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

Abstract

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.