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 μl l−1) and two air temperatures [ambient (27/19 °C day/night) and ambient + 4 °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.