Figure 1.

Mechanism of C₄ photosynthesis. (a) Simplified schematic of the C₄ syndrome, showing how the C₃ pathway is isolated from the atmosphere in most C₄ species within a specialized tissue composed of bundle sheath cells (BSC). The C₄ pathway captures CO₂ within mesophyll cells (MC) using the enzymes carbonic anyhdrase (CA) and phosphoenolpyruvate carboxylase (PEPC). It then transports fixed carbon to the BSC where decarboxylase enzymes (DC) liberate CO₂, which accumulates to high concentrations around Rubisco. (b) Transverse section of a C₄ leaf, showing the arrangement of MC and BSC. In this picture, the BSC are thicker walled, are stained darker and form rings surrounding the veins (adapted from Watson & Dallwitz [9]). Enlarged BSC relative to MC and close vein spacing are typical of C₄ leaves, and this pattern is referred to as ‘Kranz anatomy’. (c) Explanation of how variation in CO₂ and temperature favours C₃ or C₄ species [10,11]. The schematic shows the range of CO₂ partial pressures and temperatures predicted to favour the growth of either C₃ or C₄ species, based on the maximum quantum yield of photosynthesis. Quantum yield is a measure of photosynthetic efficiency, which declines in C₃ plants as photorespiration increases at low CO₂ and high temperature, (reproduced with permission from Edwards et al. [3], which was adapted from Ehleringer et al. [11]).