Physiologic and metabolic responses of wheat seedlings to elevated and super-elevated carbon dioxide

The metabolic consequence of suboptimal (400 μmol mol⁻¹ or ppm), near-optimal (1500 ppm) and supra-optimal (10,000 ppm) atmospheric carbon dioxide concentrations CO₂] was investigated in an attempt to reveal plausible underlying mechanisms for the differential physiological and developmental responses to increasing CO₂]. Both non-targeted and targeted metabolite profiling by GC–MS and LC–MS were employed to examine primary and secondary metabolites in wheat (Triticum aestivum, cv Yocoro rojo) continuously exposed to these CO₂] levels for 14, 21 and 28 days. Metabolite profile was altered by both CO₂] and physiological age. In general, plants grown under high CO₂] exhibited a metabolite profile characteristic of older plants under ambient CO₂. Elevated CO₂] resulted in higher levels of phosphorylated sugar intermediates, though no clear trend in the content of reducing sugars was observed. Transient starch content was enhanced by increasing CO₂] to a much greater extent at 10,000 ppm CO₂ than at 1500 ppm CO₂. The percentage increase of starch content resulting from CO₂ enrichment declined as plants develope. In contrast, elevated CO₂] promoted the accumulation of secondary metabolites (flavonoids) progressively to a greater extent as plants became mature. Elevated CO₂] to 1500 ppm induced a higher initial growth rate, while super-elevated CO₂] appeared to negate such initial growth promotion. However, after 4 weeks, there was no difference in vegetative growth between 1500 and 10,000 ppm CO₂-grown plants, both elevated CO₂ levels resulted in an overall 25% increase in biomass over the control plants. More interestingly, elevated atmospheric CO₂] reduced evapotranspiration rate (ET), but further increase to the supra-optimal level resulted in increased ET (a reversed trend), i.e. ET at 1500 ppm < ET at 10,000 ppm < ET at 400 ppm. The differential effect of elevated and super-elevated CO₂ on plants was further reflected in the nitrogen dynamics. These results provide the potential metabolic basis for the differential productivity and stomatal function of plants grown under elevated and super-elevated CO₂ levels.