Effects of partial rootzone drying (PRD) on adult olive tree (Olea europaea) in field conditions under arid climate

Agriculture, Ecosystems and Environment 106 (2005) 289–301 www.elsevier.com/locate/agee

Effects of partial rootzone drying (PRD) on adult olive tree (Olea europaea) in field conditions under arid climate I. Physiological and agronomic responses S. Wahbia, R. Wakrima, B. Aganchicha, H. Tahia, R. Serraja,b,* a

Laboratoire de physiologie ve´ge´tale, Faculte´ des sciences Semlalia, Marrakech, Morocco b Soil and Water Management and Crop Nutrition Section, Joint FAO/IAEA Division, Wagramer Strasse 5, A-1400 Vienna, Austria

Abstract The limited water availability in the Mediterranean ecosystems and the current and predicted decrease of water resources are leading to the urgent need to reduce water use for irrigation in the arid and semi-arid regions. This study was conducted to evaluate the effects of partial rootzone drying (PRD) irrigation technique on plant growth, yield and water use efficiency of olive tree (Olea europaea) grown under arid conditions in southern Morocco. The PRD consists in exposing one-half of the plant root system to drying soil, whereas the other half is kept in wet soil. Field experiments were conducted on adult olive trees and subjected to four irrigation treatments using a localized irrigation system: Control (irrigated with 100% of ETc on both sides of the root system), PRD1 (irrigation with 50% of the control, on one side of the root system and switched every 2 weeks), PRD2 (same switched every two irrigations: 4 weeks) and PRD3 (with same amount of water than the control applied on one side of the root system, switched every 2 weeks). The PRD treatments affected significantly olive water relations, starting with an increase in stomatal resistance, and subsequently leaf water potential (c), with a small non-significant effect on leaf relative water content. The hypothesis of a PRD-induced chemical signal was supported by the observation that stomatal closure was similar in all PRD treatments, including PRD 3, which had exactly the same level and evolution of leaf c than the control. The PRD1 and PRD2 treatments induced a slight reduction of the average shoot length, which was comparable to fruit yield reduction. Yield reduction under PRD1 and PRD2 was mainly due to a decrease in fruit numbers, whereas the average olive fruit diameter was slightly higher under PRD1 and PRD2 treatments than the control. Olive yield was significantly higher for the control and PRD3 treatments, compared to PRD1 and PRD2 treatments. The oil percentage in olive fruit and oil acidity did not show any significant differences between PRD treatments and the control. The slight PRD-induced yield reduction (15–20%) compared to the control was achieved with 50% reduction in the total amount of water applied, which resulted in a water use efficiency increase by 60– 70% under PRD1 and PRD2 treatments, compared to the control and PRD3. # 2004 Elsevier B.V. All rights reserved. Keywords: Deficit irrigation; Partial rootzone drying (PRD); Olive; Water status; Yield

* Corresponding author. E-mail address: [email protected] (R. Serraj). 0167-8809/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2004.10.015

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1. Introduction Olive tree (olea europaea) is a hardy crop that yields reasonably well even under water-limited conditions; it is also known from previous work that olive fruit and oil yields respond positively to additional water supply (Ferna´ ndez et al., 2001). However, the limited water availability in the Mediterranean ecosystems and the current and predicted decrease of water resources are leading to the urgent need to reduce water use for irrigation in the arid and semi-arid regions. Prudent management of water is, therefore, essential for a viable olive industry. Since irrigation is essential to ensure optimal yield, it is imperative to develop sound and efficient irrigation methods for olive groves, with irrigation scheduling techniques based on the plant’s actual need and optimal use of water (Ferna´ ndez et al., 2001). The olive tree (Olea europaea L.) is considered as one of the hypostomatous species best adapted to the semiarid Mediterranean environment, and is traditionally grown under dry conditions (Gimenez et al., 1997). The water absorption capacity of the root system as well as turgor maintenance have been suggested as likely explanations of this special adaptation of olive (Ferna´ ndez et al., 1991; Dichio et al., 1997). As olive is a tree with a low growth rate, physiological rather than morphological adjustments are predominant adaptation mechanisms to water deficit in the short term (Lakso, 1985). According to Xiloyannis et al. (1999), olive has three different adaptive strategies to cope with water stress: (i) by lowering the water content and water potentials of its tissues, the plant is able to establish a high potential gradient between leaves and roots, and therefore to utilize soil water up to
2.5 MPa; (ii) the tree stops shoot growth but not its photosynthetic activity under water stress conditions, which allows the continued assimilates production and their accumulation in various plant parts, in particular in the root system, creating a higher root/leaf ratio; (iii) osmotic adjustment, which was found to play an important role in maintaining cell turgor and leaf activities (Xiloyannis et al., 1999). Olive trees are generally considered as drought tolerant, and leaves can reach extremely low values of leaf water potential and relative water content (RWC)
3.5 MPa and 75–80%, respectively, before losing turgor (Hinckley et al., 1980; Lo Gullo and Salleo,

1988; Larsen et al., 1989). Giorio et al. (1999) showed that in olive trees subjected to prolonged water deficit under field conditions, leaf water potential does not control stomatal conductance which seems to be directly affected by soil moisture. This also confirmed the observation based on a split-root experiment, that the stomata closed in response to the drier part of the root system (Bongi and Palliotti, 1994). Despite the criticisms of this type of experiment in extending the results to field conditions (e.g. Kramer, 1983), the spatial distribution of both the root system and soil moisture may have a great influence on stomatal conductance and leaf water status in drip-irrigated olive trees. Good positive relationships were generally found between stomatal conductance, leaf water potential and soil moisture. This indicated that both hydraulic feedback and feed-forward mechanisms could be invoked in the response of stomata to soil drying (Giorio et al., 1999). Soil or root water status directly affecting stomata have been recognized in many plants when submitted either to split-root or to root pressurization experiments, but root to shoot chemical signaling have also been invoked to explain the independence of gs from shoot water status (Zhang and Davies, 1990). A large number of field studies have recently validated the existence of root-to shoot chemical signaling, supporting the previous studies under controlled environments (Davies et al., 2000), and opened new possibilities for agronomic applications. Partial rootzone drying (PRD) is an irrigation technique which requires that approximately half of the root system is maintained in a drying state while the remainder of the root system is irrigated. In most of the published data the PRD cycle includes 10–15 days, whereas the frequency of the switch is determined according to soil type and other factors such as rainfall, temperature and evaporative demand (Davies et al., 2000; Stoll et al., 2000; de Souza et al., 2003; dos Santos et al., 2003). This technique has been tested on several horticultural crops and fruit trees, including apple (Gowing et al., 1990), grapevine (Loveys et al., 1991), citrus (Hutton, 2000), almond (Heilmeier et al., 1990) and pear (Kang et al., 2002, 2003). In grapevine, a splitroot technique was applied to induce chemical signals in the root system of grown in field conditions (Loveys, 1991), showing that PRD reduced vine vigor and

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increased quality of fruit without affecting yield. Indeed, excessive vegetative growth was found a major problem for many fruit crops, since the use of assimilates in leaf growth restricts fruit set and development (Dry et al., 2000). Subsequent PRD field studies, mainly from Southern Australia, demonstrated that in addition to the benefit of PRD on the vineyards in terms of improvement of fruit quality and reduced canopy, crop water-use efficiency was also improved in this system (Loveys et al., 2000). These authors have also demonstrated that alternating wet and dry zones of the root system were crucial to trigger the continuous root-to-shoot signal, since the root system is not able to maintain root ABA production for long periods (Loveys et al., 2000). However, the advantage of PRD over other deficit irrigation techniques such as regulated deficit irrigation (RDI) is still debated. The aim of this work was to evaluate the effect of PRD system on the vegetative and reproductive growth, and water use efficiency of olive tree under semi-arid conditions of South Morocco. In addition to growth analysis and plant water relation parameters, yield components and fruit quality were evaluated under several PRD irrigation treatments.

2. Materials and methods 2.1. Field conditions and plant material This research was conducted during two consecutive seasons (2000–2002) in an experimental orchard at the Office Re´ gional de la Mise en Valeur Agricole (ORMVA), Station Saada (20 km west of Marrakech City), latitude: 318380 N, longitude: 88 040 W, altitude: 411.6 m). The climate is of Mediterranean type with hot and dry summers and mild winters, having an average annual rainfall of 250 mm. The soil is silty clay with an average CaCO3 content varying between 2.6 and 5.2%, poor in organic matter, with an organic matter content variable between 1.39 and 1.47% in the top soil surface layer (0–30 cm). The soil pH is slightly alkaline (pH between 8.1 and 8.5), the soil is also moderately saline (average EC around 0.73 mmho cm
1 in the top 60 cm). The olive orchard used consisted of 100 trees (O. europaea, cv, Picholine marocaine) planted in 1989 on 0.36 ha, and spaced 6 m  6 m. Standard cultural

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practices in the region were applied; trees were well fertilized each year in the beginning of January with 200 kg ha
1 potassium phosphate (45%), 138 kg ha
1 potassium sulfate (48%), and 350 kg ha
1 ammonium sulfate. N was applied thrice, with 200 kg ha
1 NH4NO3 during pre- and post-flowering stages, and 180 kg ha
1 during fruit growth. Olive trees were pruned each year in the end of December, to eliminate higher stumps. Plants were treated against olive leaf spot (Cycloconium oleaginum Cast.) in December– January by aerial spraying of a fungicide (copper oxychloride 50%), and against pests in June by application of Decis 50 (Deltamethrin). Weed control was by a combination of cultivation and a single hand weeding. 2.2. Irrigation treatments and experimental design Before the beginning of the experiment, all the plants were irrigated with the same amount of water based on the crop evapotranspiration (ETc), estimated from the potential evapotranspiration (ETo), calculated from the class A pan evaporation and using the Penman–Monteith crop coefficients (Kc = 0.7) proposed by FAO. The experiment started in March 2000 with the application of four irrigation treatments: control, irrigated with 100% of the Etc on the two sides of the trees every 2 weeks, PRD1, irrigated with 50% of control on one side, the other one kept dry, and switching every irrigation, PRD2, irrigated with 50% of the control on one side switching sides every two irrigations (4 weeks), and PRD3, irrigated with 100% of the control on one side, switching sides every irrigation. Watering was done every 2 weeks from March until October, corresponding to flowering and fruit ripening stages, respectively. Each tree was surrounded by a 3 m  3 m basin divided into two parts by a small ridge. Irrigation was conducted by filling the two sides of the basin with drip emitters placed on the two sides of the trees, at 1 m distance from the trunk, the discharge rate was 8 l/h, with two emitters per tree, one emitter per side per tree for the control, PRD1 and PRD2 and four emitters per tree, two emitters per side for PRD3. The experimental design was a split-plot, with 4 blocks and 4 treatments, each individual plot was constituted by 4 trees, the total number of trees (1 0 0) included 64 trees in the trial, and remaining 36 trees used as surrounding borders.

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2.3. Soil and plant water status

2.5. Growth, yield and fruit quality estimation

Soil water content was measured using a tethaprobe (Delta-T Devices, Cambridge, UK) on different depths, from surface to 100 cm, at three positions on each side of each olive tree. Leaf water potential (c) was measured using a Scholander pressure chamber model SKPM 1400 (Skye Instruments, Powys, UK) on three fully developed young leaves per plant with different positions in the tree, and eight replicate trees for each treatment. After cutting, the leaf was immediately enclosed in a bag filled with breathing air and the determination of the balancing pressure started in less than 1 min. This measurement was carried every 2 weeks. For diurnal leaf water potential, measurements were carried at predawn, at midday and at 16.00 PM.

Shoot vegetative growth was measured every 2 weeks during the season on four different shoots per tree, which were previously tagged. Olive fruits were harvested each year at maturity (end of October). Olive yield of the monitored trees was measured by complete hand harvesting and weighing of the fruits on a field balance. Olive fruit final diameter was also measured on 30 olives per tree on 8 replicates. Oil extraction was carried out with hexane (soxhlet) using the modified Folch method (Marzouk and Cherif, 1981). Thirty crushed olives were extracted three times for 20 min. Filtrates obtained after each extraction were mixed and washed with water. The extracted lipids were weighed after solvent removing under reduced pressure. Free fatty acids were determined according to AFNOR method (NF. T.60–204) (AFNOR, 1984).

2.4. Plant water status measurements 2.6. Data analysis Leaf stomatal resistance (Rs) was determined using a portable porometer (Delta-T AP4, Delta-T Devices, Cambridge, UK). The terminal part of the main leaf lobe was placed into the cup on the head unit which was positioned normal to the sun. Measurements were conducted during cloudless periods on exposed leaves between 10.00 h and 16.00 h. The device was calibrated before use on every occasion using the supplied calibration plate. Leaf Rs measurements were made on three different leaves per plant and eight replicate trees for each treatment, as for leaf c measurements. Three leaves per plant were detached in a similar position to determine relative water content (RWC) with eight replicate trees for each treatment. After cutting, the petiole was immediately immersed in distilled water inside a glass tube, which was immediately sealed. The tubes were then taken to the laboratory where the increased weight of the tubes was used to determine leaf fresh weight (FW). After 48 h in dim light, the leaves were weighed to obtain turgid weight (TW). Dry weight (DW) was then measured after oven drying at 80 8C for 48 h and relative water content was calculated as: RWC ¼ 100 

FW
DW TW
DW

Statistical data analysis was performed by analysis of variance (ANOVA). Bonferoni tests were carried out to test significance of differences between treatment means using the Sigmastat software (version 2.0, Jandel scientific software).

3. Results 3.1. Climate and soil water content Fig. 1 shows that the seasonal variation of rainfall and temperature followed the same pattern in the trial year than the average climate, with most of the rainfall occurring during the winter–spring seasons, between October and April, and very scarce or no rain during the summer, i.e. 57 mm in average between June and August. Similarly, minimum and maximum air temperature and evapotranspiration showed typical peak values between June and August (Fig. 1a and b). Most of the water requirements of olive trees were supplied through irrigation. The total water received by the plants, including rainfall was 23.02 m3/tree/ year for the control and PRD3 treatments and half of that (11.51 m3/tree/year) for both PRD1 and PRD2 treatments.

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Fig. 1. Average monthly air temperature (8C), evapotranspiration calculated from the class A Penman–Monteith pan evaporation, and rainfall at the experimental site during 2002 season (a), and average of 20 years (1978–1998) (b). (& Bars) Eto, ( Bars) rainfall, (!) temperature min, and (*) temperature max.

As a result, the irrigation treatments had a significant effect on soil water content (Fig. 2). During a typical PRD treatment cycle, the soil water content values decreased with soil depth for the watered sides, and increased with soil depth for the dry sides. The highest difference in soil water content was observed in the topsoil layers. The values of soil water content measured between 0 and 25 cm from the surface were significantly higher for the control than in the other treatments, varying between 0.30 and 0.37 m3 m
3 for the control and PRD watered sides and between 0.10 and 0.15 m3 m
3 for PRD dry sides. 3.2. Plant water status The diurnal variation of leaf stomatal resistance (Rs) during a typical PRD cycle showed that for all treatments, Rs values were higher in the afternoon than at noon and in the morning (Fig. 3a), where maximum of stomata opening was observed, with

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Fig. 2. Volumetric soil water contents under four treatments (control, PRD1, PRD2, PRD3), on wet side (*) and dry side (!), from surface to 100 cm depth (mean  S.E.) of eight replicates. Measurements were taken on April 25.

values ranging between 1.6–2.0, 2.5–3.1 and 3.1– 4.9 s cm
1 during the morning, noon and afternoon, respectively. No significant differences were observed between the treatments for the leaf Rs measurements made in the morning and at noon. By contrast, all PRD treatments showed higher Rs values compared to the control in the beginning of the afternoon, when the PRD-treated plants closed their stomata earlier than the control plants (Fig. 3a). The predawn measurement of leaf water potential during a typical PRD cycle showed that PRD1 and PRD2 treatments had significantly lower values compared to the control (Fig. 3b). However, as leaf water potential became more negative in the day, there was no significant difference between the treatments in the morning or afternoon. The evolution of leaf Rs measured throughout the olive growing season showed that control plants had significantly lower Rs values compared to the plants exposed to PRD treatments, including PRD3 which received the same amount of water than control, on

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Fig. 3. Diurnal evolution of leaf stomatal resistance (a) and water potential (b) of olive under four irrigation treatments (control, PRD1, PRD2, PRD3). Measurements were made at morning for Rs (0900–1000 AM) or predawn (0500 AM) for water potential (& bars), at noon (1200–1300, bars) and afternoon (1500–1600, bars) for both parameters. Values are means (S.E.) of eight replicates.

one side of the trees (Fig. 4a). Leaf water potential was not significantly affected by PRD treatments during the first 80 days after the beginning of the treatments (Fig. 4b), and then during the summer, the values of leaf water potential decreased substantially for all treatments, from values around
2.0 MPa to reach values around
3.4 MPa for PRD1 and PRD2, and
2.7 MPa for the control and PRD3 treatments, which received the same amount of water. Leaf relative water content decreased significantly from values around 99% in the beginning of the experiment to values around 60% at the end, with no significant differences between the control and PRD treatments (Fig. 4c). Despite the different responses of water relation parameters to PRD treatments, there were significant linear correlations

Fig. 4. Seasonal evolution of olive leaf stomatal resistance (a), water potential (b), and relative water content (c) during 2002 growing season, under four treatments: control (& ), PRD1 (*), PRD2 (~), PRD3 (^). Values are means (S.E.) of eight replicates.

between c and Rs (P < 0.001) and between c and RWC (P < 0.01) (Fig. 6). 3.3. Shoot and fruit growth The results in Fig. 5 show the effects of PRD treatments on the seasonal evolution of olive shoot and fruit growth, with a large decrease of shoot length under PRD, and no effect on fruit diameter. Final shoot length during the experimental season was reduced by around 35% under PRD1 and PRD2 compared to the control (Table 1), and not significantly affected under

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However, this difference was not significant. Similarly, the average fruit weight was slightly higher although not significantly, under control and PRD3 than under PRD1 and PRD2, which received half the amount of water received by the first two treatments. During the first season of experimentation (2002), olive yield was very low ranging between 25 and 38 kg per tree, and the PRD treatments did not result in any significant differences in yield, except between PRD 1 and PRD 3 (Table 2). During the second year, olive yield was almost three times higher compared to the previous year, and it was significantly higher for the control and PRD3 treatments (92.8 and 88.9 kg, respectively), compared to PRD1 and PRD2 treatments (78.7 and 74.5 kg, respectively). This PRDinduced yield reduction of 15–20% compared to the control, resulted from a 50% reduction in the total amount of water applied. Therefore, water use efficiency was increased by 60–70% under PRD1 and PRD2 treatments, compared to the control and PRD3 (Table 2). 3.4. Oil percentage and quality The oil percentage in olive fruit was slightly increased under PRD1 and PRD2 treatments, compared to the control, and PRD3 had the highest value (47.3%), although these differences were not statistically significant (Table 1). Similarly, oil acidity did not show any significant differences between PRD treatments and the control).

Fig. 5. Seasonal evolution of shoot length (a) and fruit diameter (b) during 2002 growing season, under four treatments: control (&), PRD1 (*), PRD2 (~), PRD3 (^). Values are means (S.E.) of eight replicates (30 olives per sample for fruit diameter).

PRD3 treatment which received the same amount water than the control. However, this effect was statistically significant only between the control and PRD1 and PRD2. The average olive fruit diameter was slightly higher under PRD treatments than the control (Table 1).

4. Discussion The PRD technique was recently developed in Australia and several other countries, based on the principle of irrigating only half of the root system,

Table 1 Shoot and fruit growth, oil percentage and acidity of olives exposed to four irrigation treatments (control, PRD1, PRD2, PRD3)a Parameter

Control

PRD1

PRD2

PRD3

Shoot length (cm) Fruit diameter (cm) Fruit weight (g/fruit) Oil acidity Oil % of DW

9.2  0.7 b 1.66  0.03 a 4.31  0.06 a 0.83  0.04 a 37.8  3.01a

5.9  0.4 a 1.69  0.03 a 4.27  0.04 a 0.81  0.13 a 39.6  0.8 a

6.0  0.4 a 1.71  0.03 a 4.14  0.07a 0.80  0.07 a 40.4  2.8 a

7.4  0.5 ab 1.69  0.03 a 4.32  0.06 a 0.96  0.09 a 47.5  .27 a

a

Means  S.E. For a given variable, mean values not sharing common letters are significantly different (P < 0.05).

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Table 2 Yield and water use efficiency of olive trees under four treatments (control, PRD1, PRD2 and PRD3) during 2002 and 2003 growing seasonsa Year/parameter

Control

2002/yield (kg tree
1) 2002/WUE (kg m
3) 2003/yield (kg tree
1) 2003/WUE (kg m
3)

32.0  4.2 1.4  0.2 92.8  2.5 4.0  0.1

a

PRD1 ab a a a

PRD2

24.8  2.0 2.2  0.2 78.7  1.2 6.8  0.1

a bc b b

28.6  1.7 2.5  0.1 74.5  1.9 6.5  0.2

PRD3 ab c b b

38.4  .6 b 1.7  0.1 ab 88.9  1.9 a 3.9  0.1 a

Means  S.E. For a given variable, mean values not sharing common letters are significantly different (P < 0.05).

while keeping the other half dry, and alternating the two sides during the growth season. The application of the PRD technique has already shown very promising results, first on grapes and subsequently on a variety of horticultural crops, resulting in tremendous increases in water use efficiency, compared to fully irrigated controls. The estimation of water requirements in our study was based on the Kc coefficient suggested by FAO for mature olive trees having 40–60% ground coverage by canopy (Kc = 0.70). This has resulted in total annual water supply of 6376 m3 ha
1, including the rainfall to the well-watered treatments (control and PRD3) and half of this amount to the PRD1 and PRD2 treatments. These levels of water supply allowed a clear distinction between the treatments. According to Gucci and Tattini (1997), annual water applications in olives could range from 180 to 2600 m3 ha
1, depending on rainfall, microclimate and soils of olive growing areas. In California for example, Goldhamer et al. (1993, 1994) applied a range of eight irrigation regimes on ‘Manzanillo’ olive trees based on Kc of between 0.16 and 0.85 resulting in mean seasonal irrigations of between 232 and 1016 mm. They found that there was no significant irrigation-related water stress within the Kc range of 0.65–0.85. Although, another study with mature trees irrigated at or below Kc 0.65 found that the trees were still under water stress based on their predawn leaf water potential, ant it was recommended that olives be irrigated based on Kc of 0.75 (Beede and Goldhamer, 1994). 4.1. Plant water status The PRD treatments affected significantly olive water relations, starting with an increase in stomatal resistance, and subsequently leaf water potential, with a small non-significant effect on leaf relative water

content (Fig. 4). These observations confirmed those of Bongi and Palliotti (1994) based on a split-root experiment on olive, showing that stomata closed in response to the drier part of the root system. Our results are also in agreement with the general observation that stomata often close in response to drought before any change in leaf water potential and/ or leaf relative water content is detectable (Gollan et al., 1985; Davies et al., 2000; Holbrook et al., 2002). It is also now well established that there is a droughtinduced root-to-leaf signaling, promoted by soil drying and reaching the leaves through the transpiration stream, which induces closure of stomata (Davies et al., 2000). It has also been reported that stomatal sensitivity to chemical signals was modulated by several factors including xylem pH (Wilkinson and Davies, 2002), leaf water potential (Tardieu and Davies, 1992) and plant nutritional status (Schurr et al., 1992). Most experiments investigating non-hydraulic root-to-shoot signaling during soil drying use splitroot system to compare half-dried plants to controls that receive twice as much of water (e.g. Bano et al., 1993; Stoll et al., 2000; Stikic et al., 2003). This approach usually relies on measurements of leaf water status, which, if statistically similar in the two groups, is offered in support of the conclusion that differences in gs between half-dried and fully watered plants must result from chemical signals. However, relying on measurements of leaf c and a control that receives about twice as much water as half-dried plants has serious limitations (Auge´ and Moore, 2002). A more conclusive approach is to compare treatments receiving about the same amount of water, with a different repartition. In the present study, the hypothesis of a PRD-induced chemical signal was supported by the observation that stomatal closure was similar in all PRD treatments, including PRD3, which received the same amount of water than the control, but on one side

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of its root system, and had exactly the same level and evolution of leaf c than the control (Fig. 4). Consistent with the evidence from other split root experiments (Davies et al., 2000; Dry et al., 2000), the leaf RWC of PRD plants did not differ significantly from those of well-watered controls (Fig. 4c.). These results support the hypothesis that a root-sourced signal might have triggered stomatal closure, which then resulted in maintaining leaf water status. Ferna´ ndez et al. (1997) reported that olive trees are able to prevent excessive water loss under soil water deficits or on days of high water demand by closing the stomata early in the day. Indeed, the diurnal time course of leaf c and stomatal Rs measured in our study during a typical PRD cycle showed that PRD treatments affected Rs mostly in the afternoon, while there was no significant effect on c (Fig. 3). Despite the discrepancies in the responses of water relations parameters to PRD treatments, there were significant linear correlations between c and Rs and between c and RWC (Fig. 6). It is certainly recognized that leaf water status interacts with stomatal conductance and transpiration and, under water stress, a good correlation is often observed between leaf c and stomatal conductance (Giorio et al., 1999). However, several authors reported a wide scatter when plotting gs versus c (Fereres, 1982; Ferna´ ndez et al., 1997). The stomatal response to other environmental factors (Jarvis et al., 1999) can be responsible for this scatter. In fact, the precise relationship seems to be dependent, among other factors, on the species studied, the drought history of the individuals studied, the growth conditions and the exact timing of the measurements (Tardieu and Simonneau, 1998; Flexas et al., 1999). For instance, stomatal response to vapor pressure deficit variations has been shown to have a great impact of the gs–c relationship in olive (Ferna´ ndez et al., 1997). In our study all water relations measurements were made on the same plants under the same environmental conditions, which may explain the significant correlations observed between the various water relation parameters across all irrigation treatments. 4.2. Vegetative growth, yield and WUE One important advantage of PRD is a better control of vegetative growth, without reducing

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Fig. 6. Relationship between olive leaf water potential and stomatal resistance (a) and leaf relative water content (b).

significantly fruit yield, which should lead to a considerable increase in water use efficiency (Loveys et al., 2000; Dry et al., 2001). Our results showed that PRD treatments induced a reduction of the average shoot length, which was higher than fruit yield reduction (Tables 1 and 2). However, the effect of PRD on total tree canopy could not be estimated, due to methodological difficulties, and vegetative growth was only measured by the average shoot length on tagged branches. Final shoot length was reduced by around 35% under PRD1 and PRD2, compared to the control (Table 1). On the other hand, yield reduction was mainly due to a decrease in fruit numbers (ca. 15% in average), whereas the average olive fruit diameter was slightly higher under PRD treatments

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than the control. However, this difference was only significant in case of PRD2, and there were no significant differences in fruit diameter between PRD1, PRD3 and the control. By contrast, the average fruit weight was slightly higher although not significant, under control and PRD3 than under PRD1 and PRD2, which received half the amount of water received by the first two treatments. Olive yield fluctuated between the 2 years of study (Table 2). Reasons for these fluctuations in yields of up to threefold were not obvious. The general decline in olive yield in 2002 was probably due to various factors, including disease incidence and the shift in irrigation techniques. These differences in the olive yield between seasons observed in our study had no association with water supply to the orchard, which remained largely similar between the two seasons. However, this trend of olive yield loss was generally observed across all south Moroccan regions during 2002, with national yield average around 0.764 t ha
1 versus 1.363 t ha
1 in 1998 (FAO, 2002). This could be therefore associated with seasonal yield variations typically observed in olives (Nuberg and Yunusa, 2003). Olive trees have been reported to alternate seasons of high yields with those of low yields, and the yield during the latter can be reduced by as much as 90% relative to those of the previous (Serrano, 1998). Sibbett (2002) discussed a range of possible factors that cause this phenomenon, including irrigation and time of harvest. It could also be interesting to speculate on the possible implications of plant hormonal status and its seasonal changes on the overall olive growth and fruit yield (Proietti and Tombesi, 1996). During the 2003 season, olive yield was significantly higher for the control and PRD3 treatments (92.8 and 88.9 kg/tree, respectively), compared to PRD1 and PRD2 treatments (78.7 and 74.5 kg/tree, respectively). These olive yields corresponding to as much as 25.5 t ha
1 in the control treatment were very high compared to the national average in Morocco, ranging between 0.5 and 1.6 t ha
1 under rainfed and irrigated conditions, respectively (Bamouh, 1998). However, according to FAO, good commercial olive yields under irrigated conditions are 50–65 kg/tree of fruit with a possible maximum of 100 kg/tree of fruit (http://www.fao.org/). The high yield level found in our study was also similar to

those recently reported in some Australian locations, under well-watered and well-managed conditions (Nuberg and Yunusa, 2003). Similarly in Crete, Chartzoulakis et al. (1992) reported high olive yields under well-irrigated conditions, reaching 16 t ha
1 with maximum water use. In Italy, Deidda et al. (1990), found the highest yield by table olives when irrigation was determined using a Kc of 0.66. In Greece, however, Michelakis (1990) found no difference in yields of table olives irrigated at Kc of between 0.3 and 0.6, equivalent to 260 and 570 mm of applied water, respectively. The slight PRD-induced yield reduction (15– 20%) compared to the control (Table 2) was achieved with 50% reduction in the total amount of water applied, which resulted in a water use efficiency increase by 60–70% under PRD1 and PRD2 treatments, compared to the control and PRD3 (Table 2). The average water-use efficiency based on ET was much higher during the 2003 season compared to 2002, most probably for similar reasons discussed above, for yield fluctuation. However, the PRD advantage in water-use efficiency was consistently observed during both seasons. There is a lack of information on the water-use efficiency of olives for Moroccan environments, but the amount of olive produced per unit of water used was within the range found in other regions (Michelakis, 1990; Chartzoulakis et al., 1992; Nuberg and Yunusa, 2003), and PRD effect on yield and WUE was in agreement with several previous reports on other crops. In grapevine, field results from Australia demonstrated that PRD significantly improved water-use efficiency (Loveys et al., 2000). These findings were more recently confirmed in Portugal (dos Santos et al., 2003), with results showing an increase of WUE by about 80% in both PRD and regulated deficit irrigation (RDI), when compared with fully irrigated controls, as a result of almost similar yields while reducing water supply by 50%. In olive, previous studies by Goldhamer (1999) tested the RDI strategy on olive and showed that application of 50% of the water requirement during critical growth periods did not result in any significant yield decreases. Furthermore, these studies showed the RDI improved the oil content in the fruits, without compromising total yield, thus improving total oil yield (Goldhamer, 1999).

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4.3. Oil percentage and quality In addition to the benefits in terms of reduced canopy density and consequent improvement of water-use efficiency, the advantage of PRD compared with standard conventional irrigation techniques, comes also in the improvement of fruit quality (Loveys et al., 2000). The olive fruit quality was tested in this study by analyzing oil content and acidity. The oil percentage in olive fruit was slightly increased under PRD1 and PRD2 treatments, compared to the control, although these differences were not statistically significant (Table 1). Similarly, oil acidity did not show any significant differences between PRD treatments and the control. Our results differed from previous studies that reported significant effects of irrigation treatments on olive oil content or quality (Lavee et al., 1990; Goldhamer, 1999; Motilva et al., 1999). Alegre et al. (2002) found that periods of water stress under RDI forwarded olive ripening time and seemed to increase the amount of extracted oil. However, no differences were observed in oil yield between control and RDI-25%, with RDI-50% reaching the highest yield. Similarly, d’Andria and Morelli (2002) compared oil quality between non irrigated plants and plants receiving 33, 66 and 100% of ETc, on five olive cultivars, and found that the fatty acid composition was not affected by the irrigation regime, while the polyphenol content decreased when irrigation level increased. The positive yield response to irrigation observed was attributed to increased mean fruit weight and number of fruit per plant, whereas the oil fatty acid composition was not affected by irrigation regime. These discrepancies of the effects of irrigation techniques on olive oil yield and quality might be related to variations in the olive genotypes and environmental conditions.

5. Conclusions The PRD irrigation technique is based on the basic assumption that controlling vegetative vigor by reducing the amount of water use would not automatically induce dramatic yield reductions, and thus results in higher water use efficiencies (Loveys et al., 2000). This technique has already been adapted for use in the irrigated production of various crops,

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including grapes, cereals, citrus, and vegetables (Dry and Loveys, 1999; Dry et al., 2001; Davies et al., 2000; Hutton, 2000; Stoll et al., 2000; dos Santos et al., 2003). However, little work has been done to assess the agronomic value of PRD technique under arid or semi-arid conditions. The study reported here is the first systematic evaluation of olive growth and yield under PRD regulation. Importantly, this study has confirmed the hypothetical benefits of PRD on water use efficiency in olive, by maintaining the yield and fruit quality, while reducing by half the water supply. However, the lack of comparison between PRD and RDI in this study does not allow resolving the question whether the effects observed were specifically triggered by PRD or if they were simply associated with water deficits. In addition, this study could not address all the issues related to olive yield stability and water-use efficiency, and there are still areas for which further studies are required. These include a better understanding the underlying mechanisms of yield fluctuations, and yield–water-use relationships under semi-arid conditions, and comparison of PRD with other deficit irrigation techniques, such as RDI. Further physiological studies will also help to understand the mechanisms controlling PRD grown olive plants. This knowledge will be valuable in adapting PRD irrigation techniques to local conditions, and remedy to the current lack of consistency in irrigation practices amongst olive growers in south Morocco. Acknowledgements This research was financially supported by the EU INCOMED project IRRISPLIT (ICA-1999-100008). The authors gratefully acknowledge the technical assistance provided in conducting the experiments by Mr. El Hasnaoui at ORMVA-Saada Station, Mr. Ouzzine at ORMVA-Marrakech, and the technicians of Plant Physiology Lab. at FSSM-Marrakech. References AFNOR, 1984. Recueil de normes franc¸aises des corps gras, graines ole´ agineuses et produits derives, 3rd ed. AFNOR, Paris. Alegre, S., Marsal, J., Mata, M., Arbone´ s, A., Girona, J., Tovar, M.J., 2002. Regulated deficit irrigation in olive trees (Olea europea L. cv arbequina) for oil production. Acta Hort. 586, 259–262.

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