What can the vine tell us about its water status?
Everard J. Edwards
Irrigation management
The basics:
We irrigate to produce an economically
viable crop.
Efficient irrigation is applying only the water
needed to produce the required crop.
The question:
How can we irrigate efficiently (how much
and when)?
This is why we are interested in vine water status
Plant based sensors
Most growers measure soil moisture,
But are they interested in soil moisture per
se?
We typically use soil H2O as an indicator of
the vine water status.
So why not look to the vine itself?
We can measure many aspects of plant
function, but costly and time consuming.
Interpretation requires knowledge of
physiology.
What do we mean by water status?
Water potential
Leaf:
-1.20
The plant is part of a soil-air continuum.
Water potential is the ‘suction force’
that causes water to move and is
measured as a negative pressure (MPa).
Water moves from less negative to
more negative potential.
Stem:
-0.50
Roots:
-0.05
Soil:
-0.03
Leaves are site of evaporation and have
the lowest (most negative) potential.
Lower potential = greater water
‘requirement’ in that tissue.
At night leaf water potential typically
reaches equilibrium with soil
How? When? Where?
Near infra-red spectroscopy:
leaf and destructive samples.
Balance pressure: leaf and ‘stem’.
Psychrometry:
leaf and woody tissue.
Thermal diffusivity: canes and
woody tissue.
How? When? Where?
Measures of plant function (including water potential) are
dynamic and respond to multiple environmental factors.
Leaf water potential (MPa)
-0.2
Water deficit
Well irrigated
-0.4
-0.6
Leaf water potential
≠
water stress
-0.8
-1.0
-1.2
-1.4
first light
-1.6
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
Time of day
Merlot vines in Adelaide Hills, data courtesy of Brian Loveys
Interactions
Why does the deficit irrigated vine end up with a “better”
water potential than the well watered vine?
Due to differences water loss during the day.
Controlled by pores in
the leaf: stomata.
Regulate gas exchange
between leaf and air.
Affects photosynthesis
and transpiration
(water use).
CO2
H 2O
Interactions
Low soil water availability results in stomatal closure,
reduced transpiration and less negative leaf water potential.
Leaf water potential (MPa)
-0.2
Water deficit
Well irrigated
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
first light
-1.6
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
Time of day
Merlot vines in Adelaide Hills, data courtesy of Brian Loveys
Other measures: sapflow
Vine water use can be measured directly.
Data can also be combined with information on weather to
make irrigation decisions.
1.0
a)
b)
12
-1
10
8
0.6
6
-1
0.4
4
2
0.2
0
0.0
6/1/11
Sap Flux (L d )
Sap Flux (L hr )
0.8
9/1/11
12/1/11
15/1/11
Oct 10
Dec 10
Feb 11
Apr 11
Jun 11
Other measures: dendrometers
Dendrometers measure very small variations in trunk
diameter.
More negative water potential during day results in trunk
shrinkage.
D diameter (mm)
5
RDI!1
CON1
PD1
PD2
RDI2
CON2
4
3
2
1
3/12
10/12
17/12
Date
24/12
31/12
Where is the emerging technology?
Most of these sensors/techniques have
been around for some time...
But modern manufacturing is bringing
the price down year by year, e.g. NIR.
Wireless mesh networks for the field.
Powerful IT everywhere.
Remote sensing
Technologies discussed so far are single
point (leaf/vine).
Remote sensing offers:
non-contact/non-destructive,
large area sampling,
wide range of possible wavebands.
Limitations for irrigation scheduling:
infrequent coverage,
speed of data/analysis provision,
limited view angles.
Colour and multi-spectral
images of vineyard
Remote Sensing Australia
Proximal remote sensing
Remote sensing imaging technologies
can be used at a smaller scale:
Multi-spectral, e.g. NDVI, PCD
Hyperspectral,
LIDAR, etc.
Sensors can be mounted to moving
machinery or fixed.
Many possible viewing angles.
Instant data availability (local
hardware / web-based tools).
Canopy temperature
Easy to get a value.
Lots of methods to obtain
measurements.
Popular with plant scientists – must be
good!
Leaf temperature °C
Transpiration affects leaf temperature
50
45
40
35
30
25
20
Air temperature
0
10
20
30
40
50
60
70
80
90 100
Seconds
Slide courtesy of Brian Loveys
Leaf temperature °C
Transpiration affects leaf temperature
50
45
40
35
30
25
20
Air temperature
0
10
20
30
40
50
60
70
80
90 100
Seconds
Slide courtesy of Brian Loveys
Leaf temperature °C
Transpiration affects leaf temperature
50
45
40
35
30
25
20
Air temperature
0
10
20
30
40
50
60
70
80
90 100
Seconds
Slide courtesy of Brian Loveys
Transpiration affects leaf temperature
1.7 ML/ha
0.6 ML/ha
3.3 ML/ha
Canopy temperature of deficit irrigated vines is higher
than well-watered vines.
Aerial image courtesy of Ashley Wheaton, University of Melbourne.
Using canopy/leaf temperature
40°C
28°C
50
40°C
12
45
-2
-1
28°C
10
40
8
35
6
30
4
Leaf Temperature (C)
Transpiration (mmol m s )
14
25
2
0
20
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
A small temperature difference can represent a large difference in
transpiration.
Transpiration can be a poor measure of stress in mild conditions.
Using canopy/leaf temperature
Transpiration controlled by stomata, stomata responsive to
water stress.
29.9°C
4 0 .4 ° C
40
35
Index of conductance:
30
Ig = (Tdry-Tleaf)
(Tleaf-Twet)
Irrigated
Ig = 1.80
25
2 4 .9 ° C
31.3°C
4 0 .4 ° C
40
35
30
RDI
Ig = 0.92
25
2 4 .9 ° C
Images/data courtesy of Brian Loveys
Using canopy/leaf temperature
Transpiration controlled by stomata, stomata responsive to
water stress.
So:
Integrates many signals,
including from soil and air.
Temperature →
Transpiration →
Stomatal conductance →
Water stress/status
Canopy temperature is a measure of water stress, when
considered in relation to the environment.
A simpler alternative?
Stomatal resistance
1/stomatal conductance
Leaf temp
Air temp
Dry reference
VPD
rs = -ρcprHR[s(Tl – Ta) + D]/[γρcp(Tl – Tdry)] - raW
Can get rid of wet reference if measure humidity.
Leinonen et al. 2006. Plant Cell Environment 29, 1508-1518.
A simpler alternative?
1000
IR temperature gun relatively cheap,
Box of tricks incorporating a dry
reference.
Collect data across vineyard block.
Calculated gs
800
600
400
200
0
0
200
400
Porometer gs
600
800
Images/data courtesy of Brian Loveys
Difficulties in using canopy temperature for
irrigation scheduling
Air temperature and humidity have huge effect on
leaf temperature,
Can’t compare measurements on different days.
Need additional environmental measurements to
estimate actual vine stress.
Need coverage of entire block,
Aerial imaging expensive/infrequent.
Small variation in temperature = large variation in
stomatal conductance.
What are the practical options?
You shouldn’t need a degree in plant science
to decide when to turn the tap on.
Point measurements (leaf water potential,
sapflow, trunk shrinkage) possible for very
high value crops.
Remote sensing useful for long-term effects,
e.g. differences in soil depth.
Farm-based assessment of canopy
temperature for irrigation scheduling
requires more development.
Conclusions
The reason we use soil moisture measurement is to
estimate plant water stress.
The plant lives in air and soil, therefore plant based
measures are a better estimate of this.
But,
Measuring plant function is hard (=expensive).
Interpreting plant function is complicated.
Need,
Simplified measurement, simplified indexes for
decision making.
Acknowledgements
Brian Loveys
Past and current staff of grapevine physiology group at CSIRO.
Most work described funded by GWRDC.
Plant Industry
Everard Edwards
Research Team Leader
t +61 8 83038649
e [email protected]