Functional compartmentation of C ₄ photosynthesis in the triple layered chlorenchyma of Aristida (Poaceae)

The genus Aristida (Poaceae), is composed of species that have Kranz anatomy and C₄ photosynthesis. Kranz anatomy typically consists of two photosynthetic cell types: a layer of mesophyll cells where atmospheric CO₂ is fixed into C₄ acids, and an internal, chlorenchymatous vascular bundle sheath to which C₄ acids are transferred and then decarboxylated to donate CO₂ to the C₃ cycle. The anatomy of Aristida species is unusual as it has three distinct layers of chlorenchyma cells surrounding the vascular tissue: an inner bundle sheath, an outer bundle sheath and the mesophyll cells. In this study of Aristida purpurea Nutt. var. longiseta, the functions of the three layers of chlorenchyma cells relative to the C₄ photosynthetic mechanism were determined using ultrastructural analysis, western blots, immunolocalisation of photosynthetic enzymes and starch histochemistry. The results indicate that mesophyll cells contain high levels of phosphoenolpyruvate carboxylase (PEPC) and pyruvate Pi dikinase (PPDK), and function to capture CO₂ in the C₄ cycle. The inner bundle sheath, which is high in Rubisco and contains NADP-malic enzyme and glycine decarboxylase, functions to transfer CO₂ to the C₃ cycle through decarboxylation of C₄ acids and by decarboxylation of glycine in the glycolate pathway. The outer chlorenchymatous sheath is where ADPG pyrophosphorylase is mainly located, and this cell layer functions as the primary site of starch storage. The outer sheath, which has low levels of Rubisco and PEPC, may also have a role in refixation of any CO₂ that leaks from the inner bundle sheath cells.