EPICUTICULAR CRYSTALS OF NONACOSAN-10-OL - IN-VITRO RECONSTITUTION AND FACTORS INFLUENCING CRYSTAL HABITS

The primary aerial surfaces of plant species from many families (e.g. Pinaceae, Liliaceae, Ranunculaceae, Papaveraceae) are covered by epicu ticular tubules 5-20 mu m long and 0.5 mu m in diameter. The compositi on, mechanism of growth and molecular structure of this type of epicut icular aggregates have been studied. Pure nonacosan-10-ol extracted fr om Picea pungens needle surfaces formed, in vitro, tubular crystals li ke those occurring in vivo. This crystal habit was obtained irrespecti ve of the type of solvent or substratum, if the solvent was evaporated within minutes. This shows that tubules of nonacosan-10-ol are formed in the kinetic regime of crystallization (limited by the diffusion of molecules from the solution to the crystal surface). Slow evaporation of the solvent or crystallization from the melt resulted in rhombic s cales. These planar crystals represent the thermodynamic, stable modif ication of native nonacosan-10-ol Homologous impurities in natural non acosan-10-ol (3-14%) had no effect on the formation of the tubules. Ho wever, racemic nonacosan-10-ol invariably crystallized in scales. The phase behaviour of mixtures of natural nonacosan-10-ol and its synthet ic racemate as well as synthetic (S)-nonacosan-10-ol provided evidence for the presence of the pure (S)-enantiomer on plant surfaces. The fi ndings are discussed in terms of the mechanisms leading to epicuticula r tubules consisting of nonacosan-10-ol and their molecular structure. Crystal structures for the pure enantiomer and the racemate of nonaco san-10-ol are proposed. It is concluded that the principles responsibl e for the formation of tubules are both the special molecular geometry of the naturally occurring (S)-nonacosan-10-ol and the mobility barri er of the plant cuticle. Further specific biological processes are not necessary for the formation of (S)-nonacosan-10-ol tubules. The alter ations of epicuticular structures during ageing or the impact of pollu tants are explained as spontaneous transitions between two crystal mod ifications of (S)-nonacosan-10-ol.