Effects of low and elevated CO₂ on C₃ and C₄ annuals : I. Growth and biomass allocation

In order study C₃ and C₄ plant growth in atmospheric CO₂ levels ranging from past through predicted future levels, Abutilon theophrasti (C₃) and Amaranthus retroflexus (C₄) were grown from seed in growth chambers controlled at CO₂ 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, CO₂ had no effect on the relative growth rate, total biomass or partitioning of biomass in the C₄ species. However, the C₃ species had greater biomass accumulation with increasing CO₂ partial pressure. C₃ plants grown in 15 Pa CO₂ for 35 days had only 8% of the total biomass of plants grown in 35 Pa CO₂, C₃ plants had lower relative growth rates and lower specific leaf mass than plants grown in higher CO₂ partial pressures, and aborted reproduction. C₃ plants grown in 70 Pa CO₂ had greater root mass and root-to-shoot ratios than plants grown in lower CO₂ partial pressures. These findings, support other studies that show C₃ plant growth is more responsive to CO₂ partial pressure than C₄ plant growth. Differences in growth responses to CO₂ levels of the Pleistocene through the future suggest that competitive interactions of C₃ and C₄ annuals have changed through geologic time. This study also provided evidence that C₃ annuals may be operating near a minimum CO₂ partial pressure for growth and reproduction at 15 Pa CO₂.