Olive tree shows a great capacity to tolerate the long summer water shortage by means of numerous strategic devices aimed to control water losses and increase water uptake from soil. Besides the anatomical and morphological features of leaves (small size, high specific leaf weight, thick and waxy cuticle, hairy leaf surfaces, high stomatal density), typical adaptations of drought-tolerant plants, olive presents specialized physiological and biochemical mechanisms.
Under severe drought stress, olive tree significantly lowers water content and water potentials of its tissues establishing a high potential gradient between leaves and roots (predawn LWP values of −7.0 MPa and −3.5 MPa, respectively) which allows root system to utilize water up to soil water potential of -2.5 MPa. Such value is well below the permanent wilting point, measured at -1.5 MPa for most of the fruit species. Under such conditions, and especially in soil characterized by a good water storage capacity, olive plants have access to a greater and readily available soil water (between field capacity and -2.5 MPa), so withstanding long drought period (Xiloyannis, Gucci and Dichio 2003). In olive tree, stomata progressively reduce their activity starting from predawn LWP below -0.9 MPa, and they can remain open up to -7.0 MPa (Xiloyannis et al. 1999). A progressive closure of stomata as predawn LWP decreased was observed in other fruit tree species but their stomatal closure was reached at values of predawn LWP ranging from -1.5 to -2.5 MPa (Lakso 1979; Castel and Fereres, 1982). Under stressful conditions, olive tissues are able to transpire large amounts of water, accumulated during the afternoon and night, ensuring a certain level of leaf functionality. As a matter of fact, olive leaves can give up to transpiration about 60% of the water stored in their tissues contributing to the demands of transpiration as stress increases up to extreme values (Xiloyannis et al. 1999). At predawn LWP of -6.0 MPa, olive maintains a certain transpirative and photosynthetic activity (around 10% and 20%, respectively, of that of well-watered plants), that allows the plants to produce assimilates and accumulate them in the various organs. Particularly, long-term soil water deficit reduces in young olive trees the development of the above-ground organs with respect to the under-ground part (roots and stump), so raising the under/above-ground ratio. The effect is particularly marked in leaf area, that is significantly reduced under rainfed conditions (47% lesser than irrigated plants at the seventh year from planting) (Dichio et al. 2002). Such reduction of canopy size limits the water demand for transpiration.
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Another strategy adopted by the olive tree to overcome water deficit is osmotic adjustment which consists in either active synthesis and accumulation of osmotically active compounds (carbohydrates, some aminoacids, organic and inorganic acids, cations and anions) within cells (active osmotic adjustment) or loss of water from plant cells, with the consequent increase in osmolyte concentration (passive osmotic adjustment) (Xiloyannis et al. 1999; Cataldi et al. 2000; Sofo et al. 2004b; Dichio et al. 2009). This physiological process is measured by the variation in osmotic potential within plant tissues (Dichio et al. 2007). A higher concentration of osmolytes (particularly mannitol, glucose and proline) facilitates water diffusion in cells and maintains the turgor of plant tissues essential for plant physiological activity. The maintenance of cell turgor in roots also avoids or delays the separation of these organs from the soil. Under drought conditions, olive trees activate metabolic processes to produce substances that increase cell tissue rigidity, likely by regulating some enzymes involved in lignin biosynthesis such as peroxidases (Sofo et al. 2004b). This mechanism results in an increase in elastic modulus (ε) as cell walls become more rigid or thicker. Higher ε values produces a faster turgor loss of cells for a given percentage of dehydration. An increase of cell tissue rigidity together with low values of Ψπ, due to active and passive osmotic adjustment, can be responsible for the observed high gradients of water potential between leaves and soil, and thus can facilitate water extraction from the soil.
In olive trees, the activities of some antioxidant enzymes significantly increase in leaves and roots of drought-stressed plants (Sofo et al. 2004a). These enzymes limit the cellular damages caused by AOS, so allowing the plant to maintain a photosynthetic efficiency also under severe drought conditions (Xiloyannis, Gucci and Dichio 2003). Significant increases of lipoxygenase (LOX) activity and malondialdehyde (MDA) content, two markers of oxidative stress, were also found during the progressive increment of drought stress in both leaf and root tissues of olive plants (Sofo et al. 2004a, 2004b), so suggesting that water deficit is associated with the oxidation of membrane lipids. In olive plants under drought stress, the damage of photosynthetic apparatus, and the resulting decrease in photosynthetic efficiency, occurs particularly by means of the light-dependent inactivation of the photosystem II (photoinhibition) and the oxidation of chloroplastic pigments (photo-oxidation) (Angelopoulos, Dichio and Xiloyannis 1996; Sofo et al. 2009). Although these damages, olive tree is able to recover its water status faster (5 days) than other fruit tree species even if it shows a slow recovery of photosynthesis and transpiration (Angelopoulos, Dichio and Xiloyannis 1996).
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Finally, olive tree can respond to short period stress by regulating the activity and the expression of its root water channels (aquaporins) (Tataranni 2009). As the adverse conditions continue, root suberification occurs, so avoiding dehydration. In fact, an increase of suberification process was observed in root cell walls at exodermis and endodermis level. Under such conditions, root activity recovery is preceded by the emergence of root primordia (Tataranni 2009).
Effects of irrigation management on productivity, and fruit and oil quality
Generally, irrigation raises significantly the vegetative growth of olive tree and its productive response. This leads to early bearing, steady and satisfactory yields, and improvement of fruit features. In addition, as the productive tree performances are not influenced by moderate levels of drought stress, a reduced irrigation is recommended in arid and semi-arid areas to save water. Deficit irrigation strategies in olive orchards can be applied following different approaches (Fereres and Soriano 2007).
Sustained deficit irrigation (SDI) distributes a reduced water volume, as percentage of ETc, throughout the whole irrigation season. Many studies, carried out under diverse pedo-climatic conditions, compared irrigation regimes based on different levels of ETc restitution and their influence on fruit and oil quality of different olive cultivars. Patumi et al. (2002), Magliulo et al. (2003), d'Andria et al. (2004), Grattan et al. (2006), Berenguer et al. (2006) and Dabbou et al. (2010) found that a restitution ranging from 66 to 75% of ETc is enough to obtain good yields similar to those harvested from fully irrigated trees. However, phenolic compounds in oils significantly decreased passing from the lowest to the highest irrigation levels. Although reduction in polyphenols content modified slightly sensory properties of oils decreasing their bitterness and pungency, it did not compromise oil storage capacity. Stefanoudaki et al. (2009) referred about a contradictory effect of irrigation which decreased contents of both undesirable (pungent and bitter attributes) and favourable sensory qualities (intense green notes). As a matter of fact, irrigation could be managed to meet consumer's particular needs. Patumi et al. (1999), Tovar, Motilva and Paz Romero (2001) and Tovar et al. (2002) studied the effect of several irrigation treatments on L-phenylalanine ammonia-lyase activity (PAL) in developing fruits. PAL is the key enzyme in phenolic biosynthesis and a high PAL activity is associated with the accumulation of anthocyanins and other phenolic compounds in tissues of several fruit species (Weaver and Herrmann 1997; Ryan et al. 2002). PAL activity and phenolic level decreased during fruit development and were influenced by irrigation, being lowered as the water supplied increased.
Regulated deficit irrigation (RDI), firstly proposed by Chalmers, Mitchell and van Heek (1981), reduces water supplies during specific periods characterized by a less plant sensibility to water stress with minimal effects on yield. While water deficit can reduce fruit and oil yields due to the effect on flowering, fruit set, and oil accumulation phases, many researchers agree in identifying pit hardening, generally occurring in midsummer, as the less sensitive phenological stage of olive tree (Lavee and Wodner 1991; Goldhamer 1999; Moriana et al. 2003; Orgaz and Fereres, 2004; Iniesta et al. 2009). On the other hand, in environments characterized by good spring rainfall and deep soil profiles, irrigation applied from the beginning of pit hardening to early fruit veraison could control tree vigour while maintaining crop yield and oil quality (Gómez-Rico et al. 2006; Tognetti et al. 2006 and 2007; d'Andria et al. 2009).
Partial root-zone drying (PRD) is an irrigation strategy aimed to maintain in a drying state at least half of the tree root system while the other half is kept under wet soil conditions. Such technique is based on the existence of a chemical signal between root and shoot which determines plant responses to soil drought stress limiting shoot and leaf growth. Particularly, under mild soil drought stress, abscisic acid (ABA), moving in the xylem from the roots, reaches the epigean parts of the tree, where it regulates stomatal movement and shoot meristem activity. The alternation of wet and dry conditions in the soil is a requirement to allow roots to produce ABA. Generally, a PRD cycle lasts 10 to 15 days, depending on soil type and other factors such as rainfall and temperature (Davies et al. 2000; Stoll et al. 2000; Stikic et al. 2003; Sepaskhah and Ahmadi, 2010). Wahbi et al. (2005) reported that PRD strategies slightly reduced yield (15-20%) and increased plant water use efficiency of 60-70%. Aganchich et al. (2008) showed that PRD irrigation of 'Picholine marocaine' plants, besides water saving (50%), positively affects both fruit biometric parameters and oil production (highest oil content, precocious fruit ripeness), and causes increases in total polyphenol. Instead, Fernández et al. (2006), comparing PRD and RDI treatments (50% ETc), did not find significant improvement of the physiological parameters measured. Such findings led the authors to advise against the use of PRD because of its high cost and difficulty in management.
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Moriana et al. (2003) proposed an irrigation scheduling adapted to the typical alternate bearing habit of the olive which supplies water only in 'on' years. Although this approach was successfully tested in pistachio plants (Stevenson and Shackel 1998), the authors expressed some doubts on the viability of such program for olive. As a matter of fact, an exceptional severe drought during the rainfed 'off' year, able to completely deplete water in the soil profile, could have an important impact in flowering and fruit set of the following 'on' year resulting in very low yields. On the other hand, Palese et al. (2010) reported that after a rainfed 'off' year olive trees continuously non-irrigated showed a great capacity of recovery, which led to a vegetative activity and productive response similar to those of the irrigated plants. This is due to a complete replenishment of soil water reserve following autumn-winter rains. As reported by Martín-Vertedor et al. (2011) it could be advisable the application of SDI during 'off' year when a lower water consumption occurred. Therefore, the optimal irrigation amount could be determined each year, according to crop load levels.
Strategies for rainwater capture and storage under rainfed conditions
In traditional olive cultivation areas of Mediterranean Basin, rainfall is the only source for the olive tree water needs being the use of irrigation devoted to more valuable crops. Therefore, under rainfed conditions, strategies aimed to improve the recharge of rainwater in soils by using specific soil management techniques, or to capture rainwater in collection systems (i.e. Tunisian 'jessour', hand-made stone terraces, basin at farm and hydrographic level), are recommended (Graaff de and Eppink 1999; Fleskens et al. 2005; Tubeileh, Bruggeman and Turkelboom 2009).
Among the soil management techniques, mechanical tillage is still the most common in Mediterranean olive orchards, where it is performed also as a dry farming technique with the aim of reducing soil evaporation by interrupting water capillary rise and increasing soil surface roughness (Ozpinar and Cay 2006). Furthermore, tillage should improve infiltration and percolation into soil of rainfall water but such effects often occur only for a short period of time immediately after the machine passage (Pastor et al. 2000). Unfortunately, continuous tillage may result in the degradation of soil structure which can significantly reduce water infiltration rate causing runoff, erosion processes, and fertility loss (Abid and Lal 2009). These degradation mechanisms are quickened by the high air temperatures, that induce an intense microbial biomass activity and the mineralization of the labile fraction of organic matter, the most active in the soil. A significant loss of organic matter leads to a further deterioration of soil hydraulic properties directly involved in the recharge and storage of rainfall into the soil (Lipecki and BerbeÄ‡ 1997; Strudley, Green and Ascough II 2008).
Autumn-winter cover crops, spontaneous or sown, can represent an alternative to tillage in rainfed olive orchards showing a beneficial effect in intercepting raindrops, reducing runoff, facilitating and speeding infiltration of excess surface water into the deepest soil layers even thanks to the channels left by their dense death root network (Pastor et al. 2000; Pardini et al. 2002; Hernández, Lacasta and Pastor 2005; Durán-Zuazo et al. 2009; Palese et al. 2009a). A study carried out by means of a non-invasive geophysical techniques (electrical resistivity imaging, ERI) revealed that a cover cropped mature olive orchard was more efficient to intercept and store rainwater than tilled grove, resulting in a significant water reserve at the deepest soil layers (> 1.0 m), convenient for the root system of rainfed olive trees in the driest months (Celano et al. 2011). On the other hand, cover crops show very high hydric consumptions from the soil (from 200 up to 350 mm per year) and so they could compete with olive trees for water, especially when annual rainfall is less than 500 mm (Bellini 1983; Pardini et al. 2002). Therefore, it is fundamental to choose the most opportune date for cover crops suppression (by mechanical or chemical means), avoiding the overlapping between weed growth and some critical phases for the olive productive performance such as flowering and fruit set (Orgaz and Fereres 2004).
The improvement of soil water holding capacity can be reached also by means of techniques aimed to increase and/or preserve soil carbon content. Pruned material represents an important source of dry matter internal to the olive orchard and characterised by high content of lignin, low nitrogen level (C/N > 25), and slow decomposition process (Celano, Palese and Xiloyannis 2003). Once cut and buried in the soil, pruning material is able, in the long period, to build up soil organic matter which, in turn, improve soil hydraulic features (Pastor et al. 2000; Hernández, Lacasta and Pastor 2005). The recycle of polygenic organic material inside the olive orchard (spontaneous cover crops + pruned material), offering mixed organic substrates, strongly affects the activity of soil microbial communities which show a higher complexity and diversity at genetic, functional, and metabolic levels (Sofo et al. 2010b).
Use of non-conventional water sources for irrigation
Olive trees are widely diffused in arid and semi-arid environments where water shortage and competition among the different water consumption sectors are relevant problems. For this reason, the use of low quality water for irrigation (e.g., saline water or municipal wastewater) could represent a realistic way to overcome the scarcity of 'conventional' water assigning it especially for human consumption. In addition, an increase of the irrigated olive-grown area could lead to improved farmers' income, with a general benefit to the local rural economy.
In the Mediterranean regions, large amounts of saline water (with an electrical conductivity, EC > 2.0 dS m-1) are available for irrigation. Among Mediterranean fruit tree species, olive tree is moderately salt tolerant (Ayers and Westcot 1976; FAO 1985; Rugini and Fedeli 1990), and it shows a different tolerance behaviour depending on cultivars, salt concentration (EC from 5.0 to 13.7 dS m-1, the latter identified as the tolerance limit) and salt type dissolved in the irrigation water (Rugini and Fedeli 1990; Chartzoulakis 2005). Salt tolerance in olive cultivars is basically related to salt-exclusion mechanisms occurring within roots, that prevent salt translocation rather than salt absorption by keeping Na+ and Cl- at root level and limit the accumulation of such ions into actively growing shoots. Furthermore, Ca2+ has a main role in regulating the selectivity of the ionic absorption, decreasing Na+ uptake and its transport to the shoot, and reducing toxic effects of Na+ on integrity of the plasmatic membrane in root cells (Benlloch et al. 1991; Tattini et al. 1995; Melgar et al. 2009). As a matter of fact, the increase of Ca/Na ratio by adding Ca2+ to irrigation water has been recommended to mitigate the detrimental effects of salinity stress (Rinaldelli and Mancuso 1996; Melgar et al. 2009). The correction of water irrigation, together with the use of drip irrigation and the choice of a tolerant cultivar, can be useful tools for an appropriate employment of saline water. As reported by Melgar et al. (2009), the long-term irrigation of mature olive trees of cv. 'Picual', a salt tolerant cultivar, with saline water (EC up to 10.0 dS m-1) did not affect growth and yield, and no salt accumulation was found in the upper 30 cm soil layer thanks to the ion leaching linked to the rain season (annual precipitation of 702 mm). On the other hand, irrigation with saline water could be harmful in low rainfall areas (less than 250 mm). Under such conditions, it is essential to plan a proper soil leaching management (Wiesman, Itzhak and Ben Dom 2004).
Another alternative water resource is reclaimed urban wastewater. Olive trees can lend themselves to irrigation with this low quality water because their fruits are usually harvested one month, or more, after the last water application (according to the variety and its maturation time), and they are eaten after processing (to obtain oil or table olives). Such conditions decrease risk of fruit microbial contamination. Furthermore, the use of microirrigation system avoids the contact among wastewater, fruits, and leaves allowing the production of safe high-value olive yields and avoiding health risk for the farm workers and the consumers (Palese et al. 2006; Bedbabis et al. 2009; Palese et al. 2009b). A sustainable orchard management (Figure 2) coupled to an intense water absorption by the roots of olive trees and cover crops active in the wetted soil volume, excluded water logging by runoff and percolation to deeper soil layers avoiding aquifer pollution by faecal bacteria (Palese et al. 2009b). From an agronomic point of view, wastewater is rich of mineral elements (particularly P, N and K) and organic matter, both important for yield and vegetative development of olive trees and soil fertility, and often eliminated during the sewage treatment (Ramirez-Fuentes et al. 2002; Yadav et al. 2002; Tarchouna et al. 2010a). The reduction of the treatment level decreases fertilization costs and pollution and the price of the treated water allowing, in economic terms, its sustainable reuse (Lopez et al. 2006; Palese et al. 2009b). Nutrients supplied by wastewater should be taken into account in preparing the annual fertilization plan (Palese, Celano and Xiloyannis 2008). On the other hand, reclaimed urban wastewater can be an important source of both salts and potentially toxic metals. Although urban wastewater usually shows a low concentration of heavy metals, long-term irrigation can increase their concentration into soils even if at not critical values (Ramirez-Fuentes et al. 2002; Yadav et al. 2002; Tarchouna et al. 2010b; Klay et al. 2010). Therefore, a systematic monitoring of metal content in wastewater, soil, and plant is recommended to avoid hazardous situations for populations and environment.