Evaluation of irrigation in a converted, rain fed olive orchard: the transition year
Annual Report – introducing irrigation to traditional olive orchards

Keywords: Olive, olea europea, water application, oil quality, deficit irrigation.

Abstract
Olives for oil production are traditionally grown without irrigation. In spite of this, water application during dry growing seasons has been shown to substantially increase yields and consequently, it is becoming more and more common to find irrigation systems in established orchards in traditional regions of olive cultivation. The reported research has been initiated in order to determine the response of mature 'Souri' olive (olea europea) trees, previously cultivated under rain fed conditions, to irrigation. Results from the initial growing season following introduction of treatments are presented Trees in a 15 year old orchard in Bakaa el Garbiya receive one of seven irrigation treatments (0, 25, 50, 75, 100, and 125% return of potential evapotranspiration applied throughout the season or 50% annual return concentrated during predetermined growing periods). In summer (August) samplings, mid-day leaf water potential, which ranged from -2.7 MPa in the rain-fed to -2.1 MPa in trees receiving 125% return, was negatively correlated (r2=0.75) with water dosage. Vegetative growth rate was positively (r2=0.69) correlated to the water dosage. Nutrient content in leaves was not affected by the irrigation treatment, except potassium which was found to be lower in the rain fed as compared to irrigated treatments. Fruit number was not affected by irrigation, as treatments commenced subsequent to fruit-set. Fruit growth rate was retarded in rain fed trees but, the increased size of irrigated fruit did not affect overall oil production per fruit as lower oil content was found in fruit from irrigated trees (22-23% compared to 27%). Irrigation negatively affected oil quality. Oil from irrigated trees had higher free fatty acids and lower content of polyphenol compounds. Furthermore, oil from the rain fed trees possessed higher values of positive organoleptic attributes including “fruity”, “pungent” and “bitter” than oil from trees receiving maximum irrigation return. In the initial year of water application, irrigation quantity increased vegetative growth, had no quantitative effect on oil production and appeared to negatively affect oil quality. Timing of deficit irrigation was not found to have any effect on the measured parameters.

INTRODUCTION
Olive (olea europea) trees are considered to be tolerant of drought conditions. Olive leaves being small, leathery, with a thick cuticle on the dorsal surface and sunken stomata below, may enable appreciably reduced rates of transpiration and allow the trees to sustain dry soils and severe climatic conditions. While irrigation is standard practice for canning olives, most olive cultivation for oil is rain-fed. In Israel, only 2,000 of 18,500 ha of orchards are irrigated while the rest are cultivated according to traditional practice. In the entire Mediterranean region, there has been recent interest in the irrigation of oil olives as evidence of benefits of water application during the dry summer months have been observed.
Past trials have shown supplemental water to increase yields of fruit and oil. The main traditionally grown cultivar in Israel, “Souri” was irrigated in a number of experiments in the 1950s (Spiegel 1955, Samish and Spiegel 1961). Results from these projects showed that initiation of irrigation increased vegetative growth rates and caused larger fruits and that continued irrigation into a second season resulted in larger numbers of fruit on the trees. Spiegel found reduced oil percentages in fresh olives but total oil per fruit that increased in irrigated trees. Irrigation was touted as having particular importance in “on” years as early season irrigation was claimed to stimulate growth responsible for yields in the following season and late season irrigation was found to assure large fruits even in trees with heavy fruit loads. An additional study from the 1980s found that a single 75 mm water application at the end of July doubled fruit and oil yields and reduced oil in fresh fruit by 6-8% in an orchard of “Souri” olives. The irrigation did not influence fruit size or mesocarp:endocarp ratio (Lavee et al 1990). Lavee did not find any effect of additional water applications given earlier in the season. Further experiments studied irrigation regimes in three olive cultivars; Barnea, Souri and Muhasen (Lavee et al 1997). Irrigation increased oil yields for all of the cultivars. Irrigation given throughout the growing season was found in this experiment to be advantageous to seasonal irrigation where water was applied only in the spring and fall and not during the endocarp hardening stage. Response to irrigation was found to be cultivar-dependant. Application of 250 mm water doubled yields for “Barnea” and 500 mm annual application did not further increase yields. “Souri” and “Muhasen” yields were increase 70% by 250 mm water and 80% with 500 mm seasonal application. All of these experiments applied pre-determined amounts of water without consideration for climate or actual requirement of the trees and none of them considered effect of the irrigation on oil quality.
The main objective of the current study was to determine the response of olive trees in a traditional rain-fed orchard to irrigation regimes with specific attention paid to effects on oil quality. Results of the first year of the experiment are particularly important due to the fact that all the trees began with identical history and had a similar fruit load. Subsequent years of the experiment are expected to be largely influenced by development of fruit number per tree resulting from the irrigation treatments.

MATERIALS AND METHODS
The experiment was conducted in 2 ha of a large commercial olive orchard containing 15 year old “Souri” trees planted in 10 by 10 meter spacing. The orchard is located adjacent to Bakaa El Garbiya in Israel (lat. 32o25’N; long. 35o03’E). The experimental section was divided into 30 plots, each containing 9 to 15 trees receiving a given treatment. Two central trees in each plot were used for diagnostic purposes and the rest as boundary trees. The experiment evaluated two variables (water quantity and timing of irrigation) in 4-5 replicates. Six water application levels were tested: non-irrigated, 25%, 50%, 75%, 100% and 125% return of potential evapotranspiration (ETp). Timing of irrigation was evaluated under deficit irrigation level of 50% for two treatments: continuous full season irrigation (50%) and seasonal irrigation (50b). The seasonal treatment received water during two periods: at the beginning of the growing season during budding and at the end of the season during fruit filling and ripening. During these periods, trees were applied water according to 100% return of ETp. Total annual water application for the seasonal treatment was equal to that of the 50% continuous treatment (Figure 1). Water application quantity was determined for the various treatments by multiplying ETp by percent canopy cover and by the treatment factor. Canopy cover was estimated to be 50% according to shaded area at solar noon. Potential evapotranspiration was calculated using a modified Penman equation (Monteith 1965) based on meteorological data from an on-site station. Irrigation began on the 14th of April and ended for the season on the 24th of November. Each individual tree was irrigated via nine 3.5 l/h pressure compensating drip emitters (Netafim Ltd, Tel Aviv, Israel) spaced every 0.5 m in a loop approximately 1 meter from the tree’s trunk. Water was distributed to each treatment via an independent valve and main line using an automated “Sapir”control system (Talgil, Kiryat Motzkin, Israel). Irrigation frequency was once every 3 days before June and after September and once every two days during the summer months. Total water applied by irrigation during the season ranged from 115 mm (25% return) to 530 mm (125% return).
Fertilization was according to accepted practice in rain-fed orchards. During the winter, prior to initiation of the experiment, two applications of foliar potassium chloride (3%) were given. No additional nutrients were applied during the irrigation season. Soil water content was evaluated gravimetrically in August. Soil samples were taken from three depths (0-30, 30-60, and 60-90 cm) beneath drippers in the morning immediately prior to irrigation events. Branches were covered with sealed aluminum foil bags the preceding evening were removed between 11:00 and 13:00 for determination of leaf water potential using a pressure bomb (MRC, Israel). Vegetative growth rate was measured on 4 labeled branches on each diagnostic tree. Diagnostic leaves (the newest mature leaf from the past year’s growth) were sampled on the 2nd of August and analyzed for total nitrogen, phosphorus, potassium, calcium, magnesium, iron, sodium, boron and chloride content.
Fruit growth rate was determined by monthly sampling of 10 representative fruits per tree from mid May until the harvest in October. Fruit was removed using mechanical rakes from individual trees onto netting, gathered and weighed. Mesocarp-endocarp ratio, ripening level (Anonymous 1990), and oil content of the mesocarp in chemical extraction (Avidan et al 1999) were determined for fruit from the diagnostic trees. Oil was extracted using a laboratory scale press (Oliomio, Italy) from 10 kg fruit from each tree within one week following harvest. Fruit was stored at 5oC between harvest and oil extraction. Oil samples were tested for the chemical quality parameters peroxides, free fatty acids (FFAs) and total polyphenol content (Anonymous 2003). Oil from the rain-fed treatment and the treatments receiving the greatest irrigation quantities (100 and 125%) were evaluated for organolptic properties.

RESULTS
Tangible evidence of the irrigation treatments were found in soil moisture and leaf water potential measurements. Significant positive agreement between irrigation level and soil water content was found (Figure 2). Average profile (0-90 cm) soil moisture ranged from 11% in the rain-fed treatment to 30% for 125% return. Deficit irrigation treatments (25 and 50%) had soil moisture content of around 20%. Leaf water potential was generally associated with soil water content and showed significant 2nd order negative correlation with irrigation level (Figure 3). Lowest leaf water potential (-2.7 MPa) was found for the non-irrigated treatment. The potential increased to -2.3 MPa in the 25% and 50% treatments and to -2.1 in the 125% return.
Branch expansion was affected by irrigation quantity but not by timing (Figure 4). Vegetative growth increased linearly as a function of irrigation level. Over the 4 month period May-September, branches expanded 12 cm for non-irrigated trees and 20.4 cm for the trees receiving the most irrigation water. The best fit linear relationship between water level and branch growth indicates that each 10% increase in return of ETp causes to 5.7 mm additional growth.
Average content of minerals in the leaves not significantly affected by irrigation treatments were as follows (in %): nitrogen 1.6 ± 0.02; phosphorus 0.15 ± 0.01; calcium 1.4 ± 0.2; magnesium 0.16 ± 0.02; chloride 0.27 ± 0.02; sodium 0.16 ± 0.02. Average levels of iron and boron in leaves were (in mg/kg): 82 ± 12 and 24 ± 3 accordingly. Potassium content in leaves of non-irrigated trees (1.0%) was significantly lower than in all the irrigated treatments (average of 1.2% with non significant differences between the treatments). All mineral content levels measured fall within ranges considered desirable for olives in Israel.
The experimental orchard was largely defined by the high level of variability concerning fruit production. The number of fruits per tree ranged from near zero to 50,000 and fruit yield from 0 to 70 kg. No differences were found when comparing irrigation treatments regarding fruit number or yield per tree. Fruit growth rate is presented in Figure 5. Irrigation began to affect fruit size from the beginning of August and, from the end of August, fruit from non-irrigated trees was significantly smaller (3.2 ± 0.4 g) than fruit from any of the irrigated treatments (average of 3.9 ± 0.4 g). Irrigation timing did not affect fruit size.
Oil content was determined for the mesocarp. In order to relate this to commercial oil percent the mesocarp content was multiplied by fruit weight not including the endocarp. These values are presented in Figure 6. Percent oil was found to be significantly negatively related in a logarithmic fashion to irrigation quantity. The most substantial decrease in oil occurred between the non-irrigated treatment (27% oil) and the 25% treatment (23.4%). Differences between these oil levels were significant. Further increase in irrigation caused a much less drastic decrease in oil with 21.5% oil found in fruit from trees receiving the highest irrigation level. Irrigation timing was not found to influence oil percentage in the fruit. Total oil content per fruit was determined by multiplying commercial percent oil (Figure 6) with average fruit weight (Figure 5) and is illustrated in Figure 7. Fruit oil content of around 0.85 g was found with no differentiation between irrigation regimes. While the total content per fruit was not different, there was a difference in variability of the results as the standard deviation of oil content of irrigated fruit was lower (± 0.1 g) than that of the non-irrigated fruit (± 0.2 g). In general, the non-irrigated treatment showed a pattern where in trees with higher fruit loads, oil per fruit was low (around 0.5 g/fruit) and in trees with low fruit loads, oil per fruit was higher (around 1.1 g/fruit). Ripeness level at the time of harvest and mesocarp:endocarp ratio, similarly to oil content, were not found to be a function of irrigation levels or timing.
Chemical parameters measured to assess oil quality included peroxide level, content of free fatty acids, and total polyphenol content. Peroxide levels ranged from 2.7 to 5 meq kg-1 oil and were not affected by irrigation regimes. Free fatty acids increased with increased irrigation levels (Figure 8A). FFA in oil from rain-fed trees was approximately 0.6%, within the “extra virgin” classification while irrigation caused higher FFA values reducing the oil to “virgin” classification. Only the highest irrigation level led to FFA (1.49%) that was significantly higher than the level found for rain fed trees. Similarly, content of polyphenol compounds in the oil from the rain-fed treatment was higher than that of the irrigated treatments (Figure 8B). The higher polyphenol level in the 0% irrigation treatment was significantly different from most of the irrigated treatments while no significant differences were found between those treatments themselves. No effects on oil quality were found as a function of irrigation timing. Organoleptic testing indicated substantial differences in quality between oil from the 0% and highest irrigation (100 and 125%) treatments. Oil from rain fed olives had a higher fruity aroma (level 5) compared to that from irrigated olives (level 3). Rain fed trees also produced oil that was more pungent and bitter than oil from irrigated trees.

DISCUSSION
Water quantities provided over the course of a single irrigation season well reflected the experimental plan (Figure 1) and were manifested in differences in soil water content (Figure 2), leaf water potential (Figure 3) and vegetative growth (Figure 4). As irrigation was initiated only in April of 2005 and the number of fruits per tree determined in the preceding season, irrigation could only influence the crop’s yield via fruit size and oil content. In general, these parameters demonstrated the most significantly differences when the rain fed treatment was compared to any irrigated treatment. Irrigation led to larger fruit (Figure 5) along with decreased oil content (Figure 6) such that total oil per fruit from rain-fed trees was not different from that of irrigated trees (Figure 7). Noteworthy difference in deviation from average oil per fruit was observed as deviation in rain-fed plots was substantially greater than that in irrigated plots. This finding, and the fact that trees with higher fruit loads consistently were found to have low values of oil per fruit, support Spiegel’s (1955) theory that irrigation is particularly important in “on” years when fruit load is high in order to guarantee maximum oil accumulation. The fact that results from the rain-fed treatment were significantly differentiated from all of the irrigated treatments indicates the importance of the essence of water application during the dry season and suggests that root growth and activity including water and nutrient uptake are promoted in the continuously wetted soil. Roots of drip irrigated trees are expected to concentrate in the wetted-irrigated zones of while roots of rain-fed trees will spread out throughout the entire soil in order to utilize all of the available water and minimize stress.
Mineral accumulation in leaves was not found to vary with irrigation regime and nutrient levels found were all within those considered desirable according to the World Olive Encyclopedia (1996) and we therefore conclude that mineral nutrition was not a factor in the results. The only mineral showing having levels distinguished by treatments was potassium for which higher levels were found in leaves from the irrigated trees (1.2%) as compared to leaves in the rain-fed trees (1.0%). Potassium is known as one of the more important nutrients influencing olive tree development and oil production (Fernandez-Escobar et al., 1999). Such reduced K availability for uptake and transport to leaves as a function of dry conditions has been found for other plants (Kuchenbuch et al 1986). At any rate, the lower levels of K in the rain-fed treatments did not fall from acceptable levels and did not indicate deficiencies.
Oil extracted from fruit of non-irrigated trees was of higher quality than that from irrigated trees showing lower acidity, higher phenol content and richer, fuller taste. Similar results were reported by Romero et al (2002) who did not find differences between different levels of irrigation of arbequina olives but did find significant increases in quality parameters in oil from non-irrigated trees. The increased free fatty acids found in irrigated olives could be a function of increased sensitivity to fruit, with higher water content and thinner epidermis layer, to mechanical injury caused by pressure (Patumi et al 2002). Mechanical injury accrued during harvest causes increased enzymatic processes that are detrimental to oil quality. This phenomenon increases with increased time taken between harvest and oil extraction. In the present experiment, harvested olive samples waited up to one week in refrigerated conditions. Organoleptic testing also indicated that non-irrigated olives resulted in higher quality oil. A possible explanation is that when water content of olives is greater, as in the fruit of irrigated trees, there is a greater extent of the removal hydrophilic components of taste and smell in oil during oil extraction and processing. The organoleptic differences corresponded with differences in polyphenol content of the oil. Some of the parameters of taste are polyphenol compounds themselves and it would seem that their decrease results in reduced richness of the oil’s taste.
The experiment examined the effect of timing of water application within the season in order to test a theory that if deficit irrigation is necessary, than application of the water concentrated into period of particular plant sensitivity to stress is desirable over continuous application. First year results do not support this hypothesis. None of the parameters examined indicated any difference between 50% return of ETp given continuously compared with the same total water applied only during budding and fruit filling stages.
Results reported reflect a single, transitional year wherein a rain-fed orchard was introduced to irrigation. Such a transition year is unique in that the number of fruit per tree was determined prior to instigation of irrigation, and therefore any effects of the treatments on yield could only be a function of fruit size or oil content. This year’s results showed that irrigation did not improve oil yield of the orchard and that oil quality of irrigated trees was compromised. In spite of this, the substantial increased vegetative growth that was measured gives reason to expect relatively greater numbers of fruit as a function of increased irrigation quantity in the following season.

ACKNOWLEDGEMENTS
The research receives continuing support and cooperation from The Director’s fund of Israel’s Agricultural Research Organization, the Netafim Company, and the Middle East Regional Irrigation Management Information System (MERIMIS). Special thanks to Jalaal Abu Teuma for providing the orchard, Muchamed Abu Teuma for experimental operations, Oren Hechter and Kibbutz Magal for harvest support, and to Professor Shimon Lavee for consultation and advice.

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