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Journal of Horticultural Science
ISSN: 0022-1589 (Print) (Online) Journal homepage:
Testing water potential in peach trees as an
indicator of water stress
E. Garnier & A. Berger
To cite this article: E. Garnier & A. Berger (1985) Testing water potential in peach
trees as an indicator of water stress, Journal of Horticultural Science, 60:1, 47-56, DOI:
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Date: 27 October 2017, At: 22:00
Journal of Horticultural Science (1985) 60 (1) 47-56
Testing water potential in peach trees as an indicator of
water stress
Laboratoire d'Ecophysiologie, Centre Emberger (C.N.R.S.), Route de Mende, BP 505 1,
34033, Montpellier-Cedex, France
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Water potential has been measured on transpiring ('I'L =leaf water potential) and nontranspiring ('l's =stem water potential) leaves in peach trees irrigated at 50% and 100%
actual evapotranspiration (AET). The calculated difference between 'l's and 'I'L ('l's- 'I'L
= ~\jf), which gives a measure of leaf transpiration, was studied, and only slight
differences in 'I'L values between the two treatments were observed. This non-sensitivity
of 'I'L is discussed. In contrast, before irrigation, 'l's was significantly more negative in
the 50% AET regime than in the 100% AET regime, implying a decrease in Ll\j/ values.
This suggests decreased leaf transpiration caused by stomatal closure. Differences in 'l's
values disappeared after the irrigations. The sensitivity of 'l's and Llw to alternation of
drying periods and irrigations allows the use of these parameters for the timing of
irrigation. A method involving the use of the relation between 'I'L and Ll\j/ in non-limiting
soil water conditions is proposed.
SOIL water measurements have been and still
are widely used to determine when and how
much water should be applied to meet irrigation needs (Campbell and Campbell, 1982).
However, soil parameter values used to indicate irrigation requirements may not be critical
values for the plant under all atmospheric conditions and, conversely, the plant can be
stressed before the predefined threshold value
is reached. Sensitive physiological indicators
of plant water stress, which integrate both soil
and climatic conditions, could therefore be
used to determine when to irrigate (Schmueli,
1967; Clark and Hiler, 1973; Stegman et al.,
1976). Direct measurements on plants can also
prove very useful when soil water measurements are difficult, in gravelly soils for example
(Acevedo et al., 1973). Several indicators have
been tested: leaf relative turgidity (Namken,
1965), stomatal opening (Schmueli, 196 7;
Halevy, 1972; Clark and Hiler, 1973), stem or
root diameter changes (Namken et al., 1969;
Johnson and Davis, 1973), canopy temperatures (Jackson, 1982), and in almost every
case plant water potential has been measured
as a recognized indicator of plant water status.
The design of the pressure chamber by
Scholander et al. (1965) has allowed easy
determination of plant water potential and the
extension of its measurement in field ecological
studies (see review by Ritchie and Hinckley,
19 75). As regards irrigation, one of the most
striking experiments has been conducted by
Bordovsky et al. (1974), who showed on
cotton plants that combining water potential
measurements with a crop susceptibility factor
to regulate irrigation, could improve water use
efficiency by reducing by 40% the total amount
of water applied, compared with a method
where the timing is based on the definition of a
threshold value of a soil water potential.
However, plant water potential has not
always proved sensitive to different watering
regimes, partly because of stomatal regulation
(Jones, 1983). Furthermore, although total
water potential is all-important, so far as water
movement is concerned, plant physiological
processes-and thus the determination of
water stress-seem to be more dependent on
turgor potential (Hsiao et al., 1976). Unfortunately, this parameter is not easily measured.
The purpose of the present study was to find
Water stress in peach trees
a sensitive indicator of water stress in peach
trees so as to time irrigations. Since we wanted
to retain the advantages of a simple and
valuable method of plant water status determination, but which is not always sensitive, we
measured plant water potential in two ways:
on a sunlit mature leaf and on a nontranspiring leaf close to the sunlit one. The
difference between the two values should provide a measure of leaf transpiration which is
expected to vary with soil moisture conditions.
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Experimental site and plant material
This research was conducted in the Ecole
Nationale Superieure Agronomique de Montpellier experimental orchard 8 km south-east
of Montpellier (France) (43�'N, 3�'E)
during the summer of 1983. Peach trees
(Prunus persica (L.) Batsch cv Springcrest on
GF 305 rootstock) were planted in January
1981 at 4.5 m spacing in rows 6 m apart. The
soil is alluvial with a gravelly layer at 50 em
depth. It is classified as calcic-luvisol (F.A.O.
classification). Main roots are located in the
upper clay-silt above 50 em, with some small
roots going down to 60 em.
The trees were sprinkler irrigated, and two
treatments were applied:
'Wet' treatment: eight trees were irrigated at
100% actual evapotranspiration (AET) when
half of the calculated readily available soil
water between 0 and 70 em was depleted. AET
was calculated from Brochet-Gerbier equation
(Brochet and Gerbier, 1972), corrected by a
crop coefficient of 0.6 recommended for peach
tree irrigation in the south of France during the
vegetative phase, which was the case during
the course of the present study.
'Dry' treatment: six trees were irrigated on
every other occasion when wet treatment was
applied, with the same amount of water per
irrigation as in the wet treatment. Integrated
over a complete cycle of drying and irrigation
on the dry treatment, this represents 50%
Tree water potential
Xylem water potential was measured using
a pressure chamber (PMS Instrument Co.) on
leaves treated in two ways:
'I'L (leaf water potential) on a sunlit mature
leaf selected in the middle of a shoot, which
required sampling around the tree during the
day. This was necessary because of very large
variations in water potential in different parts
of the tree (Klepper, 1968).
\fls (stem water potential) on a leaf close to
the one selected above, enclosed two hours
prior to measurement in a cellophane bag
covered with aluminium foil. This stops leaf
transpiration and enables water potential in the
xylem of the leaf to come to equilibrium with
the potential in the xylem of the stem at the
point of attachment of the petiole (Begg and
Turner, 1970; Powell, 1974; Berger, 1978; Liu
et at., 1978).
Accepting that water flow in the liquid
phase through any part of the plant conforms
to a relation analogous to Ohm's law (van den
Honert, 1948), we can write:
~'II= 'l's- 'I'L = r.T
where r = the resistance to water flow between
stem and leaf, and T = the transpiration flow
entering the leaf. If we assume that all the
water entering the leaf is transpired (steady
state conditions), then:
T = (cw-ca)/(rs+ra)
where (cw-ca) = the difference between water
vapour concentration at the leaf surface and in
the ambient air, rs = the stomatal resistance of
the leaf, and ra = the boundary layer resistance (Siatyer, 1967). Combining equations 1
and 2:
~\fl = r.(cw-ca)/(rs+ra)
Accepting that r is constant for given values of
(cw-ca) and wind speed, then the greater the
stomatal resistance, the smaller the ~'I' value.
Therefore ~'II depends on transpiration (eq. I)
and provides an image of stomatal opening
(eq. 3).
Though Powell (1974) failed to find clear
quantitative relations between ~\fl and
stomatal resistance in field apple trees, Liu et
a/. (1978) found a relationship with a
hysteresis loop between ~\fl and whole plant
transpiration in field grapevines, and
Yamamoto (1983) found a linear relationship
between sap velocity in the petiole of a pear
tree leaf and a calculated water potential
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gradient involving the use of a three compartments model (leaf, stem and fruit).
Values of ljiL, ljls and ~ljl presented in this
study are the means of three measurements
taken on the same tree for a given exposition
(at a given time). Values obtained during the
course of a day are either from one tree or
from different trees in the same plot. Indeed,
within-plot variability is low enough for these
two ways of sampling to give similar daily
station, I. 7 km south-west of the orchard.
Seasonal course of water potentials
Figure 1 shows seasonal trends of pre-dawn
water potential, minimum daily water potential
for uncovered leaves, maximum daily ~ljl
values, natural rainfall and irrigations between
10 June and 10 September.
Pre-dawn water potential (measured on uncovered leaf before sunrise): These values
should indidate the level of stress at which the
plant begins the day (Ritchie and Hinckley,
197 5). Figure 1 shows that pre-dawn water
potential is constant over a wide range of soil
moisture conditions (between 30 June and 27
July), both spatially (between treatments) and
temporarily (during a drying cycle). Two
differential irrigations had to be applied before
any difference between treatments occurred,
that is, when soil moisture under the unirrigated trees fell below the lower limit of
readily available water (see Figure 3).
Xiloyannis et a!. ( 1980) observed a similar
trend, with little difference between peach trees
irrigated at 50% and 100% AET. These
authors observed differences only with non-
Soil moisture: This was measured once a week{
from August to September with a neutron
probe (Pitman Instruments, Wallingford,
model 225) in five permanent access tubes in
each treatment. The tubes were placed 3, 1.5
and 0.75 m from the trees between rows, and
2.25 and 0.5 m from the trees within a row.
Measurements were made every I 0 em from
10 to 200 em deep. Field capacity was
estimated from a neutron probe profile
obtained 48 h after a 180-mm irrigation which
saturated the entire soil profile in January
Meteorological data on short-wave radiation,
air temperature, dew-point temperature and
wind speed were kindly provided by the MontJune
-0.25 ?: co
'; -2.0
E -2.5
____ i
a: "'"
-0.50 .;, -
c:co c:
Seasonal course of: (A) max1mum daily f-'1' values and (B) pre-dawn water potential (top) and minimum daily leaf water
potential (middle) for 50% AET (e----e) and 100% AET (x--x) treatments: arrows indicate the date from which
values of the two treatments diverge (vertical bars represent standard error (not presented when smaller than the
symbol)): the bottom of the figure shows natural ramfall (--),Irrigation on both treatments(---). and on wet treatment only( .. _.. ).
Water stress in peach trees
irrigated trees. Figure I shows recovery in the
very wet second fortnight of August.
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Minimum daily leaf water potential (uncovered leaf): We could hardly detect any
difference between the two treatments during a
drying cycle. The general trend was a steady
decline over the season (from -1.5 to -2.3
MPa for the dry treatment and to -2.2 MPa
for the wet treatment), before the wet period
where recovery occurs. A similar decline
during the dry season has been reported in
peach trees by Xiloyannis et al. ( I980) and
Proebsting and Middleton (I 980). In the
present study this minimum daily value was
not always reached at the same time of the
~\!f values: After a steady
increase at the beginning of the period (from
0.76 MPa on 28 June to 1.05 MPa on 6 July),
~\jl in the wet treatment (~\!fw) varied little
over the rest of the season, except for 4 August
when it reached a maximum value of 1.12
MPa. In contrast, ~\!f in the dry treatment
(~\!fo) was closely related to the alternation of
drying periods and irrigations. ~\!fo values
decreased as drying periods were imposed:
l.OI MPa on 6 July and 0.74 MPa on I2 July;
0.94 MPa on IS July and 0.65 MPa on 27
July, and increase.d after the irrigations.
Between 27 July and 7 August (no water
applied), ~\!fo values fluctuated around 0. 7
Maximum daily
MPa, which represents 69% of ~\!fw values
over this period.
Transpiration during a drying period is thus
strongly reduced in the dry treatment through
stomatal regulation. These variations in ~\jl
are the consequence of variations in \!fs values
since variations in \!fLare small.
Diurnal course of water potential
Figure 2 shows climatic conditions for 4
August (four days before a 35-mm irrigation)
and II August (three days after this irrigation). A I4-mm rainfall on 10 August should
be added to this irrigation. At that time the wet
treatment had received 8I mm more water
since the beginning of the season than the dry
one. Temperatures on the II th were slightly
higher, with little effect on vapour pressure
deficit (VDP) because of higher dew-point
Figure 3 shows the soil water content for the
furthest and nearest tubes from the tree trunk
in both treatments for these two days.
Humidity variations below 100 em were very
small and are not reported here. This figure
shows that irrigation affects the upper
40--50 em in both treatments. This represents
the main zone exploited by the roots. The
mean soil water contents between 0 and 70 em
(calculated with the five tubes of each treatment) were 82.2% and 6I.8% of field capacity
before irrigation and 92.5% and 80.4% after
FIG. 2
Diurnal course of climatic conditions on 4 August (x--x) and II August (e-----e).
Access tube No. 1
50 (3 m between rows)
.;::; 100/0
g. 10
Access tube No. 9 ~~
(0.50 m within a row)
....... 1
.... ~
............_, 贩贩�......... ;.
�-----. ?贩...
50 Access tube No.4
(0.50 m within a row) ~'
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Access tube No. 6
(3 m between rows)
. -.......,...._
Soil water content (cm 3 cm-3)
FJG. 3
Water content profiles for wet (left) and dry (right) treatments on 4 August (x--x) and II August (e-----).
irrigation for the wet and dry treatments,
respectively. Thus, on 4 August the soil water
content in the dry treatment was between the
lower limit of readily available water (70% of
field capacity) and the permanent wilting point
(55% of field capacity), whereas water was still
readily available in the wet treatment. After
the irrigation, water was readily available in
both treatments.
The leaf water potential ('l'L) of the two treatments followed a similar trend before irrigation (Figure 4A), except for pre-dawn and
early morning values. Recovery at the end of
the day was more rapid in the wet treatment.
Very small differences could be observed
between lOh and 18h30, except at llh when
'l'L becomes Jess negative in the wet treatment,
probably because of stomatal closure.
Irrigation did not affect 'l'L in the wet treatment (Figure 4B), and although 'l'L in the dry
treatment was less negative (0.2 MPa mean)
throughout the day than before irrigation,
values for the two treatments were very close.
The stem water potential ('l's), on the contrary,
was much more sensitive to soil water conditions: on 4 August (Figure 4A) differences
between treatments ranged from 0.3 MPa in
the morning to 0.55 MPa at 15h30, with no
overlap between the values of the two treat-
ments throughout the day. After irrigation
(Figure 4B) no difference between treatments
could be observed; 'l's in the dry treatment
reacted to water supply.
Relationships between 'l'L and ~'I' during the
Figure 5 shows the relationships between 'l'L
and ~'I' during the course of the day on 4 and
11 August. Before irrigation, for any given 'l'L
value, ~'I' values in the dry treatment were
lower than in the wet one, reflecting reduced
transpiration (eq. 1) through stomatal closure
(eq. 3 with the same atmospheric conditions).
These results are comparable to those of
Cohen et al. (1983) who showed that sap flow
velocity in citrus trees was different with the
same 'l'L values, depending on soil moisture
conditions. Figure 5 shows that, after irrigation, only slight differences between treatments could still be seen.
The data have been fitted with linear and
curvilinear relationships. Table I shows that
the curvilinear relationships fit the data slightly�
better both before and after irrigation (correlation coefficient higher in absolute value and
residual sums lower). However, the difference
between the quality of the fits is too small to
conclude definitely about the shape of the
curves. From a physiological point of view the
relationship between water potential and trans- �
Water stress in peach trees
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9 10 11 12 13 14 15 16 17 18
Time of day (T.S.T.)
Diurnal course of water potentials: (A) on 4 August. four
days before a 35 mm irrigation and (B) on II August,
three days after this irrigation. (Bars represent SE.)
1 25
0.75 ~
;.. 0.75
"' 0.50
leaf \il (MPa)
FIG. 5
Relationship between IVL and Llljl during the course of a
day (values are for 4 August-before irrigation, and II
August-after irrigation).
(C I): Llljl = (0.876.exp(-0.378.1j!L))-I; r = 0.982.
(C2): Llljl = (0.824.exp(-0.281.1j!L))-I; r = 0.972.
Dashed lines indicate 95% confidence limits of the curve
for non-limiting soil water conditions (C 1).
Parameters for the linear and curvilinear fits of the relationship between lj!L and ~'V: is the correlation coefficient;
res. sum is the residual (l:(~'V-~~ )2) and n.p. is the number of points
~'V =a. 'VL +b
res. sum.
=a. exp (b. 'Vd-1
~'V =a. 'VL +b
~IV= a. exp (b. 'VL)-1
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For Cl and C2 (soil water conditions) see Fig. 5.
piration can be either linear or curvilinear (see
Hailey et a!., 1973; Berger, 1978). From
now on in this paper, we will consider the
relationships between 'l'L and ~'l' shown in
Figure 5 to be curvilinear.
Let us write eq. I between soil and leaf:
'l'Soil-'l'L = ('l'sor'l's)+('l's-'l'L) =
k.R.T +r.T = (k.R+r).T
R = the resistance to water transport between
soil and stem, and k = the ratio between whole
plant water flow and leaf transpiration. Combining eqs 1 and 4 and rearranging:
~\jl = 'l'Soil�(r/(k.R+r))-'l'dr/(k.R+r)) (5)
The curvilinearity in the relations shown in
Figure 5 suggests that r/(k.R +r) ratio
increases-and then R.k/r decreases-as
transpiration increases. This results in a progressively more rapid increase in transpiration
when 'l'L becomes progressively less negative.
Elfving et a!. ( 1972) also found a decrease in
plant resistance in a citrus tree when transpiration, evaluated at leaf level by VPD/rs
ratio, increased.
This study shows that 'l'L in peach trees is
only slightly affected during a drying period
(Figure 1) and is not very sensitive to irrigation (Figures 4A, 4B). Studies on the effect of
irrigation on leaf water potential ('l'L) in fruit
trees do not show clear tendencies: Xiloyannis
et a/. (1980) found that in peach trees irrigated
at 50% AET, 'l'L and stomatal conductance
were slightly affected compared with trees
irrigated at 100% AET; strong differences.
appeared only with non-irrigated trees.
Acevedo et a!. (1973) did not observe any
clear trend in 'l'L values measured between
14h30 and 15h when peach trees were
subjected to drying periods.
In citrus trees Cohen et a!. (1983) showed
that 'l'L was influenced when soil water
potential below 0.45 m deep began to
decrease, and Levy (1983) found contradictory results when trees were moderately or
severely stressed. Jones et al. (1983) in apple
trees observed a rapid decline in 'l'L at the
beginning of the drying treatment and
recovery of these 'l'L values when drying treatment was carried on, which was explained by
stomatal closure. Castel and Fereres (1982)
observed an immediate and sharp decrease in
'l'L in almond trees as soon as dry treatment
started. For a clear interpretation of these
results, the rooting pattern of the trees should
be known, which was not always the case in
these studies. Evidently 'l'L in trees with
shallow root systems will be affected earlier
during a drying cycle than in trees with deeper
root systems: for example, a tree with a deep
tap-root reaching a water table can be completely insensitive to any moisture variation in
the upper soil layers.
Furthermore, 'l'L cannot be taken as a
sensitive indicator of tree water stress since
this parameter can take the same values with
different stomatal apertures (Bates and Hall,
1981; Jones, 1983 and the present study,
Figure 5), reflecting different degrees of stress.
The method employed in this study provides
a simple means to complete and clarify the
interpretation of these 'l'L values. It implies
only the use of the pressure chamber, which is
simple enough a device to be used for agronomic purpose, and allows the determination
(a) 'l's values, which seem sensitive to
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Water stress in peach trees
irrigation, and thus to soil water potent_ial.
Stomatal closure acts as a regulator of "'L? but
\jls appears to be more dependent on soil water
(b) ~"' values, directly dependent on leaf
transpiration (Liu et a/., 1978; Yamamoto,
1983). Figures 4A, 4B and 5 suggests that
transpiration is reduced in the dry treatment
even for high \j/L values, as soon as the sun
rises. This means that stomatal response is not
directly controlled by variations in leaf water
potential as defined in this study. Indeed,
stomata may be sensitive to water potential (or
its components) in the epidermal cells (Sheriff,
1977; Losch, 1979) rather than to water
potential in the xylem of the leaf. Another
hypothesis has been proposed (e.g. Bates and
Hall, 1981) which supposes that soil water
depletion induces root water stress which, in
turn, produces a change in the information
transmitted from roots to shoot; stomata
would be directly sensitive to this change,
without water potential mediation.
The validity of eq. 3 will be tested directly in
a further study by the simultaneous measurements of leaf and air temperatures, wind speed
and stomatal resistance.
An analysis of covariance (Dunn and Clark,
1974) shows that a single regression curve can
be used with a risk of 1% to fit the relation
between \j/L and ~"' (C 1 on Figure 5) for the
three following sets of data: 100% AET
regime before and after irrigation and 50%
AET after irrigation (calculated Snedecor test
F (.99,2,22) = 4.86 which is lower than 5.86,
the highest value to accept the hypothesis of a
single regression curve). This means that the
period of drought imposed on the trees of the
50% AET regime did not alter these plants as
physical systems for water movement, and
thus the two sets of trees remain comparable.
This has also been shown by Cohen et a/.
(1983) on citrus trees: a 44-day period of
drought did not modify the relationship
between sap velocity and \jlu after re-irrigation
in the dry treatment.
The use of Figure 5 provides an aid for
irrigation, as proposed by Elfving eta/. (1972).
It is possible to define a relation between \j/L
and ~"' for non-limiting soil water conditions
(C 1) with a certain confidence interval (dashed
lines in Figure 5). On any day, measurements
of \jls and \j/L at a given time permit one to
place the point (\j/L; ~\jl) on the figure. If this
point is under the predefined confidence
interval, we can consider that the tree is under
a certain degree of stress. Whether irrigation
must be started depends on the determination
of threshold values of these parameters beyond
which production can be affected.
Finally, extreme care must be taken in the
interpretation and use of leaf water potential as
a physiological indicator for irrigation. However, the simple method of measurement of
stem water potential propose~ in this study
provides a more sensitive indicator of water
stress, at least in peach trees. Threshold values
of this parameter (and consequently threshold
~\jl values) must now be defined and further
testing of the method is required for proper
timing of irrigation.
The authors thank M J. Godefroy for his
constant support during this work, M J.
Hugard for facilities to work in the
E.N.S.A.M. experimental orchard, M J. C.
Salles and M P. Oullier for valuable help in the
field, MS. Rambal and M J.P. Luc for advice
on soil moisture measurements, and M J. L.
Salager and M X. Perrier for statistical treatment. M J. Roy and M G. Heim provided
helpful comments on the manuscript.
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(Accepted 13 July 1984)
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