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 C3-C4 photosynthesis were investigated using closely related Panicum species exposed to industrial-age climate change. Panicum bisulcatum (C3), P. milioides (C3-C4), and P. coloratum (C4) were grown in a glasshouse at three CO2 concentrations ([CO2]: 280, 400, and 650"‰Î¼l l−1) and two air temperatures [ambient (27/19"‰Ã‚°C day/night) and ambient + 4"‰Ã‚°C] for 12 weeks. Under current ambient [CO2] and temperature, the C3-C4 species had higher photosynthetic rates and lower stomatal limitation and electron cost of photosynthesis relative to the C3 species. These photosynthetic advantages did not improve leaf- or plant-level water (WUE) or nitrogen (NUE) use efficiencies of the C3-C4 relative to the C3 Panicum species. In contrast, the C4 species had higher photosynthetic rates and WUE but similar NUE to the C3 species. Increasing [CO2] mainly stimulated photosynthesis of the C3 and C3-C4 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 C3 species only. Under high temperature, increasing [CO2] enhanced the biomass of the C3 and C3-C4 species to the same extent, indicating increased CO2 limitation in the C3-C4 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 C3-C4 intermediate species partially to recycle photorespired CO2 did not improve WUE or NUE relative to congeneric C3 or C4 species grown under varying [CO2] and temperature conditions.