Effects of regulated deficit irrigation on physiology and fruit quality in apricot trees

The effects of regulated deficit irrigation (RDI) were studied on 9 year-old apricot-trees (Prunus armeniaca L. cv. ‘Búlida’) grafted on ‘Real Fino’ rootstock. Two irrigation treatments were established. The first, a control treatment, was irrigated to fully satisfy the crop water requirements (100% ETc) and the second, a RDI treatment, was subject to water shortage during the non-critical periods of crop development, by reducing the amount of applied irrigation water to: a) 40% of ETc from flowering until the end of the first stage of fruit growth; b) 60% of ETc during the second stage of fruit growth and c) 50% and 25% of ETc during the late postharvest period (that starts 60 days after harvesting), for the first 30 days and until the end of tree defoliation, respectively. The results indicated that the apricot tree is an appropriate species to apply RDI thanks to the clear separation between their vegetative and reproductive growths and its ability to recover the fruit diameter reduction suffered during RDI application. Furthermore, some qualitative characteristics such as the level of soluble solids, fruit taste and the colour of the fruit are enhanced. These two reasons, together with irrigation water savings of 39%, emphasize the RDI strategies as a possible solution in areas with water shortages, like the south-eastern region of Spain. Additional key words: fruit growth, photosynthesis; Prunus armeniaca L.; regulated deficit irrigation; stomatal conductance; water relations; water stress.


Introduction
Apricot trees (Prunus armeniaca L.) are widely cultivated in Mediterranean countries, with the Murcia Region being Spain's leading apricot producer with an average annual yield in the last few years of about 85,000 tonnes of apricots, from a cultivation area of 10,500 ha (MARM, 2009); these f igures represent ~66% and ~60% of the Spanish total apricot production and cultivation area of apricots, respectively.The most important cultivar of apricot in Spain is 'Búlida', which represents ~50% and ~66% of the total production of apricots in Spain and the Murcia Region, respectively (CARM, 2009).
Apricot trees are highly sensitive to drought stress at particular phenological stages, such as stage III of fruit growth and during the 2 months after harvest (early postharvest) (Torrecillas et al., 2000;Pérez-Pastor et al., 2007, 2009).Apricot drought tolerance is mainly based on avoidance mechanisms, such as stomatal control, epinasty and limitation transpiration by reducing leaf area (Torrecillas et al., 1999;Ruiz-Sánchez et al., 2000a), together with some degree of osmotic adjustment in young apricot trees although this adjustment is not observed in adult trees (Ruiz-Sánchez et al., 2007).
The aim of this paper was to evaluate the effect of RDI on plant-water relations, yield and fruit quality in adult apricot trees (Prunus armeniaca L. cv.'Búlida').The RDI treatment was scheduled to reduce water applications during non-critical periods, satisfying the crop water requirements during the critical periods, which corresponded to the second rapid fruit growth period (stage III) and the early postharvest period (Pérez-Pastor et al., 2009).

Experimental conditions and plant material
The experiment was performed during 2008, in a 1-ha plot of a commercial orchard, located in Mula valley, Murcia, Spain (37°55' N, 1°25' W, 360 m above sea level).The soil is a clay-loam texture and classified as a Xeric Torriorthent.It is highly calcareous, has a pH of 7.8, and a low organic matter content and cationic exchange capacity.The available water capacity is about 0.31 m 3 m -3 .The climate is semiarid Mediterranean with hot and dry summers; annual evaporation calculated from reference crop evapotranspiration (ET 0 ) and rainfall was 1,055 and 318 mm, respectively.
The plant material consisted of 9-year-old apricot trees (Prunus armeniaca L. cv.Búlida, on Real Fino apricot rootstock), spaced 8 × 6 m, with an average height of 3.9 m, trunk diameter of 0.19 m, and ground cover of about 65%.Trees were drip irrigated using one drip irrigation line for each row, with five emitters per tree, each with a flow rate of 4 L h -1 .
Crop irrigation requirements were scheduled weekly according to daily ETo, calculated using the Penman-Monteith equation (Allen et al., 1998), and a local crop factor based on the time of the year (Abrisqueta et al., 2001): 0.5 February, 0.75 March, 0.8 April, 0.9 May, 0.6 June, 0.5 July-November.The correction coefficient for ground cover was 1 according to Fereres and Goldhmaer (1990).All trees received the same quantity of nutrients through the irrigation system: 110 kg N, 62 kg P 2 O 5 and 117 kg K 2 O ha -1 year -1 .Pest control was that commonly used by growers, and no weeds were allowed to develop within the orchard.
A total of 192 trees were used in this study.The experimental design of each irrigation treatment was Regulated deficit irrigation in apricot trees S87 4 standard experimental plots distributed randomly in blocks.The standard plot was made up of 24 trees, organized in 4 adjacent rows.The 8 central trees of the middle row were used for measurements, and the other 16 trees were guard trees.

Irrigation treatments
Two irrigation treatments were applied: (C), irrigated daily satisfying the estimated crop evapotranspiration (ETc) and RDI irrigated at 100% ETc during the critical periods (stage III of fruit growth and 2 months after harvest period) and was subjected to water shortage during the non-critical periods of crop development by reducing the amount of applied irrigation water to: a) 40% of ETc from flowering until the end of the first stage of fruit growth; b) 60% of ETc during the second stage of fruit growth and c) 50% and 25% of ETc during the late postharvest period (that starts 60 days after harvesting), for the first 30 days and until the end of tree defoliation, respectively (Fig. 1).This distribution of water applied during noncritical periods was based on studies by Torrecillas et al. (2000).The irrigation water was considered to be of good quality with a very low electrical conductivity (0.6 dS m -1 ).Irrigation was controlled automatically by a head unit programmer and the amounts of water applied for each irrigation treatment were measured with in-line flowmeters placed in each standard experimental plot.

Measurements
The soil volumetric water content (θ v ) of the top 0.2 m of the soil prof ile was measured by time-domainreflectrometry probes (TDR) (model 1502C, Tektronix Inc., OR.), as described by Moreno et al. (1996).The θ v content of the soil from 0.2 m down to a maximum depth of 1.0 m was measured every 0.1 m using a neutron probe (model 4300, Troxler Electronic Laboratories, Inc. NC.), in access tubes installed 1.0 m away from the trees and beside the emitters.Measurements using one neutron probe per each standard experimental plot (4 per treatment) were taken every 7 to 15 days in the morning, during the experimental period.Two TDR probes were used for each neutron probe.
Midday (12:00 h solar time) stem water potential (Ψ s ) was measured in one mature leaves per plant (6 trees per each experimental plot), taken close to the trunk.Leaves were enclosed in a small black plastic bag covered with aluminium foil for at least 2 h before measurements were made with a pressure chamber (Soil Moisture Equip.Corp, model 3000, Santa Barbara, CA, USA).Leaf water potential (Ψ l ) was measured in the same trees used for Ψ s measurements, sampling one sunny mature leaves per plant.The water potential measurements were made according to Scholander et al. (1965) and following the recommendations of Turner (1988).
The gas exchange parameters (net photosynthesis, P n, and stomatal conductance, g s ) were measured at solar midday, in a similar number and type of leaves, and in the same days and trees respectively, as for leaf water potential readings, using a field-portable photosynthesis system (LI-6400, Li-Cor, Lincoln, NE, USA).
Fruit growth was measured perpendicular to the fruit suture on 200 fruits in each treatment (50 fruits per experimental plot).Each sampling was carried out every 7-10 d from a fruit diameter minimum of 1 cm and sampling randomly 10 fruits in the canopy of 5 trees per experimental plot using digital callipers.
At harvest, 200 fruits in each treatment (50 fruits per experimental plot) were selected for their quality assessment.Skin and flesh colour, firmness, tritable acidity (TA), pH, and soluble solids content (SSC) were evaluated as quality indices.Colour values, on the surface (ground skin colour) and after peeling in the flesh, were measured with a Minolta chromameter (CR-300, Minolta, Ramsey, NJ) tristimulus colour analyzer calibrated with a white porcelain reference plate.(a*/ b*)], and chroma (a* 2 / b* 2 ) 1/2 were determined around the equatorial region in three different positions (with an average of nine times for each apricot).Fruit firmness was evaluated by compression test using a Lloyd instrument (model LR10K, Fareham Hants, UK) equipped with two flat plates (12 × 18 cm 2 ).The maximum force required to deform the fruit 5 mm at a speed of 25 mm min -1 was determined.TA was measured by titration of 5 mL of juice with 0.1 mol L -1 NaOH to pH 8.1 by an automatic titration system (AOAC, 1984).The pH values were measured using a pH-meter, and SSC was determined with an Atago N1 hand-held refractometer (Tokyo, Japan).

Statistical design and analysis
Statistical analysis was performed by weighted analysis of variance (ANOVA) using general linear model of SPSS software (version 17.0, SPSS Inc., Chicago).

Results
During the experimental period, the average values of volumetric soil water content (θ v ) from 0 to 1 m depth in C treatment was nearly constant, with values always close to field capacity (95.8% of θ v value at f ield capacity) (Fig. 2a).RDI treatment presented different θ v values (82.5% of θ v value at field capacity) between day of year (DOY) 64 and 152 and between DOY 240 and 290, both being lower than those of C treatment (Fig. 2b).The soil moisture profile in RDI treatment was characterized by the fact that during the water deficit periods, the θ v values beyond 600 mm were clearly below field capacity, indicating the non existence of drainage (data not shown).The amount of water applied was 574.0 mm and 352.4 mm in C and RDI treatments, respectively.The water saved in RDI treatment was 28.5% and 50% during the fruit development and late postharvest period, respectively (Fig. 2c).
Fruit growth, measured as fruit diameter, followed a double-sigmoid pattern (Fig. 3a).Fruits exposed to RDI had a lower but non-significant fruit diameter at the end of stage II of fruit development.When irrigation was restored in the RDI treatment, fruits of this treatment rapidly reached similar diameter values to those obtained in the C treatment due to a significant increase in the fruit growth rate (Fig. 3b).At harvest the fruit equatorial diameter was similar in both treatments and around 45 mm (Fig. 3a).
The RDI treatment induced statically significant reductions in Ψ s in all the stages during which the water deficit was imposed (Table 1).During the fruit development the values of Ψ s were nearly constant in C treatment and were about -0.65 MPa.The reduction of Ψ s in RDI treatment during this period was significant and about 47% respect to C plants at the end of stage II of fruit development (-0.94 MPa).During postharvest period, Ψ s values in C plants were lower due to increased evaporative demand, reaching an average of -0.91 MPa and -1.24 MPa during early and late postharvest periods, respectively.The decrease of Ψ s in RDI was significant and about of 26% and 18% compared to C plants during the first and second period of late postharvest respectively (-1.55 and -1.81 MPa, respectively).However, RDI treatment induced statically significant reductions in Ψ l only during the second period of late postharvest when the water deficit was more important (25% ETc).As in the case of Ψ s values, Ψ l showed a decreasing tendency as vapour pressure deficit (VPD) increased in both treatments (Fig. 2a).
The average values of Ψ l for both treatments were about -1.36 MPa, -1.65 and -2.11MPa during fruit development, early-and late-postharvest periods, respectively.Similar behaviour to that observed in Ψ l was also shown in gas exchange parameters with a significant reduction in P n and g s values only being ob- Table 1.Average values of stem (Ψ s , MPa) and leaf (Ψ l , MPa) water potential, net photosynthesis (P n , µmol m -2 s -1 ) and stomatal conductance (g s , mmol m -2 s -1 ) in each phenological period in the control (C) and regulated deficit irrigation (RDI) treatments  1).
The yield and fruit number per tree were similar between treatments (156.7 and 153.5 kg tree -1 and 2,906.7 and 3,038.4fruits per tree -1 for C and RDI treatments, respectively).Irrigation treatment affected to the main quality indices of apricot fruits.The analysis of the colour values showed that the lightness factor, L*, was similar in both treatments although slightly higher in the RDI treatment.The hue angle (Hº) in skin and flesh was significant higher in the fruits of RDI treatment.Similar behaviour observed in Hº was shown in the chroma (C*), and the fruits from the RDI treatment showed significant higher values in skin and flesh than those obtained in C treatment (Table 2).Fruit firmness significantly decreased (30%) in fruits from the RDI treatment, while SSC values were increased (9%) significantly in this treatment with respect to C treatment.However, there were no statistically significant differences between treatments as regards pH and TA (Table 3).

Discussion
The amount of water applied in the C treatment maintained high values of θ v nearest to field capacity (Fig. 2b).Similar values of θ v were measured in the RDI treatment during periods of full irrigation, being in both treatments the drainage low (lower than 10%) (data not shown) indicating that a suitable irrigation scheduling in C treatment was applied (Abrisqueta et al., 2001).During the phenological periods of water deficit in the RDI treatment, θ v decreased significantly, reaching values which provoke signif icant stress conditions in apricot trees (Ruiz-Sánchez et al., 2000).The annual water saving achieved in RDI treatment was 39% (Fig. 2c).This amount of water is similar to that obtained from RDI strategies during the two initial years by Pérez-Pastor et al. (2009).Although these authors observed signif icant reductions in plant production for both years, in our case the yield and fruit number per tree were similar between treatments.This aspect can be explained by the highest water reductions designed by Pérez-Pastor et al. (2009) during the fruit development in the initial two years (75% until the end of stage II) in contrast with our experimental conditions (50%) based on the last two years designed by those authors (Fig. 1).
The water saving during fruit development did not affect fruit growth in RDI treatment, since fruits from this treatment had a slightly lower but non-significant fruit diameter at the end of stage II of fruit development (Fig. 3a).When irrigation was restored in the RDI treatment, a compensatory fruit growth was observed (Fig. 3b) which allowed the fruit to reach a similar diameter to fruits from the C treatment (Chalmers et al., 1986) and at harvest apricot fruits were of «extra» size in both treatments (> 40 mm in diameter).This can be explained by the fact that fruit acts 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;Torrecillas et al., 2000).This behaviour has been observed in other fruit trees such as lemon (Cohen and Goel, 1984), peach (Mitchell and Chalmers, 1982) and pear (Caspari et al., 1994).This compensatory fruit growth during a recovery period of water deficit and the relative separation between shoot and fruit growth periods in apricot plants (Torrecillas et al., 2000) is essential for the successful application of RDI strategies (Goldhamer, 1989), which indicates that deficit irrigation may be applied to limit shoot growth without detrimental effects on fruit growth and yield (Chalmers et al., 1981;Mitchell and Chalmers, 1982).Plant water status (Ψ s and Ψ l ) and gas exchange parameters (P n and g s ) were affected by the RDI treatment, but not all these discontinuous water stress indicators performed in the same way.Thus, only Ψ s reflected well the effects on plant water status of the different water restrictions even under mild levels of water deficit associated at low (stage I of fruit growth) (Table 1 and Fig. 2a).For this reason, the use of Ψ s has been adopted because of its high sensitivity to water deprivation (McCutchan and Shackel, 1992;Naor et al., 1995;Remorini and Massai, 2003) and its good prediction of the yield response to deficit irrigation (Naor, 2000;Intrigliolo and Castel, 2006).The remaining water stress indicators (Ψ l , P n and g s ) depend more on the meteorological conditions (Ruiz-Sánchez et al., 2004) and in our case only presented significant differences when the water reductions in RDI treatment were very high with respect to the C treatment (25% ETc).
The water deficit imposed also affected fruit quality indices.Significant differences in fruit skin and flesh colour were found.Fruits from RDI treatment showed higher values of hue angle (Hº) and chroma (C*) (Table 2).The Hº has been described as a suitable and intuitively understandable colour index (Arias et al., 2000).The increase in this parameter in apricot fruits from RDI plants can be associated to a reduction in carotenoids accumulation attributed to the oxidation by exposure to light (Ruiz et al., 2005).This exposure to light in the fruits from RDI treatment is related to a significant reduction in the vegetative growth of the trees during fruit development (data not shown), implying a high exposure to light of fruits.Similar behaviour was observed in peach fruits under RDI (Gelly et al., 2003;Buendía et al., 2008).
One of the benefits of RDI is an improvement in fruit taste and quality (Li et al., 1989;Mills et al., 1996;Mpelasoka et al., 2000).Soluble solids content (SSC) and titratable acidity (TA) warrant particular attention due to their importance in fruit taste (Crisosto et al., 1994).In our case, SSC values were increased significantly in RDI treatment whereas TA was equal in both treatments, and therefore the SSC/TA ratio was increased significantly in RDI treatment (Table 3).This ratio affects the perception of taste (sweetness and acidity) by the consumer, thereby influencing decisions to buy again or not (Crisosto et al., 1997;Scandella et al., 1997).Thus, fruits from RDI treatment can be considered of high quality since SSC increased nearly 1º without affecting acidity (Scandella et al., 1997).
In conclusion, the results indicated that the apricot tree is an appropriate species to apply RDI thanks to the clear separation between their vegetative and reproductive growths and its ability to recover the fruit diameter reduction suffered during RDI application.Furthermore, some qualitative characteristics such as the level of soluble solids, fruit taste and the colour of the fruit are enhanced.These two reasons, together with irrigation water savings of 39%, emphasize the RDI strategies as a possible solution in areas with water shortage, like the south-eastern region of Spain.

Figure 2 .
Figure 2. (a) Daily crop reference evapotranspiration (ET 0 , mm) (solid line), mean air vapour pressure deficit (VPD, kPa) (dotted line) and rainfall (mm) (vertical bars); (b) soil volumetric water content (θ v ) to a depth of 1 m in the control (ⅷ) and RDI (⅜) irrigation treatments; (c) cumulative irrigation water applied (mm) in the control (solid line), and RDI (discontinuous line) plants along the season (DOY-day of year).The interval between vertical dotted lines from left to right represent the beginning of phases I, II and III of fruit growth, early-and late postharvest.Each value of θ v is the mean of four measurements ± SE.Asterisks indicate statistically significant differences between treatments (p < 0.001).

Figure 3 .
Figure 3. (a) Fruit diameter (mm) and (b) absolute fruit growth rate (AGR, mm d -1 ) evolution in the C (solid line, ⅷ) and RDI (discontinuous line, ⅜) irrigation treatments during the fruit growth period (DOY-day of year).The interval between vertical dotted lines from left to right represent the beginning of phases I, II and III of fruit growth.Each value is the mean of 200 measurements ± SE.Asterisks indicate statistically significant differences between treatments (p < 0.001).
served at the end of the second period of late postharvest.The decrease in P n and g s was 82 and 72%, respectively during this second period of late postharvest RDI treatment.C plants had average values of 11.3 and 167.6 in P n and g s during fruit development, respectively.These values were decreased nearly by half in the postharvest period (Table Values are the mean of 24 measurements.ns: non-significant.* p < 0.05.** p < 0.01.*** p < 0.001.

Table 2 .
Skin and flesh fruit colour values (reflectance measurements L*, Hº, C*) at harvest in the control (C) and regulated deficit irrigation (RDI) treatments