Abstract: During the last ice age, atmospheric [CO2] was 180-200 ppm compared with the modern value of 380 ppm, and temperatures were ~8 ˚C cooler. Relatively little is known about the responses of C3 and C4 species to long-term exposure to glacial conditions. Here Abutilon theophrasti (C3) and Amaranthus retroflexus (C4) were grown at 200 ppm CO2 with current (30/24 ˚C) and glacial (22/16 ˚C) temperatures for 22 d. Overall, the C4 species exhibited a large growth advantage over the C3 species at low [CO2]. However, this advantage was reduced at low temperature, where the C4 species produced 5X the total mass of the C3 species versus 14X at the high temperature. This difference was due to a reduction in C4 growth at the low temperature, since the C3 species exhibited similar growth between temperatures. Physiological differences between temperatures were not detected for either species, although photorespiration/net photosynthesis was reduced in the C3 species grown at low temperature, suggesting evidence of improved carbon balance at this treatment. This system suggests that C4 species exhibited a growth advantage over C3 species during low [CO2] of the last ice age, although concurrent reductions in temperatures may have reduced this advantage.

Key words: Abutilon theophrasti, Amaranthus retroflexus, C3 species, C4 species, climate change, low CO2, low temperature, photorespiration, Pleistocene.