Figure 1.
Schematic of leaf anatomy and photosynthetic pathway in C3, C3‐C4 and C4 grasses. In C3 plants, CO 2 assimilation via the Calvin–Benson cycle (solid black circle) and CO 2 release via photorespiration (dashed black circle) both occur in mesophyll cells (light green). C3 leaves consequently have larger areas of mesophyll tissue than bundle sheath tissue, where no photosynthetic activity occurs. C3‐C4 plants use an intermediate physiology called C2 photosynthesis, where the Calvin‐Benson cycle occurs in mesophyll cells, like in C3 plants. However, because glycine decarboxylase (GDC) is specifically localised to bundle sheath cells in these plants, the photorespiratory cycle is split across these two cell types, creating a weak CO 2‐concentrating mechanism, where CO 2 is released in the bundle sheath and can be reassimilated via the Calvin cycle. C2 photosynthesis, therefore, requires large areas of mesophyll for photosynthesis via an initial Calvin‐Benson cycle, but also close contact between mesophyll and bundle sheath cells for the photorespiratory CO 2 pump. C4 plants have a strong CO 2 concentrating mechanism whereby CO 2 is biochemically shuttled from the mesophyll into the bundle sheath. The high CO 2 concentration in the bundle sheath largely avoids oxygenation and thus, photorespiration. Photosynthesis via the C4 cycle therefore requires large areas of bundle sheath tissue, but less mesophyll, which can be achieved via the insertion of minor veins. Dark blue, bundle sheath lacking chloroplasts; dark green, bundle sheaths with chloroplasts; light green, mesophyll cells; yellow, extraxylary fibres/bundle sheath extensions; grey, epidermal cells; light blue, veins; white, metaxylem.