PHYSIOLOGICAL LIMITATIONS TO CARVONE YIELD IN CARAWAY (CARUM-CARVI L)

Although many authors have studied factors determining caraway essential oil quantity and quality, little progress has been made in improving these parameters. We will show that the lack of progress may be partly due to the misleading use of a concentration measure (g or mi of essential oil per g seed) instead of the absolute amount per seed. In the field, for both annual and biennial caraway, formation of limonene started earlier than that of carvone, but both occurred during the early stages of seed development. The content of the two compounds reached a steady level several weeks before harvest, being higher in biennial than in annual caraway, apparently because carbon partition to essential oil formation in biennial caraway is more favourable than in annual caraway. For both annual and biennial caraway there was a positive relationship between seed weight and absolute amount of essential oil per seed. But biennial caraway contained about twice as much oil as annual caraway in a similar-sized seed. However, an estimate of the volume of the essential oil ducts suggested that the potential absolute amount of essential oil per seed generally is not reached. Shading of field plots during the early stages of seed development decreased essential oil formation and the proportion of carvone in the essential oil suggesting that mainly the formation of carvone was hindered. Shading during the later stages of seed filling did not affect essential oil formation but as it did decrease 1000-seed weight, the essential oil content increased. Under laboratory conditions, volatilization of limonene was higher than of carvone but total release did not exceed 2% of the total yield of these compounds. Hence, caraway essential oil quantity and quality seem to be determined mainly by assimilate availability during the early stages of seed development. Efforts to improve carvone yield should therefore focus on this period.