Panicum milioides (C3-C4) does not have improved water or nitrogen economies relative to C3 and C4 congeners exposed to industrial-age climate change

The physiological implications of C-3-C-4 photosynthesis were investigated using closely related Panicum species exposed to industrial-age climate change. Panicum bisulcatum (C-3), P. milioides (C-3-C-4), and P. coloratum (C-4) were grown in a glasshouse at three CO2 concentrations ([CO2]: 280, 400, and 650 mu l l(-1)) and two air temperatures [ambient (27/19 degrees C day/night) and ambient + 4 degrees C] for 12 weeks. Under current ambient [CO2] and temperature, the C-3-C-4 species had higher photosynthetic rates and lower stomatal limitation and electron cost of photosynthesis relative to the C-3 species. These photosynthetic advantages did not improve leaf-or plant-level water (WUE) or nitrogen (NUE) use efficiencies of the C-3-C-4 relative to the C-3 Panicum species. In contrast, the C-4 species had higher photosynthetic rates and WUE but similar NUE to the C-3 species. Increasing [CO2] mainly stimulated photosynthesis of the C-3 and C-3-C-4 species, while high temperature had no or negative effects on photosynthesis of the Panicum species. Under ambient temperature, increasing [CO2] enhanced the biomass of the C-3 species only. Under high temperature, increasing [CO2] enhanced the biomass of the C-3 and C-3-C-4 species to the same extent, indicating increased CO2 limitation in the C-3-C-4 intermediate at high temperature. Growth [CO2] and temperature had complex interactive effects, but did not alter the ranking of key physiological parameters amongst the Panicum species. In conclusion, the ability of C-3-C-4 intermediate species partially to recycle photorespired CO2 did not improve WUE or NUE relative to congeneric C-3 or C-4 species grown under varying [CO2] and temperature conditions.