Geographical Distribution of C₄ Acid Decarboxylation Types and Associated Structural Variants in Native Australian C₄ Grasses (Poaceae)

Abstract

The native Australian C₄ grass (Poaceae) flora is estimated to comprise 347 (57%) NADP-ME (NADP- malic enzyme), 193 (32%) NAD-ME (NAD-malic enzyme), and 65 (11%) PCK (PEP carboxykinase) type species. All C₄ types are best represented in northern tropical Queensland, within the megatherm seasonal (summer) rainfall bioclimate of Nix. NADP-ME species are numerically dominant in 48 out of 73 State and Territory subdivisions, including 23 wholly or partly within the megathermlmesotherm arid bioclimate which closely corresponds to the arid and semiarid zones covering c. 80% of Australia. NAD-ME species numbers are proportionately at their highest in this bioclimate; PCK species may be the most dependent there on high soil moisture availability. The extent of the megatherm seasonal bioclimate is parallelled by the distribution of most PCK species and of many species of all C₄ types with a suberised lamella in the cell walls of their 'photosynthetic carbon reduction' (or Kranz) tissue. Whilst the physiological reasons for these correlations are unknown, it is clear that C₄ type alone is neither the sole determinant of geographical distribution nor necessarily always an adaptation to a particular bioclimatic regime. Taxonomic, ecological and historical factors in relation to C₄ type distribution are discussed.