What was done
Six different species of grass utilizing the C4 photosynthetic pathway were grown from seed in 8-liter pots in controlled environment chambers maintained at either 350 or 700 ppm CO2 under conditions of plentiful soil water and nutrients.  After 39 days of growth, the plants were removed for determination of biomass production and leaf gas exchange characteristics.

What was learned
The photosynthetic capacities of all six species were limited by the current low value of the atmosphere's CO2 concentration.  In addition, the stomatal conductances of all species declined with increasing CO2, so that their water use efficiencies all rose as the air's CO2 content climbed.

What it means
Although C4 plants are often thought to be unresponsive to atmospheric CO2 enrichment, this study demonstrates that some of them do in fact perform significantly better at higher CO2 concentrations than those characteristic of present-day earth.