The phenomenon of C3–C4 intermediate photosynthesis has now been recognised for about 25 years and has received a considerable amount of interest in that time, much of it aimed at the link that these plants might represent in the evolution of the C4 photosynthetic system from the older C3 one. A number of recent reviews have addressed evolutionary and adaptive aspects of C3–C4 photosynthesis together with the biochemistry (Peisker, 1986; Edwards & Ku, 1987; Monson, 1989; Monson & Moore, 1989; Araus et al., 1991). We intend, therefore, to confine our discussion in this article to those aspects of the C3–C4 character that have a major influence on the metabolic interactions within and between photosynthetic cells in the leaves of these plants and to compare and contrast this with what is known about photosynthetic metabolism in C3 and C4 plants.

Plants which have C3–C4 intermediate photosynthesis have been identified in seven genera across five families, including representatives from the Monocotyledoneae and the Dicotyledoneae (Edwards & Ku, 1987). Despite their wide distribution in the higher plant kingdom there are a number of well-conserved features across this group. We will describe these and then discuss their implications for inter- and intracellular metabolite transport and the metabolic regulation of photosynthesis and photorespiration.