Apricot tree response to withholding irrigation at
different phenological periods
A. Torrecillas
a,b, R. Domingo
b, R. Galego
c, M.C. Ruiz-SaÂnchez
a,*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 armeniacaL.) 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 III 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
Scientia Horticulturae 85 (2000) 201±215
*
Corresponding author. Tel.:34-968-215717; fax:34-968-266613.
E-mail address: [email protected] (M.C. Ruiz-SaÂnchez)
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 armeniacaL.) 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.
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 88 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 P2O5and 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.
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 Phenological stage of plant Dates of irrigation withholding
Start End
T-1 Flowering-fruit set Early February Early March
T-2 Stages III of fruit growth Early March Early May
T-3 Stage III of fruit growth Early May Early June
T-4 Early postharvest Early June Mid-July
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 asC, using a LI-COR LI-1600 steady-state porometer.Cmdand 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.
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 pre-dawn 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 ofÿ1.9 andÿ2.4 MPa, respectively. Leaf conductance in these treatments was very low, especially in 1995 and 1996, whenglvalues 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 ofCpdwas 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).
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 1995 1996 1997
Soil water content(mm in 1.4 m)
T-0 509.211.5 498.38.2 451.215.2 454.29.8
Leaf water potential at pre-dawn(MPa)
Fig. 2. Shoot (*) and fruit (*) growth, expressed as percentage of the maximum growth of apricot trees under control (T-0) treatment.
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
Table 3
Tree size (trunk circumference and canopy shaded area) in the different irrigation treatmentsa
Treatment Trunk circumference (cm) Shaded area (%) 1997
1994 1997b
T-0 57.081.77 77.411.96 58.040.44
T-1 58.911.42 80.251.01 53.762.38
T-2 58.411.03 73.160.96 54.392.84
T-3 55.332.21 72.002.47 53.082.23
T-4 58.651.37 76.581.21 57.172.94
T-5 59.331.07 77.581.08 57.212.13
nsc ns ns
a
Values are meanS.E.
bUsing initial trunk circumference value (1994) as a covariate. c
Non-signi®cant.
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 4
Fruit set percentage for the different irrigation treatments, during the experimental perioda
Treatment 1994 1995 1996 1997
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).
Table 5
Fruit yield (kg per tree) for the different irrigation treatments during the experimental period (1994± 1997)a
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
Treatment Diameter (mm) Volume (cm3) Weight (g) Soluble solids (%) Firmness (N) Chrome index pH
T-0 44.510.44 58.251.45 57.181.38 11.370.32 57.425.04 47.611.05 3.810.04
T-1 45.690.50 60.501.51 58.961.33 10.810.98 50.524.82 48.012.06 3.520.02
T-2 44.830.45 57.631.40 55.211.40 11.210.30 49.695.82 49.811.52 3.650.03
T-3 42.83*0.49 52.45*1.25 51.51*1.37 13.700.89 42.706.19 53.63*1.03 3.890.05
T-4 45.040.47 60.521.56 59.141.30 11.800.93 46.316.10 48.661.95 3.880.03
T-5 44.360.49 57.851.45 58.691.39 10.520.52 52.425.23 47.151.25 3.840.04
nsb ns ns
a
Values are meanS.E.
b
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).
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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 inyv,Caandglin 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.
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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