Comparison of leaf structure and photosynthetic characteristics of C3 and C4 Alloteropsis semialata subspecies

Alloteropsis semialata (R. Br.) Hitchcock includes both C-3 and C-4 subspecies: the C-3 subspecies eckloniana and the C-4 subspecies semialata. We examined the leaf structural and photosynthetic characteristics of these plants. A. semialata ssp. semialata showed high activities of photosynthetic enzymes involved in phosphoenolpyruvate carboxykinase-type C-4 photosynthesis and an anomalous Kranz anatomy. Phosphoenolpyruvate carboxylase; pyruvate, Pi dikinase and glycine decarboxylase (GDC) were compartmentalized between the mesophyll (M) and inner bundle sheath cells, whereas ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) occurred in both cells. A. semialata ssp. eckloniana also showed an anomalous non-Kranz anatomy, in which the mestome sheath cells included abundant chloroplasts and mitochondria. Rubisco and GDC accumulated densely in the M and mestome sheath cells, whereas the levels of C-4 enzymes were low. The activity levels of photorespiratory enzymes in both subspecies were intermediate between those in typical C-3 and C-4 plants. The values of CO2 compensation points in A. semialata ssp. semialata were within the C-4 range, whereas those in A. semialata ssp. eckloniana were somewhat lower than the C-3 range. These data suggest that the plants are C-3-like and C-4-like but not typical C-3 and C-4, and when integrated with previous findings, point to important variability in the expression of C-4 physiology in this species complex. A. semialata is therefore an intriguing grass species with which to study the evolutionary linkage between C-3 and C-4 plants.