The Influence of Increasing Carbon Dioxide and Temperature on Competitive Interactions Between a C₃ Crop, Rice (Oryza sativa) and a C₄ Weed (Echinochloa glabrescens)

Abstract

Many of the most troublesome weeds in agricultural systems are C₄ plants. As atmospheric CO₂ increases it is conceivable that competitive ability of these weeds could be reduced relative to C₃ crops such as rice. At the International Rice Research Institute (IRRI) in the Philippines, rice (IR72) and one of its associated C₄ weeds, Echinochloa glabrescens, were grown from seeding to maturity using replacement series mixtures (100:0, 75:25, 50:50, 25:75, and 0:100, % rice:%weed) at two different CO₂ concentrations (393 and 594 μL L^-1) in naturally sunlit glasshouses. Since increasing CO₂ may also result in elevated growth temperatures, the response of rice to each CO₂ concentration was also examined at daylnight temperatures of 27/21 and 37/29ºC. At 27/21ºC, increasing the CO₂ concentration resulted in a significant increase in above ground biomass (+47%) and seed yield (+55%) of rice when averaged over all mixtures. For E. glabrescens, the C₄ species, no significant effect of CO₂ concentration on biomass or yield was observed. When grown in mixture, the proportion of rice biomass increased significantly relative to that of the C₄ weed at all mixtures at elevated CO₂. Evaluation of changes in competitiveness (by calculation of plant relative yield (PRY) and replacement series diagrams) of the two species demonstrated that, at elevated CO₂, the competitiveness of rice was increased relative to that of E. glabrescens. However, at the higher growth temperature (37/29ºC), growth and reproductive stimulation of rice by elevated CO₂ was reduced compared to the lower growth temperature. This resulted in a reduction in the proportion of rice:weed biomass present in all mixtures relative to 27/21ºC and a greater reduction in PRY in rice relative to E. glabrescens. Data from this experiment suggest that competitiveness could be enhanced in a C₃ crop (rice) relative to a C₄ weed (E. glabrescens) with elevated CO₂ alone, but that simultaneous increases in CO₂ and temperature could still favour a C₄ species.