What was done
Twelve wild radish genotypes, of varying stomatal indices and guard cell lengths, were collected in Maine, USA, and grown in greenhouses with atmospheric CO2 concentrations of 370 and 680 ppm to study the effects of elevated CO2 on stomatal characteristics and reproductive growth.

What was learned
Elevated CO2 did not significantly affect stomatal index or guard cell length.  In fact, across all genotypes investigated, leaf surface characteristics were essentially unchanged by elevated CO2.  Among genotypes, no evidence was found for differential responses to elevated CO2 in terms of growth or reproductive characteristics.  However, in this particular study, atmospheric CO2 enrichment elicited a marginally significant decrease of 17% in flower and fruit production, which is contrary to a larger database of positive yield responses for this species (Idso and Kimball, 1989; Overdieck et al. 1988; Idso et al. 1987; Morison and Gifford, 1984; Sionit et al. 1982).

What it means
As the CO2 content of the air increases, wild radish will likely maintain its current compliment of genetic diversity with respect to stomatal indices and guard cell lengths.  Because these genotypes exhibited similar reductions in flower and fruit production, it is unlikely that elevated CO2 will selectively favor one genotype over another.  Thus, wild radish diversity should persist as the atmospheric CO2 concentration rises.

References
Idso, S.B. and Kimball, B.A.  1989.  Growth response of carrot and radish to atmospheric CO2 enrichment.  Environmental and Experimental Botany 29: 135-139.