Scented at day and night: diel variation in the floral scent chemistry of the threatened tree Amburana cearensis (Leguminosae) and effects of this variation on its pollinators

Abstract

The reproductive success of most angiosperms depends on pollinators. A combination of floral traits, including olfactory cues, are involved in attracting these animals. The emission and chemical composition of floral volatile organic compounds (VOCs) is often associated with the sensory preferences of the most frequent floral visitors. However, the relationship between VOCs and nocturnal and diurnal pollinators is poorly known in tropical dry forests. Our study describes the dynamics of floral scent chemistry throughout the anthesis of Amburana cearensis (Leguminosae) and its association with floral visitors in the Caatinga dry forest, one of the largest expanses of seasonally dry tropical forest (SDTF) in the world. The species is a threatened tree with high socioeconomic importance in the Caatinga and other SDTFs of South America (exploited for timber and medicinal uses). Its flowers emit an intense, pleasant and sweet fragrance throughout anthesis, starting at ∼16:00 h and ending at ∼15:00 h of the following day (lasting 23 h). We used dynamic headspace sampling and gas chromatography coupled with mass spectrometry (GC-MS) to collect and analyze the floral scent within a natural population. The fragrance of A. cearensis was characterized by the dominant presence of monoterpenes, especially linalool and (Z)-linalool oxide (furanoid) and (Z)-linalool oxide (pyranoid), as well as (E,E)-α-farnesene and an aromatic benzenoid, benzyl alcohol. The relative proportions of floral VOCs emitted by A. cearensis changed during anthesis, with a marked presence of linalool in daytime samples (day: 45% / night: 27%) and furanoid (Z)-linalool oxide at night (day: 12% / night: 29%). The dominance of benzyl alcohol, linalool and its oxides in the floral scent of A. cearensis is possibly related to the attraction of nocturnal and diurnal Lepidoptera as pollen vectors. Indeed, we observed six species of butterflies and seven species of moths pollinating the flowers of A. cearensis in 41 h of observations. However, other terpenoids in the fragrant blend possibly contribute to the attraction of additional diurnal floral visitors, including bees, which are also effective pollinators. The continuous scent production throughout anthesis, and the changes in floral scent with a peak of linalool production during the day, may be related to the wide spectrum of pollinators of A. cearensis. This association between floral visitors and dynamics of VOCs presentation may therefore increase the reproductive success of the species, which only blooms every two years and is strictly self-incompatible.