C₄ Carbon Fixation - C₄ Leaf Anatomy

C₄ Leaf Anatomy

The C₄ plants often possess a characteristic leaf anatomy. Their vascular bundles are surrounded by two rings of cells, the inner ring, called bundle sheath cells, contain starch-rich chloroplasts lacking grana, which differ from those in mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. This peculiar anatomy is called kranz anatomy, from the German word for wreath. The primary function of kranz anatomy is to provide a site in which CO₂ can be concentrated around RuBisCO, thereby avoiding photorespiration. In order to maintain a significantly higher CO₂ concentration in the bundle sheath compared to the mesophyll, the boundary layer of the kranz has a low conductance to CO₂, a property that may be enhanced by the presence of suberin.

Although most C₄ plants exhibit kranz anatomy, there are many species that operate a limited C₄ cycle without any distinct bundle sheath tissue. Suaeda aralocaspica, Bienertia cycloptera and Bienertia sinuspersici (all chenopods) are terrestrial plants that inhabit dry, salty depressions in the deserts of south-east Asia. These plants have been shown to operate single-cell C₄ CO₂-concentrating mechanisms, which are unique among the known C₄ mechanisms. Although the cytology of both species differs slightly, the basic principle is that fluid-filled vacuoles are employed to divide the cell into two separate areas. Carboxylation enzymes in the cytosol can, therefore, be kept separate from decarboxylase enzymes and RuBisCO in the chloroplasts, and a diffusive barrier can be established between the chloroplasts (which contain RuBisCO) and the cytosol. This enables a bundle-sheath-type area and a mesophyll-type area to be established within a single cell. Although this does allow a limited C₃ cycle to operate, it is relatively inefficient, with the occurrence of much leakage of CO₂ from around RuBisCO. There is also evidence for the exhibiting of inducible C₄ photosynthesis by non-kranz aquatic macrophyte Hydrilla verticillata under warm conditions, although the mechanism by which CO₂ leakage from around RuBisCO is minimised is currently uncertain.