Summary• The effects of elevated CO 2 are reported here on the uptake of nitrogen (N) and its relationships with growth and grain yield in rice ( Oryza sativa ).• Using free-air CO 2 enrichment (FACE), rice crops were grown at ambient or elevated ( c . 300 µ mol mol -1 above ambient) CO 2 and supplied with low, medium or high levels of N.• For the medium and high N treatments, FACE increased N uptake at panicle initiation but not at maturity. For total dry matter, as well as spikelet number and grain yield, positive interactions between CO 2 and N uptake were observed. Furthermore, spikelet number was closely associated with N uptake at panicle initiation.• These results indicate that, to maximize rice grain yield under elevated CO 2 , it is important to supply sufficient N over the whole season, in order to maintain the enhancement in dry matter production. In addition, N availability must be coordinated with the developmental stage of the crop, specifically to ensure that sufficient N is available at panicle initiation in order to maximize spikelet number and grain yield.

Summary• Here we analysed the effects of CO 2 (C a ) elevation and nitrogen availability on canopy structure, leaf area index (LAI) and canopy photosynthesis of rice ( Oryza sativa ).• Rice was grown at ambient and elevated C a ( c. 200 µmol mol − 1 above ambient, using the free-air CO 2 enrichment, FACE) and at two N availabilities. We measured leaf area, area-based leaf N contents and leaf photosynthesis, and calculated net daily canopy photosynthesis.• FACE plants had higher light-saturated rates of photosynthesis ( P max ) and apparent quantum yields than ambient plants, when measured at their own growth CO 2 . C a elevation reduced the total leaf N in the canopy ( N leaf ) but had no effect on LAI, and the average leaf N content ( N leaf /LAI) was therefore reduced by 8%. This reduction corresponded well with our model predictions. Leaf area index increased strongly with N availability, which was also consistent with our model.• Calculated canopy photosynthesis increased more strongly with N leaf under elevated than under ambient C a . This indicates that there is an N × C a interactive effect on canopy carbon gain. This interaction was caused by the increase in LAI with N availability, which enhanced the positive effect of the higher quantum yield under C a elevation.