Chemotaxonomic significance of distribution and stable carbon isotopic composition of long-chain alkanes and alkan-1-ols in C4 grass waxes

Grasses (Poaceae) are distributed across the world in broad latitudinal belts and are an important source of C-4 biomass in the geological record of soils as well as lake and marine sediments. We examined long-chain leaf wax components of thirty-five C-4 grasses of the subfamilies Aristidoideae, Chloridoideae and Panicoideae from the southern African grasslands and savannas and three C-3 grasses of the subfamily Pooideae from Peru and Australia and review the relevant botanical, phytogeographic and leaf wax compositional background information. Contents, distribution patterns and molecular stable carbon isotopic compositions of long-chain n-alkanes (n-C-27 to n-C-35) and nalkan-1-ols (n-C-22 to n-C-32) were used to estimate the chemotaxonomic relevance of wax signatures of whole plants, separately for different subfamilies and for members of the three C-4 subtypes (NADP-ME, NAD-ME and PCK). Two grass species were separated into flower heads, leaves and stems and the parts analysed separately. Grass flowers contain remarkable amounts of short-chain n-alkanes, which may have a significant influence on the chemical signature of the whole plant, whereas n-alkanol distribution patterns exhibit no systematics. The stable carbon isotopic composition of both biomarker types in different plant parts is remarkably uniform. Chemotaxonomic differentiation was not possible on a species level based on whole plant samples, but was more successful for averages of subfamily and photosynthetic subtype data. Wax signatures of C-4 grasses are generally distinguishable from those of C-3 species by heavier isotopic values, higher contents of n-C-31 and n-C-33 alkanes and the abundance of the n-C-32 n-alkanol, which is largely absent in C-3 grass waxes. Especially the waxes of the NAD-ME and PCK C-4-subtype grasses, which thrive in extremely and tropical and subtropical areas, contain high relative amounts of longer-chain n-alkane homologues. The chemical classification on a subfamily level, which is in agreement with previously reported subfamilial phylogeny of grasses, implies an evolutionary wax adaptation of C-4 grasses to warm and and habitats. Our results confirm the validity of the contents, distribution patterns and molecular stable carbon isotopic compositions of long-chain n-alkanes and alkan-1-ols as indirect proxies of continental climate conditions in environmental studies of the tropics. (c) 2006 Elsevier Ltd. All rights reserved.