Matthies and Egli (1999) grew Rhinanthus alectorolophus (a widely distributed parasitic plant of Central Europe) for a period of two months on the grass Lolium perenne and the legume Medicago sativa in pots placed within open-top chambers maintained at atmospheric CO2 concentrations of 375 and 590 ppm, half of which pots were fertilized to produce an optimal soil nutrient regime and half of which were unfertilized.  At low nutrient supply, they found that atmospheric CO2 enrichment decreased mean parasite biomass by an average of 16%, while at high nutrient supply it increased parasite biomass by an average of 123%.  Nevertheless, the extra 215 ppm of CO2 increased host plant biomass in both situations: by 29% under high soil nutrition and by 18% under low soil nutrition.

Dale and Press (1999) infected white clover (Trifolium repens) plants with Orobanche minor (a parasitic weed that primarily infects leguminous crops in the United Kingdom and the Middle East) and exposed them to atmospheric CO2 concentrations of either 360 or 550 ppm for 75 days in controlled-environment growth cabinets.  The elevated CO2 in this study had no effect on the total biomass of parasite per host plant, nor did it impact the number of parasites per host plant or the time to parasitic attachment to host roots.  On the other hand, whereas infected host plants growing in ambient air produced 47% less biomass than uninfected plants growing in ambient air, infected plants growing at 550 ppm CO2 exhibited final dry weights that were only 20% less than those displayed by uninfected plants growing in the CO2-enriched air, indicative of a significant CO2-induced partial alleviation of parasite-induced biomass reductions in the white clover host plants.

Watling and Press (1997) infected several C4 sorghum plants with Striga hermonthica and Striga asiatica (parasitic C3 weeds of the semi-arid tropics that infest many grain crops) and grew them, along with uninfected control plants, for approximately two months in controlled-environment cabinets maintained at atmospheric CO2 concentrations of 350 and 700 ppm.  In the absence of parasite infection, the extra 350 ppm of CO2 increased sorghum biomass by approximately 36%.  When infected with S. hermonthica, however, the sorghum 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 was somewhat less stressful and led to host biomass production that was about half that of uninfected controls in both ambient and CO2-enriched air.  The end result was that the doubling of the air's CO2 content employed in this study increased sorghum biomass by 79% and 35% in the C4 sorghum plants infected with S. hermonthica and S. asiatica, respectively.

Hwangbo et al. (2003) grew Kentucky Bluegrass (Poa pratensis L.) with and without infection by the C3 chlorophyllous parasitic angiosperm Rhinanthus minor L. (a facultative hemiparasite found in natural and semi-natural grasslands throughout Europe) for eight weeks in open-top chambers maintained at ambient and elevated (650 ppm) CO2 concentrations.  At the end of the study, the parasite's biomass (when growing on its host) was 47% greater in the CO2-enriched chambers, while its host exhibited only a 10% CO2-induced increase in biomass in the parasite's absence but a nearly doubled 19% increase when infected by it.

Watling and Press (2000) grew upland rice (Oryza sativa L.) in pots in controlled-environment chambers maintained at 350 and 700 ppm CO2 in either the presence or absence of the root parasite S. hermonthica for a period of 80 days after sowing, after which time the plants were harvested and weighed.  In ambient air, the presence of the parasite reduced the biomass of the rice to only 35% of what it was in the absence of the parasite; whereas in air enriched with CO2 the presence of the parasite reduced the biomass of infected plants to but 73% of what it was in the absence of the parasite.

In summary, these several observations suggest that the rising CO2 content of the air can have wide and variable effects on parasitic plants, ranging from negative to positive growth responses, depending upon soil nutrition and host plant specificity.  With respect to the infected host plants, elevated CO2 generally tends to reduce the negative effects of parasitic infection, so that infected host plants continue to exhibit positive growth responses to elevated CO2.  Thus, it is likely that whatever the scenario with regard to parasitic infection, host plants will fare better under higher atmospheric CO2 conditions than they do currently.

References
Dale, H. and Press, M.C.  1999.  Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens.  New Phytologist 140: 65-73.

Hwangbo, J.-K., Seel, W.E. and Woodin, S.J.  2003.  Short-term exposure to elevated atmospheric CO2 benefits the growth of a facultative annual root hemiparasite, Rhinanthus minor (L.), more than that of its host, Poa pratensis (L.).  Journal of Experimental Botany 54: 1951-1955.

Matthies, D. and Egli, P.  1999.  Response of a root hemiparasite to elevated CO2 depends on host type and soil nutrients.  Oecologia 120: 156-161.

Watling, J.R. and Press, M.C.  1997.  How is the relationship between the C4 cereal Sorghum bicolor and the C3 root hemi-parasites Striga hermonthica and Striga asiatica affected by elevated CO2?  Plant, Cell and Environment 20: 1292-1300.

Watling, J.R. and Press, M.C.  2000.  Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2.  Global Change Biology 6: 919-930.