GROWTH AND GAS-EXCHANGE OF OATS (AVENA-SATIVA) AND WILD MUSTARD (BRASSICA-KABER) AT SUBAMBIENT CO2 CONCENTRATIONS

A repeated sequence of monocultures and mixtures of oats (A vena 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 mumol 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 mumol m-2 s-1 at 150 mumol mol-1, respectively, to 16.1 and 15.9 mumol m-2 s-1 at 330 mumol mol-1 CO2, respectively, when measured at 1,200-1,500 mumol 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 mumol 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.