What was done
Seeds of white cockle plants (Silene latifolia Poiret) were collected from a naturally occurring population at the University of Michigan Biological Station near Pellston, Michigan, USA, germinated, and grown to maturity in sand-filled pots maintained at optimum moisture and fertility conditions in environmentally-controlled growth chambers at the Lamont-Doherty Earth Observatory, New York, USA, in which the air was continuously maintained at CO2 concentrations of either 365 or 730 ppm.

What was learned
In the words of the authors, "elevated CO2 significantly increased photosynthesis in both male and female plants and by a similar magnitude."  Vegetative mass also responded similarly in males and females, rising by 39% in both sexes in response to the doubling of the air's CO2 concentration.  Reproductive mass, on the other hand, responded much more strongly and unevenly, increasing by 82% in males and 97% in females at elevated CO2.  In the female plants, this feat was accomplished, in part, by non-significant increases of about 36% and 44% in the number and mass of seeds per plant, and by a significant increase of approximately 15% in the mass of individual seeds, in harmony with the findings of Jablonski et al. (2002) in their meta-analysis of the results of 159 CO2 enrichment experiments conducted on 79 crop and wild species of plants.

What it means
If this study is characteristic of dioecious plants in general (which comprise nearly half of all angiosperm families), we could expect to see a greater proportion of biomass allocated to reproduction in a high-CO2 world of the future.

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.