Avocado Tree Physiology –
Understanding the Basis of
Productivity
R. L. Heath, M. L. Arpaia
UC, Riverside
M. V. Mickelbart
Purdue University
Raw
Materials
Product
Labor
Light
Carbon Dioxide
Temperature
Water
Nutrients
Labor
Fruit
1
Research priorities addressed
• Canopy management, tree density, tree
architecture
• Development and refinement of a model
to predict phenological events for the
avocado
• Innovative practices to increase
efficiencies of grove operations and
orchard profits
Problem: Lowered Productivity in California
Photosynthesis is the Production Line of the “Factory”
Factory”
What factors limit
photosynthesis?
2
Problem: Lowered Productivity in California
Photosynthesis is the Production Line of the “Factory”
Factory”
What factors limit
photosynthesis?
STOMATA ⇔
CO2 & WATER FLOW
lower leaf surface of ‘Hass’ avocado leaf
stomata
[CO2 ]out
Bulk Air
[H2O ]out
Boundary Air
epidermis
[CO2]in
[H2O ]in
Light
Assimilation
carbohydrate
Internal Air
leaf
3
Problem: Lowered Productivity in California
WHY? Several reasons BUT
[1] Stomata close-down “in error”, lowering assimilation
Stomata conductance altered by water loss
[2] Light is limiting for assimilation, relative to size of canopy
Potential solutions:
[1] Increase productivity through manipulation of air relative-humidity.
[2] Alter the canopy of the trees to change air flow & light absorption
Denser canopy to control water loss.
Research Plan:
[1] Determine the assimilation efficiency upon stomata conductance
[2] Determine the variation of conductance upon relative humidity.
[3] Build a simplified model of assimilation
One Limit to Productivity: Stomata
photosynthetic assimilation (μ mol / m2 sec)
30
LiCor 6200 porometer
Field conditions
[1] During normal conditions
assimilation governed by
stomata conductance
20
10
[2] During most stress conditions
stomata conductance declines
but assimilation still governed
by stomata conductance
stomata control
of assimilation
0
-10
0
100
200
300
stomatal conductance (mmol / m2 sec)
From Mickelbart ( ) & Xuan (
400
)
4
Accomplishments to date
• Light Flecks
• Leaf area
• Leaf processes
¾Light intensity and assimilation
¾Model of productivity
• Sap flow
Light “Flecks” inside canopy
Field 10 (measured by light meter on east side, ½ way into canopy,
6 ft off ground)
960 lum/ft2
Light Intensity
Light Intensity
640 lum/ft2
Tree 9-1
6/22
7 min
Tree 8-1
6/21
20 sec
200 lum/ft2
15 lum/ft2
9 AM
Noon
9 AM
Noon
5
Speed of stomata change
Licor Experiment (Hass Leaf)
0.30
2000
Declining Intensity
Increasing Intensity
1500
0.20
Conductance
0.15
1000
Intensity
0.10
500
Light Intensity (umol/m2 sec)
Conductance (mol/m2 sec)
0.25
0.05
0.00
0
0
5
10
15
20
Time of experiment (min)
0 .3 0
Licor Experiment (Hass Leaf)
2000
Declining Intensity
Increasing Intensity
Decline
1500
Conductance
0 .2 0
Intensity
0 .1 5
1000
0 .1 0
500
0 .0 5
0 .0 0
Light Intensity (umol/m2 sec)
Conductance (mol/m2 sec)
0 .2 5
Lower intensity = closed stomata
Lower intensity & closed stomata
= lower assimilation
Lower assimilation & closed stomata
= higher internal CO2
0
0
5
10
15
20
Time o f e x p e r ime n t ( min )
6
0 .3 0
Licor Experiment (Hass Leaf)
2000
Declining Intensity
Increasing Intensity
Decline
1500
Conductance
0 .2 0
Light Intensity (umol/m2 sec)
Conductance (mol/m2 sec)
0 .2 5
Intensity
1000
0 .1 5
0 .1 0
500
0 .0 5
Lower intensity & closed stomata
= lower assimilation
Lower assimilation & closed stomata
= higher internal CO2
0
0 .0 0
5
10
15
20
Time o f e x p e r ime n t ( min )
450
ambient CO2
6
400
Internal CO2
350
4
300
2
Assimilation
250
0
internal CO2 (ppm)
0
8
Assimilation (umol/m2 sec)
Lower intensity = closed stomata
200
respiration
-2
150
0
5
10
15
20
Time of experiment (min)
0 .3 0
Licor Experiment (Hass Leaf)
2000
Declining Intensity
Increasing Intensity
Decline
Light Intensity (umol/m2 sec)
1500
Conductance
0 .2 0
Intensity
0 .1 5
1000
0 .1 0
500
0 .0 5
0 .0 0
Lower intensity = closed stomata
Lower intensity & closed stomata
= lower assimilation
Lower assimilation & closed stomata
= higher internal CO2
0
5
10
15
20
Time o f e x p e r ime n t ( min )
450
ambient CO2
6
Internal CO2
400
Increase
350
Higher intensity = higher assimilation
4
300
2
Assimilation
250
0
200
respiration
-2
internal CO2 (ppm)
0
8
Assimilation (umol/m2 sec)
Conductance (mol/m2 sec)
0 .2 5
Higher assimilation & closed stomata
= lower internal CO2
Lower internal CO2
= opening of stomata
150
0
5
10
15
20
Time of experiment (min)
7
0 .3 0
Licor Experiment (Hass Leaf)
2000
Declining Intensity
Increasing Intensity
Decline
1500
Conductance
0 .2 0
Light Intensity (umol/m2 sec)
Conductance (mol/m2 sec)
0 .2 5
Intensity
1000
0 .1 5
0 .1 0
500
0 .0 5
Lower intensity & closed stomata
= lower assimilation
Lower assimilation & closed stomata
= higher internal CO2
0
0 .0 0
5
10
15
20
Time o f e x p e r ime n t ( min )
450
ambient CO2
6
Internal CO2
400
Increase
350
Higher intensity = higher assimilation
4
300
2
Assimilation
250
0
200
respiration
-2
internal CO2 (ppm)
0
8
Assimilation (umol/m2 sec)
Lower intensity = closed stomata
Higher assimilation & closed stomata
= lower internal CO2
Lower internal CO2
= opening of stomata
150
0
5
10
Time of experiment (min)
15
20
Trigger by Light Different Speed than by CO2
So Light Flecks are not especially useful, if high light is of short duration.
Windows in canopies must give enough duration of light to allow stomata response
(30+ minutes)
8
Leaf area
• Development of a plastochron index that can be
used to describe the physiological age of each
leaf
• Based on leaf area (or even leaf length)
• Development of an “easy-to-use” program for
field measurement
Allows one to “standardize” measurements based
on leaf age – critical to reduce data variability
and to understand how flush growth influences
whole tree physiology
Plastochron Index application
The amount of carbohydrate as related to ‘Hass’ leaf age
perseitol
mannoheptulose
sucrose
Day zero = 50% of maximum leaf size
9
Stomata (regulated opening in leaf)
CO2 Transport from
air to inside leaf
CO2 Assimilation
[Photosynthesis]
H2O Transport from
inside leaf to air
H2O Loss
From Leaf
Loss of Leaf
Carbohydrate / Oil
Production
Atmospheric
Relative Humidity
Transpiration
H2O
Stomatal
Conductance
Leaf
Water Potential
H2O
Water Flow
Within The
Plant
Xylem
Conductance
H2O
Soil
Water Potential
Root
Conductance
10
10
Fuerte
Assimilation (μmol / m2 sec)
8
Hass
6
Note that assimilation is falling
because lower water potential
induces more closed stomata
and so lower conduction of
both water and CO2 flow.
4
2
0
0.0
-0.5
-1.0
-1.5
-2.0
Pre-dawn Leaf Water Potential (in MPa)
[water in soil]
California afternoon:
afternoon
AM
higher temperature and lower relative humidity
if stomata are open, higher water loss from leaf
Can the soil provide the water?
conductance
PM
Time
11
California afternoon:
afternoon
AM
higher temperature and lower relative humidity
if stomata are open, higher water loss from leaf
Can the soil provide the water?
conductance
conductance
PM
Δgs
-
Time
noon
AM
California afternoon:
afternoon
AM
Δgs
+
PM
higher temperature and lower relative humidity
if stomata are open, higher water loss from leaf
Can the soil provide the water?
conductance
conductance
PM
Δgs
-
Time
noon
AM
Conductance Difference
Δgs
+
PM
low in AM ⇒ higher in PM
null
set-point
high in AM ⇒ lower in PM
Conductance (gS) in AM (morning)
Conductance Difference = gS (in PM) – gS (in AM)
12
Δgs
Mid-Day Depression of Conductance
60
data w/nc
40
data w/chg
20
0
RH (PM) = 45%
Measurements made
with LiCor Porometer
on one leaf at a time.
-20
-40
-60
RH (PM) = 20%
-80
20
40
60
80
Stomatal Conductance in the Morning
100
RH (AM) = 45%
Assimilation Dependence Upon Light
Assimilation is Light-dependent CO2 fixation
Assimilation
Conclusion: Lowering of conductance in PM made worse by low RH
But only if conductance in AM is high.
Amax
A = ½ Amax
Light Intensity
This I = KI
A=
Amax I
[ Km + I ]
13
Assimilation is Light-dependent CO2 fixation
Amax
Assimilation
Assimilation Dependence Upon Light
A = ½ Amax
Light Intensity
This I = KI
20
Amax I
[ Km + I ]
300
Amax
Light Constant (μmol/m2 sec)
Maximum Assimilation (umol/m2 sec)
A=
15
10
5
0
0
100
200
300
400
200
150
100
50
0
Average Stomata Conductance (mmol/m2 sec)
gS
KI
250
100
200
300
400
Average Stomata Conductance (mmol/m2 sec)
For Hass Avocado
What do we know?
9Light governs assimilation but..
Stomata govern (in part) assimilation too
9Assimilation reduces internal CO2 but...
That low CO2 opens stomata
9Water loss due to transpiration changes stomata but..
Transpiration is less for high relative humidity
Soil water can mitigate this leaf water loss
Can we use these facts for a model that depends only upon the environment?
14
Model Development: Expansion
Assimilation depends upon light
That changes internal CO2
That alters the conductance
That changes Transpiration which is dependent upon vapor pressure deficit
(which is dependent upon air temperature and relative humidity)
That changes leaf water potential (which feedback upon conductance)
All in the house that Jack built!
Light (I)
Air Temperature
Relative Humidity
Vapor Pressure Deficit (VPD)
Leaf Water Potential
Assimilation
Internal CO2
gS (i)
Transpiration
Sap Flow
VPD = VPleaf - VPair
gS ( i -1)
A=
Amax I
A = gS (Co – Ci )
Ts = g’S VPD
KI + I
We still need soil water
FINALLY
Internal CO2 Controls Conductance
A
stomata
[CO2 ]in
Difference in CO2 = [CO2] outside – [CO2] inside
[CO2]in
Bulk Air
Boundary Air
[H2O ]in
Internal Air
[H2O ]out
Movement of CO2 = {conductance} x {Difference in CO2}
leaf
Movement of CO2 limits assimilation
A’max
A = ½ A’max
Internal CO2
This [CO2] = Km
Difference in CO2 (ppm)
Assimilation
300
200
100
Air CO2
0
-100
-200
-10
0
10
20
30
Assimilation (umol/m2 sec) = A
15
Validation of model: How do you measure transpiration continuously ?
Validation of model: How do you measure transpiration continuously ?
SAP FLOW (continuous heat with insulated, non-invasive probes)
Three Zutano Trees (A, B, C) in Green House
Combination of Environmental Conditions
03/30/2006
calc
B
A
C
1.5
dCO2 = 125
Comparison between the Model and the
Actual Sap Flow.
Flow of water
1
Three trees (zutano) were used in the green house to
monitor sap flow (transpiration) and environmental
parameters were also monitored. These values were
used to calculate the transpiration and that was
expanded to sap flow by the known of total leaf area
on the branch.
0.5
0
-0.5
0
500
1000
1500
2000
2500
Time of Day (hr)
16
Sap Flow Measurement
Branch
Heat carried
by sap flow
= Hs
Heat upwards =Hu
Heat in = Hi
Heat external out =Ho
Heat downwards = Hd
Heat Balance
Hi = Hu + Hd + Ho + Hs
transpiration
st e
ion
lat
u
s
in
r
ate
he
current in
C
m
Theater = 31o
A
TB = 29o
TA = 29o
B
thermocouple
voltage out
TcB
TcC
TcA
Tambient = 27o
Sap Flow
TA = 30o
TB = 28o
TcB
TcC
TcA
distance up the stem
17
Hass on Variety of Root Stocks (Duke 7 and Toro Canyon)
Young trees trimmed to only several branches; all open.
Data from Claudia Fassio, Summer 2006
500
Total Sap Flow (g)
400
300
200
Constant
Std Err of Y Est
-26.4
15.8
R Squared
No. of Observations
0.97609
30
X Coefficient(s)
Std Err of Coef.
0.938
0.028
data
fit
100
0
0
100
200
300
400
500
600
Weight Loss (g)
Other Problems: Unequal illumination of leaves on branch being tested
Age Differences of Leaves on branch
Field Sap Flow Measurements – July 2006
rooted
rooted
rooted
pollinizer
rooted
clonal
clonal
clonal
clonal
road
Downhill
Aspect
road
Figure 1
Yellow blocks = Hass
N
Kindly provided by ACW
18
Rooted vs Clonal (Duke 7)
Conductance in field (in units of cm/sec)
0.86
Clonal
0.68
0.71
Hass Trees
Rooted
0.56
0.46
0.29
Comparison of the Calculated Conductance between Rooted and Clonal Rootstock in the Field.
Sap flow (g/hr) converted by leaf area (m2) into transpiration (mmol/m2 sec). Using VPD from relative humidity and
air temperature, the transpiration is converted into conductance. These are total conductance, not those from LICOR
measurements (which misses the boundary layer).
Data are from six trees planted at a field plot at ACW, Fallbrook, spaced by 10 feet from 7/6 to 7/17/06.
To obtain “standard” conductance multiply these numbers by 250 to obtain conductance as mmol/m2 sec.
19
Rooted trees maintain morning sap flow
Afternoon / Morning
07/06
07/07
07/08
07/09
07/10
07/12
07/13
07/14
07/15
07/16
07/17
Ratio of Sap Flow - PM / AM
74%
108%
22%
24%
0.4%
Hass Trees
1.50
0.21
clonal
average PM / AM
clonal
rooted
rooted / clon
130.2%
150.3%
1.15
61.5%
106.2%
1.73
54.4%
86.0%
1.58
53.2%
84.1%
1.58
84.6%
109.9%
1.30
57.7%
67.2%
60.7%
79.3%
95.2%
64.9%
84.6%
76.8%
111.4%
122.7%
149.2%
101.7%
1.47
1.14
1.84
1.55
1.57
1.57
Sap Flow
average
std. dev.
t-test
rooted
Time of Day
Clonal compared with rooted
higher peak
afternoon fall-off
Morning = 7AM – 10AM
Afternoon = 3PM – 6PM
Less total water movement in rooted
= less total conductance in rooted
There are posters on much of this work
During the poster session, a small tree
with sap flow monitor is present
We three thank you for your
continued support!
20