A preliminary evaluation of the leaf-patchclamp-pressure probes (LPCPs) as online
tools for irrigation scheduling in South Africa
1Sebinasi
1
Dzikiti, 1Mark Gush, 1Vivek Naiken, 2Simon Rueger,
2Ulrich Zimmermann
Council for Scientific and Industrial Research
2 ZIM-Plant Technology, Hennigsdorf near Berlin, Germany
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Approaches to irrigation scheduling
A. Atmospheric measurements and water balance techniques
- most common irrigation method
ETc = Kc . ETo (FAO 56)
B. Soil moisture monitoring
- tensiometers, capacitance probes,
Sap flow
-neutron probes etc
C. Plant-based monitoring
- contact sensors
Dendrometers
- pressure chambers/ psychrometers,
dendrometers, leaf thickness sensors,
sap flow, porometers etc
- non – contact sensors
- infrared thermometers, aerial and
satellite based
Remote sensing based
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Disadvantages of currently used plant-based irrigation scheduling tools
Pressure bomb
• utterly destructive
• labor intensive
• spot measurements
Sap flow
• sophisticated
• invasive
• can be unreliable
Dendrometry
• sophisticated
• invasive
• insensitive under drought conditions
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Leaf Patch Clamp Pressure probes (ZIM-probe)
• High precision probes that measure leaf turgor pressure non-invasively and in real-time
• Data can be accessed online (PCs, smartphone, tablets etc.)
• LPCPs have been evaluated on a range of crop types… but outside RSA
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Probe operating principle
Turgor pressure (Pc) in the leaf patch is opposed
to the externally applied constant magnetic
pressure (Pclamp)
ZIM-probe measures the difference (Pp; ) between
the magnetic pressure and turgor pressure
1
a
 b 
 ⋅ Fa ⋅ Pclamp
Pp = 
 aPc + b  Zimmermann et al., 2008
Specifications:
-Sensing area: Ø 4 mm;
-Pclamp : up to 370 kPa
Pp = patch pressure
Pc = turgor pressure
Pclamp = clamp pressure
Fa = attenuation factor (compression of cuticle, cell walls and airfilled interspaces)
a, b = elasticity constants
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Measurement protocol
Aim: To evaluate ZIM-probes as online tools for irrigation scheduling in South Africa
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Tree water status: citrus
-12 yr old Satsuma mandarin
orchard at Stellenbosch University
-Data collected from 15 May – 5 Sept
2012 @ 5 min intervals
-R25 (30 MB) vodacom airtime – still
running
Microclimate: Air temp, Relative Humidity, wind speed & solar radiation
Sap flow and soil water content: Thermal dissipation probes and CS616 probes
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Response to environmental factors
4.5
Solar radiation
VPD
600
3
400
1.5
200
0
VPD (kPa)
Solar radiation (W m-2)
800
0
200
Pp (kPa)
150
100
- ZIM-probes measure
50
the integrated effects
of the microclimate
0.30
0.25
0.20
FC
FC
and the soils on the
plant
0.15
0.10
WP
12/06/2012
10/06/2012
08/06/2012
06/06/2012
Date
04/06/2012
02/06/2012
31/05/2012
29/05/2012
27/05/2012
0.05
25/05/2012
Soil water content (cm3/cm3)
0
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
ZIM-probe vs sap flow
1200
LPCP
1000
80
600
70
60
>2 h
50
800
40
600
30
400
20
200
10
80
70
500
60
< 30 min
400
50
40
300
30
200
20
100
10
19:12:00
16:48:00
Time
14:24:00
12:00:00
09:36:00
Time
07:12:00
02:24:00
0
04:48:00
0
19:12:00
16:48:00
14:24:00
12:00:00
09:36:00
07:12:00
04:48:00
0
02:24:00
0
90
Solar radiation
LPCP
Pp (kPa)
Stem sap flow
700
Solar radiation (W m-2)
1400
90
Pp (kPa)
Stem sap flow (g h-1)
1600
80
70
- Probe readings are dependent on the transpiration rates
60
Pp (kPa)
50
- Significant hysteresis effect between ZIM-probe readings and
40
the rising/ falling transpiration rates
30
20
Increasing transpiration
10
Decreasing transpiration
0
0
500
1000
Transpiration (g h-1)
1500
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Comparison of ZIM-probe measurments with pressure chamber readings
- midday leaf water potential for selected 9 days
during the period May to September 2012
- there was a fairly linear relationship between the
LWP and the average midday probe readings
Pp (kPa)
-0.5
-0.7 0
20
40
60
- a stronger correlation with xylem water potential on
LWP (MPa)
-0.9
nectarines (data not shown)
-1.1
-1.3
- range of stress values was narrow (in winter)
-1.5
-1.7
-1.9
-2.1
-2.3
y = -0.025x - 0.502
R² = 0.647
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Potted citrus tree experiments (a larger stress range)
- Monitored the midday leaf water potential until severe stress had developed (< - 3.5 MPa)
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Soil water content (cm3/cm3)
Response to severe soil drying
Beginning of stress
0.18
Irrigation
0.14
0.1
0.06
0.02
200
Pp (kPa)
160
120
80
40
24/10/2012
22/10/2012
20/10/2012
18/10/2012
16/10/2012
14/10/2012
12/10/2012
10/10/2012
08/10/2012
06/10/2012
04/10/2012
02/10/2012
30/09/2012
28/09/2012
26/09/2012
24/09/2012
22/09/2012
20/09/2012
18/09/2012
16/09/2012
14/09/2012
0
Leafless tree after re-watering
- Recovery of tree water status was incomplete under severe stress leading to higher night Pp-values
- tree lost almost all its leaves after re-watering and such stress levels are not expected in practice
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
ZIM-probe vs pressure chamber data for potted citrus
Pp (kPa)
0
20
40
60
-1.50
-2.00
-2.50
-3.50
20
40
60
80
-1.0
-1.00
-3.00
-0.5 0
80
LWP (MPa)
LWP (MPa)
-0.50
Pp (kPa)
0.0
0.00
-1.5
-2.0
-2.5
-3.0
y = -0.0384x + 0.1334
R² = 0.88
y = -0.0367x - 0.1338
R² = 0.52
-3.5
-4.0
-4.00
- Moderate stress data only
- Including severe stress levels < -3.5 MPa
Good linear correlation at moderate stress levels between the readings of the Scholander
bomb and the ZIM probes
Poorer correlation with the midday leaf water potential at severe stress levels:
•Below ca. 50 kPa turgor pressure the attenuation factor Fa becomes the important and dominating parameter (air is
accummulating in the leaf; beyond Pc=0 kPa the Pp values are meaningless. Pp reflects the temperature dependency of the
entrapped air in the leaf)
•Scholander bomb values are increasing because excessive external pressure is needed to squeeze water out of the
tissue
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Inversion of Pp under severe water stress
= normal diurnal Pp -changes /
no drought stress
= half-inversed diurnal Pp -changes
/ beginning drought stress
= inverted diurnal Pp -changes/
drought stress
- Data collected on olives under deficit irrigation by Fernandez et al., 2011 in Spain
- Grey shades depict night time periods
- First panel shows Pp values when the trees were under progressive soil drying
- Next two middle panels shows Pp under severe stress and Pp-peaking occurred at night)
- Last panel shows the recovery of the trees after re-watering (day time peaks)
Changes in the curve shape can easily be detected and can therefore very easily be
used for setting irrigation thresholds
1. Introduction
2. Leaf Patch Clamp Probes
3. Results
4. Conclusions
Conclusions
Advantages
• LPCPs are quite sensitive to small changes in plant water status;
• Potential candidate for use in precision irrigation scheduling using state-of-the –art
technology;
• Knowledge of soil moisture, microclimate and nutrients is not required if changes of turgor
pressure of the leaves can be recorded non-invasively;
• Easy to install and use, and data transfer is cheap and reliable (where there is a good
cellphone reception);
• Changes in the curve shape can easily be detected and can, therefore, used for setting
irrigation thresholds.
Disadvantages
LPCPs do not give useful info about the plant water status at severe stresses (< - 3.5 MPa for
citrus).
Thank you!!!