Physiological and Growth Responses of C3 and C4 Plants to Reduced Temperature When Grown at Low CO2 of the Last Ice Age

During the last ice age, CO2 concentration ([CO2]) was 180-200 mu mu mol/mol compared with the modern value of 380 mu mu mol/mol, and global temperatures were similar to 8 degrees C cooler. Relatively little is known about the responses of C-3 and C-4 species to long-term exposure to glacial conditions. Here Abutilon theophrasti Medik. (C-3) and Amaranthus retroflexus L. (C-4) were grown at 200 mu mu mol/mol CO2 with current (30/24 degrees C) and glacial (22/16 degrees C) temperatures for 22 d. Overall, the C-4 species exhibited a large growth advantage over the C-3 species at low [CO2]. However, this advantage was reduced at low temperature, where the C-4 species produced 5x the total mass of the C-3 species versus 14x at the high temperature. This difference was due to a reduction in C-4 growth at low temperature, since the C-3 species exhibited similar growth between temperatures. Physiological differences between temperatures were not detected for either species, although photorespiration/net photosynthesis was reduced in the C-3 species grown at low temperature, suggesting evidence of improved carbon balance at this treatment. This system suggests that C-4 species had a growth advantage over C-3 species during low [CO2] of the last ice age, although concurrent reductions in temperatures may have reduced this advantage.