The influence of the root holoparasitic angiosperm Orobanche minor Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO2 concentrations (350 and 550 μmol mol−1). Infected plants accumulated less biomass than their uninfected counterparts, although early in the association there was a transient stimulation of growth. Infection also influenced biomass allocation both between tissues (infected plants had lower root[ratio ]shoot ratios) and within tissues[ratio ]infected roots were considerably thicker before the point of parasite attachment and thinner below. Higher concentrations of starch were also found in roots above the point of attachment, particularly for plants grown in elevated CO2. Elevated CO2 stimulated the growth of T. repens only during the early stages of development. There was a significant interaction between infection and CO2 on growth, with infected plants showing a greater response, such that elevated CO2 partly alleviated the effects of the parasite on host growth. Elevated CO2 did not affect total O. minor biomass per host, the number of individual parasites supported by each host, or their time of attachment to the host root system. Photosynthesis was stimulated by elevated CO2 but was unaffected by O. minor. There was no evidence of down-regulation of photosynthesis in T. repens grown at elevated CO2 in either infected or uninfected plants. The data are discussed with regard to the influence of elevated CO2 on other parasitic angiosperm-host associations and factors which control plant responses to elevated CO2.

The outcome of dual infection of the grass Lolium perenne L. by arbuscular mycorrhizal (AM) fungi and the parasitic angiosperm Rhinanthus minor L. was investigated in a glasshouse study. Colonization of L. perenne roots by AM fungi was significantly reduced by the presence of R. minor, as was host growth which fell by 44–51%. It was concluded that these two responses were linked, with AM colonization declining in response to the reduction in availability of host carbon. Parasite growth and reproductive output rose by 58% and 47% respectively when the hosts were mycorrhizal. These trends were unrelated to the attachment success of the parasite, but were accompanied by a significant increase in the formation of secondary haustoria. The benefits afforded the parasite when the hosts were mycorrhizal were attributed to increased carbon and nutrient flux resulting from alternations in sink strength. Host responses to parasitism and mycorrhizal colonization were not affected by the interaction between the two symbionts. However, the suggestion is made that the interaction between the AM fungi and parasite might have long-term ecological implications for the host species via its impact on parasite fecundity.