Dr' Bennett copy

1
Chapter 4 Outline
I. Importance of water in plants A.
Function B. Flow (transpiration) - energy
gradients II. Components of Plant Water
Potential A. Matric B. Solute (Osmotic) C.
Gravimetric D. Turgor (Pressure) III. Water
Uptake and Movement IV. Evapotranspiration
(ET) A. Environmental Effects B. Plant
Effects C. Potential vs. Actual ET V. Effects
of Water Deficits On Plants A. Expansive
Growth (Leaf area) B. Stomata C. Biomass and
Seed Yield VI. Water Use Efficiency
2
Reading Assignments Gardner Book Chapter 4
Water Relations Sinclair and Gardner
Book Chapter 7 -Water
3
CHAPTER 4 WATER RELATIONS
- Functions of water within a plant 1.
Solvent for chemical reactions 2. Medium for
solute transport 3. Medium that provides
turgor (pressure) within cells 4. Hydration of
colloidal molecules 5. Used in some chemical
reactions (PS, hydrolytic reactions,
etc.) 6. Medium for transpiration (cooling)
4

• H bonding results in the cohesion necessary to
• maintain a water column to the top of very tall
  trees.

5
(No Transcript)
6

• WATER FLOW
• SPAC Soil-Plant-Atmosphere-Continuum
• - Water flows through the soil, plant, and into
  the
• atmosphere in response to an energy (water
• potential) gradient
• - Water loss through a plant (transpiration) on
  a
• daily basis is large relative to other uses
  of water
• in the plant.
• For example, daily transpiration (Ts) may be
• 1 to 10 times the amount of water stored in a
• plant

2H2O 4H 4e- O2
7
WATER POTENTIAL - Movement and status of water
within a plant system is determined on a
potential energy relationship. - Water Potential
( ? ) The capacity to do work (activity or
potential energy in the water molecules). -
Water will move from an area of high potential
energy to an area of low potential energy.
In an aquatic system (for example, a plant), the
potential energy is expressed by comparing it
with the potential energy of pure water. Thus, we
define POTENTIAL ENERGY OF PURE WATER AS ZERO!!!
8
WATER POTENTIAL (cont)

• So, the potential energy of any system is
• always with reference to PURE WATER, which
• we have assigned a water potential of ZERO.
• Anything that lowers the potential of pure
• water (solutes, tension, etc.) causes the
  water
• potential to become NEGATIVE indicating a
• lower water potential.

Used to designate water potential in plant and
soil systems. Units are force or pressure per
unit area (Bar or MPa or Atm)
1 Bar 105 Pa 106 dynes cm-2 0.99 Atm 1 Atm
1.0 Bar 0.1 MPa 1 Atm
9
?
10
IMPORTANT WILL BE ON EXAM!!!
-10 bars to -25 bars is a DECREASE in water
potential -25 bars is a LOWER water potential
than -10 bars - The more negative the value,
the LOWER the water potential Total water
potential in plants and soils is comprised of
several components
Matrix potential
Water potential
Pressure (turgor) potential
Gravitational potential
Solute (Osmotic) potential
11
COMPONENTS OF WATER POTENTIAL
Matric potential always (-) due to force
by which water is adsorbed to plant and soil
consti- tuents. Minor component in plants, but
major component in soils (adsorption of water
to clay particles). Solute (osmotic) potential
always (-) due to the lowering of potential
energy of water by the addition of solutes. Very
important in plants, but only important in
saline soils (interference between water
molecules). Pressure (turgor) potential
usually () due to pressure of cell contents on
cell wall. This is not a component of soil water
potential, but is extremely important in plants
for processes such as stomatal opening, cell
expansion, cell division. Gravitational
potential Significant only in tall trees (-) 1
bar with each 10 m height. Generally ignored in
short plants small component of soil water
potential.
12
COMPONENTS OF WATER POTENTIAL
Major components
Soils ?m???s?? Plants ?p ?s
13
Water loss due to decreasing water potential
Xylem (pure water) negative pressure because of
water loss to atmosphere
14
SOIL WATER
Available soil water Soil water that is
available for extraction by plant roots.
Difference between water held at field capacity
and permanent wilting point. Field capacity
Soil water held after drainage (drainage due to
gravity). Permanent wilting point Soil water
potential at which a plant permanently wilts
(will not recover even if placed in humid
atmosphere).

• Water potential in soils is affected greatly by
  the
• clay content (matric potential).

15
Defined as -15 bars
-0.1 to -0.3 bars
g water/100 g soil cm3 water/cm3 soil
Clay soil 17 cm water per meter soil
depth Sand soil 6 cm water per meter soil
depth Assume 1.5 meter root zone (soil depth),
then 25 cm (10) water held in root zone
9 cm (3.5) water held in root zone
16
Evaporation
Factors influencing the evaporation of water to
the atmosphere include Temperature - increases
the concentration of water vapor in the saturated
air layer above the water Air dryness -
concentration of water vapor in the air above the
water surface Conductivity of air - air movement
above the evaporating surface.
17
WATER LOSS IN PLANTS
Loss of water to atomosphere
Continuous water column through soil, root,
stem, leaf
Control by stomata at this point
Soil water
Think of water movement through a plant as
similar to that through a HOSE, except
that stomatal opening controls loss of water
18
(No Transcript)
19
TRANSPIRATION
continuous water column through soil, root, stem,
and leaf
20
Evaporative Demand (or Dryness of the
air) Environmental conditions dictate a certain
rate of evaporation from a free water source

• RH is primary environmental factor driving
• evaporation.

21
EVAPOTRANSPIRATION (ET)
Driven by environment but with some plant
controls built in
Evapotranspiration (ET) Evaporation
Transpiration
from soil or from surface of plants
loss of water through stomata and cuticle
22
ET is energy dependent!! Water loss is primarily
driven by environmental and plant factors
23
Boundary Layer
24
EVAPOTRANSPIRATION (cont)
25
TRANSPIRATION

• Plant Factors Affecting Transpiration
• 1. Stomatal closure
• 2. Stomatal number and size
• less influence than total opening or
  closing of stomata
• 3. Amount of leaf area (ground cover)
• 4. Leaf rolling or folding
• to eliminate absorption of radiation
• 5. Root depth and proliferation

Water (turgor pressure) in guard cells
Stomatal activity provides the major effect
26
(No Transcript)
27
(No Transcript)
28
R.H. inside the substomatal cavity of the leaf
is very Near 100
Vapor pressure gradient V.P. inside leaf V.P.
of air

• Increasing temp. increases the capacity of
• air to hold water.

Larger V.P. gradient as temp. increases
29
Open Pan
Highest evaporative demand occurs at times of
the year when solar radiation and temperature are
highest.
30
Potential ET
Potential ET (PET) ET of a short green crop
which completely covers the soil surface and is
adequately supplied with water (80 or so of pan
evaporation). A PET (maximal) value is
determined primarily by environmental
conditions!! Follows open pan evaporation
closely. Potential ET is a maximum, and actual
ET is reduced by 1. Incomplete ground
cover 2. Water stress (stomatal closure)
31
Leaf Area Index (ground cover) vs. ET
Under full canopy, optimal conditions, ET will be
ca. 80 of open pan evaporation. i.e. light
interception correlated with ET.
32
Diurnal Course of Canopy Transpiration
gt VPD during mid-afternoon when temp is highest
and RH is lowest.
33
Stomatal Closure Effects on Ts (resulting from
progressing drought)
Transpiration or Radiation
Hour of Day
34
Mediterranean climate
Continental Climate (US)
Soil storage or irrigation must make up deficit
or crops undergo water stress.
35
Sensitivity of Leaf Size to Water Stress (Leaf
growth is cell division and expansion and very
sensitive to water deficits.)
Wet
Medium stress
Severe stress
- Leaf growth at night, during severe stress.
36
Water Stress Effects on Physiological Processes
Refer to Page 85, Table 4.1 in Gardner et al.

• Cell expansion very sensitive to water
  deficits.
• Cell division somewhat less sensitive.
• Greatest expansive growth may occur at night.

37
OSMOTIC ADJUSTMENT


(-6) (-7) (-2) (3) (-11)
(-15) (-2) (6)
is probably the best indicator of stress
In above example, if only ?W is considered, the
second plant would appear to become more severely
water stressed (-6 to -11 bar). However, if??P
is considered, the plant would appear to be less
severely stressed (3 to 6 bars).
This occurs because the plants have demonstrated
OSMOTIC ADJUSTMENT (i.e., a change in osmotic
potential from -7 to -15 bars).
Osmotic adjustment is the increase in absolute
number of solutes within the cell in response to
a water stress, thus resulting in a decrease in
osmotic potential in the cell.
38
OSMOTIC ADJUSTMENT (cont)
Such accumulation will allow the plant to
maintain (or increase) ?P even though ?W
declines. Osmotic adjustment is an adaptive
mechanism which may allow the plant to maintain
turgor pressure even though
declines. - Is osmotic adjustment simply a
symptom of water stress, or is it really an
adaptive mechanism which allows the plant to
continue physiological processes at lower
water potentials?

?w
39
EFFECTS OF WATER ON STOMATA (cont)
Response of stomata to water deficits is affected
by 1. Varying environments (light,
temperature, CO2) 2. Stage of plant
development 3. Leaf position 4. Crop species
40

ABSCISIC ACID (ABA)
- A plant hormone has recently been
shown to increase in water stressed plants.
- ABA is produced in roots, which in some
way sense dry soil, and is transported via
the xylem to the shoot. - In the leaf,
ABA reduces cell expansive growth and
causes stomata to close. - ABA stimulates
root growth.
41
EFFECTS OF WATER ON STOMATA
- Water stress effects on stomatal closure are
now thought to be mediated by the plant growth
hormone, abscisic acid (ABA), as well as
reduct- ions in bulk leaf turgor. - An
increase in ABA will cause stomatal closure and
ABA has been shown to accumulate in response to
water deficits. - ABA is thought to make the
membranes of guard cells leaky to K. To remain
open, K must accumulate in the guard cells,
causing influx of water.
- A leaky guard cell membrane will not
allow maintenance of the high osmotic
concentration (primarily due to K accumulation)
necessary to cause water uptake (and turgor) and
stomatal opening.
42
ABA (cont)
43
EFFECTS OF WATER STRESS ON YIELD
Vegetative Growth Stomatal opening Leaf area
(LAI) Root growth Dry matter (PS)
Flowering and Reproductive growth Stomatal
opening Seed number Seed weight Leaf
senescence Dry matter (PS)
Interference with pollination
Expansive growth such as leaf expansion,
internode expansion, seed growth, etc., is very
sensitive to water deficits. Reductions in leaf
area and height are very obvious when stress
occurs during veget- ative growth.
44

• With time of stress post-flowering
• No. of pods
• No. seeds per pod
• Weight per pod

Soybean
Corn vs. Soybean
Indeterminate may be somewhat less sensitive
to water stress because most critical growth
stages (addition of seeds) is spread over longer
time periods.
45
Grain Crop
46
Sensitive growth stages to water
stress Soybeans - Pod filling Peanuts - Pod
addition and pod fill Corn - Tasselling and
silking Grain sorghum - Panicle initiation to
boot (heading) Small grains - Panicle initiation
to boot Generally, water stress affects tissue
that is in the most rapid stages of
expansion. Yield seeds x seed weight But,
remember, sensitive stages of growth
identified for various crops are generally those
stages during which the crops are using the
maximum amounts of water (i.e., flowering, seed
filling) resulting in,
47
g DM kg-1 water kg DM cm-1 water lb DM acre-1
inch water
Yield in relation to water used (ET) to produce
that yield
CAM increased WUE, but low yielding
48
Water use efficiency for C3 and C4 species
Grasses _ _ _ _ _ 1.49 3.14
Dicots _ _ _ _ _ 1.59 3.44
Species C3 C4
(g kg-1)
49
Examples
Environmentally dependent
13000
Humid (FL)
Grain yield (kg ha-1)
Arid (AZ)
0
0 25 50 70
ET
13000
much less efficient
Grain yield (kg ha-1)
very efficient
0 20 40
IRRIGATION
The goal for efficient irrigation management is
to attempt to supply water so that a very high
is used for T and not lost through percolation,
runoff, E, etc.
50
Example WUE ET Seed yield kg
DM Location cm kg ha-1 cm -1 Lubbock,
TX 90 9500 105 Quincy, FL 63 9500
151
43 increase in WUE by growing the crop at
Quincy where evaporative demand is lower
51
Corn WUE (Example)
Water Use
Corn Seasonal water use 25 63 cm
27,000 gal/acre inch therefore, 27,000 gal/acre
inch x 25 675,000 gal/acre to produce a corn
crop
52
Yield
Corn Grain Yield 8500 lbs ac-1 9500 kg ha-1
1 corn seed 0.32 g
WUE (on seed basis) or seeds/gallon 12,019,230
seeds 67,5000 gallons
18 seeds per gallon or 5 seeds/liter

53
Actual ET is influenced more by 1.
Atmospheric demand 2. Ground cover 3.
Water availability than by crop
species. Consumptive use coefficient (k)
actual ET potential ET
affected mainly by ground cover
54
(No Transcript)
55
How can WUE be maximized? 1. Increase crop
canopy PS 2. Obtain full canopy cover as early
as possible (reduce E, maximize T/ET) 3.
Ensure that irrigation water is used for T 4.
Grow plants in humid environments 5. Reduce
E 6. Be certain other crop growth factors
(nutrients, population, etc.) are not
limiting growth
56
(No Transcript)