Pollination biology of Theobroma and Herrania (Sterculiaceae): II. Analyses of floral oils

Abstract

Floral fragrance oils related to pollination were studied in three species of Theobroma and one Herrania species (Sterculiaceae) in Costa Rica. This study was prompted by field observations that floral odours and floral visitors are markedly different between Theobroma and Herrania, as well as between species of Theobroma.Floral fragrance in cultivated T. cacao consisted of 78 components, primarily saturated and unsaturated hydrocarbons, with 1-pentadecene the major constituent followed by n−pentadecane. The distribution of 1-pentadecene was similar at 0900 hr (43.6%), 1130 hr (52.5%) and 2000 hr (51.1%) sampling times but only 1.2% at 1600 hr. The ratio of 1-pentadecene to n−pentadecane in the 1600 hr sample, approximately 1:6, contrasted with a 3:1 ratio at 0900 and 1130 and a 6:1 ratio at 2000 hr. The greatest concentration of volatiles was found in the 1130 hr sample, with 0900 and 2000 hr concentrations being essentially equal and intermediate between the high (1130 hr) and the low (1600 hr).Of the 58 compounds detected in T. mammosum steam distillate at 0815 hr, an array of linalool oxides accounted for 12.5% of the oil followed by isoeugenol (8.9%). Saturated hydrocarbons were also present but, unlike T. cacao, n-tricosane was the major hydrocarbon (12.2%). The major constituents of T. mammosum oil clearly distinguish this species from T. cacao.A markedly different profile of major floral volatiles was found in T. simiarum: major constituents included the monoterpenoids citral, geraniol, nerol, and citronellol. Unlike the oily, hydrocarbon fragrance of T. cacao and T. mammosum, T. simiarum floral fragrance is citrus-like both in the field and laboratory. Our fragrance studies suggest considerable evolutionary divergence within Theobroma.The floral fragrance of H. cuatrecasana contained volatiles characteristic of dipteran-pollinated plant species. Unusual volatiles found in Herrania fragrance included iridomyrmecin, guaiol, and other azulenic derivatives. No such monoterpenoids and bicyclic sesquiterpenoids (with the exception of longifolene, 0.1%, in T. simiarum) were found in Theobroma. The profiles of floral fragrance chemotaxonomically support the exclusion of Herrania spp. from the genus Theobroma.