Fennel as a Crop Plant
Fennel (Foeniculum vulgare Miller) belongs to the Umbelliferae family and was originally found around the Mediterranean Basin. It has long been cultivated and introduced into many regions outside of that zone and has become naturalized in some of them. Although it can bear widely different climates and be cultivated in cold climates as well as in tropical regions, it develops best in temperate climates. In the Mediterranean climate, wild fennel often grows densely, with a potentiality of invading crops. In some areas where it has become naturalized, as in California for example, it has spread so much that in some cases it has become a weed. As in most Umbelliferae, the plant has secretory canals in all the organs and produces an essential oil rich in aromatic components. The chief constituent of the essence is anethole, which gives the plant its anise fragrance, but it also contains, in variable quantities, other elements, such as fenchone, estragole, anisaldehyde and terpenes (d-pinene, α-d-phellandrene, camphene). The genus Foeniculum is monospecific, and is represented only by the vulgare species. The latter, however, has been split into two subspecies: ssp. piperitum (Ucria) Coutinho and ssp. capillaceum (Gilib.) Holmboe. Foeniculum vulgare ssp. piperitum is a perennial type of fennel that only grows wild and can be differentiated from ssp. capillaceum by its short and rigid lobed leaves, as well as by a small number of rays in the umbels and by its very bitter fruit. The ssp. capillaceum, on the contrary, is characterized by long and supple lobed leaves as well as by a large number of rays in the umbels and less bitter fruit. In the latter subspecies, three varieties are usually distinguished. The variety vulgare (Miller) Thellung (Foeniculum vulgare Miller) (bitter fennel) comprises a group of perennial plants with rather long fruit having a more or less bitter taste. The variety dulce (Miller) Thellung ( = Foeniculum dulce Miller = Foeniculum panmorium DC.) (sweet fennel), on the contrary, consists of annuals, exceptionally biannuals, with smaller fruit, whose sweet taste is a result of the low fenchone content in the essential oil. The variety azoricum (Miller) Thellung (= Foeniculum azoricum Miller) (Italian fennel, Florence fennel), probably stemming from one of the above varieties by horticultural selection, also consists of annuals producing small sweet fruit. In these plants, however, young leaves of the rosette have hypertrophied and thick sheaves that form a “bulb” similar to that of celery.
The three varieties of ssp. capillaceum are cultivated, each with the goal of obtaining a different product. The variety dulce is essentially a condiment plant whose aromatic seeds are used to flavour food, the extracts also having medicinal properties. The variety vulgare is an industrially produced plant whose essence, extracted from the fruit, produces, after refining, pure anethole used as an aromatic primary product in the food and perfume industry. Lastly, the variety azoricum is a market garden product whose bulbs are used as a vegetable, like those of celery.
Fennel fruits, particularly those of the var. dulce, are widely used as spices in some regions of the world, particularly in Europe and Asia. In Hungary, one of the main European producers, surface areas occupied for production range from 740 to 1200 acres. Every year, India, the first Asian producer, exports more than 15000 tons of aromatic Umbelliferae fruit such as fennel, celery, coriander, cumin, aneth, and ajowan, fennel accounting for 1000 to 2000 tons. The plant is also cultivated, although on a smaller scale, around the Mediterranean Basin (Egypt, Turkey, Greece, Spain). In some countries there is a production deficit, thus West Germany imports 300 to 400 tons of seed from Hungary, France, Turkey and China each year.
Anethole is the chemical giving some plants such as star anise (Illicium verum, Illiciaceae), anise (Pimpinella anisum, Umbelliferae) and fennel their anise fragrance. This aromatic product is largely used in the food industry to flavour various drinks, cakes and sweets, as well as in the pharmaceutical industry (drugs) and perfumery (soap, toothpaste). Anethole is also used in chemisty, particularly for the synthesis of anisaldehyde, another aromatic compound (The Merck Index, 1976). Chemical production of anethole is possible, either by hemisynthesis from estragole or by complete synthesis. Most anethole produced in the USA is obtained by isomerization of estragole extracted from Pinus oil. In some countries, however, the use of synthetic anethole for food products is prohibited by law, so that its supply depends on natural product availability. For many years most natural anethole has been extracted from the star anise fruit produced in South Asia (China, Vietnam), whose oil contains approximately 88% anethole. Fennel culture was developed in Europe to counterbalance this Asian monopoly on natural anethole production. Pimpinella anisum, although having a high quality oil (90% -95% anethole), was not suitable for such a goal because of its low size and poor yields.
Pure anethole is obtained from fennel by rectification of the essential oil recovered after distillation of the plant. Ninety % of anethole of the mature plant being localized in the fruit, distillation is sometimes applied to seeds only. In this case, however, most of the small unripe fruit are lost by the harvesting machine and the final anethole yield is lower than with the entire plant. Harvesting is usually done by a crushing forage machine breaking all the aerial part of the plant into small pieces. This raw material is next submitted to continuous steam distillation in order to separate the essential oil. Vacuum distillation of oil on a large industrial column further allows the purification of its various components. Average composition of bitter fennel oil is 65% anethole, 17% terpenes, 15% fen-chone and 3% estragole.
Fennel crops grown to obtain anethole have been particularly developed in Eastern Europe and in France. In 1981, there were 3000 acres under cultivation in France with a production of 100 tons of essence and 62 tons of pure anethole, amounting to 1.3 million dollars. During the last few years, about 30% of French needs in anethole have been covered with bitter fennel. Areas for production have also been developed in the austral hemisphere (New Zealand, Tasmania).
Bulb fennel (var. azoricum) is mainly cultivated and consumed in Mediterranean countries (Italy, Spain, France, Greece, Switzerland, and North Africa). The total world production of the vegetable is estimated at 500000 tons per year, amounting to approximately 145000 dollars. Italy is the first world producer, with a cultivated surface of more than 42000 acres and a production of approximately 350000 tons per year.
Fennel is recognized by various pharmacopoeia (Russia, China, Japan, Argentina, France etc.) as a plant having medicinal properties (antispasmodic, carminative and diuretic). The different parts of the plant do not all have the same properties. The fruits are the most-used organs. It is difficult to assess precisely, for any given country, what proportion of the total consumption of fennel fruit is actually used for a medicinal purpose. In France the quantity used in the herb trade and in phytotherapy is estimated at 50 tons per year.
Pests and Parasites
The presence of larvae (Systole albipennis) inside the fruit results in large quantities of non-viable seed through lack of embryos. Moreover, during the blossoming and fructification processes, umbels can also be attacked by the caterpillars of several lepidoptera (Epinotia thapsiana, Depressaria nervosa, Papilio machaon etc.) but judicious use of insecticide can check such problems. Aphids are also to be feared, not only because of bites, but also because they are vectors of a pathogenic fungus, Phomopsis foeniculi, that causes considerable damage to the varieties dulce and vulgare (). The epidemic results in withering stems, particularly in inflorescences, and leads to a sharp drop in fructification. In 1977, 250 acres were destroyed in Southern France by this fungus. Current methods of fighting the problem consist of preventive treatments with anti-aphid and anti-fungus sprays, but the selection of tolerant cultivars is desirable. Numerous other pathogenic fungi have been depicted but, in most cases, they cause only limited damage to crops, at least in Europe. Cercosporidium punctum, which causes necrotic areas to appear in the base leaves, can substantially reduce the chlorophyllous area of these leaves. Moreover, Sclerotinia sclerotiorum, a pathogenic agent for many cultivated plants, also attacks fennel, particularly the variety azoricum, whose bulbs seem highly susceptible.
Conclusion: In Vitro Culture Impact on Fennel Improvement
Given the difficulty of vegetatively multiplying this plant with classical methods, the in vitro micropropagation technique constitutes an important progress. Plants with interesting agronomical characteristics (for example male-sterile plants) can now be kept as clones and used in improvement programmes in the form of homogeneous populations and not of unique plants. The possibility of producing variant clones from somatic embryos is also extremely interesting. Observations on somaclonal variation of plants thus obtained are, however, too recent and too fragmentary to give way to an immediate evaluation of its use in improvement programmes. Work on hybridization being at present at a stalemate due to the scarcity of homozygous material, the creation of isogenic lines through doubling androgenetic or gynogenetic haploids would be extremely useful. Unfortunately, the few attempts at anther culture have, so far, failed to achieve plant regeneration and there has been no attempt at gynogenesis. Production of plants transformed by Agrobacterium rhizogenes seems to be, at present, more interesting theoretically than practically. However, nothing precludes the possible use of the disarmed Ri plasmid as vector of outside genes intended for the improvement of the species. Considering recent progress in direct transformation of plant cells, the possibility of regenerating a high number of plants starting from protoplasts would also be desirable, but as far as we know, only one study on the isolation and culture of fennel protoplasts has been carried out. The ability of protoplasts to give embryoids without a callus stage is an interesting feature; however, regeneration rates clearly need improvement.
In conclusion, promising results have been achieved on fennel in vitro cultures, which boost expectations for future diversification and better control of the material at selectors’ and growers’ disposal.
Selections from the book: “Medicinal and Aromatic Plants II”, 1989.