What was done
Rice (Oryza sativa L. cv. IR72) was grown in open-top chambers receiving atmospheric CO2 concentrations of approximately 350 and 650 ppm during a wet and dry growing season.  In addition, plants were supplied different levels of soil nitrogen to study the effects of elevated CO2 and soil nitrogen on solar radiation interception and use in this important cereal crop.

What was learned
Early in both growing seasons, plants exposed to elevated atmospheric CO2 intercepted significantly more sunlight than plants fumigated with ambient air, due to CO2-induced increases in leaf area index.  Interestingly, this phenomenon occurred regardless of soil nitrogen content, but disappeared shortly after canopy closure in all treatments.  Later on, mature canopies achieved similar leaf area indexes at identical levels of soil nitrogen supply.  Nevertheless, mean season-long radiation use efficiency, which is the amount of biomass produced per unit of solar radiation intercepted, was 35% greater in CO2-enriched vs. ambiently-grown plants and tended to increase with increasing soil nitrogen status.

What it means
As the CO2 content of the air increases, it is likely that photosynthetic rates in rice will increase, thus supplying plants with more raw materials for achieving greater leaf areas quicker in the growing season.  This phenomenon, although transient, can cause earlier canopy closure, possibly reducing competition from weeds.  In addition, the early increase in leaf development helps to produce biomass enhancements that are sustained throughout the growing season.  Thus, rice yields will likely increase with future increases in the air's CO2 concentration.