Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol85.Issue3.Agust2000:
Scientia Horticulturae 85 (2000) 201±215
Apricot tree response to withholding irrigation at
different phenological periods
A. Torrecillasa,b, R. Domingob, R. Galegoc, M.C. Ruiz-SaÂncheza,*
a
Dpto. Riego y Salinidad, Centro de EdafologõÂa y BiologõÂa Aplicada del Segura (CSIC),
PO Box 4195, E-30080 Murcia, Spain
b
Dpto. IngenierõÂa de la ProduccioÂn Agraria, Universidad PoliteÂcnica de Cartagena (UPCT),
Cartagena, Murcia, Spain
c
Instituto de Investigaciones en Riego y Drenaje (IIRD), La Habana, Cuba
Accepted 11 November 1999
Abstract
Drip-irrigated BuÂlida apricot trees (Prunus armeniaca L.) on Real Fino apricot rootstock were
submitted, for 4 consecutive years, to water stress by withholding irrigation at different
phenological periods: during the period of ¯owering-fruit set which lasted around 1 month (T-1
treatment); during stages I II of fruit growth (including the initial exponential phase and the lag
phase of the double-sigmoid curve), which lasted around 2 months (T-2 treatment); during stage III
of fruit growth (second exponential phase) lasting around 1 month (T-3 treatment); immediately
after harvest for one and a half months (T-4 treatment); and for 2 months during late postharvest,
immediately following the T-4 treatment (T-5 treatment). These stress treatments were compared
with a control treatment (T-0), irrigated throughout the year and receiving an amount of water
equivalent to 100% of the crop evapotranspiration (ETc) demand. The greatest reduction in
volumetric soil water content, leaf water potential and leaf conductance with respect to the control
values was observed in plants from T-4 and T-5 treatments. A clear distinction could be made
between the main periods of shoot and fruit growth in apricot trees, which may be considered an
advantageous characteristic for the application of de®cit irrigation. Trunk circumference growth and
canopy shaded area were unaffected by irrigation treatments. Stressed fruits from the T-2 treatment
had a lower diameter during the water stress period, although they showed a compensatory growth
rate after irrigation, reaching a similar size to fruits from the control treatment at harvest. Two
critical periods for withholding irrigation were found. The ®rst corresponded to the second rapid
fruit growth period (T-3 treatment), in which the water stress induced a reduction in yield due to a
*
Corresponding author. Tel.: 34-968-215717; fax: 34-968-266613.
E-mail address: [email protected] (M.C. Ruiz-SaÂnchez)
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 4 6 - 6
202
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
smaller fruit size at harvest. In addition, fruits from this treatment ripened earlier. The second
critical period was immediately postharvest (T-4 treatment), in which water stress induced a
signi®cant decrease in fruit yield the following year, due to an increase in young fruit drop which
lead to a lower ®nal fruit set. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Fruit growth; Fruit quality; Fruit set; Prunus armeniaca; Vegetative growth; Water
stress
1. Introduction
Apricot (Prunus armeniaca L.) is widely cultivated in Mediterranean countries,
and the Murcia region is Spain's leading apricot producer (Burgos et al., 1993).
Future prospects for this crop are very favourable. Although apricot is considered
a drought-resistant crop and exhibits some xeromorphic characteristics, such as
the ability to endure water stress in the dry season and the loss of leaves in winter
(Torrecillas et al., 1999), commercial apricot production depends on irrigation.
Water shortage is the main feature of agriculture on the Mediterranean coast.
For this reason, in this area the optimisation of the use and ef®ciency of irrigation
by means of de®cit irrigation strategies that permit maximum yield whilst
reducing water application is of great importance. In this sense, regulated de®cit
irrigation (RDI) may offer an approach to saving water in some woody crops by
minimising or eliminating negative impacts on yield and crop revenue (Chalmers
et al., 1981; Domingo et al., 1996; Goldhamer, 1997).
In elaborating RDI strategies the key is to time the imposition of the stress to
tolerant periods in which yield and fruit quality are not adversely affected
(SaÂnchez-Blanco and Torrecillas, 1995). The effects of water stress depend on the
timing, duration and magnitude of the de®cits (Bradford and Hsiao, 1982). Some
authors indicated that ¯owering in fruit trees depends on the severity of the
postharvest water stress suffered by the plants (Ruggiero, 1986; Larson et al.,
1988; Proebsting et al., 1989). Also, Domingo et al. (1996) indicated the
importance of adequate irrigation management during the rapid fruit growth stage
of lemon in order to obtain marketable fruit size.
For these reasons the aim of this paper was to evaluate the effect of drought
stress on mature BuÂlida apricot trees, by withholding irrigation, during different
phenological periods (¯owering-fruit set, stages I II of fruit growth, stage III of
fruit growth and two consecutive postharvest periods). Soil and plant water
relations, fruit set, vegetative and fruit growth, yield and fruit quality were
measured. In this manner, it is hoped that a knowledge of the ``critical'' periods
for soil moisture stress, in which yield and/or fruit quality are adversely affected,
will contribute to irrigation management under restricted water supply conditions,
improving, thus, the elaboration of RDI strategies in apricot trees.
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
203
2. Materials and methods
2.1. Plant material and experimental site
The study was performed during 1994±1997 in a commercial orchard, located
in Mula, Murcia, SE Spain, with a clay loam texture soil. Volumetric water
content at ®eld capacity was 26% and at wilting point 13%. Plant material was 9
year-old apricot trees (Prunus armeniaca L.), cv. BuÂlida, on Real Fino apricot
rootstock. The trees were spaced 8 8 m apart and drip irrigated by a lateral per
tree row and seven emitters per tree, each with a ¯ow rate of 4 l hÿ1.
During the experimental period, the climate was typically Mediterranean, with
a maximum mean August temperature of 35.88C in 1994 and a minimum mean
January temperature of 3.98C in 1995 (data not shown). The mean daily
evaporation rate from a US Weather Bureau class A pan (on bare soil and located
on a weather station in the orchard) ranged from 1 mm per day in December±
January to 7.5 mm per day in July (Fig. 1). The annual evaporation for the
experimental period averaged 1457 mm, with only minor year-to-year deviations
from these values, while annual rainfall varied from year to year, with an average
of 282 mm, the rainiest year being 1997 with 477 mm. The usual rainy period in
this area occurs during spring and autumn (Fig. 1).
Trees were fertilised with 158 kg N, 769 kg P2O5 and 110 kg K2O, per ha and
year. A routine pesticide program was maintained.
2.2. Experimental design and irrigation treatments
The experimental design was a randomised block with three blocks. Each block
consisted of six trees. The centre four trees were used for experimental
measurements, and the others served as buffers.
Six treatments were considered: a control treatment (T-0) receiving an amount
of water equivalent to 100% of seasonal ETc demand throughout the year, in
which the amount of irrigation water to apply was determined weekly according
to the previous week's pan evaporation and rainfall and the crop coef®cients (Kc)
of Abrisqueta (unpublished data). Also, ®ve water stress treatments were applied,
in which irrigation was withheld during different phenological periods (Table 1),
while the trees were irrigated in the same way as the control treatment during the
rest of the year: T-1 treatment, during the period of ¯owering-fruit set (lasted 1
month, approximately); T-2 treatment, during stage I II of the fruit growth
(including the initial exponential phase and the lag phase of the double-sigmoid
growth curve, and lasted 2 months, approximately); T-3 treatment, during stage
III of the fruit growth (second exponential phase, and lasted 1 month,
approximately); T-4 treatment, during initial postharvest (one and a half months
immediately after harvest); T-5 treatment, during late postharvest (started when
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A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Fig. 1. Mean daily class A pan evaporation (*) and daily rainfall (bars) during the experimental
period (1994±1997).
Table 1
Description of the water stress treatments with starting and ®nishing dates of the irrigation
withholding periods during the experimental period (1994±1997)
Treatment
T-1
T-2
T-3
T-4
T-5
Phenological stage of plant
Flowering-fruit set
Stages I II of fruit growth
Stage III of fruit growth
Early postharvest
Late postharvest
Dates of irrigation withholding
Start
End
Early February
Early March
Early May
Early June
Mid-July
Early March
Early May
Early June
Mid-July
Mid-September
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
205
T-4 treatment ®nished and lasted 2 months). At the end of each stress period drip
irrigation was run continuously for 8±16 h during a period of 4±7 days
(depending on the water stress treatment), to bring the entire soil volume to ®eld
capacity.
2.3. Measurements
Volumetric soil water content (yv) was determined using a neutron probe
(Troxler mod 4300) that had been calibrated for the site previously. One 1.4 m
access tube was located in each block in the wetted area of the dripline, 2 m from
the tree trunk. Soil moisture was determined frequently at 10 cm intervals from
20 to 140 cm. Soil moisture at 10 cm was determined gravimetrically.
Pre-dawn and mid-day leaf water potentials (Cpd and Cmd) were measured
biweekly, on 16 mature leaves located on the south facing side, from the middle
third of the tree for each treatment (four leaves per tree), with a pressure chamber
(Soil Moisture Equip. Corp, model 3000), following the recommendations of
Turner (1988). Leaves were enclosed in a plastic bag and placed in the chamber
within 20 s of collection. Leaf conductance (gl) was measured, on a similar
number of leaves as C, using a LI-COR LI-1600 steady-state porometer. Cmd and
gl were measured in the same sun-exposed leaves.
The number of ¯owers per branch in Fleckinger's C±G stages (Fleckinger,
1954) was counted each year at full bloom. For this, four branches (ca. 1 m
length and 1.5 cm diameter), one growing in each compass direction, were
tagged on two trees per block of each treatment. Eight weeks afterwards,
the number of fruits per branch was counted and fruit set percentage was
determined.
The fruit diameter of 10 tagged fruits per tree was measured weekly on two
trees per block using an electronic digital calibre. The shoot length of four tagged
shoots per tree, one from each compass direction, was measured on two trees per
block every 14 days. On four trees per block, trunk circumference was measured
annually, 30 cm above the soil line. In the same trees, the canopy shaded area was
estimated each summer as the vertical projection of the tree canopy measured
across and within rows.
Apricot fruits were harvested at several commercial picking dates, depending
on the year. The total number of fruits harvested per tree on each occasion was
weighed on 12 trees per treatment (four per block). Fruit quality measurements
included diameter, volume, weight, total soluble solids (using a hand-held
refractometer ATC-1 Atago), pH (using a pH-meter Crison), ®rmness (using a
testing machine Instron Lloyd Instruments LR-10 K), peel colour (made on three
points of each fruit, using a portable tristimulus chromameter, Minolta CR-300)
was expressed in terms of chroma index (ArteÂs et al., 1999). All measurements
were made on 20 fruit samples per block from each picking.
206
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Data were analysed as a randomised block design using the GLM procedure of
the statistical analysis system (SAS Institute, 1988). Means were separated using
Dunnett's test (P < 0.05), which made comparisons of all treatments against a
control.
3. Results
3.1. Soil and plant water status
The soil water content (yv) values at the end of the different irrigation
withholding periods are shown in Table 2. The overall results indicated that T-1
and T-2 treatments produced the lowest soil water depletion levels. The low
depletion in the T-2 treatment can be ascribed to the rain, which fell during this
period in 1997 coinciding with relatively low evapotranspiration demand (Fig. 1).
T-3, T-4 and T-5 treatments presented very similar average reductions in the soil
water content with respect to control values, ranging between 51.2 and 54.4%, the
highest reduction corresponding to T-4 treatment.
Withholding water irrigation induced statistically signi®cant reductions in predawn leaf water potential (Cpd) and leaf conductance (gl) values, except in the
case of the T-2 treatment in 1997 (Table 2). The highest leaf water de®cits were
observed in the T-4 and T-5 treatments particularly the latter, with average Cpd
values at the end of the withholding periods of ÿ1.9 and ÿ2.4 MPa, respectively.
Leaf conductance in these treatments was very low, especially in 1995 and 1996,
when gl values were near stomatal closure values, that is 13 and 9 mmol mÿ2 sÿ1
for T-4 and T-5 treatments, respectively (Table 2).
Note that the soil water content level took between 4 and 7 days to reach
control values after irrigation was restarted in all the stress treatments. The
recovery of Cpd was faster than that of the soil water content, taking only between 3
and 6 days after irrigation was restored. Leaf conductance, on the other hand,
took between 8 and 15 days to reach control values, depending on the treatment.
3.2. Vegetative and fruit growth
Fig. 2 shows data for shoot and fruit growth expressed as a percentage of
maximum growth at harvest. It is clear that the ®rst phase of rapid fruit growth
started when around 85% of shoot growth was completed and the second phase of
fruit growth initiated when the 100% of shoot growth was completed. There was
an additional stage of shoot growth, occurred after harvest (data not shown).
The fruit growth curve was similar to the control in all stress treatments, except
for T-2 and T-3 treatments. For this reason, Fig. 3 only shows fruit diameter data
of the control (T-0), T-2 and T-3 treatments.
Table 2
Soil water content (yv), pre-dawn leaf water potential (Cpd) and leaf conductance (gl) at the end of
the withholding periods in the different treatments, during the experimental perioda
Treatment
1994
Soil water content (mm in 1.4 m)
T-0
509.2 11.5
T-1
352.8 6.5
*
T-0
T-2
442.6 7.2
250.6 10.5
*
T-0
T-3
518.2 21.5
251.4 4.7
*
T-0
T-4
412.0 5.6
194.7 4.2
**
T-0
T-5
450.9 5.5
212.4 6.3
**
1995
1996
1997
498.3 8.2
300.4 12.5
451.2 15.2
368.9 8.6
ns
401.1 10.2
184.6 14.5
454.2 9.8
398.4 15.6
nsb
448.7 20.5
421.9 14.0
ns
499.7 9.5
224.8 8.2
*
500.9 11.2
254.3 9.2
*
405.0 13.1
213.3 5.1
*
449.3 6.9
193.7 4.2
**
412.7 8.4
217.8 9.2
**
Leaf water potential at pre-dawn (MPa)
T-0
ÿ0.41 0.02
ÿ0.42 0.04
T-2
ÿ1.09 0.05
ÿ1.08 0.05
T-0
T-3
T-0
T-4
T-0
T-5
**
489.8 15.2
227.8 8.9
±
±
**
***
ÿ0.48 0.01
ÿ1.47 0.09
ÿ0.57 0.02
ÿ1.43 0.08
***
***
***
***
ÿ0.55 0.02
ÿ1.52 0.02
ÿ0.56 0.03
ÿ2.09 0.09
ÿ0.59 0.03
ÿ2.15 0.15
***
***
***
ÿ0.59 0.03
ÿ2.14 0.06
ÿ0.55 0.06
ÿ3.12 0.15
ÿ0.52 0.03
ÿ2.00 0.19
***
***
***
130.4 9.8
45.2 6.8
145.4 20.4
35.2 8.6
156.0 12.5
32.2 5.9
***
117.2 4.0
53.7 3.0
**
134.9 8.1
26.0 8.0
***
107.8 7.5
14.5 2.8
***
150.1 4.1
8.2 2.5
***
Values are mean S.E.
Non-signi®cant.
*
P < 0.05; ** P < 0.01; *** P < 0.001.
b
±
±
***
***
a
416.2 10.5
193.7 6.1
*
ÿ0.43 0.02
ÿ1.00 0.05
**
T-0
T-5
*
***
**
T-0
T-4
466.8 12.5
228.5 6.5
ÿ0.48 0.03
ÿ0.49 0.02
ns
ÿ0.60 0.03
ÿ1.25 0.04
Leaf conductance (mmol mÿ2 sÿ1)
T-0
98.5 4.5
T-2
54.5 3.9
T-0
T-3
**
ÿ0.41 0.03
ÿ1.01 0.08
142.1 7.5
80.6 11.5
**
157.1 18.6
29.0 5.9
***
167.9 15.6
12.4 9.8
±
±
±
±
153.9 9.75
134.1 21.3
ns
188.7 16.8
43.6 15.2
***
±
±
***
125.4 19.5
9.9 6.3
***
±
±
208
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Fig. 2. Shoot (*) and fruit (*) growth, expressed as percentage of the maximum growth of apricot
trees under control (T-0) treatment.
Fig. 3. Apricot fruit growth: (A) fruit diameter (mm) and (B) growth rate (mm per day) under T-0
treatment (*), T-2 treatment (&) and T-3 treatment (D). Each point is the average of three
replicates. Vertical bars on data points represents S.E. of the mean (not shown when smaller than
the symbols).
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
209
Table 3
Tree size (trunk circumference and canopy shaded area) in the different irrigation treatmentsa
Treatment
T-0
T-1
T-2
T-3
T-4
T-5
Trunk circumference (cm)
1994
1997b
57.08 1.77
58.91 1.42
58.41 1.03
55.33 2.21
58.65 1.37
59.33 1.07
nsc
77.41 1.96
80.25 1.01
73.16 0.96
72.00 2.47
76.58 1.21
77.58 1.08
ns
Shaded area (%) 1997
58.04 0.44
53.76 2.38
54.39 2.84
53.08 2.23
57.17 2.94
57.21 2.13
ns
a
Values are mean S.E.
Using initial trunk circumference value (1994) as a covariate.
c
Non-signi®cant.
b
Fruit growth, measured as fruit diameter, follows a double-sigmoid pattern.
Although irrigation was withheld in the T-2 treatment at the beginning of the ®rst
rapid fruit growth phase (stage I), a reduction in the diameter of fruit became
evident during the lag phase (stage II), when fruit diameter values were
signi®cantly lower than those of the control (Fig. 3A). When irrigation was
restored in the T-2 treatment, however, the fruit growth rate was higher than that
of the control one (Fig. 3B), allowing fruit to reach a similar diameter during the
second rapid fruit growth phase prior to harvest (Fig. 3A).
Fruits exposed to the T-3 treatment had a lower fruit growth rate from the
beginning of the water withholding period, which coincided with stage III
(Fig. 3B), leading to smaller fruits at harvest (Fig. 3A).
During the experimental period (1994±1997), trunk circumference increased
similarly in all treatments and the canopy shaded area was unaffected (Table 3).
3.3. Fruit set and yield
Fruit set values in the water withholding treatments were similar to those
observed in control plants (T-0 treatment) all the years studied, except for plants
from the T-4 treatment, which presented signi®cantly lower (around 9.4%) fruit
set values (Table 4) than the control throughout the experimental period.
Nevertheless, a certain alternate pattern was observed in other treatments.
The water stress to which the plants were exposed in the T-3 and T-4 treatments
signi®cantly reduced total apricot yield of all the years studied (Table 5),
particularly in the ®rst case. In 1996 the yield obtained with the T-2 treatment
was also lower than in the control treatment (Table 5) due to a failure in the
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A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Table 4
Fruit set percentage for the different irrigation treatments, during the experimental perioda
Treatment
1994
1995
1996
1997
T-0
T-1
T-2
T-3
T-4
T-5
25.70 1.80
23.52 2.65
18.59 1.95
24.25 2.36
±
±
nsb
12.53 1.75
18.79 2.65
14.25 3.52
13.59 1.52
8.88* 2.53
13.79 1.42
21.16 0.85
22.13 2.56
24.55 1.54
25.11 1.56
9.86* 2.45
25.40 2.65
17.49 1.83
15.26 0.41
18.52 2.52
13.87 0.90
9.32* 1.23
17.80 1.75
a
Values are mean S.E.
Non-signi®cant.
*
Values within a column followed by an asterisk are signi®cantly different from those of the
control treatment (T-0), according to Dunnett's test (P < 0.05).
b
irrigation system at the end of May. This coincided with the end of the
imposed stress period, and led to a substantial delay in the soil reaching its ®eld
capacity.
Table 6 shows the quality of the apricots harvested for the different treatments.
Apricot fruit quality was similar in all treatments, except the T-3 treatment, which
produced fruits of smaller size (diameter, volume and weight) than those of the
control treatment. The peel colour of the T-3 fruits also showed higher colour
intensity than those of the control treatment, as indicated by the higher chroma
index (Table 6). Although, there were no statistically signi®cant differences
between treatments as regards total soluble solids and fruit ®rmness, there was a
tendency for the soluble solids content to increase and in fruit ®rmness to
decrease in fruits from the T-3 treatment (Table 6).
Table 5
Fruit yield (kg per tree) for the different irrigation treatments during the experimental period (1994±
1997)a
Treatment
1994
1995
1996
1997
T-0
T-1
T-2
T-3
T-4
T-5
168.22 27.07
165.61 45.41
142.02 11.88
105.10* 10.45
±
±
189.48 49.14
199.29 32.14
156.58 37.93
106.79* 38.84
130.00* 37.24
201.04 50.59
203.66 8.57
205.36 22.15
164.49* 6.25
117.55* 3.25
160.76* 33.47
192.27 31.93
130.18 40.31
121.55 27.86
119.71 23.09
81.38* 25.01
86.54* 45.12
157.21 65.85
a
Values are mean S.E.
Values within a column followed by an asterisk are signi®cantly different from those of the
control treatment (T-0), according to Dunnett's test (P < 0.05).
*
Treatment
Diameter (mm)
Volume (cm3)
Weight (g)
Soluble solids (%)
Firmness (N)
Chrome index
pH
T-0
T-1
T-2
T-3
T-4
T-5
44.51 0.44
45.69 0.50
44.83 0.45
42.83* 0.49
45.04 0.47
44.36 0.49
58.25 1.45
60.50 1.51
57.63 1.40
52.45* 1.25
60.52 1.56
57.85 1.45
57.18 1.38
58.96 1.33
55.21 1.40
51.51* 1.37
59.14 1.30
58.69 1.39
11.37 0.32
10.81 0.98
11.21 0.30
13.70 0.89
11.80 0.93
10.52 0.52
nsb
57.42 5.04
50.52 4.82
49.69 5.82
42.70 6.19
46.31 6.10
52.42 5.23
ns
47.61 1.05
48.01 2.06
49.81 1.52
53.63* 1.03
48.66 1.95
47.15 1.25
3.81 0.04
3.52 0.02
3.65 0.03
3.89 0.05
3.88 0.03
3.84 0.04
ns
a
Values are mean S.E.
Non-signi®cant.
*
Values within a column followed by an asterisk are signi®cantly different from those of the control treatment (T-0), according to Dunnett's test
(P < 0.05).
b
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Table 6
Physical and chemical characteristics: diameter, volume, fresh weight, total soluble solid, ®rmness, peel colour (chrome index) and pH of BuÂlida
apricot fruits from the last pick of the 1997 harvest in the different irrigation treatmentsa
211
212
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
4. Discussion
The high pre-dawn leaf water potential values (Ca) of the control treatment
(Table 2) re¯ected adequate irrigation practices, since Ca depends on the soil
water status (Fereres and Goldhamer, 1990). On the other hand, data for soil and
plant water relations (Table 2) pointed to three levels of water stress in the studied
treatments, T-1 and T-2 producing mild water stress, and T-4 and T-5 (particularly
the latter) a severe water stress situation. An intermediate situation was observed
in plants from the T-3 treatment (Table 2). These differences can be ascribed
either to the duration or the time of application of the water withholding periods.
In this sense, the substantial reduction in yv, Ca and gl in T-4 and T-5 treatments
(Table 2) can be explained by the higher evaporative demand of the atmosphere
noted in this period (from early June to mid-July) (Fig. 1), as well as by the longer
duration of the withholding period for the T-5 treatment.
The slower recovery of leaf conductance values compared to the leaf water
potential values when full irrigation was resumed indicated that stomatal closure
was not a simply passive response to water de®cit, and may be related to
hormonal changes within the leaf (Mans®eld, 1987; Davies and Zhang, 1991).
The relative separation between shoot and fruit growth periods in apricot plants
(Fig. 2) is essential for the successful application of regulated de®cit irrigation
strategies (Goldhamer, 1989), which indicates that de®cit irrigation may be
applied to control shoot growth without detrimental effects on fruit growth and
yield. The separation between both processes was similar to that observed in
other woody plants, which have been exposed to RDI strategies (Mitchell and
Chalmers, 1982; Mitchell et al., 1984; Goldhamer, 1989; Domingo et al., 1996).
It is clear that withholding irrigation during fruit growth periods (T-2 and T-3
treatments) decreased the fruit growth rate (Fig. 3B), and led to a lower fruit
diameter (Fig. 3A). When irrigation was restored in the T-2 treatment, a
compensatory growth rate was observed in the fruits of this treatment, which
allowed the fruit to reach a similar diameter as fruit from the control treatment.
This behaviour has been observed in other fruit trees such as lemon (Cohen and
Goell, 1984), peach (Mitchell and Chalmers, 1982) and pear (Caspari et al., 1994)
and can be explained by the fact that fruits act as strong sinks of photosynthates.
These reserves are available when irrigation is restored, promoting higher fruit
growth rates (Cohen and Goell, 1984; Mills et al., 1996).
In contrast to other authors, who reported that fruit growth is less sensitive to water
stress than other above-ground portions of the tree (Irving and Drost, 1987; Forshey
and Elfving, 1989), our results indicated that trunk circumference and canopy shaded
area were not affected by water withholding in any treatment during the experimental
period (Table 3), probably because the experiment involved mature apricot trees.
The fact that fruit set in the T-4 treatment (Table 4) was signi®cantly lower than
in the control treatment indicated that the drought caused by withholding water
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
213
for one and a half months after harvest affects ¯ower bud induction and/or the
¯oral differentiation processes that occur during this period (Uriu, 1964). Such a
situation would encourage young fruit to drop and also give rise to a lower
germination potential in the pollen of the following year's bloom (Ruiz-SaÂnchez
et al., 1999).
The fruit yield obtained (Table 5) indicated the existence of two phenological
periods that are particularly sensitive or critical to water withholding. The ®rst
critical period corresponds to the second rapid fruit growth stage (T-3 treatment)
and the second to the period immediately after harvest (T-4 treatment). However
the causes for the reduction in yield were very different. The T-3 treatment
limited fruit growth (Fig. 3) and ®nal fruit size, inducing, also, earlier maturity
(Table 6). The reduction in the T-4 yield was due to an increase in young fruit
drop, which led to signi®cantly lower fruit set.
The fact that T-1 and T-2 treatments reached mild water stress, T-4 and T-5
treatments severe water stress and T-3 treatment an intermediate water stress
situation (Table 2), together with the fact that only the yield in T-3 and T-4
treatments was signi®cantly affected (Table 5), suggests that the effect of
irrigation withholding on apricot trees depends more on the physiological
processes that take place in the plant at the different times than on the water stress
level reached and/or the duration of this stress.
The above mentioned results indicate that apricot trees possess advantageous
characteristics that can be used in reduced irrigation practices. These are related
with the separation between main periods of shoot and fruit growth and with the
fact that there are several phenological periods in which irrigation withholding
does not affect yield and fruit quality. Apricot fruits also have a compensatory
capacity for growth when irrigation is restored.
Acknowledgements
The authors are grateful to J. Soto-Montesinos, M.D. Velasco and M. GarcõÂa for
their assistance. This research was supported by ComisioÂn Interministerial de
Ciencia y TecnologõÂa, CICYT (AMB95-0071) and ConsejerõÂa de Medio
Ambiente, Agricultura y Agua de Murcia (PS96-CA-d1) grants to the authors.
R. Galego was a recipient of a MUTIS research fellowship from the Agencia
EspanÄola de CooperacioÂn Internacional (AECI).
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Apricot tree response to withholding irrigation at
different phenological periods
A. Torrecillasa,b, R. Domingob, R. Galegoc, M.C. Ruiz-SaÂncheza,*
a
Dpto. Riego y Salinidad, Centro de EdafologõÂa y BiologõÂa Aplicada del Segura (CSIC),
PO Box 4195, E-30080 Murcia, Spain
b
Dpto. IngenierõÂa de la ProduccioÂn Agraria, Universidad PoliteÂcnica de Cartagena (UPCT),
Cartagena, Murcia, Spain
c
Instituto de Investigaciones en Riego y Drenaje (IIRD), La Habana, Cuba
Accepted 11 November 1999
Abstract
Drip-irrigated BuÂlida apricot trees (Prunus armeniaca L.) on Real Fino apricot rootstock were
submitted, for 4 consecutive years, to water stress by withholding irrigation at different
phenological periods: during the period of ¯owering-fruit set which lasted around 1 month (T-1
treatment); during stages I II of fruit growth (including the initial exponential phase and the lag
phase of the double-sigmoid curve), which lasted around 2 months (T-2 treatment); during stage III
of fruit growth (second exponential phase) lasting around 1 month (T-3 treatment); immediately
after harvest for one and a half months (T-4 treatment); and for 2 months during late postharvest,
immediately following the T-4 treatment (T-5 treatment). These stress treatments were compared
with a control treatment (T-0), irrigated throughout the year and receiving an amount of water
equivalent to 100% of the crop evapotranspiration (ETc) demand. The greatest reduction in
volumetric soil water content, leaf water potential and leaf conductance with respect to the control
values was observed in plants from T-4 and T-5 treatments. A clear distinction could be made
between the main periods of shoot and fruit growth in apricot trees, which may be considered an
advantageous characteristic for the application of de®cit irrigation. Trunk circumference growth and
canopy shaded area were unaffected by irrigation treatments. Stressed fruits from the T-2 treatment
had a lower diameter during the water stress period, although they showed a compensatory growth
rate after irrigation, reaching a similar size to fruits from the control treatment at harvest. Two
critical periods for withholding irrigation were found. The ®rst corresponded to the second rapid
fruit growth period (T-3 treatment), in which the water stress induced a reduction in yield due to a
*
Corresponding author. Tel.: 34-968-215717; fax: 34-968-266613.
E-mail address: [email protected] (M.C. Ruiz-SaÂnchez)
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 4 6 - 6
202
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
smaller fruit size at harvest. In addition, fruits from this treatment ripened earlier. The second
critical period was immediately postharvest (T-4 treatment), in which water stress induced a
signi®cant decrease in fruit yield the following year, due to an increase in young fruit drop which
lead to a lower ®nal fruit set. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Fruit growth; Fruit quality; Fruit set; Prunus armeniaca; Vegetative growth; Water
stress
1. Introduction
Apricot (Prunus armeniaca L.) is widely cultivated in Mediterranean countries,
and the Murcia region is Spain's leading apricot producer (Burgos et al., 1993).
Future prospects for this crop are very favourable. Although apricot is considered
a drought-resistant crop and exhibits some xeromorphic characteristics, such as
the ability to endure water stress in the dry season and the loss of leaves in winter
(Torrecillas et al., 1999), commercial apricot production depends on irrigation.
Water shortage is the main feature of agriculture on the Mediterranean coast.
For this reason, in this area the optimisation of the use and ef®ciency of irrigation
by means of de®cit irrigation strategies that permit maximum yield whilst
reducing water application is of great importance. In this sense, regulated de®cit
irrigation (RDI) may offer an approach to saving water in some woody crops by
minimising or eliminating negative impacts on yield and crop revenue (Chalmers
et al., 1981; Domingo et al., 1996; Goldhamer, 1997).
In elaborating RDI strategies the key is to time the imposition of the stress to
tolerant periods in which yield and fruit quality are not adversely affected
(SaÂnchez-Blanco and Torrecillas, 1995). The effects of water stress depend on the
timing, duration and magnitude of the de®cits (Bradford and Hsiao, 1982). Some
authors indicated that ¯owering in fruit trees depends on the severity of the
postharvest water stress suffered by the plants (Ruggiero, 1986; Larson et al.,
1988; Proebsting et al., 1989). Also, Domingo et al. (1996) indicated the
importance of adequate irrigation management during the rapid fruit growth stage
of lemon in order to obtain marketable fruit size.
For these reasons the aim of this paper was to evaluate the effect of drought
stress on mature BuÂlida apricot trees, by withholding irrigation, during different
phenological periods (¯owering-fruit set, stages I II of fruit growth, stage III of
fruit growth and two consecutive postharvest periods). Soil and plant water
relations, fruit set, vegetative and fruit growth, yield and fruit quality were
measured. In this manner, it is hoped that a knowledge of the ``critical'' periods
for soil moisture stress, in which yield and/or fruit quality are adversely affected,
will contribute to irrigation management under restricted water supply conditions,
improving, thus, the elaboration of RDI strategies in apricot trees.
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
203
2. Materials and methods
2.1. Plant material and experimental site
The study was performed during 1994±1997 in a commercial orchard, located
in Mula, Murcia, SE Spain, with a clay loam texture soil. Volumetric water
content at ®eld capacity was 26% and at wilting point 13%. Plant material was 9
year-old apricot trees (Prunus armeniaca L.), cv. BuÂlida, on Real Fino apricot
rootstock. The trees were spaced 8 8 m apart and drip irrigated by a lateral per
tree row and seven emitters per tree, each with a ¯ow rate of 4 l hÿ1.
During the experimental period, the climate was typically Mediterranean, with
a maximum mean August temperature of 35.88C in 1994 and a minimum mean
January temperature of 3.98C in 1995 (data not shown). The mean daily
evaporation rate from a US Weather Bureau class A pan (on bare soil and located
on a weather station in the orchard) ranged from 1 mm per day in December±
January to 7.5 mm per day in July (Fig. 1). The annual evaporation for the
experimental period averaged 1457 mm, with only minor year-to-year deviations
from these values, while annual rainfall varied from year to year, with an average
of 282 mm, the rainiest year being 1997 with 477 mm. The usual rainy period in
this area occurs during spring and autumn (Fig. 1).
Trees were fertilised with 158 kg N, 769 kg P2O5 and 110 kg K2O, per ha and
year. A routine pesticide program was maintained.
2.2. Experimental design and irrigation treatments
The experimental design was a randomised block with three blocks. Each block
consisted of six trees. The centre four trees were used for experimental
measurements, and the others served as buffers.
Six treatments were considered: a control treatment (T-0) receiving an amount
of water equivalent to 100% of seasonal ETc demand throughout the year, in
which the amount of irrigation water to apply was determined weekly according
to the previous week's pan evaporation and rainfall and the crop coef®cients (Kc)
of Abrisqueta (unpublished data). Also, ®ve water stress treatments were applied,
in which irrigation was withheld during different phenological periods (Table 1),
while the trees were irrigated in the same way as the control treatment during the
rest of the year: T-1 treatment, during the period of ¯owering-fruit set (lasted 1
month, approximately); T-2 treatment, during stage I II of the fruit growth
(including the initial exponential phase and the lag phase of the double-sigmoid
growth curve, and lasted 2 months, approximately); T-3 treatment, during stage
III of the fruit growth (second exponential phase, and lasted 1 month,
approximately); T-4 treatment, during initial postharvest (one and a half months
immediately after harvest); T-5 treatment, during late postharvest (started when
204
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Fig. 1. Mean daily class A pan evaporation (*) and daily rainfall (bars) during the experimental
period (1994±1997).
Table 1
Description of the water stress treatments with starting and ®nishing dates of the irrigation
withholding periods during the experimental period (1994±1997)
Treatment
T-1
T-2
T-3
T-4
T-5
Phenological stage of plant
Flowering-fruit set
Stages I II of fruit growth
Stage III of fruit growth
Early postharvest
Late postharvest
Dates of irrigation withholding
Start
End
Early February
Early March
Early May
Early June
Mid-July
Early March
Early May
Early June
Mid-July
Mid-September
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
205
T-4 treatment ®nished and lasted 2 months). At the end of each stress period drip
irrigation was run continuously for 8±16 h during a period of 4±7 days
(depending on the water stress treatment), to bring the entire soil volume to ®eld
capacity.
2.3. Measurements
Volumetric soil water content (yv) was determined using a neutron probe
(Troxler mod 4300) that had been calibrated for the site previously. One 1.4 m
access tube was located in each block in the wetted area of the dripline, 2 m from
the tree trunk. Soil moisture was determined frequently at 10 cm intervals from
20 to 140 cm. Soil moisture at 10 cm was determined gravimetrically.
Pre-dawn and mid-day leaf water potentials (Cpd and Cmd) were measured
biweekly, on 16 mature leaves located on the south facing side, from the middle
third of the tree for each treatment (four leaves per tree), with a pressure chamber
(Soil Moisture Equip. Corp, model 3000), following the recommendations of
Turner (1988). Leaves were enclosed in a plastic bag and placed in the chamber
within 20 s of collection. Leaf conductance (gl) was measured, on a similar
number of leaves as C, using a LI-COR LI-1600 steady-state porometer. Cmd and
gl were measured in the same sun-exposed leaves.
The number of ¯owers per branch in Fleckinger's C±G stages (Fleckinger,
1954) was counted each year at full bloom. For this, four branches (ca. 1 m
length and 1.5 cm diameter), one growing in each compass direction, were
tagged on two trees per block of each treatment. Eight weeks afterwards,
the number of fruits per branch was counted and fruit set percentage was
determined.
The fruit diameter of 10 tagged fruits per tree was measured weekly on two
trees per block using an electronic digital calibre. The shoot length of four tagged
shoots per tree, one from each compass direction, was measured on two trees per
block every 14 days. On four trees per block, trunk circumference was measured
annually, 30 cm above the soil line. In the same trees, the canopy shaded area was
estimated each summer as the vertical projection of the tree canopy measured
across and within rows.
Apricot fruits were harvested at several commercial picking dates, depending
on the year. The total number of fruits harvested per tree on each occasion was
weighed on 12 trees per treatment (four per block). Fruit quality measurements
included diameter, volume, weight, total soluble solids (using a hand-held
refractometer ATC-1 Atago), pH (using a pH-meter Crison), ®rmness (using a
testing machine Instron Lloyd Instruments LR-10 K), peel colour (made on three
points of each fruit, using a portable tristimulus chromameter, Minolta CR-300)
was expressed in terms of chroma index (ArteÂs et al., 1999). All measurements
were made on 20 fruit samples per block from each picking.
206
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Data were analysed as a randomised block design using the GLM procedure of
the statistical analysis system (SAS Institute, 1988). Means were separated using
Dunnett's test (P < 0.05), which made comparisons of all treatments against a
control.
3. Results
3.1. Soil and plant water status
The soil water content (yv) values at the end of the different irrigation
withholding periods are shown in Table 2. The overall results indicated that T-1
and T-2 treatments produced the lowest soil water depletion levels. The low
depletion in the T-2 treatment can be ascribed to the rain, which fell during this
period in 1997 coinciding with relatively low evapotranspiration demand (Fig. 1).
T-3, T-4 and T-5 treatments presented very similar average reductions in the soil
water content with respect to control values, ranging between 51.2 and 54.4%, the
highest reduction corresponding to T-4 treatment.
Withholding water irrigation induced statistically signi®cant reductions in predawn leaf water potential (Cpd) and leaf conductance (gl) values, except in the
case of the T-2 treatment in 1997 (Table 2). The highest leaf water de®cits were
observed in the T-4 and T-5 treatments particularly the latter, with average Cpd
values at the end of the withholding periods of ÿ1.9 and ÿ2.4 MPa, respectively.
Leaf conductance in these treatments was very low, especially in 1995 and 1996,
when gl values were near stomatal closure values, that is 13 and 9 mmol mÿ2 sÿ1
for T-4 and T-5 treatments, respectively (Table 2).
Note that the soil water content level took between 4 and 7 days to reach
control values after irrigation was restarted in all the stress treatments. The
recovery of Cpd was faster than that of the soil water content, taking only between 3
and 6 days after irrigation was restored. Leaf conductance, on the other hand,
took between 8 and 15 days to reach control values, depending on the treatment.
3.2. Vegetative and fruit growth
Fig. 2 shows data for shoot and fruit growth expressed as a percentage of
maximum growth at harvest. It is clear that the ®rst phase of rapid fruit growth
started when around 85% of shoot growth was completed and the second phase of
fruit growth initiated when the 100% of shoot growth was completed. There was
an additional stage of shoot growth, occurred after harvest (data not shown).
The fruit growth curve was similar to the control in all stress treatments, except
for T-2 and T-3 treatments. For this reason, Fig. 3 only shows fruit diameter data
of the control (T-0), T-2 and T-3 treatments.
Table 2
Soil water content (yv), pre-dawn leaf water potential (Cpd) and leaf conductance (gl) at the end of
the withholding periods in the different treatments, during the experimental perioda
Treatment
1994
Soil water content (mm in 1.4 m)
T-0
509.2 11.5
T-1
352.8 6.5
*
T-0
T-2
442.6 7.2
250.6 10.5
*
T-0
T-3
518.2 21.5
251.4 4.7
*
T-0
T-4
412.0 5.6
194.7 4.2
**
T-0
T-5
450.9 5.5
212.4 6.3
**
1995
1996
1997
498.3 8.2
300.4 12.5
451.2 15.2
368.9 8.6
ns
401.1 10.2
184.6 14.5
454.2 9.8
398.4 15.6
nsb
448.7 20.5
421.9 14.0
ns
499.7 9.5
224.8 8.2
*
500.9 11.2
254.3 9.2
*
405.0 13.1
213.3 5.1
*
449.3 6.9
193.7 4.2
**
412.7 8.4
217.8 9.2
**
Leaf water potential at pre-dawn (MPa)
T-0
ÿ0.41 0.02
ÿ0.42 0.04
T-2
ÿ1.09 0.05
ÿ1.08 0.05
T-0
T-3
T-0
T-4
T-0
T-5
**
489.8 15.2
227.8 8.9
±
±
**
***
ÿ0.48 0.01
ÿ1.47 0.09
ÿ0.57 0.02
ÿ1.43 0.08
***
***
***
***
ÿ0.55 0.02
ÿ1.52 0.02
ÿ0.56 0.03
ÿ2.09 0.09
ÿ0.59 0.03
ÿ2.15 0.15
***
***
***
ÿ0.59 0.03
ÿ2.14 0.06
ÿ0.55 0.06
ÿ3.12 0.15
ÿ0.52 0.03
ÿ2.00 0.19
***
***
***
130.4 9.8
45.2 6.8
145.4 20.4
35.2 8.6
156.0 12.5
32.2 5.9
***
117.2 4.0
53.7 3.0
**
134.9 8.1
26.0 8.0
***
107.8 7.5
14.5 2.8
***
150.1 4.1
8.2 2.5
***
Values are mean S.E.
Non-signi®cant.
*
P < 0.05; ** P < 0.01; *** P < 0.001.
b
±
±
***
***
a
416.2 10.5
193.7 6.1
*
ÿ0.43 0.02
ÿ1.00 0.05
**
T-0
T-5
*
***
**
T-0
T-4
466.8 12.5
228.5 6.5
ÿ0.48 0.03
ÿ0.49 0.02
ns
ÿ0.60 0.03
ÿ1.25 0.04
Leaf conductance (mmol mÿ2 sÿ1)
T-0
98.5 4.5
T-2
54.5 3.9
T-0
T-3
**
ÿ0.41 0.03
ÿ1.01 0.08
142.1 7.5
80.6 11.5
**
157.1 18.6
29.0 5.9
***
167.9 15.6
12.4 9.8
±
±
±
±
153.9 9.75
134.1 21.3
ns
188.7 16.8
43.6 15.2
***
±
±
***
125.4 19.5
9.9 6.3
***
±
±
208
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Fig. 2. Shoot (*) and fruit (*) growth, expressed as percentage of the maximum growth of apricot
trees under control (T-0) treatment.
Fig. 3. Apricot fruit growth: (A) fruit diameter (mm) and (B) growth rate (mm per day) under T-0
treatment (*), T-2 treatment (&) and T-3 treatment (D). Each point is the average of three
replicates. Vertical bars on data points represents S.E. of the mean (not shown when smaller than
the symbols).
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
209
Table 3
Tree size (trunk circumference and canopy shaded area) in the different irrigation treatmentsa
Treatment
T-0
T-1
T-2
T-3
T-4
T-5
Trunk circumference (cm)
1994
1997b
57.08 1.77
58.91 1.42
58.41 1.03
55.33 2.21
58.65 1.37
59.33 1.07
nsc
77.41 1.96
80.25 1.01
73.16 0.96
72.00 2.47
76.58 1.21
77.58 1.08
ns
Shaded area (%) 1997
58.04 0.44
53.76 2.38
54.39 2.84
53.08 2.23
57.17 2.94
57.21 2.13
ns
a
Values are mean S.E.
Using initial trunk circumference value (1994) as a covariate.
c
Non-signi®cant.
b
Fruit growth, measured as fruit diameter, follows a double-sigmoid pattern.
Although irrigation was withheld in the T-2 treatment at the beginning of the ®rst
rapid fruit growth phase (stage I), a reduction in the diameter of fruit became
evident during the lag phase (stage II), when fruit diameter values were
signi®cantly lower than those of the control (Fig. 3A). When irrigation was
restored in the T-2 treatment, however, the fruit growth rate was higher than that
of the control one (Fig. 3B), allowing fruit to reach a similar diameter during the
second rapid fruit growth phase prior to harvest (Fig. 3A).
Fruits exposed to the T-3 treatment had a lower fruit growth rate from the
beginning of the water withholding period, which coincided with stage III
(Fig. 3B), leading to smaller fruits at harvest (Fig. 3A).
During the experimental period (1994±1997), trunk circumference increased
similarly in all treatments and the canopy shaded area was unaffected (Table 3).
3.3. Fruit set and yield
Fruit set values in the water withholding treatments were similar to those
observed in control plants (T-0 treatment) all the years studied, except for plants
from the T-4 treatment, which presented signi®cantly lower (around 9.4%) fruit
set values (Table 4) than the control throughout the experimental period.
Nevertheless, a certain alternate pattern was observed in other treatments.
The water stress to which the plants were exposed in the T-3 and T-4 treatments
signi®cantly reduced total apricot yield of all the years studied (Table 5),
particularly in the ®rst case. In 1996 the yield obtained with the T-2 treatment
was also lower than in the control treatment (Table 5) due to a failure in the
210
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Table 4
Fruit set percentage for the different irrigation treatments, during the experimental perioda
Treatment
1994
1995
1996
1997
T-0
T-1
T-2
T-3
T-4
T-5
25.70 1.80
23.52 2.65
18.59 1.95
24.25 2.36
±
±
nsb
12.53 1.75
18.79 2.65
14.25 3.52
13.59 1.52
8.88* 2.53
13.79 1.42
21.16 0.85
22.13 2.56
24.55 1.54
25.11 1.56
9.86* 2.45
25.40 2.65
17.49 1.83
15.26 0.41
18.52 2.52
13.87 0.90
9.32* 1.23
17.80 1.75
a
Values are mean S.E.
Non-signi®cant.
*
Values within a column followed by an asterisk are signi®cantly different from those of the
control treatment (T-0), according to Dunnett's test (P < 0.05).
b
irrigation system at the end of May. This coincided with the end of the
imposed stress period, and led to a substantial delay in the soil reaching its ®eld
capacity.
Table 6 shows the quality of the apricots harvested for the different treatments.
Apricot fruit quality was similar in all treatments, except the T-3 treatment, which
produced fruits of smaller size (diameter, volume and weight) than those of the
control treatment. The peel colour of the T-3 fruits also showed higher colour
intensity than those of the control treatment, as indicated by the higher chroma
index (Table 6). Although, there were no statistically signi®cant differences
between treatments as regards total soluble solids and fruit ®rmness, there was a
tendency for the soluble solids content to increase and in fruit ®rmness to
decrease in fruits from the T-3 treatment (Table 6).
Table 5
Fruit yield (kg per tree) for the different irrigation treatments during the experimental period (1994±
1997)a
Treatment
1994
1995
1996
1997
T-0
T-1
T-2
T-3
T-4
T-5
168.22 27.07
165.61 45.41
142.02 11.88
105.10* 10.45
±
±
189.48 49.14
199.29 32.14
156.58 37.93
106.79* 38.84
130.00* 37.24
201.04 50.59
203.66 8.57
205.36 22.15
164.49* 6.25
117.55* 3.25
160.76* 33.47
192.27 31.93
130.18 40.31
121.55 27.86
119.71 23.09
81.38* 25.01
86.54* 45.12
157.21 65.85
a
Values are mean S.E.
Values within a column followed by an asterisk are signi®cantly different from those of the
control treatment (T-0), according to Dunnett's test (P < 0.05).
*
Treatment
Diameter (mm)
Volume (cm3)
Weight (g)
Soluble solids (%)
Firmness (N)
Chrome index
pH
T-0
T-1
T-2
T-3
T-4
T-5
44.51 0.44
45.69 0.50
44.83 0.45
42.83* 0.49
45.04 0.47
44.36 0.49
58.25 1.45
60.50 1.51
57.63 1.40
52.45* 1.25
60.52 1.56
57.85 1.45
57.18 1.38
58.96 1.33
55.21 1.40
51.51* 1.37
59.14 1.30
58.69 1.39
11.37 0.32
10.81 0.98
11.21 0.30
13.70 0.89
11.80 0.93
10.52 0.52
nsb
57.42 5.04
50.52 4.82
49.69 5.82
42.70 6.19
46.31 6.10
52.42 5.23
ns
47.61 1.05
48.01 2.06
49.81 1.52
53.63* 1.03
48.66 1.95
47.15 1.25
3.81 0.04
3.52 0.02
3.65 0.03
3.89 0.05
3.88 0.03
3.84 0.04
ns
a
Values are mean S.E.
Non-signi®cant.
*
Values within a column followed by an asterisk are signi®cantly different from those of the control treatment (T-0), according to Dunnett's test
(P < 0.05).
b
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
Table 6
Physical and chemical characteristics: diameter, volume, fresh weight, total soluble solid, ®rmness, peel colour (chrome index) and pH of BuÂlida
apricot fruits from the last pick of the 1997 harvest in the different irrigation treatmentsa
211
212
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
4. Discussion
The high pre-dawn leaf water potential values (Ca) of the control treatment
(Table 2) re¯ected adequate irrigation practices, since Ca depends on the soil
water status (Fereres and Goldhamer, 1990). On the other hand, data for soil and
plant water relations (Table 2) pointed to three levels of water stress in the studied
treatments, T-1 and T-2 producing mild water stress, and T-4 and T-5 (particularly
the latter) a severe water stress situation. An intermediate situation was observed
in plants from the T-3 treatment (Table 2). These differences can be ascribed
either to the duration or the time of application of the water withholding periods.
In this sense, the substantial reduction in yv, Ca and gl in T-4 and T-5 treatments
(Table 2) can be explained by the higher evaporative demand of the atmosphere
noted in this period (from early June to mid-July) (Fig. 1), as well as by the longer
duration of the withholding period for the T-5 treatment.
The slower recovery of leaf conductance values compared to the leaf water
potential values when full irrigation was resumed indicated that stomatal closure
was not a simply passive response to water de®cit, and may be related to
hormonal changes within the leaf (Mans®eld, 1987; Davies and Zhang, 1991).
The relative separation between shoot and fruit growth periods in apricot plants
(Fig. 2) is essential for the successful application of regulated de®cit irrigation
strategies (Goldhamer, 1989), which indicates that de®cit irrigation may be
applied to control shoot growth without detrimental effects on fruit growth and
yield. The separation between both processes was similar to that observed in
other woody plants, which have been exposed to RDI strategies (Mitchell and
Chalmers, 1982; Mitchell et al., 1984; Goldhamer, 1989; Domingo et al., 1996).
It is clear that withholding irrigation during fruit growth periods (T-2 and T-3
treatments) decreased the fruit growth rate (Fig. 3B), and led to a lower fruit
diameter (Fig. 3A). When irrigation was restored in the T-2 treatment, a
compensatory growth rate was observed in the fruits of this treatment, which
allowed the fruit to reach a similar diameter as fruit from the control treatment.
This behaviour has been observed in other fruit trees such as lemon (Cohen and
Goell, 1984), peach (Mitchell and Chalmers, 1982) and pear (Caspari et al., 1994)
and can be explained by the fact that fruits act as strong sinks of photosynthates.
These reserves are available when irrigation is restored, promoting higher fruit
growth rates (Cohen and Goell, 1984; Mills et al., 1996).
In contrast to other authors, who reported that fruit growth is less sensitive to water
stress than other above-ground portions of the tree (Irving and Drost, 1987; Forshey
and Elfving, 1989), our results indicated that trunk circumference and canopy shaded
area were not affected by water withholding in any treatment during the experimental
period (Table 3), probably because the experiment involved mature apricot trees.
The fact that fruit set in the T-4 treatment (Table 4) was signi®cantly lower than
in the control treatment indicated that the drought caused by withholding water
A. Torrecillas et al. / Scientia Horticulturae 85 (2000) 201±215
213
for one and a half months after harvest affects ¯ower bud induction and/or the
¯oral differentiation processes that occur during this period (Uriu, 1964). Such a
situation would encourage young fruit to drop and also give rise to a lower
germination potential in the pollen of the following year's bloom (Ruiz-SaÂnchez
et al., 1999).
The fruit yield obtained (Table 5) indicated the existence of two phenological
periods that are particularly sensitive or critical to water withholding. The ®rst
critical period corresponds to the second rapid fruit growth stage (T-3 treatment)
and the second to the period immediately after harvest (T-4 treatment). However
the causes for the reduction in yield were very different. The T-3 treatment
limited fruit growth (Fig. 3) and ®nal fruit size, inducing, also, earlier maturity
(Table 6). The reduction in the T-4 yield was due to an increase in young fruit
drop, which led to signi®cantly lower fruit set.
The fact that T-1 and T-2 treatments reached mild water stress, T-4 and T-5
treatments severe water stress and T-3 treatment an intermediate water stress
situation (Table 2), together with the fact that only the yield in T-3 and T-4
treatments was signi®cantly affected (Table 5), suggests that the effect of
irrigation withholding on apricot trees depends more on the physiological
processes that take place in the plant at the different times than on the water stress
level reached and/or the duration of this stress.
The above mentioned results indicate that apricot trees possess advantageous
characteristics that can be used in reduced irrigation practices. These are related
with the separation between main periods of shoot and fruit growth and with the
fact that there are several phenological periods in which irrigation withholding
does not affect yield and fruit quality. Apricot fruits also have a compensatory
capacity for growth when irrigation is restored.
Acknowledgements
The authors are grateful to J. Soto-Montesinos, M.D. Velasco and M. GarcõÂa for
their assistance. This research was supported by ComisioÂn Interministerial de
Ciencia y TecnologõÂa, CICYT (AMB95-0071) and ConsejerõÂa de Medio
Ambiente, Agricultura y Agua de Murcia (PS96-CA-d1) grants to the authors.
R. Galego was a recipient of a MUTIS research fellowship from the Agencia
EspanÄola de CooperacioÂn Internacional (AECI).
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