title: The relative contributions of reduced photorespiration, and improved water-and nitrogen-use efficiencies, to the advantages of C3-C4intermediate photosynthesis in Flaveria.
abstract: Analyses of carbon-assimilation patterns in response to intercellular CO2concentrations, and the photosynthetic water-and nitrogen-use efficiencies, were conducted for a C3, a C4, and three C3-C4species in the genus Flaveria in order to determine some of the advantages and disadvantages of C3-C4intermediate photosynthesis. Operational intercellular CO2partial pressures (pi), determined when the atmospheric CO2partial pressure (pa) was approximately 330 μbar, in the C3-C4species were generally equal to, or greater than, those observed in the C3species under well-watered or water-stressed conditions. This reflects equal, or lower, water-use efficiencies (WUEs) in the C3-C4species. The only case in which higher WUEs were observed in the C3-C4species, compared to the C3species, was when photosynthesis rates were limited by available nitrogen and were less than 12.5 μmol CO2m-2s-1. At higher photosynthesis rates, the C3-C4species exhibited lower values of photosynthesis rate for equal values of stomatal conductance (lower WUE), compared to the C3species. 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 C4species exhibited the highest NUE, followed by the C3-C4species, F. ramosissima, with the other two C3-C4species and the C3species being equal and exhibiting the lowest NUEs. The lack of consistent advantages in NUE and WUE in the C3-C4species F. pubescens and F. floridana suggest that in some C3-C4Flaveria 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 C4plants.
authors.affiliation: Department of Environmental, Population and Organismic Biology, University of Colorado, Box 334, 80309, Boulder, CO, USA.
authors.forename: Russell K
authors.initials: RK
authors.lastname: Monson
keywords: C3/C3 intermediate photosynthesis; Evolution of photosynthesis; Flaveria; Nitrogen-use efficiency; Photorespiration