Conclusions And Prospects For Improvement

This chapter shows that the accumulation of caraway essential oil and its composition are determined by several factors. These factors are:

1. Agricultural practices: harvest and plant density, and to a lesser extent nitrogen fertilisation

2. Environmental conditions: light intensity (=assimilate availability)

3. Endogenous factors: the activity of the enzyme limonene-6-hydroxylase determines the accumulation of carvone

How can this knowledge be exploited to improve caraway seed essential oil content and quality?

1. Careful harvesting, optimisation of plant density and nitrogen fertilisation. Of these three, careful harvesting and post-harvest handling are relatively easy to realise with good improvements of yield being possible. Optimisation of plant density and nitrogen fertilisation are more difficult as both factors also affect seed yield, to a certain extent in opposite direction (an increase in nitrogen will increase seed yield but decrease essential oil content).

2. As under 1: optimisation of plant density; breeding for improved light penetration into the crop, e.g. by breeding for absence of petals or reduction in petal size; breeding for

Figure 7 Developmental changes in limonene, carvone and total essential oil contents determined by gas chromatography (broken lines; data from Figure 5A) and calculated using a regression model with the enzyme activities from Figure 5B: (o) limonene, (A) carvone and (□) total essential oil. The activity of limonene-6-hydroxylase was corrected to account for the loss of activity during extraction. Towards maturation an increasing substrate competition was assumed to occur. (A) model describing the present situation, (B) using doubled limonene hydroxylase activity and (C) using a limonene-6-hydroxylase activity continuously at the maximum level of 14 DAP (Figure 6)

Figure 7 Developmental changes in limonene, carvone and total essential oil contents determined by gas chromatography (broken lines; data from Figure 5A) and calculated using a regression model with the enzyme activities from Figure 5B: (o) limonene, (A) carvone and (□) total essential oil. The activity of limonene-6-hydroxylase was corrected to account for the loss of activity during extraction. Towards maturation an increasing substrate competition was assumed to occur. (A) model describing the present situation, (B) using doubled limonene hydroxylase activity and (C) using a limonene-6-hydroxylase activity continuously at the maximum level of 14 DAP (Figure 6)

improved carbon allocation early during seed development to improve assimilate availability during essential oil accumulation.

3. Increasing the activity of limonene hydroxylase, decreasing competition for substrate. Competition for substrate seems to limit essential oil accumulation especially during the later stages of development. Biochemical (e.g. using inhibitors of fatty acid biosynthesis) and molecular research (inhibition of branchpoint enzymes e.g. of fatty acid biosynthesis) should show the potential benefit to essential oil accumulation of a decrease in substrate competition. We have explored the potential of changes in limonene-6-hydroxylase activity using a mathematical model (Figure 7). The in vitro enzyme activities determined in the different developmental stages from Figure 5B were used to make a model describing the accumulation of limonene and carvone depending on developmental stage. With this model we then explored the effects of changes in the activities of the individual enzymes. Figures 7B and C show that carvone content could more than double if the activity of limonene-6-hydroxylase could be enhanced. In the future we hope to substantiate this hypothesis by isolating the gene encoding the cytochrome P-450 enzyme catalysing limonene-6-hydroxylation and overexpressing it in caraway fruits to see whether indeed the carbon flux through this terpenoid biosynthesis pathway is directed more to carvone.

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