A repeated sequence of monocultures and mixtures of oats (Avena sativa L.) and wild mustard (Brassica kaber (DC.) Wheeler) was grown along a daytime gradient of CO2 concentrations ([ CO2]) from near 330 to a minimum of 150\ μ mol\ mol⁻¹ . The objectives were to determine effects of subambient [ CO2] on leaf gas exchange, biomass production, and competitive interactions of these C3 species. A decrease in stomatal conductance did not prevent a nearly linear increase in leaf internal [ CO2] and net assimilation of oat leaves as [ CO2] increased. Net assimilation of oats and wild mustard increased from 5.0 and 2.5\ μ mol\ m⁻²\ s⁻¹ at 150\ μ mol\ mol⁻¹ , respectively, to 16.1 and 15.9\ μ mol\ m⁻²\ s⁻¹ at 330\ μ mol\ mol⁻¹ CO2, respectively, when measured at 1,200-1,500\ μ mol\ m⁻²\ s⁻¹ incident light. Aboveground biomass per plant of wild mustard and oats increased 106% and 198%, respectively, and leaf area rose more than two- and threefold, respectively, from 154 to 331\ μ mol\ mol⁻¹ CO2. The CO2-induced increase in aboveground biomass of plants of each species did not vary among monocultures and mixtures. Responses of oats and wild mustard to higher subambient [ CO2] were large relative to reported responses of C3 species to comparable increases above the current atmospheric [ CO2]. This suggests that past changes in atmospheric [ CO2], including the 27% rise since the beginning of the nineteenth century, may have profoundly altered the productivity of C3 plants.
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Polley et al. (1992) studied this question.