C4 acid decarboxylation type in Eragrostis (Poaceae) patterns of variation in chloroplast position, ultrastructure and geographical distribution

Abstract We investigated the activity of C₄ acid decarboxylating enzymes, the PCR (‘photosynthetic carbon reduction’, or ‘Kranz’) bundle sheath anatomy and ultrastructure, and the geographical distribution of Australian species of the C₄ grass genus Eragrostis. Species had either the even sheath outline and centripetally located PCR cell chloroplasts characteristic of NAD-malic enzyme (NAD-ME) species (29 spp.), the uneven sheath outline and centrifugal PCR cell chloroplasts characteristic of PEP carboxykinase (PCK) species (28 spp.), or were intermediate between these types (7 spp.). The suberized lamella was present in PCR cell walls of species with PCK-like and intermediate anatomy, and absent from those of species with NAD-ME-like anatomy. Biochemical determination of C₄ type for 11 species, however, revealed only NAD-ME activity, irrespective of anatomical type; no PCK activity was detected. PCK-like species arc most numerous in northern, high rainfall, tropical Australia and also predominate in relatively humid coastal and subcoastal areas. NAD-ME-like species are numerically and proportionally dominant where rainfall is < 30 cm year^?1. Overall, as many species occur in high as in low rainfall areas. Results are discussed in relation to previously established anatomical/ultrastructural/geographical/biochemical correlations and to Infrageneric taxonomy.