Infection with the parasitic angiosperm Striga hermonthica influences the response of the C_3 cereal Oryza sativa to elevated CO_2

摘要

Upland rice (Oryza sativa L.) was grown at both ambient (350 #mu#mol mol~-1) and elevated (700 #mu#mole 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 plantsgrown 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-l,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.