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.).

The effects of elevated CO2 (650 ppm) on interactions between a chlorophyllous parasitic angiosperm, Rhinanthus minor (L.) and a host, Poa pratensis (L.) were investigated. R. minor benefited from elevated CO2, with both photosynthesis and biomass increasing, and transpiration and tissue N concentration remaining unaffected. However, this did not alleviate the negative effect of the parasite on the host; R. minor reduced host photosynthesis, transpiration, leaf area and biomass, irrespective of CO2 concentration. Elevated CO2 resulted in increased host photosynthesis, but there was no concomitant increase in biomass and foliar N decreased. It appears that the parasite may reduce host growth more by competition for nitrogen than for carbon. Contrary to expectation, R. minor did not reduce the productivity of the host-parasite association, and it actually contributed to the stimulation of productivity of the association by elevated CO2.