A plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilisation. In which of the following physiological groups would you assign this plant?

The ${\text{C}}_4$ plants avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilization.

The principal effect of the ${\text{C}}_4$ cycle is that it concentrates ${\mathrm{CO}}_2$ in the bundle-sheath cells where the enzymes of the PCR cycle are located. By shuttling the ${\mathrm{CO}}_2$ in the form of organic acids it is possible to build much higher ${\mathrm{CO}}_2$ concentrations in the bundle-sheath cells than would be possible relying on the diffusion of ${\mathrm{CO}}_2$ alone. Higher ${\mathrm{CO}}_2$ concentrations would suppress photorespiration and support higher rates of photosynthesis. Under optimal conditions, ${\text{C}}_4$ crop species can assimilate ${\mathrm{CO}}_2$ at rates two to three times that of ${\mathrm{C}}_3$ species.

In addition to the virtual absence of photorespiration, most ${\text{C}}_4$ plants tend to have a higher temperature optimum (30–45ºC) than C3 plants (20–25ºC). This difference is due primarily to the differential stability of the photorespiration relative to photosynthesis.

Photosynthesis in most situations is limited by available ${\mathrm{CO}}_2$ and water. In ${\mathrm{C}}_3$ plants, even moderate water stress will initiate closure of the stomata and reduce the available supply of ${\mathrm{CO}}_2$. The low ${\mathrm{CO}}_2$ compensation point of ${\text{C}}_4$ plants means that they can maintain higher rates of photosynthesis at lower ${\mathrm{CO}}_2$ levels. Thus, ${\text{C}}_4$ plants gain an advantage over ${\mathrm{C}}_3$ plants when the stomata are partially closed to conserve water during a period of water stress.