3.2.4.2—
The Dimorphic Chloroplasts of C4 Plants
C4 plants form oxalacetate, malate and aspartate as the primary products of their photosynthetic CO2 -fixation in contrast to the more 'usual' C3 plants whose primary fixation product is 3-phosphoglycerate. Laetsch (1974) lists the following families of flowering plants as containing C4 species: Amaranthaceae, Aizoaceae, Chenopodiaceae, Compositae, Cyperaceae, Euphorbiaceae, Gramineae, Nyctaginaceae, Portulaceae and Zygophyllaceae. All of these families also contain C3 plants and there is the case of the genus Atriplex where the C4A. rosea will hybridize with the C3A. patula (Björkman et al., 1970). Apart from the difference in primary photosynthetic CO2 -fixation products C4 and C3 plants show other substantial differences in their biochemistry, physiology, anatomy and fine structure. Maize and bean will be discussed here as typical examples of C4 and C3 plants respectively
In the maize leaf the chloroplasts are concentrated in two concentric sheaths of cells around each vascular bundle. The inner sheath of cells is described as the bundle sheath and consists of equal numbers of large barrel-shaped cells and smaller cells which can be divided into two sorts on the basis of their dimensions (Montes & Bradbeer, 1975). Laetsch (1974) points out that the thick walls of bundle sheath cells adjoining mesophyll cells in C4 grasses possess an electronopaque layer (Fig. 3.1C). The bundle sheath chloroplasts possess abundant thylakoids which do not associate into grana, (Fig. 3.1C) and they normally contain starch grains. The plant from which the material was taken for Figure 3.1C had been stored in the dark for 24 hours prior to fixation to remove the starch so as to obtain a clear electron micrograph. In most cases, the bundle sheath chloroplasts of maize have been found to be completely agranal, although by modification of the environmental conditions the formation of grana can be induced (Bradbeer & Montes, 1976). In contrast Laetsch (1974) considers it normal for bundle sheath chloroplasts to show a small amount of thylakoid appression.
The outer sheath of cells is known as the mesophyll sheath and it contains chloroplasts similar to those of C3 plants in that they have grana. They are, however, different in that they do not normally contain starch grains. Not all C4 plants show such structural dimorphism of their chloroplasts. However all chloroplasts of C4 plants possess a system of tubules, the peripheral reticulum, (PR in Fig. 3.1C), which is associated with the inner membrane of the chloroplast envelope. There are reports of peripheral-reticulum-like membrane systems in chloroplasts of some cells of C3 plants (Laetsch, 1974). Circumstantial evidence assembled by the latter author suggests that the function of the peripheral membrane in C4 plants may be to facilitate transfer of metabolites between chloroplast and cytoplasm.