Environmental and cultural factors influencing pollination and fruit set in commercial crops of fennel (Foeniculum vulgare Mill.)

Foeniculum vulgare Mill., (fennel), is grown commercially in Tasmania as an essential oil crop. Although oil is produced throughout the plant, approximately 95% is located in the fruits and fruit set directly influences oil yield. In Tasmania commercially grown fennel frequently has a percentage fruit set less than 50%. Research was undertaken into factors influencing fruit set, with the objective of improving fruit set, oil yield and economic returns to growers. The project began with an examination of flower morphology and physiology. Particular importance was placed on the fact that fennel is protandrous and therefore self-incompatible through the timing of pollen production and stigma receptivity. A field trial demonstrated that at high planting densities, fennel produces fewer higher order umbels. This reduces the time span of pollen production and reduces the quantity of pollen available for pollination of the flowers developing later in the season. The work incorporated a study of pollen viability and appeared to show that although pollen viability of glasshouse grown plants is often poor, the problem is not carried over into the field. A field trial involving the application of exogenous hormone gave rise to the hypothesis that competition within the fennel canopy causes poor fruit set. This competition could be for a number of growth factors including nutrients or carbon dioxide. The two factors studied in more detail were light intensity and water stress. Trials suggested fennel to be an obligate heliophyte. It has an open canopy as an adaptation to avoid the absorption of light in excess of that which can be utilised in photosynthesis. In the Tasmanian environment this feature is beneficial in allowing penetration of light through the canopy and prevents shading of the lower umbels. However, it makes fennel inefficient with respect to light absorption and thus possibly makes light a limiting factor. Initial experiments on the effect of water stress in fennel showed the stomata to react slowly to decreasing water availability. They remained open despite the plant wilting. Further investigation showed fennel to have a root system capable of tapping water at depth. Fennel is thought to be a phreatophyte. Fennel plants showed the ability to adjust to cycles of decreasing water potential and after a few cycles of water stress, they no longer wilted. Attempts were made to test the theory ttiat the plant adapted to water stress by increasing its osmotic potential but the methods used were inconclusive.