Photosynthetic Enzyme Activities in the C₃-C₄ Intermediate Neurachne minor S. T . Blake (Poaceae)

Abstract

The activities of eight key photosynthetic enzymes were measured in leaf blade extracts of the C₃-C₄ intermediate Neurachne minor S. T. Blake, its C₃ and C₄ relatives, C₃-C₄ Panicum milioides Nees ex Trin., and controls (all Poaceae). Phosphoenolpyruvate (PEP) carboxylase (PEPC) activity in N. minor (5.46 µmol mg Chl^-1 min^-1) is higher than previously reported for any other C₃-C₃ plant, and the ratio of ribulose-1,5-bisphosphate carboxylase activity to PEPC activity is lower than for P. milioides or C₃ species. Activity of pyruvate,P_I dikinase (up to 0.88 µmol mg Chl^-1 min^-1) is 3-5 times higher than in P. milioides. Assays of NADP-malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME) and PEP carboxykinase (PCK) show Paraneurachne muelleri (Hack.) S. T. Blake and Neurachne munroi (F. Muell.) F. Muell., N. minor's two close C₄ relatives, to be NADP-ME type, as predicted from leaf anatomy. Aspartate and alanine aminotransferase activities in these species are higher than expected, however. N. minor (C₃-C₄) exhibits higher C₄ acid decarboxylase activity than C₃ species or P. milioides, for NADP-ME only (up to 1.07 µmol mg Chl^-1 min^-1). Our results suggest that N. minor possesses a limited C₄ acid cycle, and that it is the most C₄-like C₃-C₄ intermediate grass currently identified, comparable with some of the known C₃-C₄ Flaveria (Asteraceae) species.