| || || |
| Rosemary Cineole Type Vs. Verbenone Type |
Answered by: Conrad Richter
Question from: Marika Bell
Posted on: October 31, 2007
How would one know, apart from chemical analysis, whether a rosemary plant is the cineole type. I have plants that have a relatively high cineole content, and a lower verbenone content and vice versa. Ideally one would want to know before planting, but I suppose from what I have learnt that it is impossible to know from the plant - is that so?
Cultivars with different chemical compositions are known as chemotypes. Cultivars that are different morphologically are called phenotypes. I have not grown rosemary cultivars identified as either verbenone-type or cineole-type so I do not have the benefit of direct experience of working with these chemotypes. The named varieties Richters sells are phenotypes selected mostly for their morphological characteristics such as upright or prostrate habit, leaf size and shape, leaf colour, etc.
In principle there is no reason why the different chemotypes could also also have morphological differences that can be used to tell the different types apart. As Mulas et al (Acta Hort. 576:163) showed in 2002 different clones with different leaf sizes and different growth habits can have very different chemical compositions. They found, for example, that a clone with an "intermediate/open" growth habit with "high" vigour and wide leaves had the highest 1,8-cineole content (18.7%) while a different clone with "intermediate/prostrate" habit and "low" vigour and narrow leaves had almost no 1,8-cineole (0.7%). Meanwhile another clone with an "upright" habit with "medium" vigour and leaves of intermediate width had the highest verbenone (16.3%) while yet another clone with "intermediate/open" habit and "medium" vigour and wide leaves had low verbenone (1.3%). These results clearly show that it is possible to associate visual characteristics with either verbenone or cineole types of rosemary. But these results also show that there is no inherent relationship between chemical content and the specific morphological characteristics that were studied. What this means is that it may be possible to use visual features as a way to keep track of one chemotype from another but only if you already know the chemistry of each cultivar. If you are looking at a cultivar that has not been evaluated chemically then there is no way to tell which type of rosemary you have based on its visual features alone.
It may be possible for well-trained noses to tell the difference. I don’t know of any studies that suggest that basic organoleptic characteristics such as smell and taste are distinctive enough to tell apart the two types. I do know that among the Richters varieties there are subtle and not-so-subtle differences in smell and taste; so I would not be surprised that a trained nose would be able to tell the high verbenone and high cineole types apart. But then again these chemicals have very similar olfactory features -- 1,8-cineole is known to have a eucalytpus-camphor note while verbenone has a camphor-menthol-celery note -- and I am not sure that I would be able to tell these apart among all the other odoriferous components in rosemary leaves.
A few words of caution. First, when looking at the essential oil marketing literature it is easy to get the impression that there are only two or three types of rosemary in the trade. This is a very wrong impression. Rosemary is very variable morphologically and chemically and there are untold dozens, hundreds, and probably thousands of forms in commercial use, either among the wild plants that are gathered to make essential oils or among the cultivated plants that are grown around the world. Many of these different forms may look exactly alike. So just because you have a cineole cultivar and a verbenone cultivar that that you can tell apart visually in your fields does not mean that you can identify the chemistries of plants with similar visual features in another field.
Second, you should always be wary of comparing essential oil results from different studies and different labs. A study published in the Journal of Essential Oil Research last year (McCormick et al.: findarticles.com/p/articles/mi_qa4091/is_200609/ai_n17197169/pg_1) showed that the preparation of rosemary plant samples can have a dramatic effect on the relative content of verbenone. Unless you are comparing lab results done in the same way you cannot conclude that any one cultivar is higher or lower in verbenone or cineole than another cultivar. In addition, it is likely that other key physiological parameters such as time of harvest, physiological age of the plants, harvest method and conditions, will have important effects on oil composition. So if you comparing your varieties of rosemary make sure that the test methods are consistent.