The relative contributions of reduced photorespiration, and improved water-and nitrogen-use efficiencies, to the advantages of C-C intermediate photosynthesis in Flaveria.

Analyses of carbon-assimilation patterns in response to intercellular CO concentrations, and the photosynthetic water-and nitrogen-use efficiencies, were conducted for a C, a C, and three C-C species in the genus Flaveria in order to determine some of the advantages and disadvantages of C-C intermediate photosynthesis. Operational intercellular CO partial pressures (pi), determined when the atmospheric CO partial pressure (pa) was approximately 330 μbar, in the C-C species were generally equal to, or greater than, those observed in the C species under well-watered or water-stressed conditions. This reflects equal, or lower, water-use efficiencies (WUEs) in the C-C species. The only case in which higher WUEs were observed in the C-C species, compared to the C species, was when photosynthesis rates were limited by available nitrogen and were less than 12.5 μmol CO ms. At higher photosynthesis rates, the C-C species exhibited lower values of photosynthesis rate for equal values of stomatal conductance (lower WUE), compared to the C species. Comparing slopes for the linear regions of the relationship between leaf nitrogen content and net photosynthesis rate (taken as an index of photosynthetic nitrogen-use efficiency, NUE), the C species exhibited the highest NUE, followed by the C-C species, F. ramosissima, with the other two C-C species and the C species being equal and exhibiting the lowest NUEs. The lack of consistent advantages in NUE and WUE in the C-C species F. pubescens and F. floridana suggest that in some C-C Flaveria species C-like anatomy and biochemistry do not provide the same gas exchange advantages that we typically attribute to the CO-concentrating mechanism of fully-expressed C plants.