Estimating the Excess Investment in Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase in Leaves of Spring Wheat Grown under Elevated CO2

Wheat (Triticum aestivum L.) was grown under CO₂ partial pressures of 36 and 70 Pa with two N-application regimes. Responses of photosynthesis to varying CO₂ partial pressure were fitted to estimate the maximal carboxylation rate and the nonphotorespiratory respiration rate in flag and preceding leaves. The maximal carboxylation rate was proportional to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content, and the light-saturated photosynthetic rate at 70 Pa CO₂ was proportional to the thylakoid ATP-synthase content. Potential photosynthetic rates at 70 Pa CO₂ were calculated and compared with the observed values to estimate excess investment in Rubisco. The excess was greater in leaves grown with high N application than in those grown with low N application and declined as the leaves senesced. The fraction of Rubisco that was estimated to be in excess was strongly dependent on leaf N content, increasing from approximately 5% in leaves with 1 g N m⁻² to approximately 40% in leaves with 2 g N m⁻². Growth at elevated CO₂ usually decreased the excess somewhat but only as a consequence of a general reduction in leaf N, since relationships between the amount of components and N content were unaffected by CO₂. We conclude that there is scope for improving the N-use efficiency of C₃ crop species under elevated CO₂ conditions.