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Summary The combined effects of irrigation rate and crop
load on apple yield and fruit size were examined in two com-
mercial apple orchards (cv. Golden Delicious) in a semi-arid
zone. The irrigation rates applied were 1, 3 and 7 mm day–1,
and the two fruit thinning treatments involved adjusting crop
load to 100 and 300 fruits per tree at Ortal and 50 and 150 fruits
per tree at Matityahu. Unthinned trees served as the control.
The fruit from each tree was picked separately, and fruit size
distribution was determined with a commercial grading ma-
chine. Midday stem water potentials varied from –0.9 to
–2.8 MPa, crop load varied from 80,000 to 1,900,000 fruit ha–1
and crop yield varied from 10 to 144 Mg ha–1. Midday stem
water potential decreased with increasing crop load in all irri-
gation treatments at Matityahu, but only in the 1 mm day–1
treatment at Ortal. The extent of the lowering of midday stem
water potential by crop load decreased with increasing soil wa-
ter availability. At both orchards, a similar response of total
crop yield to crop load on a per hectare basis was observed.
Mean fruit mass and relative yield of fruit > 70 mm in diameter
increased with midday stem water potential, with the low crop
loads having similar but steeper slopes than the high crop load.
The responses of mean fruit mass and relative yield of fruit
> 70 mm in diameter to midday stem water potential were simi-
lar at both orchards, perhaps indicating that thresholds for irri-
gation scheduling are transferable to other orchards within a re-
gion. Factors that may limit the transferability of these thresh-
olds are discussed.
Keywords: irrigation, Malus domestica, stem water potential,
water stress indicators.
Introduction
Dryland orchards can survive and be productive in temperate
zones without irrigation, whereas the survival of deciduous or-
chards in semi-arid zones depends on the availability of water
for irrigation throughout most of the growing season. World-
wide, the amount of fresh water available for agricultural use is
decreasing, so there is a need to increase water-use efficiency.
This goal may be achieved either by improving genetic perfor-
mance and horticultural practices, or by improving irrigation
scheduling.
Modern irrigation scheduling of deciduous orchards is
based on sets of crop coefficients derived from reference crop
evapotranspiration (Allen et al. 1998). However, published
crop coefficients may require adjustment to suit actual condi-
tions because water use in commercial orchards varies with
numerous combinations of many factors. These factors in-
clude cultivar (Robinson and Lakso 1991), rootstock ( Robin-
son and Lakso 1991, Wünsche et al. 1995, Giuliani et al.
1998), training system (Palmer 1993) and row spacing, which
affects light interception on a per hectare basis, the number of
fruit per tree and potential fruit size that, in turn, determine the
crop demand for assimilates. In addition, application effi-
ciency in commercial orchards is always lower than 1 and
there is no straightforward procedure to make site-specific es-
timates of it. The diversity in the above mentioned factors
among plots within an orchard creates considerable uncer-
tainty about optimal irrigation scheduling. Growers may over-
come most of the uncertainty in irrigation scheduling by using
assessments of tree or soil water status to adjust the irrigation
rate once it exceeds a certain threshold. However, adjusting ir-
rigation rate based on water stress assessment is not straight-
forward.
Soil water stress indicators have been proposed as a basis for
evaluating the ability of the soil to meet the peak demand for
water by the tree. Determination of soil water stress is not an
easy task because it involves the integration of soil water char-
acteristics and hydraulic properties in conjunction with the
distribution of roots (sinks) and evaporative demand. More-
over, soil water content is not uniform, even within the root
zone of a single tree. Soil water stress indicators were reported
to have greater variability than maximum daily trunk shrink-
age and midday stem water potential (Naor et al. 1995, 1999,
2000, Goldhamer and Fereres 2001, Intrigliolo and Castel
2004, Naor et al. 2006) because the tree responds to the mean
soil water availability. Thus, measuring tree water status
avoids the need to deal with the variability within the root zone
(Naor 2006). Both midday stem water potential and maximum
daily trunk shrinkage, the most popular proposed plant water
stress indicators, provide an indication of peak water stress but
their correlation with integrated daily canopy conductance and
assimilation rate may vary with climatic conditions and crop
load, and may change during the growing season (e.g., Lakso
1979, Möller et al. 2007) because of osmotic adjustment.
Recent studies indicate that midday stem water potential is a
Tree Physiology 28, 1255–1261
© 2008 Heron Publishing—Victoria, Canada
Responses of apple fruit size to tree water status and crop load
A. NAOR,1,2 S. NASCHITZ,1M. PERES3and Y. GAL3
1Golan Research Institute, P.O. Box 97, Kazrin 12900, Israel
2Corresponding author (amosnaor@research.haifa.ac.il)
3Extension Service, Ministry of Agriculture and Rural Development, Kiryat Shmona 12100, Israel
Received January 31, 2008; accepted March 31 2008; published online June 2, 2008
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relevant and reliable water stress indicator in fruit trees, but its
use is labor intensive and measurements are limited to about
2 h around midday (see review, Naor 2006). Daily trunk
shrinkage, on the other hand, is highly responsive to water
availability (Goldhamer et al. 1999, 2000, Goldhamer and
Fereres 2001, Fereres and Goldhamer 2003, Naor and Cohen
2003, Intrigliolo and Castel 2004), easy to use and yields an
analog output. However, maximum daily trunk shrinkage is
more dependent on evaporative demand than midday stem wa-
ter potential. Setting thresholds of maximum daily trunk
shrinkage for irrigation scheduling is problematic; however,
calibration of maximum daily trunk shrinkage against midday
stem water potential may provide thresholds for the use of
maximum daily trunk shrinkage for irrigation scheduling
(Naor 2006).
Setting thresholds of midday stem water potential for irriga-
tion scheduling is empirical and the question of the scale and
conditions at which thresholds are transferable among or-
chards has not been examined. The objectives of our study
were (1) to determine the responses of crop yield and fruit size
of apples to combined manipulations of irrigation and crop
load at two distant commercial orchards, and (2) to examine
the possibility of transferring thresholds of midday stem water
potential among apple orchards within a region.
Materials and methods
Experimental site
Two experiments were conducted in the northern part of Israel
during the growing season of 2006, one in the Golan Heights
(Ortal) and the other in the upper Galilee (Matityahu experi-
mental station). Both orchards are situated in a semi-arid zone
where no precipitation occurs during the summer. Reference
crop evapotranspiration was calculated from weather station
data located at 4000 m and 250 m from the experimental sites
at Ortal and Matityahu, respectively. Mean midsummer evapo-
transpiration was 6.9 and 6.6 mm day–1 at Ortal and
Matityahu, respectively. Precipitation in the 2005–2006 win-
ter was 860 and 545 mm at Ortal and Matityahu, respectively.
Experimental orchard
Plant materials were 15-year-old ‘Smoothie’ (a ‘Golden deli-
cious’ strain) apples (Malus ×domestica Borkh.) on the local
Hashabi rootstock at Ortal and 10-year-old ‘Golden delicious’
on M9 rootstock at Matityahu. Planting density was 4.5 × 2.5 m
and 3.5 × 1.5 m at Ortal and Matityahu, respectively with a
north–south row orientation in both orchards. To minimize
water percolation below the root zone, the irrigation system
consisted of three laterals of 1.6 l h–1 drippers spaced 0.5 m
apart, providing irrigation rates of 1.6 and 2.1 mm h–1 at Ortal
and Matityahu, respectively. The daily irrigation amounts
were delivered in 1-mm pulses.
Treatments
Two factors were examined, irrigation rate and crop load. The
trees were not irrigated during the cell division stage (up to the
beginning of June) and three irrigation rates were applied
thereafter, 1, 3 and 7 mm day–1. At the start of irrigation treat-
ments, the trees were hand thinned to three crop loads ~100,
~300 fruit per tree and unthinned control at Ortal and ~50,
~100 fruit per tree and unthinned control at Matityahu.
The experimental design was a split plot with irrigation as
the main plot and crop load as the sub-plot. Each main plot
comprised six measurement trees (two for each crop load) that
were surrounded by border trees and rows. Treatments at each
orchard were replicated three times.
Measurements
Midday stem water potential was measured with a pressure
chamber (Ari-Mad, Kfar Charuv, Israel or PMS, Corvallis
OR) on shaded leaves from the inner part of the canopy that
were inserted (while intact) into a plastic bag covered by alu-
minum foil for 90 min before measurements were taken. Mea-
surements were made weekly in the medium crop load treat-
ments and every 2 weeks in the low and high (unthinned) crop
loads. Six leaves were measured for each irrigation × crop load
combination. Additional midday stem water potential mea-
surements were made on July 31 at Ortal on six trees of each ir-
rigation × crop load combination (total of 54 trees). Two leaves
were measured on each tree. Two pressure chambers were
used simultaneously to shorten the measurement period. Mea-
surements started at noon and were completed within 90 min.
Harvest
The fruits were picked on September 6, 2006 at Ortal and on
September 13, 2006 at Matityahu. The fruit from each tree was
picked separately and fruit size distribution was determined
with a commercial sorting machine (Greefa, Tricht, Nether-
lands).
Results
Irrigation coefficients varied among treatments (Figure 1).
1256 NAOR, NASCHITZ, PERES AND GAL
TREE PHYSIOLOGY VOLUME 28, 2008
Figure 1. Weekly means of daily irrigation rates (fraction of evapo-
transpiration) at Ortal (filled symbols) and Matityahu (open symbols)
in 2006 in the three irrigation regimes: 1 mm day–1 (䉱,䉭); 3 mm
day–1 (䊉,䊊); and 7 mm day–1 (䊏,䊐).
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Mean coefficients were 0.15, 0.43 and 1.0 in the 1, 3 and 7 mm
day–1 treatments, respectively, at Ortal and 0.15, 0.45 and 0.98
in the 1, 3 and 7 mm day–1 treatments, respectively, at Mati-
tyahu. Cumulative irrigation rates up to harvest were similar at
both orchards, with 101, 285 and 665 mm applied in the 1, 3
and 7 mm day–1 treatments, respectively, at Ortal and the cor-
responding values at Matityahu were 80, 292 and 661 mm.
At both orchards, midday stem water potential decreased
with decreasing irrigation rate (Figure 2). Midday stem water
potential in trees at Ortal decreased with increasing crop load
in the 1 mm day–1 treatment, decreased slightly with crop load
in the 3 mm day–1 treatment and was unaffected by crop load
in the 7 mm day–1 treatment (Figure 3). In contrast, midday
stem water potential in trees at Matityahu decreased with in-
creasing crop load at all irrigation rates, with the extent of the
decrease lessening with increasing irrigation rate (Figure 3).
Fruit number per hectare was higher at Matityahu than at
Ortal (Table 1). In each orchard, fruit number per hectare was
similar in the three irrigation treatments in trees carrying low
and medium crop loads (thinned), whereas crop load varied
with irrigation treatment in the control (unthinned) trees. Total
crop yield increased with increasing irrigation rate and with
increasing crop load in both orchards (Table 1), with the
higher crop yields at Matityahu than at Ortal reflecting the
higher fruit number per hectare. In both orchards, mean fruit
mass decreased with decreasing irrigation rate and with in-
creasing crop load (Table 2). In general, trees at Ortal had
higher fruit mass than trees at Matityahu, except for trees in the
medium crop load × 1 mm day–1 irrigation treatment which
had lower fruit mass. Fruit mass of trees in the high crop load ×
3mmday–1 irrigation treatment was much lower at Matityahu
than at Ortal (Table 2).
Fruit size distribution shifted to smaller fruits with increas-
ing crop load in the 1 mm day–1 treatment, and the shift was
more pronounced at Matityahu than at Ortal (Figure 4, Ta-
ble 2). In the higher irrigation treatments, fruit size distribution
was similar in the low and medium crop loads and was shifted
to smaller fruits only in trees bearing a high crop load. Total
crop yield at Ortal and Matityahu responded similarly to crop
load (Figure 5, Table 1) except for two cases. First, trees at
Matityahu bearing a high crop load in the 3 mm day–1 treat-
ment had similar crop yield as trees at Ortal despite the large
difference in crop loads (Table 1). Second, trees at Ortal bear-
ing a medium crop load in the 1 mm day–1 treatment had lower
crop yield than trees at Matityahu despite their similar crop
loads (Table 1).
Mean fruit mass was highly correlated with midday stem
water potential in midsummer (Figure 6). Similar correlations
were apparent at both orchards with trees bearing low and me-
dium crop loads having similar responses, whereas the un-
thinned trees bearing a high crop load had lower mean fruit
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APPLE RESPONSE TO TREE WATER STATUS AND CROP LOAD 1257
Figure 2. Effects of irrigation rate and crop load on midday stem water
potentials in Malus domestica trees at Ortal and Matityahu in 2006.
The irrigation regimes were: 1 mm day –1(䊉,䊊); 3 mm day –1(䉱,䉭);
and 7 mm day –1 (䊏,䊐). The crop loads were: low, medium and high
denoted by open symbols connected by lines and filled symbols, re-
spectively. Bars denote standard error.
Figure 3. Effects of irrigation rate and crop load on mean midday stem
water potentials in Malus domestica trees up to harvest at Ortal (black
bars) and Matityahu (gray bars). Numbers on the x-axis are irrigation
rates (mm day–1) and letters denote crop loads (low, L; medium, M;
and high, H). Bars denote standard error.
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mass for each water potential value (Figure 6). The difference
in fruit mass between trees bearing a high crop load and trees
bearing the lower crop loads increased with increasing midday
stem water potential. At both orchards, the relative yield of
fruit larger than 70 mm in diameter was highly correlated with
midsummer midday stem water potential (Figure 7). Relative
yields of trees bearing the two lower crop loads responded
similarly to midday stem water potential, whereas the un-
thinned trees with the higher crop loads had lower relative
yields at each stem water potential (Figure 7).
Discussion
It is well known that fruiting deciduous orchards have higher
stomatal conductance, and thus higher transpiration rates, than
de-fruited trees (Hansen 1971, Fuji and Kennedy 1985,
DeJong 1986, Erf and Proctor 1989, Gucci et al. 1991, Wibbe
and Blanke 1995, Giuliani et al. 1997, Wünsche et al. 2000,
Marsal et al. 2005). As confirmed in Figures 2 and 3, lower
midday stem water potentials are expected at high crop loads
(Berman and DeJong 1996, Naor et al. 1997) because of the
higher transpiration and the high resistance to water flow from
the soil to the trunk xylem.
In our study, midday stemwater potential decreased with in-
creasing crop load but the response varied between orchards.
In both orchards, crop load affected midday stem water poten-
tial in the 1 mm day–1 treatment; however, at Ortal, unlike
Matityahu, no effect was apparent in trees in the 3 and 7 mm
day–1 treatments (Figure 3). In another study in the Golan
Heights, Israel where the crop load was similar to that at
Matityahu, midday stem water potential responded to crop
load at an irrigation rate of ~3 mm day –1 (Naor et al. 1997).
Higher hydraulic resistance in the M9 rootstock (Cohen et al.
2007) and the higher crop load at Matityahu may explain
the effect of crop load on midday stem water potential at
Matityahu in the 3 and 7 mm day–1 treatments. Palmer et al.
(1997) and Wünsche et al. (2000) reported an upper limit in
crop load beyond which no further increase in stomatal con-
ductance occurred in response to increased crop load, contrast-
ing with our observed stem water potential response to in-
creasing crop loads.
Midday stem water potential may decrease as a result of soil
water depletion in response to long-term higher transpiration
rates in heavily cropping trees. Under such conditions, in-
creasing crop load is expected to result in a greater decrease in
midday stem water potentials at low irrigation rates than at
high irrigation rates. The difference in midday stem water po-
tentials between the high and low crop loads at Matityahu was
–0.59, –0.52 and – 0.2 MPa in the 1, 3 and 7 mm day –1 irriga-
tion treatments, indicating that water availability plays a role
in the response of midday stem water potential to crop load.
The difference in midday stem water potential between trees
bearing high and low crop loads at Ortal was –0.48, –0.09 and
0.01 in the 1, 3 and 7 mm day–1 irrigation treatments. Similar
responses of stem water potential to crop load were apparent in
both orchards in the 1 mm day–1 treatment; however, unlike
the Matityahu trees, the Ortal trees showed practically no re-
sponse at the two highest irrigation treatments. It may be that
differences in water application efficiency between the or-
chards and therefore differences in water availability ac-
counted for the different responses of midday stem water po-
tential to crop load.
The low crop load × high irrigation rate treatment (~20 Mg
ha–1) represents non-limiting conditions where potential fruit
size is probably achieved. Fruit mass of trees in the high crop
1258 NAOR, NASCHITZ, PERES AND GAL
TREE PHYSIOLOGY VOLUME 28, 2008
Table 2. Mean fruit mass (g; ± standard error) of Malus domestica
trees at Ortal and Matityahu in 2006 in response to irrigation (1, 3 and
7 mm day–1) and fruit thinning treatments.
Irrigation up to Low Medium High
harvest (mm)
Ortal
101 116.7 (7.9) 67.4 (9.5) 51.5 (4.6)
285 171.3 (5.7) 161.8 (4.7) 98.6 (2.6)
665 204.6 (4.5) 197.3 (2.6) 122.5 (1.6)
Matityahu
80 116.2 (5.1) 101.3 (6.8) 37.8 (1.1)
292 152.1 (8.6) 132.2 (5.2) 59.4 (3.1)
661 188.8 (7.2) 169.3 (3.2) 115.8 (3.5)
Table 1. Mean (± standard error) crop load and total crop yield of Malus domestica trees at Ortal and Matityahu in 2006 in response to irrigation
(1, 3 and 7 mm day–1) and fruit thinning treatments.
Irrigation up to Fruit per hectare/ 1000 Total crop yield (Mg ha–1)
harvest (mm) Low Medium High Low Medium High
Ortal
101 87.3 (7.4) 275.5 (32.3) 913.5 (59.2) 10.1 (0.9) 17.4 (0.9) 43.0 (3.0)
285 98.8 (3.2) 276.7 (5.7) 1075 (52.8) 16.8 (0.18) 44.8 (1.7) 106.3 (6.8)
665 84.7 (1.6) 255.1 (4.5) 1177 (53.7) 17.3 (0.42) 50.3 (1.5) 144.4 (7.2)
Matityahu
80 114.3 (7.7) 282.9 (11.0) 1295 (120.4) 13.1 (0.7) 28.4 (1.7) 49.2 (5.4)
292 111.1 (7.4) 304.8 (12.7) 1630 (79.6) 16.8 (1.3) 40.3 (2.5) 96.6 (6.7)
661 117.1 (7.7) 310.5 (18.4) 1224 (121) 22.1 (1.8) 52.5 (3.0) 140.1 (10.9)
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load × low irrigation rate treatment was 38 and 52 g at harvest
at Matityahu and Ortal, respectively, and it was ~22% of the
potential fruit size, indicating a severe limitation of assimilate
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APPLE RESPONSE TO TREE WATER STATUS AND CROP LOAD 1259
Figure 4. Effects of irrigation rate and
crop load (low, 䊉; medium, 䊊; and high,
䉲) on fruit size distribution in Malus
domestica trees in 2006 at Ortal and
Matityahu. Bars denote standard error.
Figure 5. Effects of crop load on total crop yield of Malus domestica
trees in 2006 at Ortal (filed symbols) and Matityahu (open symbols)
subjected to three daily irrigation regimes (1 mm day–1 (䊉,䊊); 3 mm
day–1 (䉲,䉮); and 7 mm day–1 (䊏,䊐)) and three crop loads. Each
value represents a single tree.
Figure 6. Effects of midsummer midday stem water potential of Malus
domestica trees at Ortal (July 31, 2006; open symbols) and Matityahu
(August 15, 2006; filled symbols) bearing one of the three crop loads
(Low, L; medium, M; and high, H ) on mean fruit mass at harvest.
Symbols: L and M (䊉,䊊); and H (䉱,䉭).
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availability to the crop. In this treatment, midday stem water
potentials were lower than –2.5 MPa, a water status where
stomata are expected to be closed (Naor 1998). Both crop load
and tree water status determine fruit size (Table 2), but neither
alone is able to predict fruit size (Figures 5 and 6). In contrast,
starch content in the perennial stem predicted mean fruit mass,
independently of whether source capacity (irrigation rate) or
sink capacity (crop load) was manipulated (Naschitz et al., un-
published observations). These findings may indicate that as-
similate availability, and not changes in fruit turgor potential
with irrigation or any hormonal effect associated with varia-
tions in crop load, is the predominant mechanism through
which irrigation and fruit thinning affect fruit size.
In general, total crop yield in both Ortal and Matityahu trees
responded similarly to crop load (Figure 4, Tables 1 and 2)
with two exceptions: similar crop yields in the Ortal and
Matityahu trees in the high crop load × 3 mm day–1 treatment
despite large difference in crop loads (Figure 4); and higher
crop yield in Matityahu trees than in Ortal trees at a similar
crop load (Figure 4, ~250,000 fruit ha –1) in the 1 mm day–1 ir-
rigation treatment (Figure 4, Tables 1 and 2). These exceptions
might be explained by differences in tree water status—the
fruit mass of Ortal trees bearing a high crop load in the 3 mm
day–1 treatment was almost double that of the fruit mass of
Matityahu trees—given that midday stem water potentials
were –1.44 and –2.16 MPa in the Ortal and Matityahu trees, re-
spectively. Fruit mass of trees in medium crop load × 1 mm
day–1 treatment at Matityahu was 25% more than that at Ortal,
and midday stem water potentials were –2.71 and –2.49 MPa
in the Ortal and Matityahu trees, respectively.
Thus, the discrepancies in the response of crop yield to crop
load between orchards can be explained by tree water status,
and this is reflected in the high correlations between midday
stem water potential and mean fruit mass within each crop
load (Figure 6) and with relative yield of fruit > 70 mm in di-
ameter (Figure 7). These results indicate the importance of
tree water status in explaining the variability in crop loads and
the importance of water availability, and thus provide a means
for adjusting tree water status in orchards with variable crop
loads and water availability. These findings apply to crop loads
in the low and medium ranges (Figure 6) which cover the
common commercial apple crop yields.
Similar responses of fruit size to midday stem water poten-
tial at Ortal and Matityahu (Figures 6 and 7) were apparent de-
spite differences in rootstock, tree size, tree age, topographical
situation and row spacing between orchards, suggesting that
thresholds of midday stem water potential are transferable
among orchards at least on a within-region basis. We conclude
that transferability of thresholds of midday stem water poten-
tial is justified once the relationships between stem water po-
tential and assimilation rate are similar, because the availabil-
ity of assimilates is the predominant mechanism through
which fruit size is affected by both irrigation and crop load
(Naschitz et al., unpublished observations).
What may limit the transferability of thresholds? The rela-
tionships between water potential and stomatal conductance
change during the season (Lakso 1979, Moller et al. 2007),
probably through osmotic adjustment. Climatic conditions
may affect the degree of osmotic adjustment and therefore the
relationships between water potential and assimilation rate.
Different air temperatures and therefore respiration rates or
different solar irradiances (clouds) or day lengths (latitudes)
may affect net assimilation rate at similar tree water status.
The maximum midday stem water potential that we mea-
sured (~0.9 MPa) is close to the maximal expected value con-
sidering the evaporative demand in the region (McCutchan
and Shackel 1992), and may indicate that any deviation from
minimum water stress may decrease fruit size (Figures 6
and 7). In addition, the demand for assimilates at high crop
loads is beyond the maximum assimilate production rate under
conditions of minimum water stress and therefore potential
fruit size is not reached even at high irrigation rates. Our data
suggest that when growers are forced to use deficit irrigation
they could apply fruit thinning to minimize the reduction in
fruit size (Figures 6 and 7, Tables 1 and 2). The response
curves of mean fruit mass (Figure 6) and relative yield of
> 70 mm in diameter (Figure 7) to midday stem water poten-
tial provide growers with information on the expected com-
mercial crop yield for any given water status. It should be
noted, however, that these responses are not universal and may
change from one year to another because of differences in po-
tential fruit size resulting from variable temperature regimes
during the cell division stage (Warrington et al. 1999).
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1260 NAOR, NASCHITZ, PERES AND GAL
TREE PHYSIOLOGY VOLUME 28, 2008
Figure 7. Effects of midsummer midday stem water potential of
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