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?

The C4 cereal Sorghum bicolor was grown under either ambient (350 μmol mol−1) or elevated (700 μmol mol−1) [CO2] in either the presence or absence of the C3 obligate root hemi‐parasites Striga hermonthica or S. asiatica. Both uninfected and infected sorghum plants were taller and had greater biomass, photosynthetic rates, water‐use efficiencies and leaf areas under elevated compared with ambient [CO2]. There was no evidence of any downregula‐tion of photosynthesis in sorghum grown at elevated [CO2]. Biomass of infected sorghum was lower under both ambient and elevated [CO2], and although infected plants were larger under elevated [CO2] the relative impact of infection on host biomass was either the same (S. asiatica) or only slightly less (S. hermonthica) than under ambient [CO2]. In contrast, biomass of S. hermonthica and S. asiatica per host was lower under elevated than ambient [CO2], although rates of photosynthesis were higher at elevated [CO2] and parasite stomatal conductance was not responsive to [CO2]. Parasites emerged above‐ground and flowered earlier under ambient compared with elevated [CO2]. It appears that the mechanism(s) by which the parasites affect host growth is (are) relatively insensitive to increased atmospheric [CO2], although the parasites themselves were adversely affected by growth at elevated [CO2].