Response of Growth and Yield of Rice (Oryza sativa) to Elevated Atmospheric Carbon Dioxide in the Subhumid Zone of Sri Lanka

Increasing atmospheric CO₂ is recognized as a major aspect of global climate change that would have a significant impact on the productivity of major agricultural crops. Two field experiments were done, with the objective of quantifying the response of a short‐duration rice (Oryza sativa) variety (BG‐300) to elevated atmospheric carbon dioxide, in the low elevation, subhumid zone of Sri Lanka. The experiment contained three treatments. In the elevated CO₂ treatment, rice was grown at a CO₂ concentration of 570 µmol/mol within open top chambers (OTC s). The ambient CO₂ treatment included crops grown within OTC s, but maintained at the ambient CO₂ concentration of 370 µmol/mol. The third treatment was a crop grown in the open field under ambient CO₂ concentration. Grain yields of rice crops grown under elevated CO₂ were 24 % and 39 % greater than the respective ambient treatments in the maha (January – March 2001) and yala (May – August, 2001) seasons. Significant increases in total biomass at harvest (23 % and 39 %, respectively, in maha and yala) were more responsible for the above yield increases than the slight increases in the harvest index (4 % and 2 %). Yields of the ambient and open field treatments did not differ significantly. Among the yield components, the number of panicles per hill was significantly higher in the elevated treatment and showed significant positive correlations with grain yield in both seasons. In addition, grain yield showed significant positive correlations with the percentage of filled grains in maha and the number of grains per panicle in yala. Significant increases in the number of tillers per hill under elevated CO₂ were responsible for its greater leaf area index and the greater numbers of panicles per hill. Crops under elevated CO₂ accumulated biomass faster than those grown under ambient CO₂ during the vegetative and grain‐filling stages. The results of this study demonstrate that elevated CO₂ causes significant yield increases in rice, even when it is grown in warm, subhumid tropical climates.