Fennel swollen base yield and quality as affected by variety and source of nitrogen fertilizer

Two fennel (Foeniculum vulgare Mill.) varieties, Dolce and Azoricum, were field grown for swollen base (SB) production to select the most productive variety and the most suitable source of N fertilizers in fields supplemented with cattle manure, calcium superphosphate and potassium sulfate. Plants were then grown either with or without one of the synthetic N fertilizers, ammonium sulfate (AS), ammonium nitrate (AN) or urea applied at a rate of 170 kg N ha(-1). Plants of var. Dolce were heavier in fresh weight and produced swollen bases (SBs) of 25% higher marketability percentage, 143% yield increase and 50% more oil content as compared to Azoricum. The increased SE yield and quality in var. Dolce were due to its lower bolting tendency as compared to var. Azoricum. Among all measured parameters, bolting tendency and SE oil content proved to be variety-dependent since both parameters showed no responses to N fertilizer source. When N was applied as a synthetic source, SE yield and quality were markedly reduced with about a 36% increase in SE nitrate content as compared to the control. The lowest SE yield and quality were obtained when AN or urea was used. Although AS produced a comparable yield to that of the control, the physical quality of the harvested SBs was adversely affected. The major compound in SE essential oil, trans-anethole, increased only with AN source in var. Dolce but fluctuated in var. Azoricum. The second most common compound, para-anis-aldehyde in var. Dolce or limonene in var. Azoricum, however, was markedly reduced with the application of any synthetic N source. It is suggested, therefore, that var. Dolce is more recommended for SE production than var. Azoricum. Fertilizing with synthetic N sources during plant growth and development is not advised when the soil has already been supplemented with organic nitrogen as not only are SE yield, quality and flavor reduced but also its safety for human consumption. (C) 2001 Elsevier Science B.V. All rights reserved.