THE INFLUENCE OF INCREASING CARBON-DIOXIDE AND TEMPERATURE ON COMPETITIVE INTERACTIONS BETWEEN A C3 CROP, RICE (ORYZA-SATIVA) AND A C-4 WEED (ECHINOCHLOA-GLABRESCENS)

Many of the most troublesome weeds in agricultural systems are C-4 pla nts. As atmospheric CO2 increases it is conceivable that competitive a bility of these weeds could be reduced relative to C-3 crops such as r ice. At the International Rice Research Institute (IRRI) in the Philip pines, rice (IR72) and one of its associated C-4 weeds, Echinochloa gl abrescens, were grown from seeding to maturity using replacement serie s mixtures (100:0, 75:25, 50:50, 25:75, and 0:100, % rice:%weed) at tw o different CO2 concentrations (393 and 594 mu L L(-1)) in naturally s unlit glasshouses. Since increasing CO2 may also result in elevated gr owth temperatures, the response of rice to each CO2 concentration was also examined at day/night temperatures of 27/21 and 37/29 degrees C. At 27/21 degrees C, increasing the CO2 concentration resulted in a sig nificant increase in above ground biomass (+47%) and seed yield (+55%) of rice when averaged over all mixtures. For E. glabrescens, the C-4 species, no significant effect of CO2 concentration on biomass or yiel d was observed. When grown in mixture, the proportion of rice biomass increased significantly relative to that of the C-4 weed at all mixtur es at elevated CO2. Evaluation of changes in competitiveness (by calcu lation of plant relative yield (PRY) and replacement series diagrams) of the two species demonstrated that, at elevated CO2, the competitive ness of rice was increased relative to that of E. glabrescens. However , at the higher growth temperature (37/29 degrees C), growth and repro ductive stimulation of rice by elevated CO2 was reduced compared to th e lower growth temperature. This resulted in a reduction in the propor tion of rice:weed biomass present in all mixtures relative to 27/21 de grees C and a greater reduction in PRY in rice relative to E. glabresc ens. Data from this experiment suggest that competitiveness could be e nhanced in a C-3 crop (rice) relative to a C-4 weed (E. glabrescens) w ith elevated CO2 alone, but that simultaneous increases in CO2 and tem perature could still favour a C-4 species.