The relative contributions of reduced photorespiration, and improved water-and nitrogen-use efficiencies, to the advantages of C3-C4 intermediate photosynthesis in Flaveria.

Analyses of carbon-assimilation patterns in response to intercellular CO2 concentrations, and the photosynthetic water-and nitrogen-use efficiencies, were conducted for a C3, a C4, and three C3-C4 species in the genus Flaveria in order to determine some of the advantages and disadvantages of C3-C4 intermediate photosynthesis. Operational intercellular CO2 partial pressures (pi), determined when the atmospheric CO2 partial pressure (pa) was approximately 330 μbar, in the C3-C4 species were generally equal to, or greater than, those observed in the C3 species under well-watered or water-stressed conditions. This reflects equal, or lower, water-use efficiencies (WUEs) in the C3-C4 species. The only case in which higher WUEs were observed in the C3-C4 species, compared to the C3 species, was when photosynthesis rates were limited by available nitrogen and were less than 12.5 μmol CO2 m-2s-1. At higher photosynthesis rates, the C3-C4 species exhibited lower values of photosynthesis rate for equal values of stomatal conductance (lower WUE), compared to the C3 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 C4 species exhibited the highest NUE, followed by the C3-C4 species, F. ramosissima, with the other two C3-C4 species and the C3 species being equal and exhibiting the lowest NUEs. The lack of consistent advantages in NUE and WUE in the C3-C4 species F. pubescens and F. floridana suggest that in some C3-C4 Flaveria species C4-like anatomy and biochemistry do not provide the same gas exchange advantages that we typically attribute to the CO2-concentrating mechanism of fully-expressed C4 plants.