Classification of an ocimum germplasm collection (NCRPIS, AMES) and investigation of antifungal activity

The genus Ocimum belongs to the family Labiatae and is distributed worldwide, growing in the warmer parts of both hemispheres. It is an important genus with many medicinal, veterinary, pesticidal and culinary uses. The taxonomic classification of Ocimum is difficult and confusing. Accordingly, a joint effort began in 1999 between the PI Station at Ames, Iowa and SAC (Scottish Agricultural College) Auchincruive, Ayr to classify the Ocimum germplasm collection using classical morphological comparison coupled with chemotaxonomy (GC and GC-MS). Overall, fifty species of O. basilicum, thirteen O. americanum, seven O. tenuiflorum, three O. gratissimum and a single accession of O. selloi were characterised. Novel chemical profiles were discovered in the collection including O. americanum species containing up to 71% fenchone, 47% camphor and 30% limonene and a caryophyllene chemotype of O. tenuiflorum. Considering components ≥5%, limonene was restricted to accessions of O. americanum. Also, on examination of components ≥5%, several appeared to be species specific within this collection. High oil yields (up to 5%) were found in O. americanum species from Zambia. These yields are exceptional for plants of the genus Ocimum. Plants of O. basilicum were found to differ in phenotype while producing a similar chemical profile. Plants grown in both Iowa and the west of Scotland produced oil of almost identical quality and quantity. In contrast, O. americanum plants were phenotypically alike but produced an array of essential oil profiles. In addition, variation between essential oils from leaves and flowers of O. tenuiflorum was also found. Antifungal testing of selected O. basilicum oils and individual components demonstrated activity in vitro with concentrations as low as 2ppm, with methyl chavicol and linalol giving the largest reductions in fungal growth in vitro. In glasshouse experiments, good control of Botrytis fabae and Uromyces viciae-fabae infection of broad bean was also achieved with foliar application of selected Ocimum essential oils and individual chemical components. O. basilicum essential oils and individual components on polyamine biosynthesis, catabolism and excretion were investigated. As an initial approach to determining the mode of action of the essential oil of Ocimum basilicum, it was decided to study effects on fungal polyamine metabolism. Although test compounds did not significantly deplete intracellular levels of the major polyamines spermine, spermidine, putrecine and cadaverine, definite effects were seen on the activity of key biosynthetic and catabolic enzymes. This work provides, for the first time, a detailed chemotype analysis of the genus Ocimum and shows that the whole essential oil and some of its individual components possess fungicidal activity. With increasing interest in the use of plant essential oils as crop protection agents, the essential oil of Ocimum may provide a useful additional means of controlling plant pathogens