Further observations on rhythmic emission of fragrance in flowers

Observations regarding floral fragrance and the rhythmicity of its emission in four plant species are reported. In the case of flowers of Hoya carnosa R. Br. which are characterized by circadian rhythmicity of scentedness (R. Altenburger and P. Matile, 1988, Planta 174, 248-252), temperature compensation of the free-running period as well as persistence of oscillations in permanent darkness have been demonstrated. A hitherto unidentified component of fragrance turned out to be identical to an unusual sesquiterpene recently discovered in cardamom oil (B. Maurer et al., 1986, Tetrahedron Lett. 27, 2111-2112). In Stephanotis floribunda Brongs. the rhythmic emission of fragrance is circadian in nature, but in a constant environment the oscillations of individual components are increasingly asynchronous. In excised flowers of Odontoglossum constrictum Lindl. the diurnal oscillations observed in a natural photoperiod are abolished under constant environmental conditions. They are resumed upon the return to a 12∶12-h photoperiod. The absence of circadian control could also be demonstrated in excised flowers of Citrus medica L. In this species, too, the daily maxima of scent emission reappear upon the transfer of flowers to a 12∶12-h light/ dark cycle. Results obtained upon the comparative analysis of volatiles in the headspace above the flowers and in petal extract indicate that the relative abundance of an individual compound in the floral fragrance is not a function of differential volatility.