Transcript No Slide Title

WATER:
Properties, Role in Plants,
Watering Strategies
Water
Evaporation and Transpiration
• Evaporation - Change of liquid into gaseous
state
•Transpiration - Evaporative loss of water
from the plant.
Transpiration & Evaporation
and
Temperature
•Temperature- measure of the average velocity of
molecules (how fast they are moving) a.k.a. HEAT
• Molecule that evaporates from a surface has
enough velocity to overcome the attraction of its
neighbor.
• When water molecules escape, the temperature
of the remaining liquid decreases.
Relevance to Plants
• When water evaporates (transpires) from a
leaf, the leaf is cooled.
– Much the same as how the evaporation of
perspiration cools us.
• When water evaporates from greenhouse
cooling pads, the air is cooled which in turn
cools the plants it moves over.
• Condensation is the return of a molecule of water
to its liquid form.
• There is an EQUILIBRIUM when the rate of
condensation (return) equals the rate of escape
(evaporation)
Equilibrium
Evaporation = Condensation
No equilibrium
Evaporation < or > Condensation
•At equilibrium the atmosphere is saturated
• Relative humidity (RH) =
Actual amount of water vapor in the atmosphere
Amount of water vapor the atmosphere when saturated
• RH depends on air temperature
- warm air holds more water than cold air
e.g. 70F air at 100% RH is holding more water
than 50F air at 100% RH.
Relevance to Plants and
Greenhouses
• The higher the RH, the slower the rate of
transpiration from leaf so there is less
cooling of the leaf.
• The higher the RH, the less effective
evaporative cooling systems are in the
greenhouse.
Relative Humidity (RH) =
Can easily measure using a
Sling psychrometer
and a chart
Reading a
psychrometer chart
Example: Dry temp 15 wet temp 10
= RH 75%
Constructing your own sling psychrometer:
Tie two thermometers together, wrap the end of
one in a wet cloth. Sling around in the air for a
minute or so.
Measure air temperature (dry bulb)
Measure cooling effect of evaporation of
water (wet bulb)
Compare the readings on a chart to get
RH.
Relative humidity is not always a good
way to measure the potential for
evaporation to occur, because RH is
temperature dependent.
A better way is to measure the difference
in vapor pressures between the atmosphere
and the evaporative surface (leaf or
cooling pad).
Vapor pressure - the pressure exerted by a vapor;
often understood to mean saturated vapor pressure
(the vapor pressure of a vapor in contact with its
liquid form). Expressed as kPa
When the vapor pressure of air is less than the
surface the air is touching, there is a deficit of air
vapor pressure (VPD) relative to the surface.
The greater the deficit, the greater the rate of
evaporation from the surface.
VPD determines how fast plants use water and
how efficiently wet pads cool greenhouses.
Which in turn determine how often you have to
irrigate.
The more you understand VPD, the better you (or
the environmental control computer you
program) can decide when it’s time to irrigate.
Vapor Pressure Deficit (VPD)
Relevance to Plants
• VPD - Good way to determine watering
needs of plants
• The greater the VPD between the leaf and the
air, the more likely the plant water use will
increase.
What else makes it beneficial for you to
understand VPD?
Understanding VPD also helps you to find out if
you are wasting your money (and water resources)
using cooling pads to cool your greenhouse.
You could be better off using natural ventilation.
What else do you have to know to be
able to irrigate at the right time?
You have to understand plant water
relations and how water moves into and
through the plant.
Plant Water Relations
Starts with the concept of
Water Potential
Plant Water Relations
Water Potential () :
The difference between the activity of water
molecules in pure distilled water at 1 atm
and 30°C (standard conditions), and the
activity of water molecules in any other
system.
The activity of water molecules in a system
may be greater (positive) or less (negative)
than the activity of the water molecules
under standard conditions.
Plant Water Relations
• Water Potential ()
Defines how tightly water is held by a
system
Determines how easily water move from
one system to another
Determines which direction water flows
Plant Water Relations
• Water Potential () summary
units -- atm (atmosphere) or bar or kPa
  is 1 for pure water at sea level
For most systems,  is negative
Water moves from higher  to lower 
Think of flow of water from high to low 
as a waterfall - flowing high to low
 is greater at the top of a waterfall
than at the bottom.
Plant Water Relations
Implications for plants
Water moves into the root only if  in
root is lower (more negative) than
the soil.
Water moves through the plant in the
from higher to lower .
Components of 
T =  +  P + M
Where:
T = total potential
 = osmotic potential
P = pressure potential
M = matric potential
Components of 
T =  + P + M
Osmotic Potential 
 due to the effect of dissolved solutes
the greater the concentration of solutes, the
lower (more negative) the water potential
water moves from an area of low salt
concentration to an area of greater
concentration.
Components of 
T =  + P + M
Implications for plants
 causes the plant to have more
negative  than soil/media because of the salts
in the plant. This helps water move into the root
from the soil.
• Generally
•Applying liquid fertilizer (a.k.a. salt solution) to
a dry soil/media lowers the osmotic potential of
the media/soil. If  of the soil becomes less
than the root, water will leave the root, causing
fertilizer burn.
Components of 
T =  + P + M
Pressure Potential P
 due to the forces on water
from high water concentration in
cells
 positive value for the most
part in turgid (not wilting) plants
early stages of decreasing P
= incipient plasmolysis, useful
for controlling length of young
shoots stems
When young cells are filled with
water, the membrane presses on
the growing cell wall. The cell
walls elongate and stay relatively
thin as the cells grow and divide.
When water is slightly withheld
from young plants, the
membrane does not press on the
growing cell wall (incipient
plasmolysis). The cell walls
stay more square and thick as
the cells grow and divide.
- Cell wall
- Cell membrane
If these were stem cells, which would provide the
strongest and shortest stems which usually produce the
most durable and probably attractive plant?
If you know what you are doing, “drying
down” is one of the most effective and
cheapest ways to regulate plant height.
Have to be careful, “drying down” is only a
few minutes away from “drying up”.
Drying up can cause irreparable damage to
plants.
You do not want the plant and its young cells
to become desiccated.
Components of 
T =  + P + M
Matrix Potential M
 the adhesion of water to particles
the stronger the adhesion of water to a particle, the
lower the matrix potential
Components of 
3. Matrix Potential M
 involves potential of solid components (including
soil)
 the stronger the adhesion of water to a particle,
the lower the matrix potential
Implications for plants
The lower the M in the soil or growing media,
the more tightly the water is held by the media –
When you irrigate you are raising the M of the
media and in turn you are making it easier for
water to enter the plant.
Total water potential
T =  +  P + M
T determines
how much water enters, leaves,
and stays inside the plant.
That in turn determines how the plant grows.
You can control much of a plant’s growth by
controlling any of the T components.

Timing when water is withheld, as with every growth
regulation technique, is very important.
Triphasic growth pattern:
Typical for most
greenhouse plants.
Characterized by:
1. Slow initial growth
2. Rapid vegetative
growth and elongation
3. Slow reproductive
growth.
Growth regulation is most
effective between low and midportions of rapid growth phase
Cohesion-Tension Theory
Mechanism of water movement in xylem is
driven by changes in  from soil through plant to air
Air
Temp = 20°C = 68°F
RH

5%
-2547 bar
30% -1634 bar
50%
-943 bar
75%
-390 bar
90%
-142 bar
95%
-70 bar
98%
-27 bar
100%
0 bar
Note that even at near 100%
RH, air still more negative 
than leaf
Thus: water flows from leaf to
air
However, even at air RH
100%, the slightest air
movement across the leaf
lowers air  to less than in
leaf so water flows from leaf
to air
During all this pulling, hydrogen bonds hold water
molecules together in columns inside xylem
tubes = cohesion
The very negative 
of the air tugs on
the water column,
causing the H2O
molecules to
move up through
the plant.
Air
(Water molecules, not
Disney symbols)
Rhizoshere
(rootzone)
Cohesion/tension explains how water can travel
upwards against gravity in a plant.
Transpiration at leaves
Water molecules pulled up
stem to replace molecules
lost to air
Tension on water in xylem
Water pulled into roots
Water into the Root
Roots have evolved to increase water
absorption area by formation of root hairs.
New root hairs have to be constantly
produced to have water uptake.
Damaged or diseased roots do not produce
root hairs, severely limiting their ability to
take up water.
Disease and Water Movement
Many fungal or bacterial pathogens
cause diseases with a characteristic
symptom of wilt. The wilting comes
because the pathogen enters the
vascular tissue and as it grows, it clogs
the water-conducting vessels.
Cutting a stem and seeing discolored
vascular tissue is a good “clue” that
helps diagnose disease.
In herbaceous stems a vertical cut is
made just under the epidermis of the
stem. If there is an infection, you can
see a “streaking” in the vascular tissue.
Disease-clogged
xylem
Cavitation or Embolism
Air bubble (vapor lock) in the xylem, break in the
water chain NOT GOOD - stops water flow through
that column in its tracks and often forever
Practical application:
Cut flowers often can’t take up water because
of cavitation at cut ends of xylem - leads to
the idea of cutting stems underwater.
Water Loss from the Leaf
• Stomates- pores in the leaves, primary way
plants regulate transpiration (water loss)
Stomatal Control
1. Light
• signal stomata to open
2. Internal [CO2] (of leaf)
• independent of light
• increase in [CO2] → stomata close
• decrease in [CO2] → stomata open
O
O
C
Stomatal Control
3. Plant Water Status
• sensed by the roots
• when soil dries and soil  approaches root , roots
cannot take up water to meet plant demands, plants begin
to loose water faster than it is taken up
• in response to water loss, roots then synthesize ABA
• ABA signals stomata to close to decrease water loss
• water status is the overriding environmental
factor that controls stomatal opening/closing
Plant Adaptations to Save Water
1. Sunken Stomates
Area of higher RH develops
in the “pit” which reduces the
VPD between leaf and air.
2. Stomates on underside of leaves
The upper side of leaves are exposed to light which
warms the leaf and increases VPD causing more
water evaporating if stomates are on the upper
surface.
Plant Adaptations to Save Water
3. Hairy leaves
• hairs serve as a wind break to maintain an
undisturbed layer of air around the leaf (boundary
layer)
• reduces VPD at leaf surface
4. Osmotic adjustment
• plant will automatically add solutes to cells which
causes  to drop which draws water into the cell.
Plant Adaptations to Save Water
5. CAM Metabolism (succulents and some orchids)
• stomates closed during day, open at night
• at night CO2 enters the leaves
• CO2 then converted and stored as an acid
• during day, CO2 released and used in photosynthesis
Plant Adaptations to Save Water
6. C4 Metabolism (warm-season grasses such as
corn, turfgrasses)
• CO2 converted to acid
• acid ‘shuttled’ to special cells for photosynthesis
• CO2 released for photosynthesis
• location of special cells reduces photorespiration
which ‘wastes’ CO2 in non-C4 plants
Functions of Transpiration
1. Mineral Transport
• rate of transpiration influences uptake movement
of ions from soil and movement through xylem
2. Heat Transfer (cooling of the leaf/plant)
Caution:
You have to be careful that by limiting water you
aren’t shooting yourself in the foot by limiting heat
transfer or mineral transport.