Hmm. This article refers to that as a myth, however, it sez that increased CO2 levels could make a small difference (ordinarily, plant cells get overloaded with sugars during peak hours, shut their pores, and run on respiration):
http://environment.newscientist.com/channel/earth/climate-change/dn11655
[snip] However, it is extremely difficult to generalise about the overall impact of the fertilisation effect on plant growth. Numerous groups around the world have been conducting experiments in which plots of land are supplied with enhanced CO2, while comparable nearby plots remain at normal levels... These experiments suggest that higher O2 levels could boost the yields of non-C4 crops by around 13 per cent... However, while experiments on natural ecosystems have also found initial elevations in the rate of plant growth, these have tended to level off within a few years. In most cases this has been found to be the result of some other limiting factor, such as the availability of nitrogen or water. [end snip]