Effects of low and elevated CO on C and C annuals : I. Growth and biomass allocation.

In order study C and C plant growth in atmospheric CO levels ranging from past through predicted future levels, Abutilon theophrasti (C) and Amaranthus retroflexus (C) were grown from seed in growth chambers controlled at CO partial pressures of 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current) and 70 Pa (predicted future). After 35 days of growth, CO had no effect on the relative growth rate, total biomass or partitioning of biomass in the C species. However, the C species had greater biomass accumulation with increasing CO partial pressure. C plants grown in 15 Pa CO for 35 days had only 8% of the total biomass of plants grown in 35 Pa CO, C plants had lower relative growth rates and lower specific leaf mass than plants grown in higher CO partial pressures, and aborted reproduction. C plants grown in 70 Pa CO had greater root mass and root-to-shoot ratios than plants grown in lower CO partial pressures. These findings, support other studies that show C plant growth is more responsive to CO partial pressure than C plant growth. Differences in growth responses to CO levels of the Pleistocene through the future suggest that competitive interactions of C and C annuals have changed through geologic time. This study also provided evidence that C annuals may be operating near a minimum CO partial pressure for growth and reproduction at 15 Pa CO.