Defra Research Project NF0526 Summary for publishing on the Defra website.
Following the decline in the market for hops, UK hop growers were encouraged to switched from growing hops to growing other essential oil crops, which can be processed using the same equipment (e.g. at Botanix, Paddock Wood, Kent). There are two species of Lavandula that are grown for oil. L. angustifolia which produces a high quality low yield oil rich in linalyl acetate and L. x intermedia, a hybrid of L. angustifolia x L. latifolia, a non-hardy Mediterranean species, which produces large yields of a low quality oil containing less linalyl acetate and the two undesirable chemicals, camphor and 1,8-cineole. In lavender the oil harvested is from flower spikes not leaves, oil glands are sited on the flower calyx. L. x intermedia flowers a month after L. angustifolia, so most growers grow both species to optimise the use of equipment.
This project aimed to use two different techniques, chromosome doubling and induced mutation, to attempt to produce more competitive clones of Lavender for UK growers. All the cultivars currently available are French and not ideally adapted to UK conditions.
Plants are much more flexible in the number of chromosome sets they can have than mammals which can only ever have two sets. Plants with four sets (tetraploids) usually have slightly larger flowers than the same plant with just two sets (diploids), and there are reports of enhanced synthesis of secondary products in tetraploid plants. The chromosome number can be doubled in the laboratory by limited exposure to a herbicide called oryzalin. L. angustifolia and L. x intermedia both behave as diploids although they have a complicated cytometry with variations in the number of chromosomes. Tetraploid plants may produce more oil as a result of increased flower size or enhanced oil synthesis; also tetraploid L. x intermedia crossed with the best L. angustifolia clone would produce a triploid with two sets of L. angustifolia chromosomes and one set of L. latifolia chromosomes diluting any remaining camphor and cineole. In addition, triploids are naturally sterile plants the flowers of which are expected to stay open longer enabling a spike with all flowers open to be produced. This would give a wider harvest window and should improve oil yield. Also, triploid flowers should be slightly larger than those of the diploid, with more oil glands.
Tetraploid clones were successfully produced of three L. angustifolia cultivars and of three L. x intermedia cultivars. Both diploid and tetraploid clones of the L. x intermedia cultivars were planted in the field in Autumn 2006 and flowered this summer 2007. The spikes of tetraploid clones had larger florets than those of the same diploid clone giving a higher oil yield per spike but the tetraploid plants only produced about half as many spikes as the equivalent diploid clone giving a lower oil-yield per plant. However, the difference in flower spike number may not be due to differences in ploidy but a result of husbandry this year, therefore performance of diploid and tetraploid plants raised from cuttings will be compared again next year. Now that tetraploid clones have been produced it will be possible to produce triploid clones by crossing a diploid and a tetraploid.
If the two unacceptable chemicals could be knocked out of L. latifolia and/or L. x intermedia, then it might be possible to produce a high-yielding lavender with high quality oil. Protocols were established for irradiating lavender seed and leafless nodes of lavender. A low-cineole clone of one L. x intermedia was obtained using Cobalt 60 irradiation. Low camphor clones of L. latifolia were obtained by selection of seedlings.
These results suggested that it should be possible to produce a hybrid lavender giving high yield and high quality oil. The production of tetraploid clones, which had lower yield than the equivalent diploid, does allow for the production of triploid clones. In the Ukraine, Rabotyagov(1983) found L. x angustifolia (4x) x L. latifolia (2x) the most productive.