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

The genus Aristida (Poaceae), is composed of species that have Kranz anatomy and C photosynthesis. Kranz anatomy typically consists of two photosynthetic cell types: a layer of mesophyll cells where atmospheric CO is fixed into C acids, and an internal, chlorenchymatous vascular bundle sheath to which C acids are transferred and then decarboxylated to donate CO to the C 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 C 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 CO in the C cycle. The inner bundle sheath, which is high in Rubisco and contains NADP-malic enzyme and glycine decarboxylase, functions to transfer CO to the C cycle through decarboxylation of C 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 CO that leaks from the inner bundle sheath cells.