What was done
Two crops of rice (Oryza sativa L., var. BG300) were grown in the field at the Rice Research and Development Institute, Batalagoda, Ibbagamuwa, Sri Lanka, from January to March (the maha season) and from May to August (the yala season) in open-top chambers maintained at either the ambient atmospheric CO2 concentration or ambient plus 200 pppm CO2.

What was learned
The three-month duration rice variety exhibited a number of positive responses to the extra CO2 to which it was exposed.  The CO2-enriched plants produced more leaves per hill, more tillers per hill, more total plant biomass, greater root dry weight, more panicles per plant and had harvest indices that were increased by 4% and 2%, respectively, in the maha and yala seasons, which led to ultimate grain yield increases of 24% and 39% in those two periods.  The authors report the yield increases came about primarily "because of significant increases in the number of panicles per hill and the percentage of filled grains."

What it means
In the words of the authors, "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."  In fact, when linearly scaled to a 300-ppm increase in the air's CO2 concentration, the results of this study translate into grain yield increases of 36% and 58%, which are to be compared to a mean CO2-induced increase of 38% in the mass of rice grains produced in the 58 studies reviewed by Jablonski et al. (2002).  These results are indeed impressive.

Reference
Jablonski, L.M., Wang, X. and Curtis, P.S.  2002.  Plant reproduction under elevated CO2 conditions: a meta-analysis of reports on 79 crop and wild species.  New Phytologist 156: 9-26.