Olea europaea (PROSEA)
- Protologue: Sp. pl.: 8 (1753).
- Family: Oleaceae
- Chromosome number: 2n= 46
- Olive (En). Olivier (Fr)
- Indonesia: zaitun
- Malaysia: zeitun
- Philippines: oliva.
Origin and geographic distribution
The olive is a characteristic fruit tree of the Mediterranean. Wild olive (oleaster) is a typical component of the Mediterranean shrub vegetation and the most likely progenitor of the cultivated olive trees. First domestication is associated with early civilizations in the eastern Mediterranean. Archaeological evidence of olive cultivation dates back to the fourth millennium BC. The Phoenicians and Greeks in particular contributed to the expansion of olive cultivation around the Mediterranean Sea during the last millennium BC. In the Roman empire of the 2nd Century AD, olive oil became one of the most economically important commodities. Eastwards, olive cultivation spread up to north-western India and the Caucasus. Olive cultivation was introduced to the new world (Peru, Chile, Argentina, Mexico and United States (California)) in the 16-18th Centuries by the Spanish, to Australia and South Africa by Italian and Greek immigrants and to Japan and China from France in the 19th Century. Nevertheless, about 97% of the world’s 800 million olive trees are still grown in the Mediterranean region.
The main product of the olive tree is the edible oil extracted from the fruit’s mesocarp and commonly used as a cooking and salad oil and in the preservation of various foods. It is much appreciated for its specific flavour and supposedly beneficial effects on health due to the high concentration of monounsaturated fatty acids and polyphenolic anti-oxidants. Lower grade olive oils are used in the manufacturing of soap, cosmetics and lubricants. Traditionally, olive oil also has various pharmaceutical applications and has served as lamp oil.
Fruits are processed into green and black table olives (whole, sliced, minced or paste). The presscake is not a very suitable livestock feed, but can be used as fuel or fertilizer. Other useful products from the olive tree include the leaves as cattle feed, valuable timber from the stem, and firewood from the branches. Olive trees are planted for ornamental purposes, as firebreaks and to control soil erosion.
Production and international trade
Average world production of olive oil during the period 1995-2000 was 2.3 million t/year, almost all from the Mediterranean region. The biennial bearing habit of the olive tree and variable weather conditions cause considerable fluctuations in annual world production (1.6-2.8 million t). The total area planted with olive trees is estimated at 9.1 million ha in 21 countries. The principal olive oil producing countries are: Spain (32%), Italy (21%), Greece (17%), Tunisia (8%), Syria (4%), Turkey (4%), Morocco (3%) and Algeria (1%), which together account for 90% of the world supply. About 400 000 t per year reach the international vegetable oil market; the European Union and United States are the main importers of olive oil.
The 1.1 million t of table olives produced annually represent about 8% of total olive fruit yields. Spain is the largest producer of table olives (25%) followed by the United States (14%), Turkey, Morocco, Syria, Greece and Italy (6-9% each).
Mature olive fruits consist of mesocarp (pulp) 70-90%, endocarp (stone) 9-29% and seed 1-3%. Per 100 g fresh edible portion, the mesocarp contains: water 60-70 g, oil 15-30 g, carbohydrates 3-6 g, crude protein 1-2 g, cellulose 1-4 g, phenolic compounds 1-3 g, ash and other substances 1-3 g. The fatty acid composition of the oil is: palmitic acid 7.5-20%, palmitoleic acid 0.3-3.5%, stearic acid 0.5-5%, oleic acid 55-83%, linoleic acid 3.5-20%, linolenic acid 0-1.5%, arachidic acid 0.1-0.6% and traces of gadoleic, behenic and lignoceric acids. The anti-oxidant effect of the phenolic compounds (50-400 ppm) and the high oleic content combine to give an oil of exceptional stability even during deep frying. Olive oil is classified into two main quality classes: cold-pressed or virgin oil and refined olive oil. Virgin olive oil is one of the few vegetable oils that is traded and consumed without any refinement and contains its full complement of secondary compounds. Mainly oleuropein but also other phenolic compounds are responsible for the intense bitterness of olive fruits, as well as for fruit blackening and inhibition of microorganisms during processing. The bitterness in table olives is largely removed in the early stages of processing. The weight of a fresh fruit is 1-12 g.
Evergreen tree, up to 10-(15) m tall, with broad multibranched crown, trunk thick, up to 1-2 m in diameter, often gnarled, or densely branched shrub, up to 2(-5) m tall. Root system extensive with main roots thickened due to fasciation, up to 6 m deep in porous soils and spreading laterally with dense mat of feeder roots; protuberances (spheroblasts) at the base of trunk with additional lateral roots. Twigs grey, subterete and thornless in cultivars, 4-angled and thorny in wild forms. Leaves opposite, simple, entire, coriaceous, subsessile; blade elliptical to lanceolate, 1-8 cm × 0.5-2.5 cm, mucronate at apex, dark grey-green and glabrous above, densely silvery lepidote beneath. Inflorescence an axillary panicle, shorter than the leaves, 3-5 cm long with 10-40 bisexual, white, fragrant flowers 3-4 mm long; pedicel short; calyx cup-shaped with 4 teeth, persisting in fruit; corolla with short tube and 4 valvate lobes; stamens 2, filaments short, anthers large; pistil with superior, 2-loculed ovary, a short style and a 2-lobed stigma. Fruit a subglobose to ellipsoid drupe, 0.5-4 cm × 0.5-2.5 cm, bright green, turning purple-black, brown-green or ivory-white at maturity; mesocarp rich in oil; endocarp stony, usually containing 1 seed. Seed ellipsoid, 9-11 mm long with straight embryo and copious endosperm. Seedling with epigeal germination.
Growth and development
Practically all olive trees in the world are grown from clonal cultivars. Seeds germinate within 25-50 days after sowing, but seed viability of cultivated olives is generally low. Olive seedlings have a distinct juvenile phase lasting 4-9 years and characterized by strong vegetative growth and profuse branching. Plants raised from cuttings have a more adult growth habit with monopodial branching and may start flowering within 3-7 years after field planting. The life of leaves is 2-3 years. Flowering occurs annually in spring on branch segments formed during the previous season, with 50-80% of the leaf axils developing inflorescences. Wind pollination and cross-fertilization are the rule due to self-incompatibility. Even under optimum conditions of pollination and initial fruit set, generally only 1-5% of the flowers will develop into mature fruits due to severe early (up to 50%) and late physiological fruit abscission, water stress, diseases and pests. In a year of profuse flowering, such low fruit set still represents a large crop. Olive is a strongly biennial bearer, because a heavy fruit load in one year inhibits adequate shoot extension necessary for the following year’s bearing wood and vice versa. Olive fruit development takes 6.5-7 months from anthesis to harvesting, the last 20-40 days being essential for oil formation in the mesocarp.
The commercial life span of an olive tree is about 50 years, but individual trees can become very old (hundreds of years). Very often, old trees are hollow, usually because during its history, fungus diseased wood has been cut away repeatedly. Such old, gnarled trees are often also twisted and slanting, giving the tree a peculiar appearance: abundant, fresh, lively, young, green sprouts on an old, grey, twisted, gnarled and slanting, hollow cylinder.
Other botanical information
The O. europaea taxonomy is confusing and many names and classifications exist. Usually, 2 groups are distinguished: cultivated and wild, and a selection of scientific names encountered for each group is listed:
- the cultivated olive: O. europaea L. var. europaea , O. europaea L. subsp. europaea , O . gallica Miller, O. hispanica Miller, O. lancifolia Moench, O. sativa Gaterau, O. europaea L. var. sativa Lehr., O. europaea L. var. sativa Loud., O. europaea L. subsp. sativa Arcang.
- the wild olive: O. europaea L. var. sylvestris (Miller) Lehr., O. sylvestris Miller, O. europaea L. subsp. sylvestris (Miller) Rouy, O. europaea L. subsp. sylvestris (Miller) Hegi, O. europaea L. var. sylvestris Brot., O. oleaster Hoffmannsegg & Link, O. europaea L. subsp. oleaster (Hoffmannsegg & Link) Negodi.
Instead of classifying the cultivated olive into a system developed for wild plants, it is better to distinguish cultivars and cultivar groups. More than 2500 cultivars are known, but an overall cultivar classification system has not yet been developed. According to their use, three groups of cultivars can be distinguished:
- Cultivars for oil extraction, e.g. "Picual", "Arbequina" and "Blanqueta" in Spain; "Frantoio" and "Leccino" in Italy and "Koroneiki" in Greece.
- Cultivars for fruit consumption, e.g. "Gordal Sevillana" and "Manzanilla de Sevilla" in Spain, "Conservolea", "Kalamata" and "Chaldiki" in Greece, "Picholine du Languedoc" in France, "Manzanillo" and "Mission" in the United States and "Oliva di Spagna" and "Oliva di Cerignola" in Italy.
- Dual-purpose cultivars (for oil extraction and fruit consumption), e.g. "Hojiblanca", "Manzanilla Cacereña" and "Aloreña" in Spain, "Tanche" in France, "Picholine marocaine" in Morocco, "Dan" in Syria and "Arauco" in Argentina.
It is generally believed that the cultivated olive originates from the wild form of O. europaea (often called "oleaster") by selection of high oil yielding genotypes and their vegetative propagation. Oleaster is distributed similarly to the cultivated olive in the Mediterranean. It can be distinguished by its obovate leaves (shorter than 4 cm), its thorny, quadrangular lower twigs and its small fruits (up to 1.5 cm long). Oleaster fruits have always been used for their edible oil. Sometimes, it is difficult to decide to which group a plant belongs: in disturbed habitats (e.g. near the edge of olive plantations), intermediate forms exist because the two are interfertile. Many cultivars are genetically heterozygous and they have often been grafted on wild oleaster stock.
Olea L. comprises about 35 species but in the Mediterranean only O. europaea occurs. Several non-Mediterranean wild Olea species, however, are closely related to and sometimes interfertile with O. europaea. Some examples are: O. chrysophylla Lamk (synonyms: O. cuspidita Wall ex G. Don, O. africana Miller, O. verrucosa Link) occurring in southern Iran, southern Arabia and eastern Africa; O. ferruginea Royle, occurring in South Asia; O . laperrini Battand. & Trab., occurring in the mountains of the Sahara; and O. cerasiformis Webb & Berth., occurring in the Canary Islands and Madeira. These taxa are so closely related to O. europaea that they have also been classified as its subspecies (subsp. cuspidata (Wall. ex G. Don) Cif. and subsp. cerasiformi s (Webb & Berth.) Kunkel & Sunding). Their geographic isolation from the location of early domestication of the olive makes them unlikely progenitors, but they could become useful genetic resources for the improvement of the cultivated olive, e.g. for disease and pest resistance and adaptation to new ecosystems.
The olive tree is well adapted to the seasonal and relatively dry climate of the Mediterranean region. Worldwide cultivation is concentrated between 30-45°latitudes in the northern and southern hemispheres (excluding the tropics), from sea level to 900 m altitude on south-facing slopes (higher than 1200 m in Argentina). Frost in spring can damage young shoots and flowers, and the ripening fruits in late autumn. Olive trees are fairly frost-hardy during winter, tolerating -8°C to -12°C. For flower initiation, most olive cultivars require a vernalization period of 6-11 weeks below 9°C which ends 40-60 days before anthesis. Optimum temperatures for shoot growth and flowering are 18-22°C. Temperatures above 30°C in spring can damage flowers, but the tree can withstand much higher temperatures in summer. The xerophytic physiology of olive trees makes them highly tolerant of long period of water stress, but for economic yields, low and irregular rainfall (less than 300 mm) should be supplemented by irrigation during critical growth stages to 500-800 mm per year.
Soils should be light-textured (less than 20% clay), well-drained and have a depth of at least 1.5 m. Olives can do well on very poor soils, except when these are waterlogged, saline or too alkaline (higher than pH 8.5).
Propagation and planting
The main method of propagation of olive is based on rooting of semi-hardwood cuttings prepared from one-year old branches (10-12 cm long with 4-5 nodes and two pairs of leaves). Propagation by seed is possible but gives rather variable seedlings because of cross fertilization. It is mostly used for breeding purposes. In vitro micro-propagation of olive explants has not yet passed the experimental stage, partly because of large variation in rates of success between different cultivars. Somatic embryogenesis is very difficult to achieve from adult tissues and cannot be used for propagation purposes. Traditional methods of clonal propagation are: large hardwood cuttings, grafting on seedlings or mature trees, grafting on wild olive trees and rooting of fragments of protuberances with a shoot attached. Protuberances can also be used for in situ rejuvenation of very old and decaying olive trees.
Plants from rooted cuttings are raised in beds or polythene bags in nurseries for 1.5-2 years prior to planting in the field in spring. They are planted in large holes (40 cm × 40 cm × 60 cm) which are later refilled with topsoil, organic compost and fertilizers, especially P and K. Plant densities traditionally vary from 40-60 trees/ha in very dry areas to 300-400 trees/ha under optimum soil conditions and water availability (more than 600 mm) and using cultivars with more compact and erect growth habit. Field experiments with high density olive orchards (up to 2000 trees/ha planted in hedges) are in progress in Spain and France. The majority of olive orchards in the Mediterranean region have traditional densities of 100-250 trees/ha. Planting along contour lines or in terraces is necessary in sloping terrain to prevent soil erosion. Leguminous and cereal crops have been planted as intercrops in olive groves.
The olive tree requires pruning to shape it into the desired main frame and crown, to maintain a proper balance between vegetative growth and fruit production and so reduce biennial bearing and to rejuvenate senescent trees. There is a long tradition of manual pruning methods and some are region specific. Mechanized maintenance pruning is done in modern olive orchards, but requires adaptation of tree shape and careful management to prevent excessive branch damage and subsequent disease problems.
Regular fertilizer application is needed for sustained fruit production, but type and rate vary with local climate, soil condition and agronomic practice. Foliar analysis provides information on the nutrient status of olive trees. Nutrients removed by 3 t of fruit amount to about 19 kg N, 9 kg P2O5and 25 Kg K2O. A general fertilizer recommendation would be: annual applications of 0.8 kg N (in 2-3 split applications), 0.3 kg P2O5and 0.9 kg K2O per tree at medium planting density (150 trees/ha). These correspond to 120 kg N, 45 kg P2O5and 135 kg K2O per ha. Occasional correction of calcium, magnesium and boron deficiencies may also be needed. Triennial application of organic manure or compost (50 kg/tree) is recommended to improve soil texture and fertility. It can also be done before planting.
Only 15% percent of areas planted with olive trees worldwide are actually irrigated but this is steadily increasing. Surface, sprinkler or drip irrigation are some of the methods applied to supplement deficient rainfall in intensive olive cultivation. Correctly timed and dosed irrigation is required to produce economic responses in yield and fruit quality. Irrigation combined with ground cover positively influence olive production and soil conservation.
Diseases and pests
Leaf spot or peacock spot caused by Spilocaea oleagina ( Cycloconium oleaginum ) is the most common disease in olive cultivation. Methods of control include preventive copper-based fungicide sprays and host resistance. Copper sprays also have a tonic effect of promoting longer leaf retention. Other diseases are sooty mould caused by secondary infection of Alternaria, Capnodium and Cladosporium spp. following black scale infestation, Verticillium wilt caused by Verticillium dahliae and bacterial canker or olive knot caused by Pseudomonas syringae pv . savastanoi .
There are numerous pests, which generally cause much more economic harm to olive cultivation than diseases. The most damaging insect pests are the olive fly ( Bactrocera oleae ) and olive moth or kernel borer ( Prays oleae , synonym Prays oleellus ) on fruits, black scale ( Saissetia oleae ) on branches, jasmin moth ( Margaronia unionalis ) on young shoots, bark beetles ( Hylesinus oleiperda and Phloeotribus scarabaeoides ) on branches and trunk, psyllids ( Euphyllura olivina ) sucking on flowers, mites ( Aceria oleae ) on leaves and fruits and thrips ( Liothrips oleae ) on flowers and young leaves. Insect control in olive cultivation is increasingly based on systems of integrated pest management including monitoring, pheromone trapping, promoting or releasing natural enemies, Bacillus thuringiensis -based insecticides and cultural measures such as pruning and irrigation.
Olives for oil are harvested at full maturity in late autumn or early winter, either mechanically or with the use of rakes, beating poles and collecting nets. Table olives are harvested by hand; mature green fruits in early autumn and black olives in late autumn. Manual fruit picking (capacity about 80 kg/person per day) accounts for 50-60% of field production costs. Machines developed to reduce harvesting costs include trunk and branch shakers in combination with inverted umbrellas or rolling canvas frames to catch the fruits. Self-propelled overhead harvesting machines in olive orchards planted in hedge rows and the application of chemicals (e.g. ethephon) to promote fruit abscission shortly before harvesting are still in the testing stage.
World average yield in 1999 was 1.7 t of olive fruits per ha. Fruit yield per ha varies from less than 1-3 t in traditional olive groves to 4-10 t under irrigation and optimum agronomic practices (e.g. in Italy at 280 trees per ha). In well-managed plantings under rainfed conditions, fruit yield is 2-5 t/ha. There is always considerable year-to-year variation in productivity. About 5-6 kg of fruits are needed to produce 1 kg oil. The world’s average oil production in 1999 reached about 300 kg/ha.
Handling after harvesting Oil extraction should start within 3-4 days after fruit harvesting to avoid a change in flavour and increase in free fatty acid content. The fruits are washed, crushed and mashed into a paste, from which the oil is cold-extracted by mechanical pressing. The "margine", or mixture of water and oil, is allowed to settle and the oil is separated by decantation, centrifugation and filtration. Oil prepared exclusively by this process, i.e. by physical means only and without any heating, is called virgin olive oil. In the European Union, virgin olive oil is graded into 4 classes based on many characteristics of which the most important ones are free fatty acid content and organoleptic test score: extra virgin oil, virgin oil, standard and "lampante" virgin oil. "Lampante" virgin oil and oil obtained by heating or solvent extraction are either used industrially or have to be refined by neutralization, bleaching and deodorization to produce refined olive oil. Further solvent extraction may be done to produce an industrial grade oil from the cake.
Preservation of table olives starts with soaking fruits in an alkaline solution to reduce the bitterness before pickling in brine (Spanish-style and Californian-style). The Greek-style preservation of fully ripe, black olives involves pickling in brine without alkaline pre-treatment.
The numerous traditional olive cultivars (estimated at 2000) are gradually disappearing because of abandonment of marginal groves and urbanization or replacement by modern cultivars. Programmes to collect and preserve this valuable olive germplasm are in progress with the support of the International Olive Oil Council (COI) and the European Union. In addition to the Olive World Collection at Cordoba (Spain) with 310 accessions, there are 73 collections of olive germplasm in 23 countries and a project of a second world collection at Marrakech (Morocco).
Olive improvement has a long tradition of clonal selection. Breeding programmes based on inter-varietal crosses followed by selection within segregating seedling populations are of fairly recent date. The long juvenile phase of olive seedlings has been an impediment to breeding, but forcing methods and existing genetic variation in length of juvenile phase have contributed to shorter breeding cycles. Main criteria of selection in the olive are fruit yield, regular production, cold tolerance, early first bearing, compact growth, oil content of the fruit mesocarp, quality of the oil and resistance to diseases and pests. Quality of olive oil is determined by standard physical and chemical analyses and sensory assessment of taste and flavour. Host resistance to Spilocaea oleagina has been reported in Israel and to Pseudomonas syringae pv. savastanoi in Portugal. Progress is also being made with the application of molecular biology in the olive, including molecular markers for cultivar identification, the construction of a linkage genome map and marker assisted selection. There are no crossing barriers for introgression of desired characters from the oleaster olive and some related species.
Increasing interest in the olive as a source of high quality and healthy vegetable oil may have a positive effect on world production, notwithstanding its high production costs in relation to other vegetable oils. The olive also contributes considerably to environmental protection (soils, flora and fauna) in dry and hilly areas, particularly of the Mediterranean region. There are prospects for olive cultivation in Australia, India, China and possibly northern Thailand but possibilities in South-East Asia are very limited.
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N. Moutier & H.A.M. van der Vossen