COMPARISON OF MODERN AND FOSSIL PLANT CUTICLES BY SELECTIVE CHEMICAL-EXTRACTION MONITORED BY FLASH PYROLYSIS-GAS CHROMATOGRAPHY MASS-SPECTROMETRY AND ELECTRON-MICROSCOPY

In order to investigate the preservation processes influencing the occ urrence of plant cuticles (and hence leaves etc.) in the fossil record we have undertaken a comparative study of modern and fossil Ginkgo cu ticles by chemical and microscopical methods. Cuticles are stripped or released from modern leaf tissue with hydrogen peroxide in aqueous ac etic acid. The polysaccharide component and lignin can be selectively removed by acetyl bromide in acetic acid, and the cutin (polyester) by saponification. These treatments reveal the presence of a non-saponif iable residue of a resistant biomacromolecule with a characteristic do minantly aliphatic pyrolysis pattern as demonstrated and named cutan i n the prototypical Agave americana cuticle. However, the same chemical treatments of recent Ginkgo biloba, the untreated cuticle of which sh ows an aliphatic signature upon pyrolysis, results in complete solubil isation of the sample with no resistant residue. The pyrolysis pattern s can be clearly related to electron microscopic observations of the c uticles at different stages of chemical treatment. In particular the i nitial presence and extent of extracuticular cellular material, and it s subsequent removal by the acetylation treatment can be visualised an d explained. The saponifiable cutin polyester has a structural functio n even when associated with a resistant biomacromolecule in the Agave americana cuticle because electron microscopy shows that the resistant residue consists only of cuticle fragments. Fossil cuticles of Ginkgo huttonii were examined by Py-GC-MS and electron microscopy. These con sist of extensive cuticle sheets on which SEM reveals gross morphology closely comparable to that in the modern cuticle whilst TEM shows tha t extra cuticular cellular material is lacking but that the outermost amorphous cuticle zone is preserved. An aliphatic pattern of alkene/al kane doublets has been identified in all of the samples. In addition, phenolic compounds have been found in the modern and fossil Ginkgo cut icle. The presence of a series of alkene/alkane doublets and some phen olic compounds in the fossil sample, combined with preservation of out er cuticle morphology and ultrastructure suggests that a less resistan t, saponifiable polymer like that in modern Ginkgo cuticle is one poss ible source of the morphologically-preserved organic matter in the fos sil. A highly resistant macromolecule is not responsible for the prese rvation of fossil Ginkgo cuticles. (C) 1997 Elsevier Science B.V.