Teacher: What is the formula for water?
Student: H, I, J, K, L, M, N, O
Teacher: That's not what I taught you.
Student: But you said the formula for water was...H to O.
Water cycling in ecosystems
What’s water potential?
- Water potentials (ψ) are a way of measuring the free-energy (G) of
water. Water will flow spontaneously from a high potential to a
low potential, like a ball rolling down a hill.
• What forces cause water to move?
– Pressures
• Gravity
• Forces created by organisms (turgor pressure)
– Osmotic gradients (concentration of solutes)
Water Cycle:
•These forces also operate in the hydrologic (water) cycle.
•Remember in the hydrologic cycle that water runs downhill (likewise it
falls from the sky, to get into the sky it must be acted on by the sun and
evaporated, thus needing energy input to power the cycle)
What’s the deal with water potential?
- Plants need water (as a solvent, chemical reactant in hydrolysis, for
cell shape, to counteract lost water from transpiration-necessary for
CO2 uptake and photosynthesis)
- The water potential at any given point, ψ, is the
combined effect of all the factors that make water
move.
• Water potential is usually negative so it moves
from -1 to -4 (high to low).
What factor’s affect water potential
• Its concentration (chemical potential): water will diffuse from a
dilute solution to concentrated solution (osmosis).
• Its pressure (mechanical potential): water will from from a high
pressure system (hose-pipe) to a low pressure system (vacuum).
• Its height (gravitational potential): water will flow downhill.
• Its charge (electrical potential): water is uncharged, so we can
ignore this.
• These factors sum together to give the overall free energy of a
mass of water.
Plant-available water:
Difference between field capacity and permanent
wilting point
Coarse
Particle size
Fine
Components of Water Potential
• Pressure potential (ψp).
– Difference between pressure of the water and that of the atmosphere. Most
healthy cells exhibit a slightly + pressure potential due to the outward push
of water (within vacuoles & cytoplasm) on the membrane or cell wall=
turgor pressure
• Osmotic pressure (solute potential) (ψs).
– Affect of concentrations of solutes inside vs outside the cell that influence
osmosis
• Matric potential (in soil)
– Affect of adhesion sticking water to soil surfaces. Measures how likely it is
water becomes “unstuck”
• Vapor potential
• Gravitational potential
Pressure Potential
• Turgid cell (left) have a high
positive pressure potential (the
water presses out on the cell
wall, and the wall presses
back on the water with an
equal but opposite force).
• Cells that are flaccid or
plasmolysed (right) have a
pressure potential of zero.
• We assume a pressure
potential of zero when
measuring water potential in
our experiment.
Osmotic/ Solute Potential
• ψs = − C R T
• C, concentration of solutes (M).
•
R, Universal gas constant
(8.314472 J K−1 mol−1).
•
T, absolute temperature (K).
– Higher temp means more rapid
molecular movt. More rapid
osmosis.
• Considered primary factor affecting
our experiment, often critical in
living systems,
• Solute potential of pure water is 0
Matric Potential
• Not used in our study b/c
these interactions are more
common in soils
• Adhesive forces between
water and soil particles
influence the “readiness” of
water to leave soil pores.
• As soil becomes drier matric
potentials increase & water is
less likely to flow into roots.
Gravitational Potential and Vapor
potential
• Their effects are negligible at the cellular level
within a small plant.
• Gravitational potential explains why condensed
water falls as rain or precipitation.
– Higher up you go, the higher the
gravitational potential
– Can also be a factor in moving water up tall
trees
• Vapor potential -don’t worry about it, except
that it becomes more important when
explaining water movement from leaves-->
atmosphere
Homeostasis
• In plant cells (and animals) equilibrium is achieved when
water potential in/out of cell are same:
• Concentration of solutes inside and out of cell are equal
(facilitated largely by osmosis)
• Pressures are same (of water inside and outside cell)
• For working, growing plants most cells are in dynamic
(every changing) equilibrium…in this way they can
change with environmental conditions and move water
and nutrients as needed.
Water moves along a pressure gradient
Transpiration is a major driving
force
• Dry warm air has low water potential & pulls water from cells
with higher potential.
• Water is lost under these conditions when pores on the leaf
surface are open for gas exchange for photosynthesis
• Water moves in continuous column from soil particles to leaf
cells
• Plant spends no energy in transporting water
– Passive transport driven be transpiration
What about Capillary Action?
• Cohesive and adhesive forces of capillary action
help HOLD water in plant tissues
• Can only move water against gravity ~1/2 meter
on their own.
• Capillary action also explains how water is held
in soils
What about vessel size?
• Transpiration causes the difference in water potentials
necessary to move large quantities of water from root to
leaf in large plants.
• Flow rate increases in proportion to capillary radius
(larger is faster)
– 45 meters per hour vs. 6 per hour
– Larger vessels = larger risk. Water columns break as they carry
larger volumes. The drier the plant becomes the more frequent
the breaks.
– Most plant have large and small vessels (xylem)
Root Pressure
• Differences in concentrations of salts in soil or
sugars in roots can influence water flow.
• In spring, sugar concentration is high in roots.
This difference in concentration increases
osmotic potential in cells.
• Water can then “push” into the roots and fill
empty vessels from last season or new
growing tissue. The vessels must be full
before cohesion and transpiration can cause a
steady pull of water up the plant.
Water potential and water flow in
plants in a nutshell
• Water moves from one part of the plant to another down
a water potential gradient. Different components of ψ are
important at different stages.
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Soil to roots: matric potential.
Roots to stems: pressure potential.
Stems to cells: osmotic potential.
Cells to stomata: vapor potential.
Where did our lab focus?
Precipitation (Yg)
Overland flow
(erosion, nutrients)
Throughfall)
Infiltration
Percolation
Evapotranspiration
Transpirational water loss
to the atmosphere
Water transport
through the plant
Water uptake by
plant roots
Evaporation from
leaf surfaces
Evaporation from
soil
Water in the soil-plant-atmosphere continuum J = L (Dyt/l)
Atmosphere
Yt = - 30 MPa
Water moves along a
gradient of decreasing
water potential
Leaves
Yt = - 1.5 MPa
Surface roots
Surface soil
water Yt = - 0.8 MPa
Yt = - 1.1 MPa
What happens
at night?
Stomata close –
which term does
this affect?
How would water
potential gradient
respond?
Relatable topics
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You should be able to use water potential in discussions regarding:
Photosynthesis and transpiration
The water cycle
The chemical importance of water to life on earth
Plant physiology
Cell homeostasis and water balance
Membrane function and cell wall function
Plant evolution and adaptations of higher (vascular plants)
Form and function of membranes and vacuoles
Kinetics, free energy change
Diffusion/ Osmosis