Another row to hoe

Published April 1, 2000

Hundreds of studies each year find that most plants produce more fruit, seem more resistant to environmental stresses, and use water more efficiently in a world of higher atmospheric carbon dioxide (CO2).

Still, some people find it hard to accept that a byproduct of human activity could actually benefit the biota. They argue that weeds or other parasitic plants might also gain from elevated CO2, and possibly even develop a competitive advantage over the more desirable members of the botanical fraternity.

Turns out they’ve got another row to hoe. Parasitic plants do not thrive as their useful or delicious cousins do when the CO2 level rises, according to three new studies.

Certain weeds, for example, acquire carbon, nutrients, and water from their sorghum hosts and would be expected to reduce the growth rates of the infected plants. But in one experiment, Watling and Press introduced these parasitic weeds into the chambers of some of the sorghum plants they were growing with atmospheric CO2 at 350 parts per million (ppm)–the ambient, or natural level–and 700 ppm. They found that increased atmospheric CO2 nearly doubled sorghum’s photosynthesis rate, weeds or no.

When comparing only the uninfected sorghum, Watling and Press found a 36 percent increase in plant biomass thanks to the extra CO2. One of the weeds reduced normal CO2 plant growth by 68 percent, but elevated CO2 plant growth by only 57 percent–meaning CO2 enrichment alleviated some of the growth reduction brought about by the weed.

More interestingly, elevated CO2 reduced the growth of the weeds by well over 50 percent, delaying their emergence and completely stifling their flowering. While sorghum loved CO2, the weeds apparently hated it!

In another experiment, Matthies and Egli grew a parasitic plant in isolation and in combination with grass and legumes with CO2 levels at 375 ppm and 590 ppm, also varying the amount of fertilizer. The many variables made for complex interactions between hosts, parasites, nutrient levels, and different concentrations of atmospheric CO2. But one thing was clear: Elevated CO2 did not substantially influence the effect of the parasite on its host. Instead, both the host plants and the parasitic weeds thrived in elevated CO2.

Dale and Press studied the interaction between leguminous plants and a weed common in the United Kingdom and Middle East. Under ambient conditions, the weed reduced its host’s biomass by 47 percent. But under elevated CO2 conditions (550 ppm), the plants lost only 20 percent of their dry weight due to the weed. And CO2 had no positive effect on the weeds themselves. Elevated CO2 acted to alleviate biomass reductions in the parasites’ host plants.

Plant species’ interactions may complicate the overall biological response to actual elevated CO2 levels. But these three articles suggest there is no reason to believe that weeds will gain any advantage in the years to come; in fact, the evidence suggests parasites are in for a tough future.

Robert C. Balling Jr., Ph.D. is director of the Laboratory of Climatology at Arizona State University and coauthor of The Satanic Gases.


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

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

Watling, J.R., and M.C. Press, 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.