What was done
Striga hermonthica and Striga asiatica are important parasitic C3 weeds in the semi-arid tropics that infect many grain crops, including maize, sorghum, millet, and rice.  Like many parasitic species, S. hermonthica and S. asiatica depend upon their hosts to acquire carbon, nutrients, and water to support their own growth and development.  Thus, infected host plants typically exhibit reductions in growth relative to uninfected host plants.  In this paper, the authors grew the C4 crop Sorghum bicolor with and without infection by S. hermonthica and S. asiatica for about two months in controlled environment cabinets fumigated with atmospheric CO2 concentrations of 350 and 700 ppm to determine the effects of elevated CO2 on these parasite-host interactions.

What was learned
Elevated CO2 significantly increased photosynthetic rates in sorghum; and plants grown at 700 ppm CO2 exhibited rates that were almost twice as great as those displayed by ambiently-grown plants, regardless of parasitic infection.  In the absence of infection, this photosynthetic increase contributed to a 36% increase in plant biomass.  With infection, however, plant growth decreased in both a CO2- and a parasitic species-dependent manner.  When infected with S. hermonthica, for example, plants grown at ambient and elevated CO2 concentrations only produced 32 and 43% of the biomass displayed by their respective uninfected controls.  Infection with S. asiatica, however, led to host biomass production that was about half that of uninfected controls, regardless of CO2 concentration.  Thus, on an absolute basis, atmospheric CO2 enrichment alleviated some of the growth reducing effects of S. hermonthica on sorghum, but had no impact on those resulting from S. asiatica infection.  However, on a percentage basis, elevated CO2 increased biomass by 79 and 35%, respectively, for plants infected with S. hermonthica and S. asiatica.

As far as the parasitic plants are concerned, even though atmospheric CO2 enrichment significantly increased their numbers per host, their total biomass per host was only about 35% of what it was at ambient CO2.  Thus, elevated CO2 significantly reduced the growth performance of both parasitic species.  In addition, elevated CO2 delayed the emergence of parasitic species from the soil by seven days, relative to that observed under ambient CO2 conditions, and completely inhibited their flowering during the two-month experiment.

What it means
As the CO2 content of the air continues to rise, it is likely that these two C3 parasitic plants will exhibit decreases in their growth, and thus reduce their deleterious impacts on C4 host crops.  Specifically, greater atmospheric CO2 concentrations will probably alleviate some of the absolute growth-reducing effects of S. hermonthica on sorghum.  In addition, sorghum plants infected with either of these parasites will likely fair much better than they do presently, as the air's CO2 concentration rises and increases its beneficial stress-ameliorating and growth-promoting effects on vegetation.  In the absence of any parasitic infection, sorghum should exhibit substantial gains in yield, thus demonstrating (once again) that C4 plants can and do respond favorably to the aerial fertilization effect brought about by the rising atmospheric CO2 concentration.