Taxonomy of Basil
As with many plants, the assignment of taxonomic systems to Ocimum species has been an active and contentious area. The genus was named by Swedish naturalist Carl Linneaus in 1753 and enumerated into five species. In the past 250 years, there have been a number of taxonomic systems proposed, greatly expanding the species number of Ocimum. These systems were based on the structure of the plants, particularly those of the flower parts. The most recent system in common use is that of British plant taxonomist Alan Paton and his fellow researchers from 1999, in which Ocimum basilicum is placed within the Ocimum section of this genus.
The morphology of the plants can vary greatly, however, given the crossbreeding between various cultivars and species. The science of chemistry was in its infancy when the original Ocimum taxonomic schemes were proposed. Since then, the advent of analytical chemistry has enabled the study of the numerous aromatic compounds found in all species of Ocimum, particularly those of the essential oils of these plants. This has enabled researchers to classify types of basil based on their chemical profile—a field called chemotyping. One problem with this avenue of research is that the particular chemicals in a given plant can vary widely depending on the environmental conditions, seasons, and soil and region in which the plant is grown.
Other methods of taxonomic classification of basils include examining the geographic origin of the plants and whether or not they can be crossed with one another. With the discovery of DNA and cell nuclei, karyotyping became a viable method of analysis. This technique involves determining the number and appearance of chromosomes per cell in an organism. One problem with using this technique in basil is that researchers have found the chromosome count to vary widely within the same species.
The advent of molecular biological techniques, such as the polymerase chain reaction (PCR), have ushered in a new era in plant taxonomy. A class of techniques based on PCR amplification of small amounts of DNA takes advantage of minor changes in the structure of genes. This enables genetic fingerprinting to be performed in plants and gives a much more rigorous analysis of genetic relatedness. Croatian scientist Klaudija Carovic-Stanko and coworkers published such research in 2010 comparing 28 accessions of basil, including 22 of Ocimum basi- licum. In addition to PCR techniques that had been previously used in studies of basil taxonomy, along with chromosome counting, they also examined nuclear DNA content.
Of the two PCR methods used, AFLP (amplified fragment length polymorphism) was successful in separating all of the accessions from one another. The commonly used technique RAPD (random amplified polymorphic DNA) did not distinguish between all of the accessions. In tandem, both of these techniques enabled the researchers to successfully classify all of the plants tested.
The researchers found a high level of genetic diversity among the basil accessions, which adds credence to the taxonomic system of Paton with its large number of species. They were able to clearly separate taxa, with a grouping of Ocimum basilicum type species and one of Ocimum americanum. All of the Ocimum basilicum cultivars and varieties fit within that species’ group. Surprisingly, Ocimum minimum fit within the group also and could be a subspecies or variety of Ocimum basilicum. An additional surprise was that two Russian accessions of Ocimum basilicum ssp. purpurascens were found to be genetically identical to the Ocimum americanum group, which has a different number of chromosomes.
The current classification of basil includes several varieties and a very large number of cultivars. The primary variety of Ocimum basilicum in cultivation is basilicum, in particular cultivars Sweet Basil and Genovese. Also notable are varieties purpurascens Benth. (purple basil), and difforme Benth.