Nitrogen and water use efficiency of C4 plants

Species with the C4 photosynthetic pathway have evolved biochemical CO2 concentrating mechanisms that allow Rubisco to function in a high CO2 environment. This increases both their nitrogen and water use efficiency compared to C3 species. A comparison between Australian C4 grasses and global data (Glopnet) reveals that C4 species have greater rates of CO2 assimilation than C3 species for a given leaf nitrogen when both parameters are expressed either on a mass or an area basis. The comparison also revealed that although the range in leaf N content per unit area is less in C4 compared to C3 species, the range in leaf nitrogen concentration per unit dry mass is similar for both C4 and C3 species. While C3 and C4 species invest a similar fraction of leaf N into photosynthetic components, C4 species allocate less to Rubisco protein and more to other soluble proteins and thylakoid components. Hence, the driving force that increases CO2 assimilation rate per unit leaf nitrogen in C4 species is greater catalytic turnover rate of Rubisco in vivo. This is exemplified by the fact that differences in photosynthetic nitrogen use efficiency amongst C4 species of the NAD-ME and NADP-ME decarboxylation types is linked to variation in Rubisco kinetic properties amongst these species. Improved leaf and plant water use efficiency in C4 species is due to both higher photosynthetic rates per unit leaf area and lower stomatal conductance. By contrast, leaf and plant water use efficiency is increased in C4 plants under elevated CO2 because of reduced stomatal conductance. The geographic distribution of the different C4 subtypes is strongly correlated with rainfall. One might expect that these distribution differences are linked to differences in water use efficiency. The convergence found in water use efficiency and leaf gas exchange characteristics under most growth conditions, between NAD-ME and NADP-ME grasses, is therefore a curious reminder that geographic distribution may not be related fully to the physiology of photosynthesis.