A study of some factors affecting the yield and composition of fennel oil (Foeniculum vulgare Mill.)

The aim of this research programme was to investigate factors influencing the production of essential oil from fennel (Foeniculum vulgare Mill.). Fennel is a perennial plant belonging to the family Apiaceae, formerly referred to as the Umbelliferae. This plant is cultivated in Europe, and more recently in Tasmania, for its essential oil. The essential oil is comprised of anethole (65 to 70 percent), fenchone (15 to 17 percent), estragole (2 to 3 percent) and terpenes, mainly limonene and beta-phellandrene. The oil is present within secretory canals in most parts of the plant. However, in mature plants approximately 95 percent of the total oil is present in the seed. Therefore, an understanding of the physiological factors controlling umbel initiation and seed development are of utmost importance, when considering the production of essential oil from fennel. The components of oil yield in fennel include: the number of umbels, the number of rays per umbel, the number of seeds per ray and the oil yield per seed. The anethole content of the oil largely determines the commercial value of fennel oil. In order to manipulate these components, it is necessary to understand the physiological factors which control these components. Detailed studies on the reproductive physiology of fennel have not been previously reported. Examination of differentiating meristems by scanning electron microscopy revealed that floral initiation occurred when the daylength exceeded 13 hours. Within 5 days of primary umbel initiation, secondary and tertiary umbel initiation was observed. Glasshouse trials using night break treatments indicated that the daylength response in fennel was a phytochrome mediated response rather than merely a response to longer photosynthetic periods. A minimum of 10 long day inductive cycles resulted in initiation of primary and secondary umbels. Many plants are not responsive when exposed to inductive conditions during early growth stages. This juvenile phase may extend from days to several months. In fennel, it was apparent that such a juvenile phase exists, and continues until the main stem has differentiated 12 nodes and has produced 8 fully expanded leaves. Under field conditions, perennial fennel plants reach this \ripe to flower\" stage prior to the onset of inductive photoperiods. Once induced to flower fennel plants commence a period of rapid stem elongation. No reversion of floral primordia was observed in any of the experiments conducted. In commercial fennel fields it is often difficult to control excessive vegetative vigour. This results in very tall plants which are low yielding and difficult to harvest. The plant response to application of exogenous gibberellins indicated that this group of phytohormones was a major regulator of this rapid stem elongation. To investigate whether such gibberellin induced stem elongation could be suppressed without adversely affecting initiation a gibberellin biosynthesis inhibitor (flurprymidol) was applied. When applied at the appropriate concentration and timing this growth retardant suppressed stem elongation without adversely affecting flowering and oil yield. Approximately 90 to 95 percent of fennel oil is contained within the umbels. Of the total oil yield from umbels 84 percent is contributed by the secondary and tertiary umbels. Harvesting is timed to coincide with the maximum oil yield of these umbels. The supply of photosynthate is known to be important in the biosynthesis and interconversion of essential oil components. Maximum net CO\\(2\\) fixation occurred at high light intensities (800 ˜í¬¿mole m\\(^{-2}\\)s\\(^{-1}\\) ) and between 20 to 25¬¨‚àûC. From data collected on net CO\\(2\\) exchange characteristics of umbels and plant defoliation studies it was apparent that maturing umbels were capable of supplying a significant proportion of their required photosynthate. Studies utilizing \\(^{14}\\)CO\\(2\\) and autoradiographic techniques clearly indicated that the umbels are photosynthetically active units and that during seed development the leaves play a minor role in photosynthate supply to the maturing seed. Competition for assimilates between the various umbel orders was observed the lower the umbel order the lower the sink strength."