As the CO2 content of the air increases, sunflower plants will likely display enhanced rates of photosynthetic carbon uptake.  In the study of Sims et al. (1999), for example, exposure to twice-ambient atmospheric CO2 concentrations enhanced rates of net photosynthesis in individual upper-canopy sunflower leaves by approximately 50%.  Similarly, Luo et al. (2000) reported that sunflowers grown at 750 ppm CO2 displayed canopy carbon uptake rates that were fully 53% greater than those exhibited by plants grown at 400 ppm CO2.

Because elevated levels of atmospheric CO2 stimulate rates of photosynthesis in sunflower plants, they should also increase biomass production in this important agricultural species.  The study of Zerihun et al. (2000) suggests this inference is correct, as they reported that twice-ambient CO2 concentrations increased whole plant biomass in sunflowers by 44, 13 and 115% when the plants were simultaneously exposed to low, medium and high levels of soil nitrogen.

In summary, it is clear that as the CO2 content of the air increases, sunflower plants should exhibit enhanced rates of photosynthesis and biomass production, which should consequently lead to greater seed yields in this important agricultural crop.

For more information on sunflower growth responses to atmospheric CO2 enrichment see Plant Growth Data: Sunflower (dry weight, photosynthesis).

References
Luo, Y., Hui, D., Cheng, W., Coleman, J.S., Johnson, D.W. and Sims, D.A.  2000.  Canopy quantum yield in a mesocosm study.  Agricultural and Forest Meteorology 100: 35-48.

Sims, D.A., Cheng, W., Luo, Y. and Seeman, J.R.  1999.  Photosynthetic acclimation to elevated CO2 in a sunflower canopy.  Journal of Experimental Botany 50: 645-653.

Zerihun, A., Gutschick, V.P. and BassiriRad, H.  2000.  Compensatory roles of nitrogen uptake and photosynthetic N-use efficiency in determining plant growth response to elevated CO2: Evaluation using a functional balance model.  Annals of Botany 86: 723-730.