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aData after Zijlstra (1916).

aData after Zijlstra (1916).

content at one of the four locations, there were no consistent significant differences between the varieties. However, there were clear differences in carvone content between locations with the light sandy soil giving an average carvone content of 1.6%, the sea clay 2.1% and the sandy loam and river clay 2.4 and 2.5%, respectively. No clear explanation for these differences was given but they may have been caused by the soil properties or other differences between the locations (Toxopeus and Lubberts 1994).

6.3.1.2. Soil Properties and Fertilisation

For several essential oil producing species authors have reported a negative effect of nitrogen on essential oil contents (e.g. Mihaliak and Lincoln 1985, Ross and Sombrero 1991). One of the explanations for this phenomenon may be that due to the limitation of plant growth by a lack of nitrogen, the excess carbon may be used to produce the high-energy, non-nitrogen containing terpenes (which may also protect against herbivores the little vegetative matter produced) (Baas 1989). The effects of fertilisation on caraway essential oil content reported in literature are difficult to interpret. Already in the first decades of this century it was hypothesised that fertilisation increases caraway seed yield but decreases essential oil content (Dafert and Scholz 1927/28, Potlog 1938). However, Boshart (1942) showed that this effect varied from year to year: sometimes fertilisation indeed decreased, sometimes, however, it increased essential oil content. Kordana et al. (1983) varied NPK fertilisation in a pot experiment and although they found large effects on seed yields, essential oil contents were not affected. Both Van Roon (1959) and Nordestgaard (1986) report a slightly negative relationship between nitrogen rate and caraway essential oil content. For the closely related dill, Bali et al. (1992) report no effect of nitrogen rate on essential oil content or quality, whereas Wander and Bouwmeester (1997) report a significant negative correlation between nitrogen rate and carvone content, with 1.75% for 0kg N/ha and 1.59% for 120kg N/ha. In the latter experiment, also the amount of carvone per seed decreased with increasing nitrogen rate, excluding the possibility that nitrogen affected essential oil content through an effect on seed filling. Interestingly, there was a highly significant negative correlation between the amounts of nitrogen and carvone per seed.

6.3.1.3. Plant Density

Zijlstra (1916) hypothesised that differences in essential oil content between two caraway production areas in The Netherlands were caused by differences in plant density. He assumed that with a lower plant density, assimilate availability would be better which should lead to a higher essential oil content. He tested his hypothesis in a field trial with four plant densities and indeed found a negative correlation between plant density and essential oil and carvone content (Table 3). Hegnauer and Fluck (1949) mentioned the results of Zijlstra to explain the high essential oil contents usually reported for wild caraway plants (see Table 2), which usually grow as single plants and thus at a very low plant density. In an experiment with annual caraway we found a significant negative linear relation between plant density (20, 80 and 140 plants/m2) and seed essential oil content or amount of essential oil per seed (Bouwmeester and Loman, unpublished results).

Table 3 Effect of sowing distance on caraway seed essential oil and carvone content"

Table 3 Effect of sowing distance on caraway seed essential oil and carvone content"

aData after Zijlstra (1916).

However, results of other studies are less consistent: Hornok and Csâki (1982) found slightly lower (although apparendy not significant) essential oil contents at both sub- and supra-optimal plant densities, and Wander (pers. comm.) found no significant correlation between plant density and carvone content.

Several authors have reported an effect of harvest time on essential oil and/or carvone content. Zijlstra (1940) performed a series of trials over a period of eight years in which he harvested on several dates before, at and after the optimal harvest time. In all eight years essential oil content decreased with harvest date. Toxopeus and Lubberts (1994) reported a significant decrease from 2.4 or 2.5% to 2.2% in carvone content, but an increase from 2.0 to 2.1% or no change in limonene content. When harvest was postponed for one week, Wander (pers. comm.) also found a decrease in essential oil content of only 0.1%. All these data are difficult to interpret as contents were expressed as a percentage of seed weight. If seed filling continues during the last stages of seed ripening, and essential oil formation does not, this may indeed lead to a decrease in essential oil content. Also see above the paragraph Essential Oil Accumulation During Seed Development.

The harvesting method, however, clearly affects caraway (and dill) essential oil content. Essential oil content of hand-harvested caraway seeds was 5.9% and of seeds threshed at 450rpm 5.5% (Wander and Bouwmeester, in prep.). Increasing the threshing velocity up to 1080rpm gradually further decreased essential oil content to 5.2%. There was a significant negative correlation between threshing velocity and essential oil content. The same significant negative correlation was found for dill with the negative effect of high threshing velocities being even higher than for caraway (4.4% at 1200rpm compared with 5.3% for hand-harvested seeds).

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