Water‐use efficiency and nitrogen‐use efficiency of C 3 ‐C 4 intermediate species of Flaveria Juss. (Asteraceae)

Plants using the C 4 pathway of carbon metabolism are marked by greater photosynthetic water and nitrogen‐use efficiencies (PWUE and PNUE, respectively) than C 3 species, but it is unclear to what extent this is the case in C 3 ‐C 4 intermediate species. In this study, we examined the PWUE and PNUE of 14 species of Flaveria Juss. (Asteraceae), including two C 3 , three C 4 and nine C 3 ‐C 4 species, the latter containing a gradient of C 4 ‐cycle activities (as determined by initial fixation of 14 C into C‐4 acids). We found that PWUE, PNUE, leaf ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) content and intercellular CO 2 concentration in air ( C i ) do not change gradually with C 4 ‐cycle activity. These traits were not significantly different between C 3 species and C 3 ‐C 4 species with less than 50% C 4 ‐cycle activity. C 4 ‐like intermediates with greater than 65% C 4 ‐cycle activity were not significantly different from plants with fully expressed C 4 photosynthesis. These results indicate that a gradual increase in C 4 ‐cycle activity has not resulted in a gradual change in PWUE, PNUE, intercellular CO 2 concentration and leaf Rubisco content towards C 4 levels in the intermediate species. Rather, these traits arose in a stepwise manner during the evolutionary transition to the C 4 ‐like intermediates, which are contained in two different clades within Flaveria .