Water-use efficiency and nitrogen-use efficiency of C3-C4 intermediate species of Flaveria Juss. (Asteraceae)

Abstract

Plants using the C4 pathway of carbon metabolism are marked by greater photosynthetic water and nitrogen-use efficiencies (PWUE and PNUE, respectively) than C3 species, but it is unclear to what extent this is the case in C3-C4 intermediate species. In this study, we examined the PWUE and PNUE of 14 species of Flaveria Juss. (Asteraceae), including two C3, three C4 and nine C3-C4 species, the latter containing a gradient of C4-cycle activities (as determined by initial fixation of 14C into C-4 acids). We found that PWUE, PNUE, leaf ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) content and intercellular CO2 concentration in air (Ci) do not change gradually with C4-cycle activity. These traits were not significantly different between C3 species and C3-C4 species with less than 50% C4-cycle activity. C4-like intermediates with greater than 65% C4-cycle activity were not significantly different from plants with fully expressed C4 photosynthesis. These results indicate that a gradual increase in C4-cycle activity has not resulted in a gradual change in PWUE, PNUE, intercellular CO2 concentration and leaf Rubisco content towards C4 levels in the intermediate species. Rather, these traits arose in a stepwise manner during the evolutionary transition to the C4-like intermediates, which are contained in two different clades within Flaveria. © 2011 Blackwell Publishing Ltd.