Insight into the chemical composition of the foliar structure in extinct and extant taxa belonging to cycadophytes

LAFUENTE DIAZ, M; ZODROW E L; D`ANGELO J A; DEL FUEYO, G

Simposio; XVIII Simposio Argentino de Paleobotánica y Palinología; 2022

The molecular characterization of Cycas rumphii (extant Cycadales) leaves using semi-quantitative Fourier Transform Infrared (FTIR) spectroscopy is performed. The resulting chemical information is compared to that of fossil leaves of Cycadales and Bennettitales. Thus, the aim of this contribution is to study functional groups; i.e., structural compounds as well as primary and secondary metabolites making up Cycadophytes plants. In this way, FTIR data obtained for foliar specimens of Cycas rumphii (extant Cycadales; NY 581/2006-F) are compared with those of Pseudoctenis ornata (extinct Cycadales; BA Pb 1217, 1220, 1222) and Ptilophyllum micropapillosum (Bennettitales; MPM Pb 15355) samples. The fossil taxa are foliar compressions with well-preserved cuticles which were collected from two different geological units outcropping in the Santa Cruz province (Argentina). Particularly, Pseudoctenis ornata was recovered from the Baqueró Group (Punta Del Barco Formation, Aptian), whereas Ptilophyllum micropapillosum from the Springhill Formation (Hauterivian–Barremian). Fossil and extant specimens were spectroscopically analyzed using two sample forms: 1) complete leaf (mesophyll + cuticles) and 2) cuticles. Semi-quantitative IR-data were analyzed using a multivariate statistical method (principal component analysis). The spectroscopic pattern of C. rumphii resembles that of the cuticles of P. micropapillosum. The cuticles of these two taxa show a high content of aromatic functional groups (C=C contribution), a relatively low contribution of aliphatic- and oxygen-containing compounds, and low cross-linking of polymeric structures (high CHal/C=O values). The relatively strong aromatic contribution recorded for C. rumphii leaves is mainly related to cutin/cutan monomers (phenylpropanoids and aromatic domains) and phenolic compounds (e.g., lignins, tannins), which are present in cuticles (including lignified stomata) and in mesophyll (e.g., hypodermic fibers, transfusion tissue, conspicuous midribs). Additionally, C. rumphii spectra exhibit distinctive peaks related to cell wall components (i.e., pectic polysaccharides, hemicelluloses, and celluloses) in the 1200-800 cm-1 region. On the other hand, P. ornata differs from the remaining taxa by having the highest aliphatic contents. Chemical differences between the fossil and extant Cycadales samples may be more related to the original chemistry of P. ornata rather than to their systematic affinity. However, the previously reported natural oxidation as a consequence of the volcanic activity during the Baqueró Group sedimentation where the parent plants of P. ornata inhabited cannot be ruled out. In conclusion, the microanatomical differences that allow the distinction between Cycadales and Bennettitales are not reflected as chemical differences. In contrast, FTIR spectroscopic data support the morphological similarities between Cycadales and Bennettitales, being Cycas rumphii and Ptilophyllum micropapillosum, chemically similar in terms of functional-groups contributions.