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Contents lists available at ScienceDirect
Agricultural Water Management
journal homepage: www.elsevier.com/locate/agwat
Comparative monitoring of temporal and spatial changes in
tree water status using the non-invasive leaf patch clamp
pressure probe and the pressure bomb
S. Rüger a , W. Ehrenberger a , M. Arend b , P. Geßner a , G. Zimmermann a , D. Zimmermann c ,
F.-W. Bentrup d , A. Nadler e , E. Raveh f , V.L. Sukhorukov a , U. Zimmermann a,∗
a
Lehrstuhl für Biotechnologie, Biozentrum, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
Eidg. Forschungsanstalt für Wald, Schnee und Landschaft, WSL, Umweltwandel und Ökophysiologie, Zürcherstr. 111, Ch-8903 Birmensdorf, Switzerland
c
Abteilung für Biophysikalische Chemie, Max-Planck-Institut für Biophysik, Max-von-Laue-Str. 3, D-60439 Frankfurt a. M., Germany
d
Abteilung für Pflanzenphysiologie, Universität Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria
e
Institute of Soil, Water and Environmental Sciences, ARO Volcani Center, Bet Dagan 50250, Israel
f
Gilat Research Center, ARO Volcani Center, Negev 85280, Israel
b
a r t i c l e
i n f o
Article history:
Received 11 June 2010
Accepted 27 August 2010
Available online xxx
Keywords:
Leaf patch clamp pressure probe
Pressure chamber
Cell turgor pressure probe
Trees
Leaf water potential
Xylem pressure
a b s t r a c t
Real-time monitoring of plant water status under field conditions remains difficult to quantify. Here we
give evidence that the magnetic-based leaf patch clamp pressure (LPCP) probe is a non-invasive and
online-measuring method that can elucidate short- and long-term temporal and spatial dynamics of
leaf water status of trees with high precision in real time. Measurements were controlled remotely by
telemetry and data transfer to the Internet. Concomitant measurements using the pressure chamber
technique (frequently applied for leaf water status monitoring) showed that both techniques yield in
principle the same results despite of the high sampling variability of the pressure chamber data. There
was a very good correlation between the output pressure signals of the LPCP probe and the balancing
pressure values (on average r2 = 0.90 ± 0.05; n = 8), i.e. the external pressure at which water appears at the
cut end of a leaf under pressure chamber conditions. Simultaneously performed direct measurements
of leaf cell turgor pressure using the well-established cell turgor pressure probe technique evidenced
that both techniques measure relative changes in leaf turgor pressure. The output pressure signals of the
LPCP probe and the balancing pressure values were inversely correlated to turgor pressure. Consistent
with this, the balancing pressure values and the cell turgor pressure values could be fitted quite well by
the same firm theoretical backing derived recently for the LPCP probe (Zimmermann et al., 2008). This
finding suggests that use of the LPCP probe technique in agricultural water management can be built up
on the knowledge accumulated on spot leaf or stem water potential measurements.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Irrigation scheduling involves determining precisely both the
timing of irrigation and the quantity of water to apply. Irrigation
scheduling decisions are frequently based on the determination of
soil moisture content or soil moisture tension. Local measurements
of soil water status have, however, the drawback that they do not
give direct information about the water needs of a plant (Naor et
al., 2008 and literature quoted there). By contrast, determination of
plant-based water stress indicators allows optimum irrigation and
∗ Corresponding author. Tel.: +49 931314508; fax: +49 9313184509.
E-mail address: [email protected]
(U. Zimmermann).
thus high efficiency use of the worldwide scarce water resources
(Steduto et al., 2007).
Irrigation scheduling is usually based on soil water measurements. An inherent problem determine the water content is that
changes in the bulk soil water content do not necessarily reflect
plant water demands (Jones, 2004). Higher precision in the application of irrigation is expected by use of plant-based methods, i.e.
by methods which directly measure the water supply of plants.
There are several plant-based approaches that are used for monitoring water stress in trees such as psychrometers (McBurney,
1988), sap flow techniques (Fernández et al., 2001, 2006; Green
et al., 2003; Smith and Allen, 1996), water content variations in
stems by time domain reflectometry (Nadler et al., 2003, 2006)
and stem dendrometers (Goldhamer and Fereres, 2001; Zweifel
et al., 2000, 2001). These methods have found broad applications
0378-3774/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.agwat.2010.08.022
Please cite this article in press as: Rüger, S., et al., Comparative monitoring of temporal and spatial changes in tree water status using the
non-invasive leaf patch clamp pressure probe and the pressure bomb. Agric. Water Manage. (2010), doi:10.1016/j.agwat.2010.08.022
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in basic research, but are too sophisticated for routine applications by growers (at least at the present). Therefore, leaf or stem
water potential, measured with the pressure chamber (Scholander
et al., 1965) at midday, has been proposed as standard parameter to determine the plant water status for irrigation scheduling of
fruit trees (Fereres and Goldhamer, 2003; Naor, 2001). Advantage
of the pressure chamber is that no sophisticated instrumentation
is required and that measurements are easy to be performed. Balancing pressure, Pb , values are measured by this technique, i.e. the
external pressure is recorded at which water appears at the cut end
of a leaf or a leafy twig kept at atmosphere. Pb values are usually
in the megapascal range. Drawback of the pressure chamber technique is that it is destructive, slow, labour intensive and unsuitable
for automation. Thus, spot measurements are usually only possible
which may prevent precise water management of crops and fruit
trees. Moreover, Pb values determined on various leaves or twigs
taken at the same time show very often a large scatter. Among
other things, values can also depend on the microclimate at the
measuring site (Zimmermann et al., 2004).
The disadvantages of the pressure chamber are not shared by
the recently developed online-measuring leaf patch clamp pressure
(LPCP) probe (Zimmermann et al., 2008). The online-measuring
LPCP probe measures plant water status in real time and is characterised by high precision, operating convenience, automation
suitability and minimum costs. Multiple probes can, in principle,
be clamped on leaves over the entire height of a plant for unravelling the dynamics of leaf water status at different sites of a plant.
For real-time evaluation data can be sent via wireless telemetric
units to a data logger or more conveniently to a GPRS (General
packet radio service) modem linked to an Internet server via a
mobile phone network. The entire setup allows simultaneous data
processing of multiple probes.
The probe technique measures the pressure transfer function
of an intact leaf, i.e. the attenuation of an externally applied
clamp pressure by the leaf tissue. The clamp pressure is generated
by springs or – more user-friendly – by magnets. Its magnitude
depends on leaf-specific structural features. Theory and experiments on grapevine, banana and lianas have shown (Westhoff et
al., 2009; Zimmermann et al., 2008, 2010) that the pressure transfer
through a leaf patch is dictated predominantly by turgor pressure.
High turgor pressure prevents pressure transfer through the leaf
and, in turn, the output pressure Pp measured by the probe is small.
At very low turgor pressure the transfer function assumes values
close to unity, i.e. the applied pressure is transferred to the pressure
sensor at most and Pp assumes a maximum value (Zimmermann
et al., 2008). By contrast to the balancing pressure values Pp values are much smaller ranging between 5 kPa (corresponding to
maximum turgor pressure) to 200 kPa (corresponding to minimum
turgor pressure).
To assess the potential of the LPCP probe as a novel and universally usable water stress monitoring method for agricultural water
management and for the elucidation of the mechanisms of water
ascent in tall trees, we have applied the LPCP probe to leaves of
various tree species. Among other things, an important aim was
to figure out whether the LPCP probe can replace the pressure
chamber for measuring water relations of trees. Despite large differences in morphology, compressibility and turgescence of the
leaves of the various species we will demonstrate that the LPCP
probe technique measures similar diurnal changes in leaf water
status as the pressure chamber technique, but continuously and
at a much higher resolution and accuracy. We will further show
by direct cell turgor pressure measurements using the cell turgor pressure probe (Zimmermann et al., 2004) that the agreement
between the pressure chamber and probe data are due to the fact
that both techniques ultimately monitor relative changes in leaf
turgor pressure.
2. Materials and methods
2.1. Plant material
Pressure chamber measurements on excised twigs and online
leaf patch clamp pressure (LPCP) probe were performed on eucalyptus (Australia and Israel), avocado trees (Australia), beeches
(Germany), pomelo (Israel) and oaks (Switzerland). Trees were
exposed to relative favourable water conditions. Parallel to the
chamber and probe measurements ambient air temperature, Ta ,
and relative humidity, RH, were measured using data loggers
(Tinytag; RS Components GmbH, Mörfelden-Walldorf, Germany).
Additionally, a manual Ta and RH measuring instrument (testo 625)
was also used.
Eucalyptus pilularis: Measurements were performed during the
end of February and beginning of March 2007, on a ca 42 m tall
tree growing in a forest close to Port Macquarie, NSW, Australia
(31◦ 27 57.0 S, 152◦ 48 50.4 E). Access to the top of the tree was
gained by an 80-ton crane equipped with a gondola (provided by
Mid Coast Cranes, Port Macquarie). Measurements were also conducted on up to 2 m tall sucker sprouts growing on stumps of trees
felled a year before. The experimental area was close to the first
one. The concomitant measurements of Pb and Pp were performed
on sunny days (Ta = up to 37 ◦ C, minimum RH = ca 50%).
Eucalyptus gomphocephala A.DC.Tuart: Measurements were performed at the beginning of October 2007 (Ta = up to 31 ◦ C, minimum
RH = ca 40%) as well as at the end of March 2009 (Ta = up to 30 ◦ C,
minimum RH = ca 32%) on young leaves of ca 1 m tall suckers
growing on branches of ca 18 m tall trees at 1–2 m height (Campus of the Hebrew University, Rehovot, Israel; 31◦ 54 21.25 N,
34◦ 48 6.77 E).
Persea Americana (avocado): Measurements were performed
during the end of January until mid March 2008, on a plantation
at Red Hill close to Port Macquarie, NSW, Australia (31◦ 17 14.00 S,
152◦ 46 20.90 E). Spacing between the 5–6 m tall trees was ca 4 m
and between the rows ca 6 m. Ta and RH profiles and irrigation
events, respectively, during the measuring periods are given in
Figs. 1 and 2. The concomitant measurements of Pb and Pp as well
as the multiple probe recordings of eight LPCP probes in the four
directions of the compass at lower (1.5 m) and upper (4 m) height
were performed on sunny days (Ta = up to 31 ◦ C, minimum RH = ca
53%).
Citrus maxima × Citrus grandis (pomelo): Measurements were
performed at the beginning of April 2009, on ca 3 m tall
trees planted at the Experimental Research Station, Gilat, Israel
(31◦ 19 28.15 N, 34◦ 39 20.29 E). Spacing between the trees was ca
3 m and between the rows ca 6 m. Measurements were performed
on sunny days (Ta = up to 27 ◦ C, minimum RH = ca 40%). Plants were
not irrigated at this time of the year.
Fagus sylvatica (beech): Measurements were performed during
the summer season, 2008, at the Kranzberg Forest research site of
the Department of Ecology, Technische Universität München (near
Freising, Germany, 48◦ 25 11.42 N, 11◦ 39 46.68 E) in a mixed 60year-old stand (closed canopy) with about 27–30 m high European
beech and Norway spruce trees. Scaffolding provided access to the
crowns. Measurements were performed on sunny days (Ta = up to
27 ◦ C, minimum RH = ca 58%).
Quercus robur (oak): Measurements were performed at the
beginning of June 2009, on 5-year-old trees planted in the
Model Ecosystem Facility of the Swiss Federal Institute for Forest, Snow and Landscape Research (Birmensdorf, Switzerland,
47◦ 21 43.38 N, 8◦ 27 19.16 E). Plants were protected against rainfall by movable glass roofs, which were closed by rain. During the
measurement period the trees were irrigated daily in the afternoon.
Measurements were performed on sunny days (Ta = up to 28 ◦ C,
minimum RH = ca 36%).
Please cite this article in press as: Rüger, S., et al., Comparative monitoring of temporal and spatial changes in tree water status using the
non-invasive leaf patch clamp pressure probe and the pressure bomb. Agric. Water Manage. (2010), doi:10.1016/j.agwat.2010.08.022
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Fig. 1. Typical 2-month online recordings of the output clamp pressure, Pp (upper
panels) together with the corresponding ambient air temperature, Ta , and relative
humidity, RH (solid and dotted lines, respectively, middle panel) and vapour pressure deficit, VPD (lower panel) on a leaf of a Persea americana tree. Measurements
were performed on a south-directed leaf of a 5–6 m tall tree at a height of 1.5 m (plantation close to Port Macquarie, Australia; (A) 24 January until 15 February 2008; (B)
21 February until 14 March 2008; EST = Australian Eastern Standard Time). The tree
was irrigated at 26 February as well as at 3 and 7 March. r = rain; h.r. = heavy rain.
2.2. Leaf patch clamp pressure (LPCP) probe
The measuring principle of the non-invasive, online-monitoring
LPCP probe together with the theoretical background are described
in details elsewhere (Westhoff et al., 2009; Zimmermann et al.,
2008, 2010). Briefly, an intact leaf is positioned in the space between
the two planar circular pads of the probe. One of the pads contains
a miniaturised, temperature-independent pressure sensor chip. A
constantly kept, external clamp pressure, Pclamp is applied by two
magnets to the clamped leaf patch. The fraction of the clamp pressure, i.e. the output pressure, Pp , measured by the sensor depends
on the pressure transfer function of the clamped leaf patch. The
transfer function is related to the compressibility of the leaf patch
and therefore varies exclusively with cell turgor pressure, Pc , of
the clamped leaf. If the compressibility is increasing due to turgor
pressure loss, the pressure transfer function and thus the output pressure Pp increases. Theory shows (Westhoff et al., 2009;
Zimmermann et al., 2008, 2010) that Pp as a function of Pc is given
by Eq. (1):
Pp =
b
aPc + b
1/a
Fa Pclamp
(1)
3
Fig. 2. Diurnal Pp changes measured on upper (A) and lower (B) leaves located on
the east (red line), north (grey line), west (black line) and south (blue line) side of
a 5 m tall P. americana tree (28 January 2008, plantation close to Port Macquarie,
Australia). Pp values of the lower and upper leaves were normalised to the Pp range
of the respective probes located on the north side. Measurements were performed
on a sunny day (maximum Ta = 31.4 ◦ C, minimum RH = 52.5%). Sun-exposure dictated the Pp response of the leaves. Upper leaves (except leaves on the west side)
were more or less sun-exposed over the entire day, where the lower leaves were
only temporarily sun-exposed (arrows). It should be noted that similar results were
obtained by measurements on the following three sunny days.
where a and b are constants and equal or larger than unity. If
a = 1 and b Pc , Eq. (1) turns over into Pp = (b/Pc )Fa Pclamp , i.e. Pp
is inversely coupled with Pc . Fa is the attenuation factor accounting
for turgor pressure independent losses due to the compressibility
of the silicone of the sensor chip and of leaf-specific structural elements (e.g. air-filled spaces, cuticle and cell walls). In the case of
the rigid leaves of the trees investigated here Fa was ca 0.2–0.3 as
demonstrated by control experiments (data not shown).
Real-time recording of the LPCP values occurred by batterypowered wireless telemetric transmitters (teleBITcom GmbH,
Teltow, Germany) which sent the data together with the transmitter ID-code every 5 min via the ISM band of 433 MHz to a
receiver base station which logged and transferred the data with
time stamps to a GPRS modem linked to the Internet server of
the University of Würzburg (NTBB Systemtechnik GmbH, Zeuthen,
Germany).
2.3. Cell turgor pressure probe
The principle of the cell turgor pressure probe is described
in detail elsewhere (Zimmermann et al., 2004). The probe was
inserted from the adaxial or abaxial side of the leaves into the
parenchyma cells close to the midrib. Adaxial and abaxial measurements yielded similar results. Therefore, the data were pooled.
Please cite this article in press as: Rüger, S., et al., Comparative monitoring of temporal and spatial changes in tree water status using the
non-invasive leaf patch clamp pressure probe and the pressure bomb. Agric. Water Manage. (2010), doi:10.1016/j.agwat.2010.08.022
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Table 1
Characteristic parameters of the diurnal Pp changes measured on upper and lower leaves of Persea Americana on sunny days (mean values ± SD; n = 4; 28–31 January 2008).
East
North
Upper leaves
tr (min)a
Pp /t (%/min)b
tpeak (min)c
(min)d
a
b
c
d
25
1.57
281
44
±
±
±
±
11
0.55
52
7
Lower leaves
27
0.92
336
62
±
±
±
±
10
0.11
91
5
West
Upper leaves
25
0.61
409
71
±
±
±
±
6
0.12
36
9
Lower leaves
17
0.42
500
39
±
±
±
±
9
0.05
20
2
South
Upper leaves
19
0.14
591
31
±
±
±
±
11
0.02
28
2
Lower leaves
19
0.14
538
16
±
±
±
±
9
0.02
14
3
Upper leaves
25
0.63
364
40
±
±
±
±
3
0.08
56
6
Lower leaves
36
0.30
383
48
±
±
±
±
5
0.05
150
13
Rise time given in minutes after sunrise (06:11 h EST)
Percentage increase of Pp per min after sunrise.
Time of Pp peaking around noon given in minutes after sunrise.
Relaxation time of the exponential turgor pressure recovery phase in the afternoon.
Data were recorded on leaves located close to the LPCP probes.
Cell turgor pressure measurements were repeated on several
days.
2.4. Balancing pressure measurements
Balancing pressure values, i.e. determination of the pressure at
which the cut end became wetted, were determined in parallel to
online leaf patch clamp pressure measurements using Scholandertype pressure chambers equipped with a digitised manometer
(Scholander et al., 1965). Samples were excised in air from locations near those where the LPCP probe and cell turgor pressure
measurements were performed, and were sealed inside the pressure chamber within 1 min after excision. Pressure was increased at
a rate of ca 0.3 MPa/min, and the pressure at which liquid appeared
at the cut surface of the twig was taken to be the Pb value.
3. Results
3.1. Temporal and spatial diurnal changes of the output patch
pressure Pp
Strong winds, heavy rains as well as thunderstorms (or sandstorms at Gilat, Israel) did not affect the LPCP probe readings on the
various tree species. Probes normally operated over more than 2–3
months without adverse side effects to the clamped leaf. Removal of
the probes and microscopic inspection of the leaves showed sometimes slight impressions of the probes, but no necrosis, lesions or
structural changes. Sometimes the area beneath the pads of the
probes was somewhat faded after long-term clamping compared
to the surrounding tissue, due to some decrease in the chlorophyll
concentration.
An example for long-term LPCP probe measurements is given in
Fig. 1 together with the concomitant recordings of Ta , RH and VPD
(vapour pressure deficit). It is evident that the diurnal changes in Pp
of the leaves of P. americana depended strongly on the weather conditions. During rainy days diurnal changes in Pp were low, whereas
on sunny days, particularly at relatively high temperatures, Pp
reached partly very high peak values at noon.
Large temporal and spatial variations in the Pp values were
recorded on the avocado trees after sunrise over the entire trees.
Typical multiple probe recordings are depicted in Fig. 2. Probes
were clamped in the four directions of the compass at upper (4 m;
Fig. 2A) and lower height (1.5 m; Fig. 2B). Among other things, it
can be seen that continuous sun-exposure resulted in a continuous
increase of the Pp values, whereas temporary sun-exposure led to
a transient increase in the Pp values (see arrows in Fig. 2). Evaluation of complete Pp diurnal data sets recorded by the eight probes
between 28 and 31 January 2008, are given in Table 1. The analysis
of the curves in Fig. 2 together with the average data of the characteristic Pp variables in Table 1 gives some insight into the temporal
and spatial dynamics of turgor pressure loss after sunrise (P < 0.05).
The rise time, tr , of Pp at the different measuring sites upon sunrise (06:11 h) was almost identical (on average 20 min), except of
tr measured on lower leaves on the south side. Here, onset of turgor pressure loss upon sunrise occurred significantly later (by ca
10 min) compared with the other sites. The following increase in Pp ,
i.e. the percentage increase of Pp per min, Pp /t, was generally
larger at upper than at lower leaves. Pp /t assumed maximum
values on the east side and minimum values on the west side. Consistent with this, the time, tpeak , at which Pp peaking occurred after
sunrise, was significantly smaller on the east side compared to the
west side (ca 5 h versus ca 9 h). Intermediate tpeak values were measured on the north and south side. Generally, Pp peaking occurred
earlier at the upper than at the lower leaves. Significant differences
(P < 0.05) were found for the relaxation time, , of the exponential Pp decreasing phase (i.e. turgor pressure recovery phase) in
the afternoon. On the west side turgor pressure recovery occurred
much faster than at the other sides. of leaves of the east side was
of the same order of magnitude than the values of leaves on the
north and south side. of upper and lower leaves were significantly
different, but did not show a clear-cut trend with height.
Similar diurnal changes of Pp in dependency of the exposure of
the leaves to sunshine were also measured on leaves of the other
species (data not shown).
3.2. Leaf patch clamp pressure probe versus pressure chamber
measurements
Typical plots of diurnal changes of the output pressure, Pp , and
of balancing pressure values, Pb , measured on leaves of the five
species in parallel by the LPCP probe and chamber technique are
given in Fig. 3 (A and B: P. americana, leaves on the east and west
side, respectively; C: C. maxima × C. grandis; D: Q. robur; E: F. sylvatica (at 25 m height); F: E. gomphocephala; G and H: E. pilularis,
measurements performed on a 2 m tall sucker and at the top of a
42 m tall tree, respectively). Due to the limited amount of leaves
(particularly in the case of Q. robur) comparative measurements
were made only up to noon. Inspection of the figures shows that
the scatter of the balancing pressure data was usually quite high.
In the light of Fig. 2 this is not very surprising because the Pb values reflect variations between different leaves, while the Pp values
reflect variations within the same leaf. Nevertheless, despite this,
it is clear that the overall trend of the Pb changes with progressing day was comparable with that measured for Pp . Interestingly,
sometimes the increase of the Pb values after sunrise lagged significantly behind the corresponding increase of the Pp values (e.g. see
the measurements performed on the P. americana leaf located on
the east side; Fig. 3A).
Fig. 4 shows the dependency of Pp on Pb for the five species
calculated from the data given in Fig. 3 and from concomitant measurements on other days. There were obviously a linear dependency
between Pp and Pb (on average r2 = 0.90 ± 0.05; n = 8) over a large
range of Pb values. In one case (Fig. 4A) a plateau value of Pp values
was reached at relatively high Pb values.
Please cite this article in press as: Rüger, S., et al., Comparative monitoring of temporal and spatial changes in tree water status using the
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Fig. 3. Concomitant measurements of output pressures, Pp (solid line), and balancing pressure, Pb (open circles), values (using the Scholander pressure chamber) on leaves
of P. americana (A: east-directed leaf, 28 January 2008; B: west-directed leaf, 28 January 2008; EST), C. maxima (C: 31 March 2009; EET), Q. robur (D: 4 June 2009; CET), F.
sylvatica (E: 25 July 2008; measurements performed at 25 m height; CET), E. gomphocephala (F: 26 March 2009; EET) and E. pilularis (G: 2 m tall sucker, 28 February 2007; H:
top of a 42 m tall tree, 27 February 2007; EST). Note that the increase of the Pb values upon sunrise could lag sometimes (e.g. A) significantly behind the increase of the Pp
values.
3.3. Concomitant cell turgor pressure probe measurements
Because of plant-specific structural peculiarities (thick cuticle, etc.) and unfavourable size of the leaf cells of some of the
investigated species turgor pressure measurements could be only
performed on the leaves of E. gomphocephala, C. maxima × C. grandis and Q. robur using the cell turgor pressure probe. Measurements
on these species were also extremely difficult, particularly during
the phase of rapid turgor pressure changes after the onset of transpiration. When turgor pressure, Pc , dropped below ca 150 kPa it
became more and more difficult to insert the microcapillary into
a cell because of enlargement of the intercellular air spaces of
the leaves with proceeding day. Microcapillary tips filled with air
prevented correct Pc measurements. Furthermore, because of leaf
movements the duration of the Pc measurements was rather short
(ca 4 min). In Fig. 5 the Pp and Pb values are plotted versus the turgor
pressure values measured at the same time of the day. Data were
pooled from measurements performed over two to three consecutive days. Inspection of the figure shows that at sunrise Pc was
of the order of 550 kPa for C. maxima × C. grandis (Fig. 5A) and Q.
robur (Fig. 5B), but much less for E. gomphocephala. Both during
March (Fig. 5C) and at the beginning of October (inset in Fig. 5C)
turgor pressure did not exceed ca 350 kPa. With ongoing transpiration the turgor pressure of the leaves of the three species dropped
to ca 50 kPa on sunny days. The decrease in turgor pressure was
accompanied by a corresponding increase in the Pp and Pb values.
Both the Pp and Pb values in the dependency on Pc could be fitted by Eq. (1) using appropriate values for the parameters a and
b. However, the data points measured on C. maxima × C. grandis
(Fig. 5A) and on E. gomphocephala (Fig. 5C) during spring could
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Fig. 4. Plot of the Pp values against the corresponding Pb values measured on P. Americana (A, B), C. maxima (C), Q. robur (D), F. sylvatica (E), E. gomphocephala (F) and E.
pilularis (G, H). Data were taken from Fig. 3 (see details) and from data measured on the following days. Bars of the Pb values represent the SD of at least three measurements.
Pb values measured on Q. robur represent single data points because the relatively small size of the plants only allowed a few measurements. Data were fitted by the least
squares method; r2 : 0.97 (A), 0.97 (B), 0.91 (C), 0.90 (D), 0.83 (E), 0.86 (F), 0.91 (G), 0.85 (H) on average r2 = 0.90 ± 0.05; n = 8.
also be fitted quite well with a straight line (using the least square
method).
4. Discussion
There is no doubt that continuous, temporarily and spatially
resolved measurements of leaf water status are very useful for agricultural water management. Together with continuous sap flow
measurements in plant stems and/or roots (Burgess et al., 2000;
Fernández et al., 2001, 2006; Green et al., 2003) and other automatable techniques (e.g. TDR for monitoring water content in stems,
Nadler et al., 2003, 2006), high resolution measurements of turgor
pressure will certainly unravel the diurnal whole-plant water use
needed for precise irrigation decisions.
The data reported here highlight that the leaf patch pressure
clamp probe is an all-purpose high precision tool to elucidate short-
and long-term temporal dynamics in leaf water status of trees
in dependence on microclimate and soil factors in real time. At
days of high amount of rainfall or at low temperatures and high
relative humidity, when transpirational water loss and, in turn,
turgor pressure changes are small, changes in the output pressure Pp were minimal. By contrast, on sunny days diurnal changes
of Pp were much more pronounced than on cloudy or rainy days
because of large transpirational water loss associated with a significant decrease in turgor pressure (Fig. 1). As shown in Fig. 2,
the short- and long-term spatial dynamics of leaf water status at
the whole tree level can also be resolved by multiple probe readings. Longer exposure of the leaves to sun resulted in an immediate
and steady increase of Pp , whereas temporary sun-exposure led
to a transient change in Pp (see arrows in Fig. 2A and B). Accordingly, the evaluation of the rise time of Pp after sunrise, tr , the
rate of turgor pressure loss ( = the percentage increase of Pp per
Please cite this article in press as: Rüger, S., et al., Comparative monitoring of temporal and spatial changes in tree water status using the
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Fig. 5. Plot of Pp (triangles) and Pb (circles) values versus the turgor pressure values,
Pc , measured at the same time of the day by using the cell turgor pressure probe. Data
were pooled from measurements performed on two to three consecutive days. (A) C.
maxima, measurements were performed on up to 4 m tall trees at March/April 2009,
(B) Q. robur, measurements were performed on up to 2 m tall trees at the beginning
of June 2009, (C) E. gomphocephala, measurements were performed on suckers at
March 2009, and at the beginning of October 2007 (inset). The straight lines in A
and C were fitted by the least squares method (r2 = 0.93 and 0.90, respectively),
whereas the corresponding dependency of Pp on Pc in B and in the inset of C were
fitted by Eq. (1) (Fa = 0.3, Pclamp = 252 kPa, a = 6.8, b = 49.9 kPa; r2 = 0.93 and Fa = 0.2,
Pclamp = 416 kPa, a = 1.47, b = 35.99 kPa; r2 = 0.91, respectively). Bars of the Pb values
represent the SD of at least four measurements.
min, Pp /t) during the morning hours, the time of Pp peaking
after sunrise, tpeak , and the relaxation time, , of the exponentially occurring turgor pressure recovery in the afternoon shows
partly large differences between the upper and lower leaves and
with the compass direction (see Table 1). Most interestingly, turgor pressure recovery was much faster on the west side than on the
other sides. Obviously, leaf water supply and transpirational water
loss in the various regions of a tree occur quite independently of
each other during the day. Local variations in transpirational water
loss are expected because of differences in the microclimate and
because of stomata oscillations (Dzikiti et al., 2007; Zimmermann
et al., 2010) and the fact that not all stomata are open at the
same time to the same extent (so-called patchy stomatal closure)
(Cardon et al., 1994; Haefner et al., 1997; Larcher, 1995). This
leads to a non-linear water-flow pattern that is difficult to pre-
7
dict, but measurable by multiple LPCP probes as demonstrated
here.
Similar dependencies of Pp on environmental factors as shown
for P. americana in Figs. 1 and 2 were also found for the other five
trees investigated here. This is important to note because morphology, structural properties, stiffness and thickness of the leaves of
the various species were quite different.
A further interesting result of the measurements reported here
is the comparison of the LPCP probe with the Scholander pressure
chamber technique used currently in agricultural water management for detection of water stress symptoms. A plot of the output
pressure values Pp determined by the LPCP probe against the balancing pressure values Pb determined by the Scholander pressure
chamber yielded a linear relationship between the two parameters over a large range of turgescence. In one case (Fig. 4A) the
Pp values reached a plateau value at relatively high Pb values. A
similar dependency of Pp on Pb was found recently for leaves of
grapevines (Westhoff et al., 2009) and is expected if turgor pressure has approached zero, i.e. the lower limit of the measuring
range of the LPCP probe. As outlined previously (Westhoff et al.,
2009; Zimmermann et al., 2004) beyond this point the excess chamber pressure is obviously used to shift remaining, cell-bound water
against high unbalanced osmotic pressure to the cut end of the leaf
petiole or twig.
The regression coefficient of the straight lines was on average
r2 = 0.90 ± 0.05 (n = 8; for details, see legend of Fig. 4). Keeping in
mind that the probe measurements are performed on a single leaf
level, whereas the pressure chamber data are based on multiple
leaves, the correlation between Pp and Pb seems to be very good.
This obviously arises from the fact that both parameters are correlated inversely with relative changes in cell turgor pressure, Pc
(Fig. 5) as evidenced by isochronous cell turgor probe measurements. The Pp and the Pb data in dependency of Pc measured on
leaves of Q. robur (Fig. 5B) and of E. gomphocephala suckers at
the beginning of October could be fitted quite well by Eq. (1) (see
inset of Fig. 5C). A similar agreement between theory and experiments was found for leaves of the liana Tetrastigma voinierianum
and grapevine (Westhoff et al., 2009; Zimmermann et al., 2008).
The corresponding data measured on leaves of C. maxima × C. grandis and of the young leaves of E. gomphocephala suckers during
spring could best be fitted by a straight line. One possible reason for these findings could be the large amount of air which was
detected regularly in the leaves of these species during insertion of
the microcapillary of the turgor pressure probe once turgor pressure has dropped below ca 150 kPa. In the derivation of Eq. (1) it was
assumed (Zimmermann et al., 2008) that the bulk volumetric elastic
modulus of the cell walls (which correlated inversely with the bulk
compressibility) depends linearly on turgor pressure. Enlargement
of air-filled spaces within the leaves due to turgor pressure loss
interfere with this assumption and may change the compressibility
of the system significantly. Another explanation could be that the
assumption that the attenuation factor Fa in Eq. (1) is constant does
no longer hold upon approaching the plasmolytic point because of
the large compressible air spaces.
From a scientific viewpoint it is quite interesting to note that the
finding that the pressure chamber measurements obviously reflect
relative changes in turgor pressure is at variance with the common
interpretation of balancing pressure values. In the literature it is
usually assumed (e.g. Boyer, 1967; Koch et al., 2004) that the balancing pressure values reflect absolute negative values of xylem
pressure or absolute values of leaf water potential (Pc − ). By taking into account the corresponding in Fig. 5C the cellular (leaf)
osmotic pressure in dependency on the progressing day can be
calculated provided that these assumptions are true. Calculations
show that the osmotic pressure must increase from sunrise to noon
by ca 33% for C. maxima × C. grandis ( = 982 kPa to = 1361 kPa),
Please cite this article in press as: Rüger, S., et al., Comparative monitoring of temporal and spatial changes in tree water status using the
non-invasive leaf patch clamp pressure probe and the pressure bomb. Agric. Water Manage. (2010), doi:10.1016/j.agwat.2010.08.022
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by ca 66% for E. gomphocephala ( = 906 kPa to = 1545 kPa) and
by ca 150% for Q. robur ( = 777 kPa to = 2038 kPa). The dramatic increase in osmotic pressure must be related with a similar
decrease of cell volume (if an ideal osmometer is assumed for a first
approximation). These changes in osmotic pressure or volume are
extremely unlikely and indicate that the balancing pressure values
cannot be set numerically equal to xylem pressure or to leaf water
potential.
Future work on other plants is certainly necessary in order to
demonstrate whether the interpretation of the pressure chamber
data has to be refined or not. For monitoring of leaf water status by using the LPCP probe and thus for irrigation decisions it is
only important to keep in mind that both techniques measure in
principle the same relative leaf water status parameter. However,
it must be emphasised that in contrast to the pressure chamber
the LPCP probe is characterised by high precision, high temporal
and spatial resolution as well as by operational convenience and
the possibility of remote control. These features of the LPCP probe
may lead in future to the replacement of the pressure chamber as a
monitoring device for routine determination of water stress symptoms under field conditions. The only disadvantage of the LPCP
probe is that metal tools should be kept in a certain distance from
the probes. Because of the magnetic attraction the probe will be
removed instantaneously and the measurement is interrupted. In
this case, the probe has to be reclamped.
5. Conclusion
In the light of the data presented here we can conclude that the
miniaturised, non-invasive LPCP probe is a high precision method
for online monitoring of the water supply of higher plants including tall trees in real time. The possibility of remote and continuous
data acquisition of plant water status in real time over long measuring periods opens up new avenues for improvement of current
irrigation protocols. Because of the high temporal and spatial resolution of the water supply of leaves thresholds for irrigation can
easily be found by this non-invasive technique. Setting of thresholds for irrigation scheduling will occur on precise data sets and
not empirically as in the case of the Scholander pressure chamber.
However, it is important to note that concomitant spot measurements of leaf water status using the pressure chamber technique
performed here have shown that both techniques yield comparable results. As evidenced by direct turgor pressure measurements
using the cell turgor pressure probe, both techniques monitor relative changes in turgor pressure. Therefore, data measured by the
two techniques are mutually transferable.
Acknowledgements
This work was supported by a grant from the AIF (no. KF
0054703WM8) to U. Z. We would like to thank Ewa Stepien-Bötsch
for her great help in evaluation of the LPCP datasets and Zipora Hillman Bronshtain for her great organisation, support and control of
the field experiments at Gilat, Israel.
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Please cite this article in press as: Rüger, S., et al., Comparative monitoring of temporal and spatial changes in tree water status using the
non-invasive leaf patch clamp pressure probe and the pressure bomb. Agric. Water Manage. (2010), doi:10.1016/j.agwat.2010.08.022