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.  Leaf net photosynthetic rates were significantly higher in the CO2-enriched chambers than in the ambient-air chambers: 51-75% greater in the maha season and 22-33% greater in the yala season.  Likewise, leaf chlorophyll contents were significantly greater in the CO2-enriched chambers, ranging from 1-9% higher in both seasons.

Leaf stomatal conductances also exhibited favorable responses, being reduced in the CO2-enriched chambers by 15-52% in the maha season and by 13-19% in the yala season.  These responses led to significant reductions in leaf transpiration rate per unit leaf area in both growing seasons.  However, because of the significantly greater leaf area in the CO2-enriched chambers, total canopy transpiration rate per unit land area did not differ significantly between the two CO2 treatments.  In addition, leaf water potentials were higher (less negative) in the CO2-enriched chambers.

What it means
In the words of the authors, "the significant increase of leaf net photosynthetic rate is responsible for the observed increase in total biomass and yield of rice under elevated CO2," which enhancement (24% and 39% increases in grain production in the maha and yala seasons, respectively) are reported in their companion paper (De Costa et al., 2003).  Hence, if the air's CO2 content continues to rise as it has over the past few decades, the future should see substantially more food produced on each hectare of land with little to no change in per-hectare water requirements.

Reference
De Costa, W.A.J.M., Weerakoon, W.M.W., Herath, H.M.L.K. and Abeywardena, R.M.I.  2003.  Response of growth and yield of rice (Oryza sativa) to elevated atmospheric carbon dioxide in the subhumid zone of Sri Lanka.  Journal of Agronomy and Crop Science 189: 83-95.