Infection with the parasitic angiosperm Striga hermonthica influences the response of the C 3 cereal Oryza sativa to elevated CO 2

Summary Upland rice ( Oryza sativa L.) was grown at both ambient (350 μmol mol −1 ) and elevated (700 μmol mol −1 ) CO 2 in either the presence or absence of the root hemi‐parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO 2 alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO 2 was 35% that of uninfected, control plants, while at elevated CO 2 , biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO 2 supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO 2 . The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO 2 . There were significant CO 2 and Striga effects on photosynthetic metabolism and instantaneous water‐use efficiency of O. sativa . The response of photosynthesis to internal [CO 2 ] ( A / C i curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO 2 had significantly lower CO 2 saturated rates of photosynthesis, carboxylation efficiencies and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO 2 ‐grown O. sativa . In contrast, infection with S. hermonthica prevented down‐regulation of photosynthesis in O. sativa grown at elevated CO 2 , but had no impact on photosynthesis of hosts grown at ambient CO 2 . At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO 2 had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO 2 . The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO 2 or infection effects on the concentrations of soluble sugars in leaves of O. sativa , but starch concentration was significantly lower in infected plants at both CO 2 concentrations. These results demonstrate that elevated CO 2 concentrations can alleviate the impact of infection with Striga on the growth of C 3 hosts such as rice and also that infection can delay the onset of photosynthetic down‐regulation in rice grown at elevated CO 2 .