Infection with the parasitic angiosperm Striga hermonthica influences the response of the C-3 cereal Oryza sativa to elevated CO2

Upland rice (Oryza sativa L.) was grown at both ambient (350 mu mol mol(-1) ) and elevated (700 mu mol mol(-1)) CO2 in either the presence or absence o f the root hemi-parasitic angiosperm Striga hermonthica (Del) Benth. Elevat ed CO2 alleviated the impact of the parasite on host growth: biomass of inf ected rice grown at ambient CO2 was 35% that of uninfected, control plants, while at elevated CO2, biomass of infected plants was 73% that of controls . This amelioration occurred despite the fact that O. sativa grown at eleva ted CO2 supported both greater numbers and a higher biomass of parasites pe r host than plants grown at ambient CO2. The impact of infection on host le af area, leaf mass, root mass and reproductive tissue mass was significantl y lower in plants grown at elevated as compared with ambient CO2. There wer e significant CO2 and Striga effects on photosynthetic metabolism and insta ntaneous water-use efficiency of O. sativa. The response of photosynthesis to internal [CO2] (A/C-i curves) indicated that, at 45 days after sowing (D AS), prior to emergence of the parasites, uninfected plants grown at elevat ed CO2 had significantly lower CO2 saturated rates of photosynthesis, carbo xylation efficiencies and ribulose-1,5-bisphosphate carboxylase/oxygenase ( Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO2-grown O. sativa . In contrast, infection with S. hermonthica prevented down-regulation of p hotosynthesis in O. sativa grown at elevated CO2, but had no impact on phot osynthesis of hosts grown at ambient CO2. At 76 DAS (after parasites had em erged), however, infected plants grown at both elevated and ambient CO2 had lower carboxylation efficiencies and Rubisco contents than uninfected O. s ativa grown at ambient CO2. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concen tration of O. sativa leaves, both before and after parasite emergence. Ther e were no significant CO2 or infection effects on the concentrations of sol uble sugars in leaves of O. sativa, but starch concentration was significan tly lower in infected plants at both CO2 concentrations. These results demo nstrate that elevated CO2 concentrations can alleviate the impact of infect ion with Striga on the growth of C-3 hosts such as rice and also that infec tion can delay the onset of photosynthetic down-regulation in rice grown at elevated CO2.