Water-use efficiency and nitrogen-use efficiency of C3-C4 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 C-14 into C-4 acids). We found that PWUE, PNUE, leaf ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) content and intercellular CO2 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 CO2 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.