to the soil particles.
• This water forms very thin films around soil particles and is not available to the
plant. The water is held so tightly by the soil that it can not be taken up by roots.
• not held in the pores, but on the particle surface. This means clay will contain
much more of this type of water than sands because of surface area
differences.
• Gravity is always acting to pull water down through the soil. However, the force
of gravity is counteracted by forces of attraction between water molecules and
soil particles and by the attraction of water molecules to each other.
3- Capillary water: Water filled in the small spaces/interstices of particles, easily
get to the surface of water by the force of capillarity.
• Most, but not all, of this water is available for plant growth
• Capillary water is held in the soil against the pull of gravity
Forces Acting on Capillary Water
•Capillary water is held by cohesion (attraction of water molecules to each other)
and adhesion (attraction of water molecule to the soil particle).
•The amount of water held is a function of the pore size (cross-sectional diameter)
and pore space (total volume of all pores)
Field capacity:
• Field capacity is the water content of a
soil after it has been saturated with water
and excess water has been allowed to
drain away due to the force of gravity.
• Field capacity is large (40%) for clay soils
and soils that have a high humus content
and much lower (3%) for sandy
Water absorption by the root
Water Moves through the Soil by Bulk
Flow
• Water moves through soils predominantly by
bulk flow driven by a pressure gradient, although
diffusion also accounts for some water
movement.
• As a plant absorbs water from the soil, it
depletes the soil of water near the surface of the
roots.
Root tip—the water absorption
zone
The overall scheme of water
movement through the plant
1- From soil to root epidermis
– Diffusion to the intercellular space
• Capillary movement of soil water to plant
roots. Plant root removes water. Tension in
the soil right around the root increases
gradient flow of water from low tension to high.
This keeps a source of capillary water flowing
to the plant root.
– Osmosis to the epidermis cells
2- From epidermis to and through cortex
1- Apoplast pathway: water moves exclusively through
the cell wall without crossing any membranes. (The
apoplast is the continuous system of cell walls and
intercellular air spaces in plant tissues.)
2- Symplast pathway: water moves through the symplast,
traveling from one cell to the next via the
plasmodesmata (The symplast consists of the entire
network of cell cytoplasm interconnected by
plasmodesmata.)
3- Transmembrane pathway: water sequentially enters a
cell on one side, exits the cell on the other side. In this
pathway, water crosses at least two membranes for each
cell in its path.
Symplast pathway and transmembrane pathway are two
components of cellular pathway,
Transversing endodermis
• Casparian strip?
– Casparian strip is a band
of cell wall material deposited
on the radial and transverse
walls of the endodermis, which
is chemically different from the
rest of the cell wall. It is used
to block the passive flow of
materials, such as water and
solutes into the stele of a plant.
– To transverse casparian strip,
apoplast pathway does not
work (blocked), only cellular
pathway works
Stele is the central part of the root or stem containing the tissues derived from
the procambium. These include vascular tissue, in some cases ground tissue
(pith) and a pericycle, which, if present, defines the outermost boundary of the
stele. Outside the stele lies the endodermis.
3- From endodermis to root vessel
apoplast pathway and cellular pathway
(diffusion or osmosis)
4- From root vessel to stem vessel to leaf
vessel
apoplast pathway (mass flow)
5- From leaf vessel → leaf mesophylls and
intercellular space→stomatal
cavity→stomata →air (diffusion or osmosis)
Driving Forces of Water absorption
and movement
1- Root Pressure
2- Transpiration pull
1- Root Pressure
• Solute Accumulation in the Xylem Generates
“Root Pressure”
• The root absorbs ions from the dilute soil
solution and transports them into the xylem. The
buildup of solutes in the xylem sap leads to a
decrease in the xylem osmotic potential (Ψs)
and thus a decrease in the xylem water potential
(Ψw). This lowering of the xylem Ψw provides a
driving force for water absorption.
Guttation
Dew?
Appearance of xylem sap drops
on the tips or edges of leaves e.g.
grasses
•Sugars, mineral nutrients and
potassium
•Transpiration stops at night time due to stomata closing
• High soil moisture level
• Lower root water potential
•Accumulation of water in plants
•Plants will start bleeding through leaf tips and edges
2-Transpiration Pull
Transpirationcohesion theory
Transpiration is the loss of water through
the stomata in leaves. This loss of water
causes an area of low pressure within the
plant and water moves from where it is at
high pressure to low pressure. The
cohesion part is what allows water to do
this against gravity.
How do we genetically
manipulate plant water relations?
Arabidopsis as example!!!
Mutation in MRH2 Kinesin (ARM domain-containing
kinesin-like protein) Enhances the Root Hair Tip
Growth Defect
Stomata fail to close under scarce water conditions
Arabidopsis PARG1
mutants
Knockout of PARG-1 gene causes Arabidopsis plants to
wilt earlier than the wild type under drought stress
STOMAGEN positively regulates stomatal development.
Knockout
Overexpressing
Further Readings
• Chapter 4, Plant Physiology by Taiz and
Zeiger, 3rd ed.